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A Milliohm Adaptor For Digital Multimeters

Do you occasionally need to measure very low resistances accurately but don't have access to an expensive benchtop Milliohm Meter or DMM? This low-cost adaptor will let you use almost any DMM to make accurate low-resistance measurements.

By Jim Rowe

When it comes to measuring low resistances (ie, below about 10Ω) with any significant accuracy, very few standard handheld digital multimeters are of much use. Only the top-of-the-range models offer any real performance in this area.

And when you want to measure even lower resistances – less than one ohm – even some of these drop out of contention. It’s really only the most expensive benchtop models that will provide milliohm-level measurements as a matter of course.

Click for larger image
The top diagram (Fig.1a) shows the way resistance is measured in "normal" meters (ie, two-terminal). The lower diagram (Fig.1b) shows how higher accuracy is achieved with four-terminal measurement, especially for low resistances. This is the approach taken in this adaptor.

This doesn’t pose much of a problem for most of us, most of the time, because accurate low-value resistance measurements are not needed very often.

But sometimes you do: matching the values of low-value resistors used for current sharing in amplifier output stages, for example, or when you need to make up a low resistance current shunt for a panel meter.

That’s when you need this Milliohm Meter Adaptor. It’s self-contained and designed to act as a very low resistance measuring ‘front end’ for almost any standard DMM.

It works by converting low resistance values into a directly proportional DC voltage (nominally 0-1.000V), so the DMM is simply set for its 1V or 2V DC voltage range, the range where most DMMs have their highest accuracy.

So when the adaptor is being used to measure a very low resistance, the resistance value is simply read out on the DMM in millivolts.

Actually the adaptor provides two measurement ranges, one a ‘0-1.0Ω’ range where it converts milliohms directly into millivolts (so 125mΩbecomes 125mV, for example) and the other a ‘0-10Ω’ range where it converts tens of milliohms into millivolts – so 2.2Ω (ie, 2200mΩ) becomes 220mV.

So reading the low value resistances on your DMM doesn’t require much mental arithmetic.

Now at this stage you’re probably thinking this: if a low-cost adaptor like the one we’re describing here can make this kind of very low resistance measurement relatively easily, why don’t most DMMs provide such ranges?

That’s because there is a catch: in order to measure low resistances accurately, you have to use a four-terminal measurement approach rather than the two-terminal approach used in the majority of DMMs.

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