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United States Patent
4,808,914
Talmor
February 28, 1989
Resistance measuring ohms converter circuit employing a constant
current source
Abstract
Inventors:
Talmor; Shlomo S. (Syosset, NY)
Assignee:
Harris Corporation (Melbourne, FL)
Appl. No.:
788236
Filed:
October 17, 1985
Current U.S. Class:
324/705; 324/607; 324/710
Intern'l Class:
G01R 027/02
Field of Search:
324/62,64 307/296 R,297
References Cited [Referenced By]
U.S. Patent Documents
2995704
Aug., 1961
Norgaard
324/62.
3365662
Jan., 1968
Wereb, Jr.
3633098
Apr., 1972
Westlund.
3646436
Feb., 1972
Chan et al.
3711850
Jan., 1973
Kelly.
3919601
Nov., 1978
Suko et al.
3978472
Aug., 1976
Jones.
4088949
May., 1978
Goldish et al.
4092591
May., 1978
Lozowski.
4104578
Aug., 1978
Thuot.
4109196
Aug., 1978
Carmody.
4228294
Oct., 1980
Crosby.
4242631
Dec., 1980
Hall.
4323972
Apr., 1982
Winter.
4349777
Sep., 1982
Mitamura
324/62.
Primary Examiner: Eisenzopf; Reinhard J.
Assistant Examiner: Snow; Walter E.
Attorney, Agent or Firm: Scully, Scott, Murphy & Presser
Claims
What is claimed is:
1. An ohms converter circuit for measuring
an unknown resistor Rx by passing a current of known magnitude Iref therethrough
and measuring the voltage drop thereacross, comprising:
a. a scaler
resistor Rscaler, coupled in series with the unknown resistor Rx, having the
current of known magnitude Iref passing therethrough to develop a voltage drop
thereacross;
b. a floating potential connected in a loop circuit;
c. a differential amplifier coupled with the voltage across the scaler
resistor Rscaler placed across the output and one differential input of the
differential amplifier and the voltage across the floating potential placed
across the output and a second differential input of the differential amplifier
to form said loop circuit, to differentially measure the voltages across the
scaler resistor and said floating potential and provide an output indicative
thereof which determines Iref, such that the voltage drop across the floating
potential and the current Iref through said scaler resistor remains constant at
a known magnitude, said current of known magnitude Iref also passing serially
through the unknown resistor Rx;
d. an inverting amplifier coupled
between the output of said differential amplifier and a commom coupling to said
scaler resistor Rscaler and said floating potential, for providing enhanced
voltage compliance and for voltage isolation in the event of the accidental
application of a high voltage to the circuit;
e. a voltage measuring
means for measuring the voltage across Rx to provide a precise measurement of
Rx, by knowing Vx and Ix which is equal to Iref;
f. a reference resistor
Rref coupled in series with the unknown resistor Rx; and
g. means for
switching said voltage measuring means across either the unknown resistor Rx or
said reference resistor Rref, for measuring the voltage across either the
unknown resistor Rx or said reference resistor Rref, with the measurement of the
voltage across Rref providing a precise measurement of Iref, by knowing Vref and
Rref, and the measurement of the voltage across Rx providing a precise
measurement of Rx.
2. An ohms converter circuit for measuring an unknown
resistor Rx by passing a current of a known magnitude therethrough and measuring
the voltage drop thereacross, as claimed in claim 1, the unknown resistor Rx
being coupled to the ohms converter circuit through a serially connected
blocking diode which provides reverse high voltage overload protection for the
ohms converter circuit.
3. An ohms converter circuit for measuring an
unknown resistor Rx by passing a current of a known magnitude therethrough and
measuring the voltage drop thereacross, as claimed in claim 2, said loop circuit
further comprising two clamping diodes coupled between the circuit loop and
ground which provide high voltage overload protection for the loop circuit by
clamping the anodes thereof to a given positive voltage.
4. An ohms
converter circuit for measuring an unknown resistor Rx by passing a current of a
known magnitude therethrough and measuring the voltage drop thereacross, as
claimed in claim 3, further including a third clamping diode, coupled between
the unknown resistor Rx and ground, to limit the voltage compliance of the ohms
converter circuit, when an extremely large resistor or an open circuit is
presented across the input terminals where Rx is normally connected.
5.
An ohms converter circuit for measuring an unknown resistor Rx by passing a
current of a known magnitude therethrough and measuring the voltage drop
thereacross, as claimed in claim 1, said floating potential comprising a stable
current source I and a floating potential resistor Rfp coupled in series with
the stable current source I to provide a voltage across said floating potential
resistor Rfp which is placed across said second differential input of the
differential amplifier.
6. An ohms converter circuit for measuring an
unknown resistor Rx by passing a current of a known magnitude therethrough and
measuring the voltage drop thereacross, as claimed in claim 5, wherein said
inverting amplifier is a transistor amplifier, and further including a voltage
follower circuit coupled to said floating potential resistor to prevent the
current through said floating potential resistor from flowing to the collector
of said inverting amplifier which is a transistor inverting amplifier.
7. An ohms converter circuit for measuring an unknown resistor Rx by
passing a current of a known magnitude therethrough and measuring the voltage
drop thereacross, as claimed in claim 6, the unknown resistor Rx being coupled
to the ohms converter circuit through a serially connected blocking diode which
provides reverse high voltage overload protection for the ohms converter
circuit.
8. An ohms converter circuit for measuring an unknown resistor
Rx by passing a current of a known magnitude therethrough and measuring the
voltage drop thereacross, as claimed in claim 7, said loop circuit further
comprising two clamping diodes coupled between the circuit loop and ground which
provide high voltage overload protection for the loop circuit by clamping the
anodes thereof to a given positive voltage.
9. An ohms converter circuit
for measuring an unknown resistor Rx by passing a current of a known magnitude
therethrough and measuring the voltage drop thereacross, as claimed in claim 8,
further including a third clamping diode, coupled between the unknown resistor
Rx and ground, to limit the voltage compliance of the ohms converter circuit,
when an extremely large resistor or an open circuit is presented across the
input terminals where Rx is normally connected.
10. An ohms converter
circuit for measuring an unknown resistor Rx by passing a current of a known
magnitude therethrough and measuring the voltage drop thereacross, as claimed in
claim 9, said stable current source I including an MOS amplifier, and said
differential and inverting amplifiers and said voltage follower circuit also
comprising MOS circuits.
11. An ohms converter circuit for measuring an
unknown resistor Rx by passing a current of a known magnitude therethrough and
measuring the voltage drop thereacross, as claimed in claim 10, said stable
current source including a Zener diode, coupled across the inputs of said stable
current source MOS amplifier, for providing temperature compensation therefor.
12. An ohms converter circuit for measuring an unknown resistor Rx by
passing a current of a known magnitude therethrough and measuring the voltage
drop thereacross, as claimed in claim 5, wherein said inverting amplifier
comprises an inverting transistor amplifier coupled to the output of said
differential amplifier, and a voltage follower circuit coupled to said floating
potential resistor to prevent the current through said floating potential
resistor from flowing to the collector of said inverting transistor amplifier.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to an ohms converter circuit for
measuring the resistance of an unknown resistor, and more particularly pertains
to an ohms converter circuit employing a highly stable current source which is
protected against the accidental application thereto of high voltages.
2. Discussion of the Prior Art
In a digital multimeter, an ohms
measurement is commonly performed by measuring the voltage drop produced across
an unknown resistor with the application thereto of a known current flow. In
order to get an accurate reading, two measurements must be performed
simultaneously, namely the voltage drop across and the current flow through the
unknown resistor. This measurement technique generally utilizes either a sample
and hold circuit or two A/D converters. However, both approaches are generally
unacceptable because mismatches and drift create errors in the measurements.
In lieu of performing simultaneous measurements, the approach of an ohms
converter circuit can be employed, which utilizes a highly stable current
source. Most ohms converter circuits regulate the voltage drop across the
emitter resistor of a transistor, the collector of which is the output. With
this approach, the beta of the output transistor changes with temperature and
with changes of V.sub.CE, which causes I.sub.C to change, thereby creating an
inherent error. The ohms converter of the present invention eliminates this
basic design flaw by monitoring the actual output current of the current source.
SUMMARY OF THE INVENTION
Accordingly, it is a primary object of
the present invention to provide an ohms converter circuit for measuring the
resistance of an unknown resistor which monitors the actual output current of
the current source employed therein to ensure its accuracy.
A further
object of the subject invention is the provision of an ohms converter circuit as
described hereinabove which employs a highly stable current source which is
protected against high voltage overloads. A practical requirement of an ohms
converter circuit is that the circuitry should not be damaged by the accidental
application thereto of a high voltage, as during measurements. The ohms
converter of the present invention detects the application of a voltage
overload, and in response thereto shuts off the current source. This also
prevents the reference resistor from dissipating unnecessary power, thereby
prolonging its life and maintaining its accuracy. Reverse or negative voltage
overload protection for the ohms converter circuit is provided by a blocking
diode which blocks an accidental application of a negative voltage across the
input terminals from being applied to the ohms converter circuit. Additionally,
positive voltage overload protection is provided by two clamping diodes, coupled
between the circuit loop and ground, which provide high voltage overload for the
circuit loop by clamping the anodes thereof to a given positive voltage.
Additionally, a third clamping diode is provided such that when an extremely
large resistor or an open circuit is presented across the input terminals of the
circuit, the third clamping diode limits the voltage compliance of the ohms
converter.
In accordance with the teachings herein, the ohms converter
of the present invention measures an unknown resistance Rx by passing a current
of known magnitude Iref through the unknown resistance and measuring the voltage
drop thereacross, thereby allowing the resistance Rx to be determined by Ohm's
law R=V/I.
The current of known magnitude Iref is passed without
branching through a series connection of Rx and a reference resistor Rref.
Relays are utilized to switch between measurements of the voltage across Rx and
the voltage across Rref. The voltage across Rref is checked periodically to
update the precise magnitude of Iref. The exact value of Iref is not important,
but its precise value must be known and is determined by this measurement. Since
the current supply provided by the present invention is very stable, the voltage
across Rref has to be checked less frequently to verify Iref, and therefore the
relays have to be switched less frequently, resulting in longer lives therefor.
The current of known magnitude Iref is delivered by a highly stable current
source, which avoids a prior art problem with the V.sub.CE and temperature
dependence of an output transistor. The ohms converter of the present invention
eliminates this basic design problem by monitoring the actual output current of
the current source.
In accordance with the teachings herein, the ohms
converter circuit of the present invention passes the current of known magnitude
Iref through a scaler resistor to develop a voltage drop thereacross, which is
connected with a floating potential in a loop circuit. A differential amplifier
is coupled to differentially measure the voltages across the scaler resistor and
the floating potential, and provides an output indicative thereof, which
controls the generation of Iref. In this arrangement, the voltage drop across
the scaler resistor equals the voltage drop across the floating potential, and
the current Iref through the scaler resistor remains constant at a known
magnitude. The current of known magnitude Iref is passed serially through the
unknown resistor Rx, and a voltage measuring circuit measures the voltage across
Rx to provide a precise determination of Rx, by knowing Vx and Ix, which is
equal to Iref.
In greater detail, in one disclosed embodiment the
floating potential preferably comprises a stable current source I and a floating
potential resistor Rfp. An inverting amplifier is coupled to the output of the
differential amplifier for providing enhanced voltage compliance and for
isolation in the event of the accidental application of a high voltage to the
circuit. The inverting amplifier is a transistor amplifier, and a voltage
follower circuit is coupled to the floating potential resistor Rfp to prevent
the current therethrough from flowing to the collector of the transistor
inverting amplifier. Preferably, the stable current source includes an MOS
amplifier, and the differential and inverting amplifiers and the voltage
follower circuit also comprise MOS circuits.
BRIEF DESCRIPTION OF THE
DRAWINGS
The foregoing objects and advantages of the present invention
for an ohms converter circuit may be more readily understood by one skilled in
the art with reference being had to the following detailed description of
several preferred embodiments thereof, taken in conjunction with the
accompanying drawings wherein like elements are designated by identical
reference numerals throughout the several views, and in which:
FIG. 1 is
a schematic illustration of a simplified conceptual embodiment of an ohms
converter circuit constructed pursuant to the teachings of the present
invention;
FIG. 2 illustrates a conceptual embodiment of an ohms
converter circuit similar to that of FIG. 1, wherein an inverting amplifier has
been added to the output of the differential amplifier;
FIG. 3
illustrates a conceptual embodiment of an ohms converter circuit similar to that
of FIG. 2, wherein a stable current source and floating potential resistor have
replaced the floating potential battery; and
FIG. 4 is a schematic
drawing of a preferred embodiment of an ohms converter circuit, similar in
concept to FIG. 3, and illustrating fully the complete details of the circuit.
DETAILED DESCRIPTION OF THE DRAWINGS
Referring to the drawing in
detail, the ohms converter of the present invention measures an unknown
resistance Rx by passing a current of a known magnitude Iref through the unknown
resistance and measuring the voltage drop thereacross, thereby allowing the
resistance Rx to be determined by Ohm's law R=V/I.
FIG. 1 is a schematic
illustration of a simplified conceptual embodiment of an ohms converter circuit
pursuant to the teachings of the present invention. In this circuit, the voltage
across a scaler resistor Rscaler is established by a current of known magnitude
Iref passing therethrough. The voltage across the scaler resistor Rscaler is
compared with a voltage supplied by a floating potential, generated in this
embodiment by a battery B. A differential amplifier U1 compares the two voltages
as inputs thereto, and controls the generation of the current of known magnitude
Iref in accordance with the differential measurement of the voltages. In
explanation of the operation of this circuit, any increase in the voltage drop
across the scaler resistor Rscaler forces the noninverting input of U1 to become
more positive than the inverting input thereof, causing the output of U1 to
become more positive, such that the voltage drop across the scaler resistor is
less and equals the voltage drop supplied by the floating potential, such that
Iref=B/Rscaler.
The current of known magnitude Iref passes from Rscaler
without branching through a series connection of Rx and a reference resistor
Rref. Relays R are utilized to switch between measurements of the voltage across
Rx and the voltage across Rref. The voltage across Rref is checked periodically
to update the precise magnitude of Iref. The exact value of Iref is not
important, but must be known and is determined by this measurement. Since the
current supply provided by the present invention is very stable, the voltage
across Rref has to be checked less frequently to verify Iref, and therefore the
relays R have to be switched less frequently, resulting in longer lives
therefor.
FIG. 2 illustrates a conceptual embodiment of an ohms
converter circuit similar to that of FIG. 1, wherein an inverting amplifier has
been added to the output of the differential amplifier therein. In this circuit,
the inputs to the differential amplifier U1 are reversed because of the
introduction of an inverting amplifier Q1 within the circuit loop. In some
circuits, an inverting amplifier transistor Q1 is essential to provide enhanced
voltage compliance and the capability of withstanding the accidental application
of high voltages thereto.
FIG. 3 is a slightly more complex circuit than
FIG. 2 in which the battery B is replaced by a stable current source I supplying
a stable current to a floating potential resistor Rfp to provide a stable
floating potential thereacross. In this circuit, U2 is a voltage follower
preventing the current from I from flowing to the collector of Q1, thereby
providing a buffering or isolation function in the circuit.
FIG. 4 is a
schematic drawing of a preferred embodiment of an ohms converter circuit,
similar in concept to FIG. 3, and illustrating fully the complete details of the
circuit. In this embodiment, the stable current source I is provided by U3,
which is an MOS amplifier, and a temperature compensated Zener diode D1. The
stable current source I provides a stable current through the floating potential
resistor Rfp to establish a stable bias voltage drop thereacross. U3 produces a
constant current=(R3.Vz)/(R2.R6). The use of a temperature compensated Zener
reference diode D1 reduces the temperature coefficient of this current source to
just a few parts per million.
In this circuit, Iref is determined
primarily by the floating potential and the magnitude of the current scaler
resistor Rscaler, which is composed of Rscaler a and Rscaler b, and can be
selectively changed by relay switch S1. When relay switch S1 is open, Rscaler
consists of only Rscaler a which is 1 Mohms. When relay switch S1 is closed,
Rscaler comprises both Rscaler a and Rscaler b in parallel, which is
approximately equal to Rscaler b or 1 Kohms because of the much greater value of
Rscaler a. In this embodiment, Iref can be selected between approximately 0.7
mAmps and 0.7 uAmps under control of relay R1.
In the embodiment of FIG.
4, Rref is actually formed by five interconnected resistors as shown for ease of
construction, precision and switching, but could be just one resistor. In the
designed embodiment Rref is either 1 Kohms or 1 Mohms, depending upon the
desired value of Iref, which depends upon the anticipated value of Rx and the
range of the voltmeter reading the voltage across either Rref or Rx. In this
circuit, Rref consists of Rref a, which is 999 Kohms, and Rref b, which is 1
Kohms. When relay S2 is open, Rref is Rref a and Rref b, and when relay S2 is
closed, Rref=Rref b only, such that Rref is precisely 1 Kohms or 1 Mohms.
Switching by relay S2 is provided in order to maintain a low voltage drop over
Rref. If, for instance, Rref=1 Mohms, and Iref=0.7 uAmps, then the voltage drop
across Rref=0.7 volts, and when Iref is switched to 0.7 mAmps, the voltage over
Rref will equal 700 volts. Therefore, it is desirable to scale Rref when Iref is
scaled, such that relays S1 and S2 are switched simultaneously, as indicated by
the dashed connection, with both being on or both being off.
Relays S3
and S4 are utilized to switch between measurements of the voltage across Rx and
the voltage across Rref. The voltage across Rref is checked periodically to
verify the precise magnitude of the current of predetermined value Iref. Since
the current supply provided by the present invention is very stable, the voltage
across Rref has to be checked less frequently to verify Iref, and therefor the
relays S3 and S4 have to be switched less frequently, resulting in longer lives
therefor. As illustrated, the voltmeter preferably includes an A to D converter
for providing a digital output reading.
An inverting amplifier Q1 is
connected in the loop circuit for providing the current of known magnitude Iref
through the scaler resistor Rscaler, and establishes a voltage drop IrefRscaler
thereacross equal to the voltage drop across Rfp. The inverting amplifier Q1 is
coupled to the output of the differential amplifier U1 for providing enhanced
voltage compliance and for isolation in the event of the accidental application
of a high voltage to the circuit. The inverting amplifier is a transistor
amplifier, and a voltage follower circuit U2 is coupled to the floating
potential resistor Rfp to prevent the current therethrough from flowing to the
collector of the transistor inverting amplifier Q1.
The ohms converter
of the present invention detects the application of a voltage overload, and in
response thereto shuts off Q1. This also prevents the reference resistor Rref
from dissipating unnecessary power, thereby prolonging its life and maintaining
its accuracy. Negative or reverse voltage overload protection for the ohms
converter circuit is provided b a blocking diode D6, which blocks an application
of a negative voltage to the circuit. Additionally, positive voltage overload
protection is provided by two clamping diodes D3 and D4, coupled between the
circuit loop and ground, which protect against positive high voltage overloads
for the circuit loop by clamping the anodes thereof to a given positive voltage.
Diodes D3 and D4 are low leakage (less than 1 pico amp.) diodes. A further
clamping diode D5 is provided such that when Rx is an extremely large
resistance, e.g. hundreds of Mohms, or an open circuit is provided instead of
Rx, clamping diode D5 prevents the voltage at node 50 from being pulled down
below -4.7 V to the lower potential of -9.9 V, and thereby possibly damaging a
circuit under test. Diode D5 has its anode at -4.3 V, and will start conducting
with a -0.4 V thereacross, such that the cathode thereof is clamped to -4.7 V.
In summary, the scaler resistor Rscaler is connected in a loop circuit
in which the voltage drop across Rscaler is maintained equal to the voltage drop
across a floating potential resistor Rfp. The resistor Rfp is connected to a
stable constant current generator, such that the voltage drop across Rfp is
maintained constant, and the voltage drop across Rscaler is thereby maintained
constant, which results in a highly stable current through Rscaler. For reasons
explained hereinbelow, the current through Rscaler does not branch at node 50 or
otherwise, but proceeds as Iref through diode D6, through Rx and Rref.
The inputs of U1, U2 and U4 are MOS FET transistors, so the currents
through R12 and R11 are very small, making the voltage drop over R12 and R11
less than 0.4 uV.
Therefore:
V70=V40 (voltage at node 70 equals
voltage at node 40)
V100=V50
U2 is a voltage follower with no
voltage drop thereacross, therefore:
V80=V70
V90=V80+RfpIfp
RfpIfp=0.7 V
U1 loop is closed via R10, Q1, Rscaler and R12 and
it stabilized when:
V100=V90
which can also realized as:
V50=V90
V50=V80+0.7 V V50=V40+0.7 V V50-V40=0.7 V
so
when Rscaler=1 Kohms, the circuit will output 0.7/1000=0.7 mA, and when
Rscaler=1 Mohms, the circuit will output 0.7/1000=0.7 uA.
Q1, D5 and D6
are the only high voltage withstanding components, selected to withstand a
minimum of 350 volts at the input terminals which are also the digital
multimeter input terminals. D6 will block an application of -5 to -350 volts.
When a positive voltage is applied to the output terminals, D6 conducts,
and since the current source initially continues to operate, it will require
over 1.2 volts to bring V50 to a voltage more positive than ground. U4 is used
as a comparator, and will flip to output -9.9 V (in normal operation V50 is
always more negative than ground so the U4 output is at +4.7 V), node 180 will
change from +2.35 V to -4.95 V, D2 will conduct and node 70 will follow node 180
to -4.3 V. Node 90 will then be -4.3 V+0.65 V=3.65 V, with +0.4 V at the inv.
input and -3.65 V at the noninv. input, the output of U1 will go all the way to
the negative rail, and Q1 will be turned off.
The maximum power that
Rref will be dissipating, in the Mohms range with +350 V at the MDMM input, is
((350/(1000+360,000+l,000,000)).sup.2.1,000,000=66 mW.
In ranges where 1
Kohms is being utilized, V50 follows the input with a minimal voltage drop over
Rref, hence V50 can get as high as 350 V, therefore D5 is a high voltage diode
with low reverse leakage, less than 500 pA at 25.degree. C. and full reverse
voltage.
In accordance with the teachings herein, the present invention
provides an ohms converter circuit designed to produce an output constant
current of 0.7 mA or 0.7 uA, with a voltage compliance of 4.7 Volts. The
constant current flows through a reference resistor Rref (1 Kohms or 1 Mohms)
with no possible current branching (Iscaler=Iref).
The current source is
electrically and electrostatically isolated (floating), and can withstand an
application of up to +/-350 volts to its output terminals.
The circuit
eliminates any possible current branching between Rscaler and Rx, and prevents
damages to Rref when a high voltage is connected to the input of the digital
multimeter (DMM) while in an ohms measurement mode.
While several
preferred embodiments of the present invention for an ohms converter circuit are
described in detail herein, it should be apparent that the disclosure and
teachings of the present invention will suggest many alternative designs to
those skilled in the art.
* * * * *
United States Patent
4,808,914
Talmor
February 28, 1989
Abstract
Inventors:
Talmor; Shlomo S. (Syosset, NY) | ||
|---|---|---|
Assignee: |
Harris Corporation (Melbourne, FL) |
|
Appl. No.: |
788236 |
|
Filed: |
October 17, 1985 |
Current U.S. Class: |
324/705; 324/607; 324/710 |
Intern'l Class: |
G01R 027/02 |
Field of Search: |
324/62,64 307/296 R,297 |
| 2995704 | Aug., 1961 |
Norgaard |
324/62. |
| 3365662 | Jan., 1968 |
Wereb, Jr. |
|
| 3633098 | Apr., 1972 |
Westlund. |
|
| 3646436 | Feb., 1972 |
Chan et al. |
|
| 3711850 | Jan., 1973 |
Kelly. |
|
| 3919601 | Nov., 1978 |
Suko et al. |
|
| 3978472 | Aug., 1976 |
Jones. |
|
| 4088949 | May., 1978 |
Goldish et al. |
|
| 4092591 | May., 1978 |
Lozowski. |
|
| 4104578 | Aug., 1978 |
Thuot. |
|
| 4109196 | Aug., 1978 |
Carmody. |
|
| 4228294 | Oct., 1980 |
Crosby. |
|
| 4242631 | Dec., 1980 |
Hall. |
|
| 4323972 | Apr., 1982 |
Winter. |
|
| 4349777 | Sep., 1982 |
Mitamura |
324/62. |
Primary Examiner: Eisenzopf; Reinhard J.
Assistant Examiner: Snow; Walter E.
Attorney, Agent or Firm: Scully, Scott, Murphy & Presser
Claims
What is claimed is:
1. An ohms converter circuit for measuring
an unknown resistor Rx by passing a current of known magnitude Iref therethrough
and measuring the voltage drop thereacross, comprising:
a. a scaler
resistor Rscaler, coupled in series with the unknown resistor Rx, having the
current of known magnitude Iref passing therethrough to develop a voltage drop
thereacross;
b. a floating potential connected in a loop circuit;
c. a differential amplifier coupled with the voltage across the scaler
resistor Rscaler placed across the output and one differential input of the
differential amplifier and the voltage across the floating potential placed
across the output and a second differential input of the differential amplifier
to form said loop circuit, to differentially measure the voltages across the
scaler resistor and said floating potential and provide an output indicative
thereof which determines Iref, such that the voltage drop across the floating
potential and the current Iref through said scaler resistor remains constant at
a known magnitude, said current of known magnitude Iref also passing serially
through the unknown resistor Rx;
d. an inverting amplifier coupled
between the output of said differential amplifier and a commom coupling to said
scaler resistor Rscaler and said floating potential, for providing enhanced
voltage compliance and for voltage isolation in the event of the accidental
application of a high voltage to the circuit;
e. a voltage measuring
means for measuring the voltage across Rx to provide a precise measurement of
Rx, by knowing Vx and Ix which is equal to Iref;
f. a reference resistor
Rref coupled in series with the unknown resistor Rx; and
g. means for
switching said voltage measuring means across either the unknown resistor Rx or
said reference resistor Rref, for measuring the voltage across either the
unknown resistor Rx or said reference resistor Rref, with the measurement of the
voltage across Rref providing a precise measurement of Iref, by knowing Vref and
Rref, and the measurement of the voltage across Rx providing a precise
measurement of Rx.
2. An ohms converter circuit for measuring an unknown
resistor Rx by passing a current of a known magnitude therethrough and measuring
the voltage drop thereacross, as claimed in claim 1, the unknown resistor Rx
being coupled to the ohms converter circuit through a serially connected
blocking diode which provides reverse high voltage overload protection for the
ohms converter circuit.
3. An ohms converter circuit for measuring an
unknown resistor Rx by passing a current of a known magnitude therethrough and
measuring the voltage drop thereacross, as claimed in claim 2, said loop circuit
further comprising two clamping diodes coupled between the circuit loop and
ground which provide high voltage overload protection for the loop circuit by
clamping the anodes thereof to a given positive voltage.
4. An ohms
converter circuit for measuring an unknown resistor Rx by passing a current of a
known magnitude therethrough and measuring the voltage drop thereacross, as
claimed in claim 3, further including a third clamping diode, coupled between
the unknown resistor Rx and ground, to limit the voltage compliance of the ohms
converter circuit, when an extremely large resistor or an open circuit is
presented across the input terminals where Rx is normally connected.
5.
An ohms converter circuit for measuring an unknown resistor Rx by passing a
current of a known magnitude therethrough and measuring the voltage drop
thereacross, as claimed in claim 1, said floating potential comprising a stable
current source I and a floating potential resistor Rfp coupled in series with
the stable current source I to provide a voltage across said floating potential
resistor Rfp which is placed across said second differential input of the
differential amplifier.
6. An ohms converter circuit for measuring an
unknown resistor Rx by passing a current of a known magnitude therethrough and
measuring the voltage drop thereacross, as claimed in claim 5, wherein said
inverting amplifier is a transistor amplifier, and further including a voltage
follower circuit coupled to said floating potential resistor to prevent the
current through said floating potential resistor from flowing to the collector
of said inverting amplifier which is a transistor inverting amplifier.
7. An ohms converter circuit for measuring an unknown resistor Rx by
passing a current of a known magnitude therethrough and measuring the voltage
drop thereacross, as claimed in claim 6, the unknown resistor Rx being coupled
to the ohms converter circuit through a serially connected blocking diode which
provides reverse high voltage overload protection for the ohms converter
circuit.
8. An ohms converter circuit for measuring an unknown resistor
Rx by passing a current of a known magnitude therethrough and measuring the
voltage drop thereacross, as claimed in claim 7, said loop circuit further
comprising two clamping diodes coupled between the circuit loop and ground which
provide high voltage overload protection for the loop circuit by clamping the
anodes thereof to a given positive voltage.
9. An ohms converter circuit
for measuring an unknown resistor Rx by passing a current of a known magnitude
therethrough and measuring the voltage drop thereacross, as claimed in claim 8,
further including a third clamping diode, coupled between the unknown resistor
Rx and ground, to limit the voltage compliance of the ohms converter circuit,
when an extremely large resistor or an open circuit is presented across the
input terminals where Rx is normally connected.
10. An ohms converter
circuit for measuring an unknown resistor Rx by passing a current of a known
magnitude therethrough and measuring the voltage drop thereacross, as claimed in
claim 9, said stable current source I including an MOS amplifier, and said
differential and inverting amplifiers and said voltage follower circuit also
comprising MOS circuits.
11. An ohms converter circuit for measuring an
unknown resistor Rx by passing a current of a known magnitude therethrough and
measuring the voltage drop thereacross, as claimed in claim 10, said stable
current source including a Zener diode, coupled across the inputs of said stable
current source MOS amplifier, for providing temperature compensation therefor.
12. An ohms converter circuit for measuring an unknown resistor Rx by
passing a current of a known magnitude therethrough and measuring the voltage
drop thereacross, as claimed in claim 5, wherein said inverting amplifier
comprises an inverting transistor amplifier coupled to the output of said
differential amplifier, and a voltage follower circuit coupled to said floating
potential resistor to prevent the current through said floating potential
resistor from flowing to the collector of said inverting transistor amplifier.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to an ohms converter circuit for
measuring the resistance of an unknown resistor, and more particularly pertains
to an ohms converter circuit employing a highly stable current source which is
protected against the accidental application thereto of high voltages.
2. Discussion of the Prior Art
In a digital multimeter, an ohms
measurement is commonly performed by measuring the voltage drop produced across
an unknown resistor with the application thereto of a known current flow. In
order to get an accurate reading, two measurements must be performed
simultaneously, namely the voltage drop across and the current flow through the
unknown resistor. This measurement technique generally utilizes either a sample
and hold circuit or two A/D converters. However, both approaches are generally
unacceptable because mismatches and drift create errors in the measurements.
In lieu of performing simultaneous measurements, the approach of an ohms
converter circuit can be employed, which utilizes a highly stable current
source. Most ohms converter circuits regulate the voltage drop across the
emitter resistor of a transistor, the collector of which is the output. With
this approach, the beta of the output transistor changes with temperature and
with changes of V.sub.CE, which causes I.sub.C to change, thereby creating an
inherent error. The ohms converter of the present invention eliminates this
basic design flaw by monitoring the actual output current of the current source.
SUMMARY OF THE INVENTION
Accordingly, it is a primary object of
the present invention to provide an ohms converter circuit for measuring the
resistance of an unknown resistor which monitors the actual output current of
the current source employed therein to ensure its accuracy.
A further
object of the subject invention is the provision of an ohms converter circuit as
described hereinabove which employs a highly stable current source which is
protected against high voltage overloads. A practical requirement of an ohms
converter circuit is that the circuitry should not be damaged by the accidental
application thereto of a high voltage, as during measurements. The ohms
converter of the present invention detects the application of a voltage
overload, and in response thereto shuts off the current source. This also
prevents the reference resistor from dissipating unnecessary power, thereby
prolonging its life and maintaining its accuracy. Reverse or negative voltage
overload protection for the ohms converter circuit is provided by a blocking
diode which blocks an accidental application of a negative voltage across the
input terminals from being applied to the ohms converter circuit. Additionally,
positive voltage overload protection is provided by two clamping diodes, coupled
between the circuit loop and ground, which provide high voltage overload for the
circuit loop by clamping the anodes thereof to a given positive voltage.
Additionally, a third clamping diode is provided such that when an extremely
large resistor or an open circuit is presented across the input terminals of the
circuit, the third clamping diode limits the voltage compliance of the ohms
converter.
In accordance with the teachings herein, the ohms converter
of the present invention measures an unknown resistance Rx by passing a current
of known magnitude Iref through the unknown resistance and measuring the voltage
drop thereacross, thereby allowing the resistance Rx to be determined by Ohm's
law R=V/I.
The current of known magnitude Iref is passed without
branching through a series connection of Rx and a reference resistor Rref.
Relays are utilized to switch between measurements of the voltage across Rx and
the voltage across Rref. The voltage across Rref is checked periodically to
update the precise magnitude of Iref. The exact value of Iref is not important,
but its precise value must be known and is determined by this measurement. Since
the current supply provided by the present invention is very stable, the voltage
across Rref has to be checked less frequently to verify Iref, and therefore the
relays have to be switched less frequently, resulting in longer lives therefor.
The current of known magnitude Iref is delivered by a highly stable current
source, which avoids a prior art problem with the V.sub.CE and temperature
dependence of an output transistor. The ohms converter of the present invention
eliminates this basic design problem by monitoring the actual output current of
the current source.
In accordance with the teachings herein, the ohms
converter circuit of the present invention passes the current of known magnitude
Iref through a scaler resistor to develop a voltage drop thereacross, which is
connected with a floating potential in a loop circuit. A differential amplifier
is coupled to differentially measure the voltages across the scaler resistor and
the floating potential, and provides an output indicative thereof, which
controls the generation of Iref. In this arrangement, the voltage drop across
the scaler resistor equals the voltage drop across the floating potential, and
the current Iref through the scaler resistor remains constant at a known
magnitude. The current of known magnitude Iref is passed serially through the
unknown resistor Rx, and a voltage measuring circuit measures the voltage across
Rx to provide a precise determination of Rx, by knowing Vx and Ix, which is
equal to Iref.
In greater detail, in one disclosed embodiment the
floating potential preferably comprises a stable current source I and a floating
potential resistor Rfp. An inverting amplifier is coupled to the output of the
differential amplifier for providing enhanced voltage compliance and for
isolation in the event of the accidental application of a high voltage to the
circuit. The inverting amplifier is a transistor amplifier, and a voltage
follower circuit is coupled to the floating potential resistor Rfp to prevent
the current therethrough from flowing to the collector of the transistor
inverting amplifier. Preferably, the stable current source includes an MOS
amplifier, and the differential and inverting amplifiers and the voltage
follower circuit also comprise MOS circuits.
BRIEF DESCRIPTION OF THE
DRAWINGS
The foregoing objects and advantages of the present invention
for an ohms converter circuit may be more readily understood by one skilled in
the art with reference being had to the following detailed description of
several preferred embodiments thereof, taken in conjunction with the
accompanying drawings wherein like elements are designated by identical
reference numerals throughout the several views, and in which:
FIG. 1 is
a schematic illustration of a simplified conceptual embodiment of an ohms
converter circuit constructed pursuant to the teachings of the present
invention;
FIG. 2 illustrates a conceptual embodiment of an ohms
converter circuit similar to that of FIG. 1, wherein an inverting amplifier has
been added to the output of the differential amplifier;
FIG. 3
illustrates a conceptual embodiment of an ohms converter circuit similar to that
of FIG. 2, wherein a stable current source and floating potential resistor have
replaced the floating potential battery; and
FIG. 4 is a schematic
drawing of a preferred embodiment of an ohms converter circuit, similar in
concept to FIG. 3, and illustrating fully the complete details of the circuit.
DETAILED DESCRIPTION OF THE DRAWINGS
Referring to the drawing in
detail, the ohms converter of the present invention measures an unknown
resistance Rx by passing a current of a known magnitude Iref through the unknown
resistance and measuring the voltage drop thereacross, thereby allowing the
resistance Rx to be determined by Ohm's law R=V/I.
FIG. 1 is a schematic
illustration of a simplified conceptual embodiment of an ohms converter circuit
pursuant to the teachings of the present invention. In this circuit, the voltage
across a scaler resistor Rscaler is established by a current of known magnitude
Iref passing therethrough. The voltage across the scaler resistor Rscaler is
compared with a voltage supplied by a floating potential, generated in this
embodiment by a battery B. A differential amplifier U1 compares the two voltages
as inputs thereto, and controls the generation of the current of known magnitude
Iref in accordance with the differential measurement of the voltages. In
explanation of the operation of this circuit, any increase in the voltage drop
across the scaler resistor Rscaler forces the noninverting input of U1 to become
more positive than the inverting input thereof, causing the output of U1 to
become more positive, such that the voltage drop across the scaler resistor is
less and equals the voltage drop supplied by the floating potential, such that
Iref=B/Rscaler.
The current of known magnitude Iref passes from Rscaler
without branching through a series connection of Rx and a reference resistor
Rref. Relays R are utilized to switch between measurements of the voltage across
Rx and the voltage across Rref. The voltage across Rref is checked periodically
to update the precise magnitude of Iref. The exact value of Iref is not
important, but must be known and is determined by this measurement. Since the
current supply provided by the present invention is very stable, the voltage
across Rref has to be checked less frequently to verify Iref, and therefore the
relays R have to be switched less frequently, resulting in longer lives
therefor.
FIG. 2 illustrates a conceptual embodiment of an ohms
converter circuit similar to that of FIG. 1, wherein an inverting amplifier has
been added to the output of the differential amplifier therein. In this circuit,
the inputs to the differential amplifier U1 are reversed because of the
introduction of an inverting amplifier Q1 within the circuit loop. In some
circuits, an inverting amplifier transistor Q1 is essential to provide enhanced
voltage compliance and the capability of withstanding the accidental application
of high voltages thereto.
FIG. 3 is a slightly more complex circuit than
FIG. 2 in which the battery B is replaced by a stable current source I supplying
a stable current to a floating potential resistor Rfp to provide a stable
floating potential thereacross. In this circuit, U2 is a voltage follower
preventing the current from I from flowing to the collector of Q1, thereby
providing a buffering or isolation function in the circuit.
FIG. 4 is a
schematic drawing of a preferred embodiment of an ohms converter circuit,
similar in concept to FIG. 3, and illustrating fully the complete details of the
circuit. In this embodiment, the stable current source I is provided by U3,
which is an MOS amplifier, and a temperature compensated Zener diode D1. The
stable current source I provides a stable current through the floating potential
resistor Rfp to establish a stable bias voltage drop thereacross. U3 produces a
constant current=(R3.Vz)/(R2.R6). The use of a temperature compensated Zener
reference diode D1 reduces the temperature coefficient of this current source to
just a few parts per million.
In this circuit, Iref is determined
primarily by the floating potential and the magnitude of the current scaler
resistor Rscaler, which is composed of Rscaler a and Rscaler b, and can be
selectively changed by relay switch S1. When relay switch S1 is open, Rscaler
consists of only Rscaler a which is 1 Mohms. When relay switch S1 is closed,
Rscaler comprises both Rscaler a and Rscaler b in parallel, which is
approximately equal to Rscaler b or 1 Kohms because of the much greater value of
Rscaler a. In this embodiment, Iref can be selected between approximately 0.7
mAmps and 0.7 uAmps under control of relay R1.
In the embodiment of FIG.
4, Rref is actually formed by five interconnected resistors as shown for ease of
construction, precision and switching, but could be just one resistor. In the
designed embodiment Rref is either 1 Kohms or 1 Mohms, depending upon the
desired value of Iref, which depends upon the anticipated value of Rx and the
range of the voltmeter reading the voltage across either Rref or Rx. In this
circuit, Rref consists of Rref a, which is 999 Kohms, and Rref b, which is 1
Kohms. When relay S2 is open, Rref is Rref a and Rref b, and when relay S2 is
closed, Rref=Rref b only, such that Rref is precisely 1 Kohms or 1 Mohms.
Switching by relay S2 is provided in order to maintain a low voltage drop over
Rref. If, for instance, Rref=1 Mohms, and Iref=0.7 uAmps, then the voltage drop
across Rref=0.7 volts, and when Iref is switched to 0.7 mAmps, the voltage over
Rref will equal 700 volts. Therefore, it is desirable to scale Rref when Iref is
scaled, such that relays S1 and S2 are switched simultaneously, as indicated by
the dashed connection, with both being on or both being off.
Relays S3
and S4 are utilized to switch between measurements of the voltage across Rx and
the voltage across Rref. The voltage across Rref is checked periodically to
verify the precise magnitude of the current of predetermined value Iref. Since
the current supply provided by the present invention is very stable, the voltage
across Rref has to be checked less frequently to verify Iref, and therefor the
relays S3 and S4 have to be switched less frequently, resulting in longer lives
therefor. As illustrated, the voltmeter preferably includes an A to D converter
for providing a digital output reading.
An inverting amplifier Q1 is
connected in the loop circuit for providing the current of known magnitude Iref
through the scaler resistor Rscaler, and establishes a voltage drop IrefRscaler
thereacross equal to the voltage drop across Rfp. The inverting amplifier Q1 is
coupled to the output of the differential amplifier U1 for providing enhanced
voltage compliance and for isolation in the event of the accidental application
of a high voltage to the circuit. The inverting amplifier is a transistor
amplifier, and a voltage follower circuit U2 is coupled to the floating
potential resistor Rfp to prevent the current therethrough from flowing to the
collector of the transistor inverting amplifier Q1.
The ohms converter
of the present invention detects the application of a voltage overload, and in
response thereto shuts off Q1. This also prevents the reference resistor Rref
from dissipating unnecessary power, thereby prolonging its life and maintaining
its accuracy. Negative or reverse voltage overload protection for the ohms
converter circuit is provided b a blocking diode D6, which blocks an application
of a negative voltage to the circuit. Additionally, positive voltage overload
protection is provided by two clamping diodes D3 and D4, coupled between the
circuit loop and ground, which protect against positive high voltage overloads
for the circuit loop by clamping the anodes thereof to a given positive voltage.
Diodes D3 and D4 are low leakage (less than 1 pico amp.) diodes. A further
clamping diode D5 is provided such that when Rx is an extremely large
resistance, e.g. hundreds of Mohms, or an open circuit is provided instead of
Rx, clamping diode D5 prevents the voltage at node 50 from being pulled down
below -4.7 V to the lower potential of -9.9 V, and thereby possibly damaging a
circuit under test. Diode D5 has its anode at -4.3 V, and will start conducting
with a -0.4 V thereacross, such that the cathode thereof is clamped to -4.7 V.
In summary, the scaler resistor Rscaler is connected in a loop circuit
in which the voltage drop across Rscaler is maintained equal to the voltage drop
across a floating potential resistor Rfp. The resistor Rfp is connected to a
stable constant current generator, such that the voltage drop across Rfp is
maintained constant, and the voltage drop across Rscaler is thereby maintained
constant, which results in a highly stable current through Rscaler. For reasons
explained hereinbelow, the current through Rscaler does not branch at node 50 or
otherwise, but proceeds as Iref through diode D6, through Rx and Rref.
The inputs of U1, U2 and U4 are MOS FET transistors, so the currents
through R12 and R11 are very small, making the voltage drop over R12 and R11
less than 0.4 uV.
Therefore:
V70=V40 (voltage at node 70 equals
voltage at node 40)
V100=V50
U2 is a voltage follower with no
voltage drop thereacross, therefore:
V80=V70
V90=V80+RfpIfp
RfpIfp=0.7 V
U1 loop is closed via R10, Q1, Rscaler and R12 and
it stabilized when:
V100=V90
which can also realized as:
V50=V90
V50=V80+0.7 V V50=V40+0.7 V V50-V40=0.7 V
so
when Rscaler=1 Kohms, the circuit will output 0.7/1000=0.7 mA, and when
Rscaler=1 Mohms, the circuit will output 0.7/1000=0.7 uA.
Q1, D5 and D6
are the only high voltage withstanding components, selected to withstand a
minimum of 350 volts at the input terminals which are also the digital
multimeter input terminals. D6 will block an application of -5 to -350 volts.
When a positive voltage is applied to the output terminals, D6 conducts,
and since the current source initially continues to operate, it will require
over 1.2 volts to bring V50 to a voltage more positive than ground. U4 is used
as a comparator, and will flip to output -9.9 V (in normal operation V50 is
always more negative than ground so the U4 output is at +4.7 V), node 180 will
change from +2.35 V to -4.95 V, D2 will conduct and node 70 will follow node 180
to -4.3 V. Node 90 will then be -4.3 V+0.65 V=3.65 V, with +0.4 V at the inv.
input and -3.65 V at the noninv. input, the output of U1 will go all the way to
the negative rail, and Q1 will be turned off.
The maximum power that
Rref will be dissipating, in the Mohms range with +350 V at the MDMM input, is
((350/(1000+360,000+l,000,000)).sup.2.1,000,000=66 mW.
In ranges where 1
Kohms is being utilized, V50 follows the input with a minimal voltage drop over
Rref, hence V50 can get as high as 350 V, therefore D5 is a high voltage diode
with low reverse leakage, less than 500 pA at 25.degree. C. and full reverse
voltage.
In accordance with the teachings herein, the present invention
provides an ohms converter circuit designed to produce an output constant
current of 0.7 mA or 0.7 uA, with a voltage compliance of 4.7 Volts. The
constant current flows through a reference resistor Rref (1 Kohms or 1 Mohms)
with no possible current branching (Iscaler=Iref).
The current source is
electrically and electrostatically isolated (floating), and can withstand an
application of up to +/-350 volts to its output terminals.
The circuit
eliminates any possible current branching between Rscaler and Rx, and prevents
damages to Rref when a high voltage is connected to the input of the digital
multimeter (DMM) while in an ohms measurement mode.
While several
preferred embodiments of the present invention for an ohms converter circuit are
described in detail herein, it should be apparent that the disclosure and
teachings of the present invention will suggest many alternative designs to
those skilled in the art.
* * * * *
1. An ohms converter circuit for measuring an unknown resistor Rx by passing a current of known magnitude Iref therethrough and measuring the voltage drop thereacross, comprising:
a. a scaler resistor Rscaler, coupled in series with the unknown resistor Rx, having the current of known magnitude Iref passing therethrough to develop a voltage drop thereacross;
b. a floating potential connected in a loop circuit;
c. a differential amplifier coupled with the voltage across the scaler resistor Rscaler placed across the output and one differential input of the differential amplifier and the voltage across the floating potential placed across the output and a second differential input of the differential amplifier to form said loop circuit, to differentially measure the voltages across the scaler resistor and said floating potential and provide an output indicative thereof which determines Iref, such that the voltage drop across the floating potential and the current Iref through said scaler resistor remains constant at a known magnitude, said current of known magnitude Iref also passing serially through the unknown resistor Rx;
d. an inverting amplifier coupled between the output of said differential amplifier and a commom coupling to said scaler resistor Rscaler and said floating potential, for providing enhanced voltage compliance and for voltage isolation in the event of the accidental application of a high voltage to the circuit;
e. a voltage measuring means for measuring the voltage across Rx to provide a precise measurement of Rx, by knowing Vx and Ix which is equal to Iref;
f. a reference resistor Rref coupled in series with the unknown resistor Rx; and
g. means for switching said voltage measuring means across either the unknown resistor Rx or said reference resistor Rref, for measuring the voltage across either the unknown resistor Rx or said reference resistor Rref, with the measurement of the voltage across Rref providing a precise measurement of Iref, by knowing Vref and Rref, and the measurement of the voltage across Rx providing a precise measurement of Rx.
2. An ohms converter circuit for measuring an unknown resistor Rx by passing a current of a known magnitude therethrough and measuring the voltage drop thereacross, as claimed in claim 1, the unknown resistor Rx being coupled to the ohms converter circuit through a serially connected blocking diode which provides reverse high voltage overload protection for the ohms converter circuit.
3. An ohms converter circuit for measuring an unknown resistor Rx by passing a current of a known magnitude therethrough and measuring the voltage drop thereacross, as claimed in claim 2, said loop circuit further comprising two clamping diodes coupled between the circuit loop and ground which provide high voltage overload protection for the loop circuit by clamping the anodes thereof to a given positive voltage.
4. An ohms converter circuit for measuring an unknown resistor Rx by passing a current of a known magnitude therethrough and measuring the voltage drop thereacross, as claimed in claim 3, further including a third clamping diode, coupled between the unknown resistor Rx and ground, to limit the voltage compliance of the ohms converter circuit, when an extremely large resistor or an open circuit is presented across the input terminals where Rx is normally connected.
5. An ohms converter circuit for measuring an unknown resistor Rx by passing a current of a known magnitude therethrough and measuring the voltage drop thereacross, as claimed in claim 1, said floating potential comprising a stable current source I and a floating potential resistor Rfp coupled in series with the stable current source I to provide a voltage across said floating potential resistor Rfp which is placed across said second differential input of the differential amplifier.
6. An ohms converter circuit for measuring an unknown resistor Rx by passing a current of a known magnitude therethrough and measuring the voltage drop thereacross, as claimed in claim 5, wherein said inverting amplifier is a transistor amplifier, and further including a voltage follower circuit coupled to said floating potential resistor to prevent the current through said floating potential resistor from flowing to the collector of said inverting amplifier which is a transistor inverting amplifier.
7. An ohms converter circuit for measuring an unknown resistor Rx by passing a current of a known magnitude therethrough and measuring the voltage drop thereacross, as claimed in claim 6, the unknown resistor Rx being coupled to the ohms converter circuit through a serially connected blocking diode which provides reverse high voltage overload protection for the ohms converter circuit.
8. An ohms converter circuit for measuring an unknown resistor Rx by passing a current of a known magnitude therethrough and measuring the voltage drop thereacross, as claimed in claim 7, said loop circuit further comprising two clamping diodes coupled between the circuit loop and ground which provide high voltage overload protection for the loop circuit by clamping the anodes thereof to a given positive voltage.
9. An ohms converter circuit for measuring an unknown resistor Rx by passing a current of a known magnitude therethrough and measuring the voltage drop thereacross, as claimed in claim 8, further including a third clamping diode, coupled between the unknown resistor Rx and ground, to limit the voltage compliance of the ohms converter circuit, when an extremely large resistor or an open circuit is presented across the input terminals where Rx is normally connected.
10. An ohms converter circuit for measuring an unknown resistor Rx by passing a current of a known magnitude therethrough and measuring the voltage drop thereacross, as claimed in claim 9, said stable current source I including an MOS amplifier, and said differential and inverting amplifiers and said voltage follower circuit also comprising MOS circuits.
11. An ohms converter circuit for measuring an unknown resistor Rx by passing a current of a known magnitude therethrough and measuring the voltage drop thereacross, as claimed in claim 10, said stable current source including a Zener diode, coupled across the inputs of said stable current source MOS amplifier, for providing temperature compensation therefor.
12. An ohms converter circuit for measuring an unknown resistor Rx by passing a current of a known magnitude therethrough and measuring the voltage drop thereacross, as claimed in claim 5, wherein said inverting amplifier comprises an inverting transistor amplifier coupled to the output of said differential amplifier, and a voltage follower circuit coupled to said floating potential resistor to prevent the current through said floating potential resistor from flowing to the collector of said inverting transistor amplifier.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to an ohms converter circuit for measuring the resistance of an unknown resistor, and more particularly pertains to an ohms converter circuit employing a highly stable current source which is protected against the accidental application thereto of high voltages.
2. Discussion of the Prior Art
In a digital multimeter, an ohms measurement is commonly performed by measuring the voltage drop produced across an unknown resistor with the application thereto of a known current flow. In order to get an accurate reading, two measurements must be performed simultaneously, namely the voltage drop across and the current flow through the unknown resistor. This measurement technique generally utilizes either a sample and hold circuit or two A/D converters. However, both approaches are generally unacceptable because mismatches and drift create errors in the measurements.
In lieu of performing simultaneous measurements, the approach of an ohms converter circuit can be employed, which utilizes a highly stable current source. Most ohms converter circuits regulate the voltage drop across the emitter resistor of a transistor, the collector of which is the output. With this approach, the beta of the output transistor changes with temperature and with changes of V.sub.CE, which causes I.sub.C to change, thereby creating an inherent error. The ohms converter of the present invention eliminates this basic design flaw by monitoring the actual output current of the current source.
SUMMARY OF THE INVENTION
Accordingly, it is a primary object of
the present invention to provide an ohms converter circuit for measuring the
resistance of an unknown resistor which monitors the actual output current of
the current source employed therein to ensure its accuracy.
A further
object of the subject invention is the provision of an ohms converter circuit as
described hereinabove which employs a highly stable current source which is
protected against high voltage overloads. A practical requirement of an ohms
converter circuit is that the circuitry should not be damaged by the accidental
application thereto of a high voltage, as during measurements. The ohms
converter of the present invention detects the application of a voltage
overload, and in response thereto shuts off the current source. This also
prevents the reference resistor from dissipating unnecessary power, thereby
prolonging its life and maintaining its accuracy. Reverse or negative voltage
overload protection for the ohms converter circuit is provided by a blocking
diode which blocks an accidental application of a negative voltage across the
input terminals from being applied to the ohms converter circuit. Additionally,
positive voltage overload protection is provided by two clamping diodes, coupled
between the circuit loop and ground, which provide high voltage overload for the
circuit loop by clamping the anodes thereof to a given positive voltage.
Additionally, a third clamping diode is provided such that when an extremely
large resistor or an open circuit is presented across the input terminals of the
circuit, the third clamping diode limits the voltage compliance of the ohms
converter.
In accordance with the teachings herein, the ohms converter
of the present invention measures an unknown resistance Rx by passing a current
of known magnitude Iref through the unknown resistance and measuring the voltage
drop thereacross, thereby allowing the resistance Rx to be determined by Ohm's
law R=V/I.
The current of known magnitude Iref is passed without
branching through a series connection of Rx and a reference resistor Rref.
Relays are utilized to switch between measurements of the voltage across Rx and
the voltage across Rref. The voltage across Rref is checked periodically to
update the precise magnitude of Iref. The exact value of Iref is not important,
but its precise value must be known and is determined by this measurement. Since
the current supply provided by the present invention is very stable, the voltage
across Rref has to be checked less frequently to verify Iref, and therefore the
relays have to be switched less frequently, resulting in longer lives therefor.
The current of known magnitude Iref is delivered by a highly stable current
source, which avoids a prior art problem with the V.sub.CE and temperature
dependence of an output transistor. The ohms converter of the present invention
eliminates this basic design problem by monitoring the actual output current of
the current source.
In accordance with the teachings herein, the ohms
converter circuit of the present invention passes the current of known magnitude
Iref through a scaler resistor to develop a voltage drop thereacross, which is
connected with a floating potential in a loop circuit. A differential amplifier
is coupled to differentially measure the voltages across the scaler resistor and
the floating potential, and provides an output indicative thereof, which
controls the generation of Iref. In this arrangement, the voltage drop across
the scaler resistor equals the voltage drop across the floating potential, and
the current Iref through the scaler resistor remains constant at a known
magnitude. The current of known magnitude Iref is passed serially through the
unknown resistor Rx, and a voltage measuring circuit measures the voltage across
Rx to provide a precise determination of Rx, by knowing Vx and Ix, which is
equal to Iref.
In greater detail, in one disclosed embodiment the
floating potential preferably comprises a stable current source I and a floating
potential resistor Rfp. An inverting amplifier is coupled to the output of the
differential amplifier for providing enhanced voltage compliance and for
isolation in the event of the accidental application of a high voltage to the
circuit. The inverting amplifier is a transistor amplifier, and a voltage
follower circuit is coupled to the floating potential resistor Rfp to prevent
the current therethrough from flowing to the collector of the transistor
inverting amplifier. Preferably, the stable current source includes an MOS
amplifier, and the differential and inverting amplifiers and the voltage
follower circuit also comprise MOS circuits.
BRIEF DESCRIPTION OF THE
DRAWINGS
The foregoing objects and advantages of the present invention
for an ohms converter circuit may be more readily understood by one skilled in
the art with reference being had to the following detailed description of
several preferred embodiments thereof, taken in conjunction with the
accompanying drawings wherein like elements are designated by identical
reference numerals throughout the several views, and in which:
FIG. 1 is
a schematic illustration of a simplified conceptual embodiment of an ohms
converter circuit constructed pursuant to the teachings of the present
invention;
FIG. 2 illustrates a conceptual embodiment of an ohms
converter circuit similar to that of FIG. 1, wherein an inverting amplifier has
been added to the output of the differential amplifier;
FIG. 3
illustrates a conceptual embodiment of an ohms converter circuit similar to that
of FIG. 2, wherein a stable current source and floating potential resistor have
replaced the floating potential battery; and
FIG. 4 is a schematic
drawing of a preferred embodiment of an ohms converter circuit, similar in
concept to FIG. 3, and illustrating fully the complete details of the circuit.
DETAILED DESCRIPTION OF THE DRAWINGS
Referring to the drawing in
detail, the ohms converter of the present invention measures an unknown
resistance Rx by passing a current of a known magnitude Iref through the unknown
resistance and measuring the voltage drop thereacross, thereby allowing the
resistance Rx to be determined by Ohm's law R=V/I.
FIG. 1 is a schematic
illustration of a simplified conceptual embodiment of an ohms converter circuit
pursuant to the teachings of the present invention. In this circuit, the voltage
across a scaler resistor Rscaler is established by a current of known magnitude
Iref passing therethrough. The voltage across the scaler resistor Rscaler is
compared with a voltage supplied by a floating potential, generated in this
embodiment by a battery B. A differential amplifier U1 compares the two voltages
as inputs thereto, and controls the generation of the current of known magnitude
Iref in accordance with the differential measurement of the voltages. In
explanation of the operation of this circuit, any increase in the voltage drop
across the scaler resistor Rscaler forces the noninverting input of U1 to become
more positive than the inverting input thereof, causing the output of U1 to
become more positive, such that the voltage drop across the scaler resistor is
less and equals the voltage drop supplied by the floating potential, such that
Iref=B/Rscaler.
The current of known magnitude Iref passes from Rscaler
without branching through a series connection of Rx and a reference resistor
Rref. Relays R are utilized to switch between measurements of the voltage across
Rx and the voltage across Rref. The voltage across Rref is checked periodically
to update the precise magnitude of Iref. The exact value of Iref is not
important, but must be known and is determined by this measurement. Since the
current supply provided by the present invention is very stable, the voltage
across Rref has to be checked less frequently to verify Iref, and therefore the
relays R have to be switched less frequently, resulting in longer lives
therefor.
FIG. 2 illustrates a conceptual embodiment of an ohms
converter circuit similar to that of FIG. 1, wherein an inverting amplifier has
been added to the output of the differential amplifier therein. In this circuit,
the inputs to the differential amplifier U1 are reversed because of the
introduction of an inverting amplifier Q1 within the circuit loop. In some
circuits, an inverting amplifier transistor Q1 is essential to provide enhanced
voltage compliance and the capability of withstanding the accidental application
of high voltages thereto.
FIG. 3 is a slightly more complex circuit than
FIG. 2 in which the battery B is replaced by a stable current source I supplying
a stable current to a floating potential resistor Rfp to provide a stable
floating potential thereacross. In this circuit, U2 is a voltage follower
preventing the current from I from flowing to the collector of Q1, thereby
providing a buffering or isolation function in the circuit.
FIG. 4 is a
schematic drawing of a preferred embodiment of an ohms converter circuit,
similar in concept to FIG. 3, and illustrating fully the complete details of the
circuit. In this embodiment, the stable current source I is provided by U3,
which is an MOS amplifier, and a temperature compensated Zener diode D1. The
stable current source I provides a stable current through the floating potential
resistor Rfp to establish a stable bias voltage drop thereacross. U3 produces a
constant current=(R3.Vz)/(R2.R6). The use of a temperature compensated Zener
reference diode D1 reduces the temperature coefficient of this current source to
just a few parts per million.
In this circuit, Iref is determined
primarily by the floating potential and the magnitude of the current scaler
resistor Rscaler, which is composed of Rscaler a and Rscaler b, and can be
selectively changed by relay switch S1. When relay switch S1 is open, Rscaler
consists of only Rscaler a which is 1 Mohms. When relay switch S1 is closed,
Rscaler comprises both Rscaler a and Rscaler b in parallel, which is
approximately equal to Rscaler b or 1 Kohms because of the much greater value of
Rscaler a. In this embodiment, Iref can be selected between approximately 0.7
mAmps and 0.7 uAmps under control of relay R1.
In the embodiment of FIG.
4, Rref is actually formed by five interconnected resistors as shown for ease of
construction, precision and switching, but could be just one resistor. In the
designed embodiment Rref is either 1 Kohms or 1 Mohms, depending upon the
desired value of Iref, which depends upon the anticipated value of Rx and the
range of the voltmeter reading the voltage across either Rref or Rx. In this
circuit, Rref consists of Rref a, which is 999 Kohms, and Rref b, which is 1
Kohms. When relay S2 is open, Rref is Rref a and Rref b, and when relay S2 is
closed, Rref=Rref b only, such that Rref is precisely 1 Kohms or 1 Mohms.
Switching by relay S2 is provided in order to maintain a low voltage drop over
Rref. If, for instance, Rref=1 Mohms, and Iref=0.7 uAmps, then the voltage drop
across Rref=0.7 volts, and when Iref is switched to 0.7 mAmps, the voltage over
Rref will equal 700 volts. Therefore, it is desirable to scale Rref when Iref is
scaled, such that relays S1 and S2 are switched simultaneously, as indicated by
the dashed connection, with both being on or both being off.
Relays S3
and S4 are utilized to switch between measurements of the voltage across Rx and
the voltage across Rref. The voltage across Rref is checked periodically to
verify the precise magnitude of the current of predetermined value Iref. Since
the current supply provided by the present invention is very stable, the voltage
across Rref has to be checked less frequently to verify Iref, and therefor the
relays S3 and S4 have to be switched less frequently, resulting in longer lives
therefor. As illustrated, the voltmeter preferably includes an A to D converter
for providing a digital output reading.
An inverting amplifier Q1 is
connected in the loop circuit for providing the current of known magnitude Iref
through the scaler resistor Rscaler, and establishes a voltage drop IrefRscaler
thereacross equal to the voltage drop across Rfp. The inverting amplifier Q1 is
coupled to the output of the differential amplifier U1 for providing enhanced
voltage compliance and for isolation in the event of the accidental application
of a high voltage to the circuit. The inverting amplifier is a transistor
amplifier, and a voltage follower circuit U2 is coupled to the floating
potential resistor Rfp to prevent the current therethrough from flowing to the
collector of the transistor inverting amplifier Q1.
The ohms converter
of the present invention detects the application of a voltage overload, and in
response thereto shuts off Q1. This also prevents the reference resistor Rref
from dissipating unnecessary power, thereby prolonging its life and maintaining
its accuracy. Negative or reverse voltage overload protection for the ohms
converter circuit is provided b a blocking diode D6, which blocks an application
of a negative voltage to the circuit. Additionally, positive voltage overload
protection is provided by two clamping diodes D3 and D4, coupled between the
circuit loop and ground, which protect against positive high voltage overloads
for the circuit loop by clamping the anodes thereof to a given positive voltage.
Diodes D3 and D4 are low leakage (less than 1 pico amp.) diodes. A further
clamping diode D5 is provided such that when Rx is an extremely large
resistance, e.g. hundreds of Mohms, or an open circuit is provided instead of
Rx, clamping diode D5 prevents the voltage at node 50 from being pulled down
below -4.7 V to the lower potential of -9.9 V, and thereby possibly damaging a
circuit under test. Diode D5 has its anode at -4.3 V, and will start conducting
with a -0.4 V thereacross, such that the cathode thereof is clamped to -4.7 V.
In summary, the scaler resistor Rscaler is connected in a loop circuit
in which the voltage drop across Rscaler is maintained equal to the voltage drop
across a floating potential resistor Rfp. The resistor Rfp is connected to a
stable constant current generator, such that the voltage drop across Rfp is
maintained constant, and the voltage drop across Rscaler is thereby maintained
constant, which results in a highly stable current through Rscaler. For reasons
explained hereinbelow, the current through Rscaler does not branch at node 50 or
otherwise, but proceeds as Iref through diode D6, through Rx and Rref.
The inputs of U1, U2 and U4 are MOS FET transistors, so the currents
through R12 and R11 are very small, making the voltage drop over R12 and R11
less than 0.4 uV.
Therefore:
V70=V40 (voltage at node 70 equals
voltage at node 40)
V100=V50
U2 is a voltage follower with no
voltage drop thereacross, therefore:
V80=V70
V90=V80+RfpIfp
RfpIfp=0.7 V
U1 loop is closed via R10, Q1, Rscaler and R12 and
it stabilized when:
V100=V90
which can also realized as:
V50=V90
V50=V80+0.7 V V50=V40+0.7 V V50-V40=0.7 V
so
when Rscaler=1 Kohms, the circuit will output 0.7/1000=0.7 mA, and when
Rscaler=1 Mohms, the circuit will output 0.7/1000=0.7 uA.
Q1, D5 and D6
are the only high voltage withstanding components, selected to withstand a
minimum of 350 volts at the input terminals which are also the digital
multimeter input terminals. D6 will block an application of -5 to -350 volts.
When a positive voltage is applied to the output terminals, D6 conducts,
and since the current source initially continues to operate, it will require
over 1.2 volts to bring V50 to a voltage more positive than ground. U4 is used
as a comparator, and will flip to output -9.9 V (in normal operation V50 is
always more negative than ground so the U4 output is at +4.7 V), node 180 will
change from +2.35 V to -4.95 V, D2 will conduct and node 70 will follow node 180
to -4.3 V. Node 90 will then be -4.3 V+0.65 V=3.65 V, with +0.4 V at the inv.
input and -3.65 V at the noninv. input, the output of U1 will go all the way to
the negative rail, and Q1 will be turned off.
The maximum power that
Rref will be dissipating, in the Mohms range with +350 V at the MDMM input, is
((350/(1000+360,000+l,000,000)).sup.2.1,000,000=66 mW.
In ranges where 1
Kohms is being utilized, V50 follows the input with a minimal voltage drop over
Rref, hence V50 can get as high as 350 V, therefore D5 is a high voltage diode
with low reverse leakage, less than 500 pA at 25.degree. C. and full reverse
voltage.
In accordance with the teachings herein, the present invention
provides an ohms converter circuit designed to produce an output constant
current of 0.7 mA or 0.7 uA, with a voltage compliance of 4.7 Volts. The
constant current flows through a reference resistor Rref (1 Kohms or 1 Mohms)
with no possible current branching (Iscaler=Iref).
The current source is
electrically and electrostatically isolated (floating), and can withstand an
application of up to +/-350 volts to its output terminals.
The circuit
eliminates any possible current branching between Rscaler and Rx, and prevents
damages to Rref when a high voltage is connected to the input of the digital
multimeter (DMM) while in an ohms measurement mode.
While several
preferred embodiments of the present invention for an ohms converter circuit are
described in detail herein, it should be apparent that the disclosure and
teachings of the present invention will suggest many alternative designs to
those skilled in the art.
* * * * *
The foregoing objects and advantages of the present invention for an ohms converter circuit may be more readily understood by one skilled in the art with reference being had to the following detailed description of several preferred embodiments thereof, taken in conjunction with the accompanying drawings wherein like elements are designated by identical reference numerals throughout the several views, and in which:
FIG. 1 is a schematic illustration of a simplified conceptual embodiment of an ohms converter circuit constructed pursuant to the teachings of the present invention;
FIG. 2 illustrates a conceptual embodiment of an ohms converter circuit similar to that of FIG. 1, wherein an inverting amplifier has been added to the output of the differential amplifier;
FIG. 3 illustrates a conceptual embodiment of an ohms converter circuit similar to that of FIG. 2, wherein a stable current source and floating potential resistor have replaced the floating potential battery; and
FIG. 4 is a schematic drawing of a preferred embodiment of an ohms converter circuit, similar in concept to FIG. 3, and illustrating fully the complete details of the circuit.
