Low-Ohm Meter

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If you need a rough check on the value of a resistors the ohms ranges of your analogue multimeter will provide it, so long as the resistance value is between 10 ohms and one or two megohms, but the non-linear scales reduce the accuracy of the higher readings. A digital multimeter will provide greater accuracy - perhaps 1% but neither meter can be used for values below 1 ohm. It may be argued that one does not very often need to measure such values, but the emitter resistors of output transistors in power amplifiers fall into this category, and so do many coil and transformer resistances. It is also useful to be able to check the resistance between switch contacts, especially if you suspect that a switch is faulty.

There are a number of ways of measuring low resistance, and for many years I used a shunt ohmmeter with mid-scale readings of 10 and 1 ohms on its two ranges. I described this in a workshop article in the January 1984 Newsletter (No. 72).

On the ohms ranges of a normal multimeter the unknown resistance is connected in series with a battery and the meter and the scale reads backwards. A variable resistor is included in the circuit so that the meter can be adjusted to read full scale with the test terminals shorted (Fig 1). In the shunt ohmmeter a battery and variable resistor are connected across a milliammeter and the resistor is adju ted so that the meter reads full scale with the test terminals unconnected (Fig.2). Any resistance across the test terminals will bypass some current so the meter reading will fall. No commercial meter that I have encountered uses this circuit. In my case the shunt ohmmeter uses the two lower ranges of a three range milliammeter (Fig 3).

P11_F1 P11_F2 P11_F3
In April 1981 a design for a low ohmmeter by Ray Marston was published in "Electronics Today International". I built a modified version of this meter and found it very satisfactory. It will measure resistance from 100 ohms down to a few milliohms, and has a linear scale. A short while ago I was asked about measuring low resistance by a B.A.E.C. member and recommended this circuit to him. I thought it might interest other members so I obtained permission from the editor of ETI to publish details in the Newsletter - and here they are.

The meter contains two independent and independently powered circuits: a multi-range constant current generator and a D.C. millivoltmeter with a full scale sensitivity of 10mV. The generator is used to apply a known fixed current to the resistor being measured, and the millivoltmeter measures the voltage developed across the resistor, eliminating the effects of the test leads.

The complete circuit is shown in Fig 4. A 5V regulated supply feeds the constant current generator and the output test current is determined by resistors R1 to R4. On each range the value of Rx is very low in relation to the current limiting resistor, and the full scale test voltage (lOmV) is very small compared with the 5V regulated supply. As a result, the test current is virtually independent of the effects of lead resistance's etc.; on the most sensitive range (lOOmV) one ohm of lead resistance will introduce a maximum full scale error of 2%. On the other ranges the effect of lead resistance is negligible. In practice, the reading errors are primarily determined by the accuracy of the resistors R1 to R4.

The D.C. millivoltmeter is based on a CA3140 operational amplifier, which can respond to inputs down to zero volts. To allow the output to go slightly negative for zero setting, a -600mV supply rail is generated by R11 and D1. The sensitivity of the meter is variable over a limited range by the calibration control VR1; zero setting is provided for by the multiturn potentiometer VR2.

Components list

R1            47 ½W VR1   47K horizontal preset
R2           470    VR2   10K multiturn preset
R3, 10, 11   4K7 
R4,  6       47K     C1   330n polyester 
R5            1M     C2    10n polyester
R7           82K      
R8            1K    IC1   78L05
R9           10K    IC2   CA3140 
D1        1N4148     S1   1 pole 4 way rotary switch
Resistors R1 to R4 should preferably be of 1% accuracy.			
P12_F6 P12_F5
The PCB track layout is shown in Fig.5, and Fig.6 shows the component positions I used a 1¼" multiturn pot for VR2 but the commoner size is ¾", which would require slight repositioning of the pads for this component. R1 to R4 can conveniently be mounted on S1.

If batteries are used to power the meter, B1 must be capable of providing a current of l00mA on the lowest range, and a PP9 is recommended. The op-amp current is only around 5mA so here a PP3 would suffice. To power my version of this meter I use a power supply which provides 5V and 9V regulated supplies; in this case all the components to the left of the dotted line in Figs. 4, 5 and 6 can be omitted. The 5V supply should be connected at the points marked x.

The meter M should have a full scale reading of 1V; the 1v range of a multimeter is perfectly satisfactory. If the meter has a sensitivity of 10K/V or better, R9 should be l0K as shown; for lower sensitivities R9 should have twice the ohms-per-volt value of the meter.

The various flexible leads from the ohmmeter should be terminated appropriately. Those for connection to B1 and B2 should have battery clips (or suitable plugs if a mains power supply is used). The MS leads can have plugs to fit the meter sockets. Probably the simplest way to terminate the V and I leads is to use crocodile clips, though these are liable to come off at a vital moment.

When construction is finished, connect batteries or power supply to the unit and connect the M leads to a 1V D.C meter or multimeter. Short the V leads together, and adjust VR2 for a zero reading on the meter. Then turn S1 to the 100 ohm range and connect the I leads across a 100 ohm resistor (1% if possible). Connect the V leads across the resistor on the resistor side of the I leads, with like polarities, and adjust VR1 to get a full scale reading on the meter. The unit is now calibrated and ready for use on all ranges.