Consider testing a bridge diode inside a device, or before installation, to avoid a power failure. In general, diodes act as electrical current valves only allowing electricity to flow through them in one direction. A bridge diode, also called a rectifier, consists of four diodes. In addition to being a current valve, the rectifier converts alternating current to direct current for use by a machine, circuit or vehicle. A diode only transits current in one direction, so a resistance test indicates if a diode is bad. Resistance measures how well a device limits electrical current flow.
Turn on the digital multimeter. Connect the red probe to the positive port and the black probe to the negative port.
Rotate the multimeter measurement dial to the resistance setting. This is often denoted by the capital Greek letter "Ω." It stands for the unit of resistance — the ohm.
Remove power to the bridge diode if it is installed in a device. Do this by disconnecting the device's battery. Pull the bridge diode from its circuit. Four leads are exposed. Two are labeled with a "~" for AC current; one is labeled "+"; and the last is labeled with a "-." The "+" and "-" are the DC current leads.
Connect the red probe of the multimeter to one of the AC current leads. Connect the black probe of the multimeter to the "+" lead. Now reverse the connections. If you get the same ohm reading each time, that diode is bad. Replace the entire bridge diode. Repeat this process connecting to the same "~" lead and the "-" lead.
Connect the red probe of the multimeter to the other AC current lead. Connect the black probe of the multimeter to the "+" lead. Now reverse the connections. If you get the same ohm reading each time, that diode is faulty. Replace the entire bridge diode. Repeat this process, connecting to the same "~" lead and the "-" lead.
Note on some multimeter models the negative port is labeled "common."