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How To Use A Multimeter To Troubleshoot Your Sensor

multimeter dial closeupYou’re having problems with the signal from your sensor. Maybe it only works occasionally, maybe there’s too much noise to establish a strong connection, or maybe you just don’t know what is wrong. Grab your multimeter, and let’s see what we can figure out.

What’s that? You’re not sure what a multimeter is, or how to use it? Let’s take a quick look then. After all, we’ve got a sensor to troubleshoot.

What is a Multimeter?

A multimeter is an electrical instrument that is used to test circuits. Multimeters can measure voltage, current, resistance, and continuity, thus the name: multi-meter. A multimeter is crucial for troubleshooting. When a circuit or device malfunctions, testing for continuity (i.e., is the circuit continuous from source to sensor and back) and measuring voltage/current/resistance can help locate and identify problems.

On a multimeter you will find several settings available to test for different areas. The most common settings are:

  • for current, both alternating (AC) and direct (DC), measuring from micro- or milli-amps to amps;
  • for voltage, both AC and DC, measuring from millivolts to hundreds of volts;
  • for resistance, measuring from ohms to megaohms.

More advanced models have additional settings to measure capacitance, decibels, frequency, inductance and/or temperature.

How does a Multimeter work?

Magic miniature elves.

Or not. We haven’t been able to reach them for comment.

Until we hear from the elves, we’ll have to assume that multimeters are designed using fundamental electric circuit theory. (I know, it’s nowhere near as fun as magic elves.) Ohm’s Law states the fixed relationship between voltage, current, and resistance between any two points in a circuit: I=V/R (i.e., current is equal to voltage divided by resistance). Multimeters, like any good math student, use two known quantities to solve for the third, unknown, quantity:

  • To measure resistance, the change in voltage created by a small current is measured.
  • To measure voltage, the movement created by a quantifiably small current through known resistance is measured.
  • To measure current, a similar movement is measured through a resistance at a specific ratio to the current in question.

Further quantities mentioned above (capacitance, etc.) are measured using similar techniques.

How is a Multimeter used?

So, you’ve got your multimeter in your hands. Now what? Let’s run three simple tests that will help us pinpoint the problem.

We’ll start with Circuit Continuity. We want to make sure that all the wires are connected correctly.

sample circuit for multimeter test connections

  1. Disconnect the wires for the sensor at its power source (Point A in the diagram).
  2. Plug the black probe into the COM (common) port on your multimeter. Plug the red probe into the VΩ port.
  3. Set your multimeter to Continuity - the symbol looks a bit like this: •))).
  4. Connect the red probe to the + wire going to the sensor, and connect the black probe to the ground wire going to the sensor.
  5. Note: Communication wiring is often more complicated than a + wire and a – wire, and will vary according to the output of your sensor, and your control system. Please consult your sensor’s user manual or manufacturer for more information.

  6. If the multimeter registers a reading, your circuit wiring is intact. If the multimeter does not register a reading, then there is something wrong with the wiring. Repeat these steps along the various sections of the circuit between the source and sensor to isolate the problem.
  7. This process can (and should!) also be performed with your sensor’s communication wiring.

Having established the continuity of the circuit, let’s check the source voltage, but not at the source.

  1. Reconnect the sensor’s power source.
  2. Disconnect the power wires at the sensor (Point C in the diagram) or connection point closest to the sensor (Point B, if the cable to your sensor cannot be disconnected at the sensor).
  3. Maintain the same probe – multimeter connections.
  4. Connect the red probe to the incoming + wire, pin, or terminal, and the black probe to the ground wire/pin/terminal.
  5. Select the DCV value on the multimeter that is closest to, yet bigger than, the source voltage.
  6. Turn on the power source.
  7. Verify that the voltage at the sensor is within the range suggested in your user manual. If so, we’ve eliminated source voltage as the problem. If not, the voltage source is at least a problem, if not the problem. (And either way, turn the power source back off!)

Next, we’ll check circuit impedance or resistance*. In general, circuit impedance is only critical for communication circuits (Modbus, Hart, etc.), but checking can still be instructive for other circuits.

  1. Reconnect the power wires at the sensor.
  2. Disconnect the communication wires for the sensor at the source (Point A).
  3. Maintain the same probe – multimeter connections.
  4. As before, connect the red probe to the + wire going to the sensor, and connect the black probe to the ground wire going to the sensor.
  5. Many sensors that use communication protocols require a minimum of 150Ω to 180Ω, so choose the Ohm value on the multimeter that is closest to, yet bigger than, 200Ω. If the circuit impedance is less than that recommended by your user manual, then add an appropriate amount of resistance to the circuit.
  6. If the multimeter doesn’t register the impedance, select the next highest denomination of Ohms. If the circuit’s impedance is too high (and not infinite), something will need to be removed from the circuit (switch to a smaller wire size, too many intermediate junctions, etc).

If these steps have not helped you identify and isolate the problem, there may well be a problem with your sensor. Contact your vendor or manufacturer for help with your sensor. Of course, we’re always available to support our customers. Let us know if we can do anything for you.

*Yes, I am aware that there is a difference between impedance and resistance (X=R+jωL). However, I’m also aware that the difference is only critical for AC circuitry at high frequency. But even for this DC circuit, the total opposition to current flow is called impedance rather than resistance.


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top photo credit: Andrew Mason via flickr cc

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