100 AMP Multimeter Upgrade




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The goal of this ible is to show you how to make a small device called a shunt that will allow your multimeter to read amperages in the 100+ amps. I will explain the principles behind this, and you can apply them to measure any amperage you want, even in the thousands of amps if you need it (not sure where you would find that though).

Step 1: How It Works

Measuring current with a shunt is fairly simple. We start with a known resistance, and measure the voltage difference from the beginning to the end of that resistance (known as voltage sag). The formula is fairly simple,

voltage = current * resistance

You can see an example of this in my spot welder ible, but this time I wanted one I could keep separate from my build. What's special about this build though is the resistance we will be using and how cheap and easy to find it is. Normally, I was using metric volumes and metric measures of copper wires to determine what I needed to build shunts of a given resistance, but Lucien from RCGroups pointed out that if you use the imperial figures, 1000 feet of 10 Ga wire is almost exactly 1 ohm. What we get from this is that 12 inches and 1/8th of 10 gauge wire is almost exactly 0.001 ohms. When we plug this into the formula, it becomes evident that what we obtain is

mV = current

That means if we read our voltage in millivolts, whatever we read is the current going through the circuit.

If you need to go to much higher currents, a 1 GA wire will afford you about 4x the capacity, and at 10 inches will give you an easy read of 0.1 mv per amp, so the reading is almost as easy since you just multiply by 10; 40mv would be 400 amps for example.

As a side note, some might be wondering why I am saying that you can take a 100 amp measurement on wiring that has code limits of 30-55 amps (depending on the different codes). The explanation is fairly simple, it comes down to duty cycle and length. I certainly would not recommend having a circuit built around the 10 GA for 100 amps, but those ratings are meant for continuous usage and long distances. When it comes to a short duty cycle for a readout (seconds per minutes) on only 1 foot, you can probably go even higher. It's why you can have dozens of amps rushing through electronic components with a standard 1/16 pin; the distance is so short that it can use 16 gauge aluminium legs to carry the current. On the other hand, running 30+ amps through even a few feet of 16 gauge aluminium wiring would be a sure recipe for melting.

Step 2: What You Need

What you need for this is

Step 3: The Build

Make sure your wire is flat so that you get a good measurement. You can test this by rolling it and see if it wobbles. Strip away the ends of your wire so that you are left with exactly 1 foot of insulated copper. This does not include the bits we just stripped. We will ignore the 1/8th, because when we then connect our multimeter, we are likely not to be exactly flush on the sheathing.

Once that is done, install your clamps, and you are good to insert it in your circuit for testing.

Step 4: Test 1

I first tested it on my DIY 10 watt flashlight, and as expected I get a reading of 0.5 amps. So far so good, the measurements are calibrated.

As you can see, I clamped the multimeter to both ends of the insulation jacket, and I used the big clamp in line with the circuit. Normally I would not be using thin wire like that in line, I would just clamp between two large connectors. This is why I chose to set it up to go in the circuit and be tapped from there to the multimeter, rather than meant to go on the multimeter and tapped into the circuit. If you are trying to run that current through inadequate wires, it can be dangerous, but the millivolt readout on the other hand is very safe.

Step 5: Test 2

Next, I torque out a drill by misusing a 1-1/4" auger bit with it. The current reading reaches 37.2 mv or 37.2 amps as the anemic alligator wires I was using melted.

That's it. An easy add-on for your multimeter.

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    5 Discussions


    2 years ago

    The temperature coefficient: 0.393%/°C will not be helpful either :-/

    Constantan is the way to go for shunts (almost no temp. dependence and much higher resistance), if you want to be able to rely on the numbers and if some temperature dependence is OK, I usually go for 18/8 stainless (way higher resistance than copper as well).

    Have a nice day :)

    1 reply

    Reply 2 years ago

    Right, but stainless requires more than 50 cents, and is not as simple to make into getting a 1:1 readout for simplicity. I have other shunts, but this is certainly going to be my go-to for testing stuff in my workshop. Powertools, welder, and e-bikes aren't going to go belly up on a 5% misread.


    2 years ago

    I've seen these type shunts built before, and in most everyone they used exactly one foot. Reason why is because when you attach clips, you just changed the total resistance to the fact that one foot will yield a closer true results in the final annalists. But since you are merely getting a close reading anyways, a foot and a 1/16" isn't going to make a huge difference. Good project all the same. And a good safe test for such a shunt is using a vehicle headlight bulb of any type. It will demonstrate the current very well and still be safe.

    1 reply

    Reply 2 years ago

    the read out is actually going to happen where your multimeter intersects with the shunt, the voltage drop is only occurring between those two leads, so the extra resistance added by the clamps won't play into the voltage drop measurement. circtuit-------+---shunt---+------- where + is where the multimeter reads in, as long as the clamps are before and after it, it will not be part of the shunt.


    2 years ago

    #10 AWG specs:


    0.10190", 2.58826 mm

    Ohms per 1,000 feet: 0.9989

    Ohms per km: 3.276392

    Max. amps for wiring:55.0

    Good to know stuff, thanks for the post. ☺