How to Use a Multimeter to Measure Current
This article will examine and discuss how to measure current with a multimeter, using the standard in-line current measuring method.
Using a multimeter to measure current may not be the most frequently used function, but it can be very important. Current is what alerts you that something dynamic is happening in the circuit, electricity is moving. This measurement can give you critical information about the circuit, but also comes with some warnings. It can be dangerous if the meter is used improperly.
Examples of DC Current
Voltage measurements are the most common function in a multimeter for good reason. Voltage is the potential for energy to be consumed in a circuit. Yet having potential does not mean that the energy is being used properly, or used at all. Another electrical property, current, is only present when electricity is in motion. Understanding current can give insights that voltage alone cannot often show.
Multimeter-electronic measuring device for measuring current, voltage, electricity resistance, temperature, for checking and repairing electronic equipment.
As an example of where current might be a helpful property, imagine testing a failed 24-volt relay coil in a circuit. If the relay has gone bad, there is a good chance the coil is open, or infinite resistance. A voltmeter alone would show 24 volts across the coil, but that voltage would exist as well, even if the coil was in perfect working order. A quick current measurement would show 0 amps in the failed relay, but the proper current in a working device, possibly 0.1 to 0.2 amps or more.
There are often alternatives to measuring current, many of them intentionally designed. Most technicians will stay away from the current measurement and try to obtain that information with other methods. The procedure for measuring current is tricky, and if done improperly can damage the multimeter.
How Do You Measure Current With a Multimeter?
One important note first - this article is not addressing clamp or fork-style current meters. These models have an important function within industrial settings by removing many of the safety problems. But for now, this article will examine only the standard in-line current measuring method.
On the front of most meters, there will be a set of test lead connections. The black one will always be the COM or common connection point for all measurements. One red will be for voltage, resistance, and other tests. A third and usually fourth point will be reserved and marked for mA and A, or milliamp and amp current tests.
Many meters will sound an alarm if the function dial is set to the amps check, while a test lead is not in the correct port or the other way around. The connections and functions must match. If this beeping tone sounds, check to make sure the leads and the function set are correct.
Most of the time, one of the current test ports, the one marked mA, will also include a maximum safe number of milliamps. If you are measuring a small current, use this port. If you are measuring larger amperages, use the one marked A for amps. This will be discussed further a bit later in the article - it’s a very important topic.
To measure the current, the circuit must be disconnected so that the meter actually becomes a part of the circuit. Current is the rate of electrical flow, so it’s like measuring liquid or gas flow with a turbine flow gauge. It must be directly in the path of the flow. This is very different from voltage, which must only touch two exposed metal parts in the circuit to obtain a relative voltage between them.
With the circuit broken open, the red positive lead of the meter is placed on the exposed connection closer to the +v side of the circuit, and the black COM lead closer to the common ground. If an AC measurement is being obtained, the polarity does not matter, but for the sake of understanding, the black line wire is considered the source, and the white Neutral is considered the common return.
A measurement with a positive sign will indicate current flowing properly from + to - through the meter. If the number is negative, the current is flowing in reverse - perhaps the meter was connected improperly, or there is a problem in the circuit construction causing current to flow in reverse through that particular point.
Cautions in Current Measurement
Since the meter must withstand the entire flow of the circuit, the meter must be protected from dangerous current levels. A set of fuses protects the delicate internal circuitry.
Usually, the mA port is protected to around 400 mA and the A port can withstand up to about 10 A, although the exact values are printed on each meter. If that current is exceeded, the fuse will fail and become open.
This is a danger because current will no longer flow in the meter, giving the technician a false sense of safety. There appears to be no current, but in reality, the circuit is only waiting for that meter to be removed and current will instantly flow once again. There usually isn’t an indication on the LCD screen showing a failed fuse, so there is no way to know.
An assortment of various test leads and attachments.
The other safety concern arises from the fact that the user must break open the circuit and make a new failure point. The user is right in the path of the flow and must exercise extreme caution to not let the other meter lead touch anything other than the appropriate point. It’s not just a matter of bad measurements, it could even cause extreme disasters. This is why it’s avoided when possible.
If you must measure current with an in-line meter, always use the higher capacity Amps plug first. If it shows a value below the mA limit, then that plug should be safe to use as well. Test the meter in a known working circuit. This will ensure that the fuses are intact.
Remove power to make the connections, using terminal blocks or other connection terminals - try not to just cut wires. Re-apply power, make the test, then remove power to repair the circuit. It can be more time consuming to measure current, and safety must be observed, but it can lead you to information that voltage tests are incapable of providing without a lot of extra work.