Multimeters 101

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Multimeters are used by anyone who deals with electricity. They’re very versatile and can be used to characterize an embedded system or troubleshoot a non-functioning lightbulb in your house. Let’s discuss what it is and how to use it.

What is a Multimeter?

Multimeter - also known as multitester or VOM (Volt-Ohm-Milliammeter)
Multimeter - also known as multitester or VOM (Volt-Ohm-Milliammeter)

Roughly, meter means measure and multi means many. So, a multimeter is something that measures many things. In this case, it means that it measures multiple electrical related things. Specifically, it measures:

  • Volts (DC and AC)
  • Current (DC and AC)
  • Ohms
  • Diode continuity
  • Frequency of signals
  • Many, many other, but less common, things.
Digital multimeters not just measure voltage, current and resistance but also capacitance, diode condition, frequency, transistor type and more.
Digital multimeters nowadays are not just limited to voltage, current, and resistance measurement.
They can also measure capacitance, diode condition, frequency, transistor type, and more.

You may have heard of ohmmeters (measures resistance) and ammeters (measures current) and voltmeters (measures voltage). A multimeter does all of these in a single device and they’re not only fairly common, they can be quite inexpensive.

Types of Multimeters and Features

Different types of Multimeters - Analog and Digital (Clamp, Pen type, SMD tester, Benchtop, etc.)
Different types of Multimeters - Analog and Digital (Clamp, Pen type, SMD tester, Benchtop, etc.)

Multimeters come in a variety of sizes and packages. And until the last couple of decades, all multimeters were analog but now digital multimeters are much more common. You can also find portable multimeters that are battery operated and can be taken anywhere. These are more likely to be for use by electricians but that 1) doesn’t mean they still can’t be very useful with electronics and 2) have lots of exceptions of portable multimeters that are perfect for lab usage. More commonly in the lab, though, will be a benchtop multimeter that is wall powered. Again, these are generalizations, but benchtop multimeters tend to be more expensive and “better” - though they can be better in different ways.

So high level options we have are:

  • Analog multimeters
  • Digital multimeters

and

  • Benchtop
  • Handheld

If you spend more on a multimeter, what should you expect to get? Depending on what you’re looking for, you can get any of the following things:

Digital multimeters can have features like data logging, USB interface, etc.
Digital multimeters can come with more features like data logging, USB interface, etc.
  • More features
    • RMS Voltages
    • LCR readings, meaning measuring the values of inductors, capacitors, and resistors (this is quite specialized)
    • Autoranging, meaning the multimeter can switch ranges such as microvolts to kilovolts (this is pretty common)
  • Higher precision
  • Higher accuracy and/or certified calibration
  • More stability across temperatures ranges and as it ages
  • Faster response times
  • Less effect on the DUT

How to use a Multimeter

When measuring resistance, connect the probes of the multimeter in parallel with the resistor or load and ensure that you're not touching the probes or load.
When measuring resistance, connect the probes of the multimeter in parallel with the resistor or load that you're testing and make sure you're not touching the probes or the load to get a more accurate result.
  • For everything except measuring current, you will place the probes across the circuit you’re testing. Place the probes on both sides of what you’re trying to measure. Watch what ports your probes are plugged into - you want them in the high-impedance ports where minimal current will flow through the multimeter. If you happen to have the probes plugged into the low-impedance ports and connect it to a low-impedance source, you will blow a fuse or destroy your multimeter. Please be careful.
Ensure that the probes are in the right socket, especially when measuring high voltages.
Make sure that the probes are in the right socket, especially when measuring high voltages.
  • For measuring current, you want to put your probes in series with the circuit you’re testing. That means you’ll want to be on one side or the other of the circuit you’re testing - either watching the current flow in or flow out. In this case, you will have your probes connected to the low-impedance port - it will say something along the lines of “A” “mA” “μA”.
When measuring current, the multimeter must be connected in series with the load or circuit being tested
When measuring current, the multimeter must be connected in series with the load or circuit being tested. Also, you will notice that most multimeters have a separate socket for larger currents.

Important notes:

Like any measurement device, a multimeter also changes the device under test (DUT).

  • In high-impedance mode, it’s very large, but the impedance is still finite. This means that you will be affecting the voltage and resistance measurements. Again, very slightly, but if you need extreme accuracy and precision, you may notice it.
  • In low-impedance mode, it’s very small, but the impedance is still there. If you’re measuring current (which, really, is the only time you should be in low-impedance mode) then the minimal resistance will slightly decrease the current you’re reading. Very slightly.

Measurement of devices in circuitry

  • You probably don’t want to measure things that are hooked up to power. Not only will it make the measurements practically useless, it will also be dangerous.
  • Measuring devices while they’re in the circuit, even unpowered, could give very confusing and/or useless readings. Pay attention to what nodes you’re testing and the others connections to that node. Do a common-sense test of whether or not you’re wasting your time.

Electrical outlets - again.

  • Electrical outlets are hooked up to mains and are a low-impedance output, meaning they can source a lot of current. Electrical outlets and batteries. Amplifiers, etc.
  • Don’t hook up a low-impedance load to a low-impedance output unless you want a huge amount of current, fire and death. At the least, blown fuses.
  • Why do I care? When I was a kid, my older brother melted the probes on his multimeter because he had it setup to measure current and then stuck them into the electrical outlet. I was too young to understand what he had done wrong but it terrified me for a decade or two until I learned how to use a multimeter myself. I’m trying to transfer this awareness of danger to you. If you pay attention and know what you’re doing, though, you’ll be perfectly safe.
  • Something that we do, and is generally a good plan, is to never leave your probes plugged into their low-impedance ports. After we make a current measurement and are putting away a multimeter, we switch the probes back to the high-impedance ports. Just in case we’re not paying attention the next time we use the multimeter.

Summary

That should be it! The most important thing about using a multimeter is surviving the experience. Whenever dealing with electrical outlets (mains voltage), please be very careful and if you do not feel confident about what you’re doing, don’t do it. That being said, play around with batteries and low voltage sources at first and you’ll be fine. You can still spark and melt things with batteries but you won’t kill yourself unless you light something on fire. Okay, I’m just going to be quiet now.

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