low ohms adaptor

Multimeter Low Ohms Adapt or Introduction Most digital multimeters (DMMs) have problems reading low resistances. The Fluke 179 for example (not an inexpensive meter) has a resistance accuracy of +/-1% which sounds ok but it also has an uncertainty of +/-2 least significant digits. For example, when measuring a 0.8 Ohm resistance with the Fluke 179 the resistance value displayed could be anywhere between 0.6 and 1.0 Ohms as the meter's resolution (least significant digit) is 0.1 Ohms on the lowest range. The following device will enable even cheap multimeters able to accurately measure milli-Ohm resistances. The Circuit The circuit is adapted from a design by Peter Chamberlain. The circuit is based around an LM317T adjustable voltage regulator configured as a 100mA constant current source. The 100mA current is applied across a low-value resistor of unknown value (i.e. the resistance to be measured) and the resulting voltage drop can then be measured accurately by a digital multimeter. The circuit is short circuit proof as the maximum current into a short is 100mA. No on/off switch is required since no current will be drawn when no external resistance or meter is connected. NOTE: Before using the adaptor, check that your meter is not likely to be damaged by having the full output (6 to 8V+) applied when it is set to a low voltage range.

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Most digital multimeters (DMMs) have problems reading low resistances. The Fluke 179 forexample (not an inexpensive meter) has a resistance accuracy of +/-1% which sounds ok but it alsohas an uncertainty of +/-2 least significant digits.For example, when measuring a 0.8 Ohm resistance with the Fluke 179 the resistance valuedisplayed could be anywhere between 0.6 and 1.0 Ohms as the meter's resolution (least significantdigit) is 0.1 Ohms on the lowest range.The following device will enable even cheap multimeters able to accurately measure milli-Ohmresistances.

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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.