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
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.