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Re: DCmA readings of Multimeter

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Posted by Indiana Ken on November 11, 2012 at 18:17:44 from (

In Reply to: DCmA readings of Multimeter posted by Dell (WA) on November 11, 2012 at 16:51:30:

You cannot do much with 200 ma around a tractor. Many VOMs have a 10 amp scale where you put the red wire in a socket labeled 10 Amp. If your meter does not have that you can use what is called a shunt. The shunt is placed in the circuit you wish to measure the current draw (amps) and the meter is used to read the voltage drop across the shunt in milli-volts. The current is calculated using Ohms law.

It is not that hard: To make a shunt take a length of 14 Ga Romex (house wiring) cable and remove one of the wires. You will need a length of 47.5" for the shunt and add two inches at each end to allow for connecting into the various circuits you want to measure. Strip the insulation for 2.5" at each end and mark two places exactly 47.5" apart on the bare wire. The marked positions are where the meter leads are placed.

The resistance of 47.5" of 14 Ga wire is 10 milli-ohms, this is your shunt. Ohms law states: I = V / R, where I is the current in amps, R is the resistance in milli-ohms and V is the voltage in milli-volts measured across the shunt. For example: Lets assume you measure a voltage of 10 milli-volts across the shunt. From ohms law, I = 10 milli-volts / 10 milli-ohms which equals a current of 1 amp for the circuit being measured. In other words each 10 milli-volts equals 1 amp, therefore 200 milli-volts would indicate a current of 20 amps, for example.

To use the shunt simply wire it in the head light circuit, for example, and turn on the lights. Set the meter on the milli-volt scale, place the meter leads at the 47.5" marks and read the milli-volts. Calculate the amps. using ohms law.

One nice thing about the shunt is if you place it in a dead short or a very high ampere circuit by mistake the shunt may be toast but your meter is not harmed. If the shunt becomes hot, you have too high an ampere circuit for the shunt.

Good Luck.


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