Measuring a 4-20mA signal without blowing the fuse in your meter

Published on Apr 12, 2021
The first step to measuring a 4-20mA signal without blowing your fuse is understanding how it works.

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A 4 to 20 mA loop will have a output device that produces or regulates a milliamp loop and an input that will read the value. When using a 4-20mA source the positive terminal of the output will go to the positive terminal of the input and the negative terminal of the input will go back to the negative terminal of the analog output.

Examples of outputs could be the position of a valve, a flow meter, temperature, pH, or ORP. Pretty much anything where you need to make a measurement that isn't simply on/off or good/not good. Examples of inputs could be a PLC, display meter, data logger, or a speed control on a drive.

Where a 4-20 mA loop gets tricky is that you can have multiple input devices connected to an output as long as they are all in the loop. So say the input on the right is a PLC and someone wanted a local display. They would cut this wire then connect the plus of the milliamp output to the plus of the local display then connect the minus of the local display to the plus of the next input device.

When troubleshooting a milliamp circuit your first inclination will be to grab your amp meter. It makes sense but this is made to measure big amps, in this ones case, big amps. We need to measure milliamps which is a much smaller meter. If you google “4-20mA meter” then you are going to come up with some expensive solutions such as this Fluke 772. But don't fret. You typical affordable meter such as this one can measure a 4-20mA signal.

One thing you must pay close attention to when testing a mA signal is down near the terminal post where it says fused. Nearly all volt meters are fused and the post common way to blow a fuse in a volt meter is when checking milliamps.

I've heard of instructors that would give students a lower grade if they blew the fuse in their meters. Students, if this happened to you then sorry about your luck but trust me, you just got a valuable lesson that will help you in the real world. Instructors, if you are doing this will you please stop. If your budget doesn't allow for you have replacement fuses then call us and we will donate some to you.

Now before we can understand how to prevent blowing fuses in our volt meter we need to understand how they get blown. So we take our positive terminal of our volt meter and connect it to the positive terminal of our mA output just like we typically would when checking voltage. Then we take the minus of our voltmeter and connect it to the minus terminal of our volt meter. When reading mA on our voltmeter there is a small fuse in the circuit and for our purposes there is no other resistance in the circuit so we are dealing with a short circuit. So the moment touch these two leads to the circuit we blow the fuse in the meter.

In order to properly test a 4 to 20 mA loop you must be part of the loop. So instead of connecting it across the circuit you must remove one of the wires. It could the positive or the negative wire. Then connect your meter through the loop. Then your meter will only see between 4 and 20 milliamps preventing the fuse from blowing.

Now we are ready to measure our milliamp signal. First we will turn our volt meter to mA, we'll make sure we have the leads in the proper position for measuring a milliamp signal, then we will turn on our SIM-ALP2 which will generate a 4-20 mA signal.

We will select a current source and we see that the black post is our common so we will connect it to our multimeters black lead and the red post is our PLC input or in this case the red lead of our meter.

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