Testing of a diode:
A diode can be open circuited or short circuited when damaged. It can be tested using a multimeter by following the steps given below:
1. Insert the probes into the required sockets: The digital multimeter will have several sockets for the test probes. Insert these probes and check if they are already in the correct sockets. Typically, these are labeled COM for common and the others for current or voltage. This is normally combined with the voltage measurement socket.
2. Turn on the multimeter and select the maximum resistance range.
3. Check resistance in forward and reverse direction. Place the red probe on diode anode and black probe on the cathode to measure the forward resistance. Place the red probe on diode cathode and black probe on anode to measure the backward resistance. The forward resistance must be very small in few ohms while backward resistance must be very high in the range of mega ohms. If forward resistance is very high the diode is open circuited and if backward resistance is very small diode will be short circuited.
4. Another way is to select diode on the multimeter. Place the red probe on diode anode and black probe on the cathode and of the multimeter beeps then it indicates a short circuit otherwise it is open. Place the red probe on diode cathode and black probe on the anode and if the multimeter does not beep then it indicates an open circuit otherwise if it beeps the diode is short.
5. Turn off the multimeter: Once the resistance measurement has been made, the multimeter can be turned off to preserve the batteries. It is also wise to turn the function switch to a high voltage range. In this way, if the multimeter is used again for another type of reading then no damage will be caused if it is inadvertently used without selecting the correct range and function.
source: engineersgarage.com
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