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Multimeter Low Ohms Adaptor

IntroductionMost digital multimeters (DMMs) have problems reading low resistances. The Fluke 179 ore!ample (not an ine!pensive meter) has a resistance accurac" o #$%1& which sounds ok but it also

has an uncertaint" o #$%' least signiicant digits.

For e!ample when measuring a .* +hm resistance with the Fluke 179 the resistance valuedispla"ed could be an"where between ., and 1. +hms as the meter-s resolution (least signiicant

digit) is .1 +hms on the lowest range.The ollowing device will enable even cheap multimeters able to accuratel" measure milli%+hm

resistances.

The CircuitThe circuit is adapted rom a design b" eter /hamberlain.

The circuit is based around an 0M17T ad2ustable voltage regulator conigured as a 1m3

constant current source. The 1m3 current is applied across a low%value resistor o unknown value(i.e. the resistance to be measured) and the resulting voltage drop can then be measured accuratel"

b" a digital multimeter. The circuit is short circuit proo as the ma!imum current into a short is

1m3.

4o on$o switch is re5uired since no current will be drawn when no e!ternal resistance or meter isconnected.

4+T6 8eore using the adaptor check that "our meter is not likel" to be damaged b" having the

ull output (, to *#) applied when it is set to a low voltage range.

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Construction and AlignmentThe circuit is constructed on an * b" : hole piece o eroboard. The output can be soldered to a :1connector or multimeter probes.

+nce complete ad2ust the trimpot (;1) to its mid range. Then connect the DMM and set it to D/

current range. /onnect the 9 batter" and with no resistance connected ad2ust ;1 until the meterreads 1m3.

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+nce the circuit is ad2usted connect the meter on the voltage range. The ull output voltage will be

displa"ed. This can be used as an indication o batter" condition.

/onnect a resistance and read the D/ voltage

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The resistance is calculated b" dividing the displa"ed voltage b" 1m3. +n the m range o this

meter this means dividing b" 1 or shiting the decimal point to the let two places to give .*1'+hms.