# How to calculate shunt resistor value??



## lymph (Sep 13, 2004)

My VOM (Triplett 310) only measures up to 600mA of DC current. If I wanted it to measure up to, say, 3A, how do I calculate what value of resistor to use as a shunt? Is putting the correct resistor in parallel with the meter all there is to it?

Thanks!


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## evan9162 (Sep 13, 2004)

A better solution is to use an 0.1 ohm resistor, then measure the voltage across the resistor with the 2V scale of your DMM. With an 0.1 ohm resistor, 0.1V = 1A

Place the 0.1 ohm resistor in series (so in the current path) with whatever you're measuring. For instance, to measure the current in a Mag lite, take off the tail cap, turn on the switch, short the negative end of the cell to the end of the body tube, then measure the voltage across the 0.1 ohm resistor.


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## andrewwynn (Sep 14, 2004)

great plan... that's actually all the high-current meters do.. they just have that .1? resistor inside (also an inline fuse)... and getting the shunt on the other side of the meter leads increases the accuracy dramatically.


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## lymph (Sep 14, 2004)

Thanks for the answer - clever! I could use a 1 ohm resistor to read up to 3A on my 3v scale... Basically I'm looking to be able to measure around 1A for running Luxeon LEDs.


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## evan9162 (Sep 14, 2004)

The problem with using a 1 ohm resistor is that it causes a much larger voltage drop and will affect your circuit - causing the measurement you take with the shunt resistor to not be the same as how the circuit is behaving without the shunt (and with the tailcap in place).

A 1 ohm resistor will cause a 1V drop at 1A. A 1V drop can drastically affect the behavior of the circuit. You may measure 500mA with the shunt resistor in place, only to replace the tail cap and actually have over 1A or more of current flowing. Thus, you may measure current to be a safe level with the shunt resistor, but when the tailcap is replaced, 2-3 times as much current may flow, severely overdriving the luxeon.

Thus, it is imperative to use a very low value resistor - 0.1 ohms is perfect for this.


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## lymph (Sep 14, 2004)

OK, makes sense. The only problem I can see is that the smallest scale on my VOM is 3v, and reading around .1V (analog meter) is going to be difficult. Maybe I need to get a DMM. /ubbthreads/images/graemlins/frown.gif


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## evan9162 (Sep 14, 2004)

Sure - you can get a pretty good quality DMM for about $3-$10.

If there's a Harbor Freight around, they have DMM meters with a 10A current measuring capability for about $3 that many folks here use - they seem to be accurate enough.


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## andrewwynn (Sep 14, 2004)

1) you should definitely have a DMM.. simply silly not to.

2) that still doesn't make the best current measurement solution... I would still either use the 0.1? resistor solution or make (as i did) some very heavy-duty very short test leads

-awr


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