![]() For a 2-wire measurement, the injected current and the observed voltage are both done through the same probes. This is a simpler measurement to make and the probes are less complicated and expensive. If you’re not worried about the added resistance of the probe wires, taking a 2-wire resistance measurement will be sufficient. For this reason, there are two different modes for measuring resistance: 2-wire mode and 4-wire mode. Since even wires have resistance, the wires of the probes can actually add to the observed resistance measurement. The known current, and the resulting voltage drop are then used to calculate the resistance using Ohm’s Law, V=I^2*R. Multimeters measure resistance by injecting a small current into the circuit, and then measuring the voltage drop across those points in the circuit. Measuring resistance with a multimeter can be done a couple of different ways, depending on the level of accuracy needed in the measurement. Apply power to the circuit or device under test and probe points on circuit The positive probe should be connected to the “INPUT HI” port, while the negative probe should be connected to the “INPUT LO” port. To measure AC voltage using a benchtop multimeter, select the “AC V” mode and connect your probes. Measuring AC voltage is almost identical to measuring DC voltage, however this mode is used for measuring the voltage potential between two points of an AC or “alternating current” circuit. Apply power to the circuit or device under test and probe points on circuit. To measure DC voltage using a benchtop multimeter, once you've turned it on, select “DC V” mode.Ĭonnect your probes to your multimeter the positive probe should be connected to the “INPUT HI” port, while the negative probe should be connected to the “INPUT LO” port. That difference in potential is measured in units of. ![]() The DC voltage measurement is used to determine the difference in electrical potential between two points in a DC or “direct current” circuit. Let's start with DC voltage, one of the simplest and most-used multimeter measurements. For this purpose of this article, we’ll assume a benchtop multimeter is being used. Benchtop multimeters tend to offer higher precision than their smaller handheld counterparts. Multimeters can be both handheld or benchtop units. This includes measurements like voltage, current, resistance, etc. Multimeters are used for taking basic electrical measurements related to Ohm’s Law. If you still see "OL", or infinite resistance on the switch then it's most likely faulty.Reprinted with permission from Evaluation EngineeringĪ multimeter or digital multimeter (DMM) is one of the most essential and common pieces of lab equipment. In my case, I logged 0.4 ohm for an old 3-way switch I know works just fine. If the light switch is in good order the multimeter will now read a resistance value close to zero. Now flip the switch on and take the same measurement. That makes sense because the switch is off. This means there is infinite resistance on the switch circuit. You should see "OL" on the multimeter's display. For this test it doesn't matter which probe connects to which switch terminal. With the switch in the off position, touch one probe to the switch's "COM" black terminal and the other to the brass terminal next to it. Switch the multimeter dial to the resistance (ohms) mode and turn it on. ![]() Use the probe port/terminal positions as before (black on COM, red on V/ohms). Disconnect the switch from wiring in its electrical box. Shut down power to the switch at the panel. If you have a light switch you suspect is bad, you can use your multimeter to confirm its condition. (see this for a detailed how to article on swapping light switches.)Ĭonfirm if a suspect switch is working or has failed. Then, take a look inside the box by pulling the connected switch out of it carefully. To troubleshoot the light switch, first shut off power at your main breaker panel. Let's say a light fixture on the ceiling failed - when you flip the switch on and off, nothing happens. If there's no sound then you've got a dead bulb. When the red probe makes contact, the multimeter will sound a tone if the light bulb's circuitry checks out. Now touch the tip of the red probe to the bulb's electrical foot contact (that's a fancy way to say the metal circle at the bottom of the bulb). It helps to put the probe tip inside a screw thread groove for extra stability. Touch the black probe to the outside of the bulb's metal threaded end. If you have a box of old bulbs in unknown conditions, use your multimeter to test which ones are good and which ones are blown.Ĭonfirm your meter is set in continuity test mode (see step 2) and is switched on. This trick will only work reliably on simple incandescent bulbs. ![]()
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