A multimeter is a device that can measure several key details of an electrical circuit. These units are commonly used by professional electricians as well as hobbyists, and they can be purchased from retail stores such as Radio Shack. A modern digital multimeter makes it easy to measure the current flow, expressed in amps; voltage can also be determined. Once the voltage and current are known, you can calculate watts using a simple formula.
Select the "Current" setting on the main dial of the multimeter. Choose a current range high enough for the circuit being tested. For example, if the circuit has a current that you estimate to be around five amps, select the "10 amp" setting instead of the "1 amp" option. Choosing a setting that is too low can overload the multimeter.
Insert the leads into the multimeter connections labeled "current." Typically, the black lead should be connected to the "common" port, while the red lead is connected to a port that matches the selected current range. This arrangement varies, depending on the unit. Consult the multimeter labels carefully to confirm that the configuration is correct for measuring current.
Place the multimeter leads into the circuit using a series configuration. The electricity must be redirected to flow completely through the multimeter to obtain an accurate reading. Current should usually move into the red lead and exit the black lead.
Read the amount of amperage displayed on the multimeter. Remember to consider this number in the context of the selected current range. For example, the number "10.00" may indicate 10 amps on one setting but only 0.01 amps if the multimeter is set to the smaller "milliamps" range.
Choose the "Voltage" setting on the multimeter dial. As you did when measuring the current, ensure that the appropriate voltage range is selected. It is usually a good idea to select the highest possible voltage range to prevent an overload.
Configure the multimeter leads into the ports marked for "voltage." The black lead can usually remain in the "common" position. Move the red lead to the port labeled for the voltage range being tested.
Connect the leads to the circuit in a parallel arrangement. The black lead should link to a negative or grounded point, while the red lead should be touched to a point of the circuit you wish to measure. Unlike a current measurement, a voltage test does not need to be completely redirected through the multimeter.
Read the number on the multimeter display. As with current, remember to consider the context of the multimeter range setting.
Multiply the amount of current and the amount of voltage in a circuit to determine the watts. For example, a motor circuit that uses 5 amps and 12 volts has 60 watts of power.
- Always observe electrical safety procedures when using a multimeter. Avoid causing a short circuit with the test leads. Confirm that the multimeter is on the correct setting before use. Read the owner's manual and follow the instructions carefully.
Items you will need
- Polka Dot Images/Polka Dot/Getty Images