How does my multimeter work?

Most people who work around electricity are familiar with multimeters. They have been around since the 1920s and used to only be available in analog form with a moving needle that pointed to several scales. Originally multimeters only measured current or voltage, but with the developement of disposable batteries it became easy to also determine resistance. By using series or shunt resistances more than one range of voltage, current or resistance could be accurately measured.

Digital multimeters made measurements more accurate than their analog counterparts and multimeters are much less costly than they used to be, but much of the technology and techniques remain the same. Some things to remember about the use of standard multimeters: When measuring voltage a current is passed through a known resistance to determine the voltage drop. It is important to select the correct range and AC or DC, but typically you can’t easily damage the meter or yourself unless you come into contact with the conductor. Remember that you are testing a hot circuit by necessity and take appropriate precautions. When measuring resistance you are using the power supply within the meter, a battery. As long as there is no power on the circuit or device under test, again it is difficult to damage either yourself or the the meter. Measuring current requires the disconnection of a conductor from the circuit so the meter can be placed in series with the current flow. This presents several potential problems: you have to have some idea of the amount of current you are looking for before energizing the circuit, and of course it is not always easy to place the meter in series easily. In this case it is much easier to damage both yourself and the instrument. The thing to remember is that the only limit to the current is the circuit under test, the meter places as little resistance as possible in series with the measurement.

So why is it important to know how the multimeter works? As long as it does its job, why should we care? I’d like to share an experience I had about 15 years ago. I was on a startup at a large facility in Canada. There were a lot of different typical startup and troubleshooting activities going on including powering up of different sections of the production line. I was working on my laptop next to a large multidoor enclosure when a young guy asked if he could borrow my meter. I said sure and he proceeded to begin working on something inside the cabinet. About 2 minutes later I heard a loud BOOM, the guy stepped back out of the cabinet and looked at me and his eyes were as big as golf balls. There was also a black smudge across the side of his face.

You guessed it, he had set the meter to amps and placed the leads between two phases of 480VAC. It blew the face of the meter off right by his head. It is safe to say he probably never did THAT again, but a little understanding of exactly what the meter was doing might have stopped him from doing what he did.

Be safe!