Resistance measuring ohms converter circuit employing a constant current source

An ohms converter circuit for measuring the resistance of an unknown resistor Rx which monitors the actual output current of the current source employed therein to ensure its accuracy. The ohms converter circuit employs a highly stable current source which is protected against high voltage overloads. A 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 verify the magnitude of the current of predetermined value Iref. The ohms converter establishes a loop circuit in which the current of known magnitude Iref passes 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 determines 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 also passes serially through the unknown resistor Rx, and a voltage measuring circuit measures the voltage across Rx to provide a precise determination thereof.

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.

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.