How to Use a Simple Multimeter
DEAR TIM: I’ve got a few perplexing problems at my house and have no idea how to solve them. A friend tried to change 3-way switches for me at the top and bottom of my steps and now they don’t work right. My doorbell all of a sudden stopped working. A friend at work said I could buy an inexpensive digital multimeter to help me troubleshoot these vexing electrical problems saving me lots of money. Is this true? Are the meters hard to use? Are they dangerous? - Michelle C., Riverside, CA
DEAR MICHELLE: Your friend is correct. You can purchase a very useful tool called a multimeter. I have several and professionals use these tools everyday in their work. They can save you enormous amounts of frustration and can give you the information you need to know when dealing with invisible electricity.
The multimeters are not hard to use. They all come with two probes that have pointed metal tips. One of the probes is black and the other one is red. This is by design. The black probe connects to the common port on the multimeter and the red one connects to the V or port with the Greek Omega symbol. The probes and cables are identical, the color just helps you and a professional connect them to the right places.
Multimeters are like any tool. There are simple ones that are homeowner grade and then there are ones that are typically used only by professionals. As you might expect, the pro-grade multimeters can cost in the hundreds of dollars while you can get a homeowner meter for less than $10.00!
The homeowner multimeters are great at measuring three things around your home. The first thing they’ll measure is both AC and DC voltage. The tools also allow you to check for continuity. You absolutely must have this capability in the tool you purchase as you need to check for that to diagnose both of your problems. Finally, the multimeters measure resistance.
The multimeters are not dangerous. However, if you use one to check household AC voltages of 120 or 240 volts, you need to be alert, paying attention, and trained in what to do. It’s just a common sense issue. You wouldn’t want to touch an energized bare wire and you most certainly don’t want to cause a dead short or be working around energized wires without realizing you could get shocked by a mistake or you might cause a dead short that would send molten metal into your eyes or skin. Always wear safety glasses when working with a multimeter around energized wires.
It’s not hard to get the training to discover how to use the tools safely. You can watch videos online or you can ask for some help from an electrician or ham radio hobbyist that uses these tools all the time. I highly recommend you get some training and practice how to use the tools.
The easiest and safest way to train with a multimeter, in my opinion, is to just take a standard AA battery and check its voltage. Many of the modern multimeters can automatically sense the difference between AC and DC voltage. Simple household batteries are DC.
All you have to do is set the multimeter to check voltage, usually there’s a large capital “V” on a dial or a button. The LCD screen on the tool should show a V on the screen and some zeros.
Touch the end of the red probe to the positive end of the battery and the black probe to the negative end of the battery. If all is well, you’ll see a measurement of 1.5 on the LCD screen of the tool. That’s what most AA batteries put out, 1.5 volts DC current.
When checking for continuity, the multimeters are sensing the amount of resistance in the circuit. If you have a short piece of solid wire with each end bare and set the multimeter to check for continuity, when you touch the probes to the end of the wire, there’s very little resistance and the multimeter might emit a tone, a small light lights up or you see a bunch of zeros and a tiny resistance reading like 0.3.
You’ll use the multimeter to help you check the continuity and voltage of the tiny low voltage wires that are connected to the transformer that powers your doorbell. You can also use the multimeter to check the continuity and voltage of the wires at your 3-way switches so you can get them working correctly.
To understand how to properly wire your 3-way switch, I suggest you watch a video I taped about this topic. It shows you all the wires and exactly how they should be connected at each switch. Go to: 3-Way Switch Video