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Low-Ohm Meter

Low-Ohm Meter

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.

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.

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