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Build A Microcurrent DMM Adaptor

You might not be aware of it but your digital multimeter is unable to make accurate current measurements in low-voltage circuits because of its "burden voltage". This precision current adaptor solves that problem and greatly improves the measurement accuracy, as well.

By David L. Jones

Don’t most multimeters already have current measurement ranges? Well, of course they do. But most multimeters, be they a no-name $10 hardware store throwaway model or a $1000 highly-accurate brand-name meter, all suffer from two rather annoying issues with their current measurement ranges – burden voltage and reduced accuracy.

The biggest problem with current measurement ranges is burden voltage. This is the voltage that the internal current shunt resistor drops as the circuit’s current passes through it.

The burden voltage is typically specified in millivolts per Amps (mV/
A). The value will change for different current ranges, so you might have 1mV/A, 1mV/mA and 1mV/μA for example.

Normally, you may not give burden voltage a second thought, as like many, you probably think it’s fairly insignificant in most applications. In fact, most people would be hard-pressed to tell you what the burden voltage of their particular multimeter actually is. It’s usually buried away in the user manual, if it’s mentioned at all. Next time you borrow a colleague’s meter, ask them what the burden voltage is, and watch their reaction!

At small displayed currents, the burden voltage is usually not an issue but at larger displayed currents (relative to full-scale) the burden voltage can be very high, even in the order of several volts! This can often force you to use a higher current range (with a lower-value shunt resistor), with subsequent loss of resolution and (often) accuracy.

You may in fact have encountered this many times, with your circuit either not working or “playing up” on too low a current range. That’s the burden voltage at work, starving your circuit of the voltage it needs to function correctly. You usually have no option but to reluctantly switch to a higher current range to lessen the effect.

The problem can also be highlighted with the many 4½-digit or “10000 count” meters on the market. In theory, they allow you to get an extra digit of resolution over a 3½-digit meter. But you may now find yourself trying to measure, for example, 990.0μA on the 1mA range with a burden voltage of just under 1V. Can your circuit really handle a 1V drop?

The burden voltage of a multimeter is determined primarily by the shunt resistor used for measurement. However, on the higher current ranges (mA & A) it also includes the protection fuse resistance and, to a much lesser extent, any switch and test lead contact resistance. Some manufacturers will specify it as a total or just the shunt resistor, or in many cases not mention it at all!

Some meters will specify it as a maximum voltage drop only. For example, “300mV max”. In this case, to get the mV/A value, you simply divide that voltage by the full-scale range current.

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