DIY Shunt Resistor Increases Amp Current Limits of MultiMeter DMM

Ingenuity reins when access to expensive tech is shunned or restricted. You may find this alternative shunt resistor useful when the amps to be measured exceed the capacity of your multimeter. Many digital multimeters (DMM) are designed to handle up to 10 amps. You may need to measure a 30A solar array or a 800A automotive starting or 90A charging system,

When you need to measure amps greater than the maximum capacity of your multimeter or DMM, you need an ammeter or shunt resistor. Shunts need to be able to carry the amps you will be measuring and should be of low resistance so as the circuit is minimally disturbed. The epiphany: Why not use a cable that is already conducting the current as a shunt resistor? Resistance of the cable we can measure or lookup.

A limitation of this po-boy shunt: is that the leads of our meter have to reach exposed ends of a cable that carries the current we want to measure. In addition you will need to know the material of the conductor, as in copper or aluminum, and its length. Then thanks to the 150 year old telegraph and Georg Simon Ohm we can use the millivolt meter of our multimeters to measure amps.

To solve for Ohm’s law, V = I x R, we need to know volts and ohms to shake out the unknown amps. Shunts provide a know constant resistance expressed in ohms. Cable resistance is proportional to the mass, length, and material of the conductor. Thank you PowerStream for this voltage drop calculator, which will tell you what voltage drop you can expect over a run of your cable. The calculator will double the cable run length you enter to account for the return cable, so you have to enter half the length of your alternative shunt resistor. You have to guess at the maximum amps you expect to measure.

For example, we have a solar array we expect to generate 28A @ 24V in full midday sun. We have 4 ft of AWG #6 copper cable between the charge controller and battery bank. So off we go to the online calculator where we enter copper #6  24V 2ft and 30 Amps. The calculator tells us to look for a voltage drop of 49 mV over 4 feet of cable when 30 amps are flowing. Now with the DMM set to 200mV scale and connected to both ends of the 4ft cable, it reads 44mV.

The simplified math uses the ratio (mV  measured / mV expected at full amps) times full amps, or (44 / 49) x 30 = 26.9 amps.

Now maybe you don’t need to sweat the decision whether to buy the $60 dc clamp ammeter or the $200 one.


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