A diode is a type of check-valve that will only allow electrical current to flow in one direction. As a general rule, this device does not breakdown gradually over a period of time, but rather, it continues to function normally until it stops working all together. You can usually test a diode for functionality by performing two simple procedures using an ordinary digital or analog multi-meter to test for resistance in both directions of the device. Although the testing procedure varies slightly depending on which type of multi-meter you are using, the basic steps are the same.
You must be certain of the polarity of the test leads of your particular multi-meter before you can accurately test a diode. When using an analog multi-meter in the diode or ohms mode, the normal polarity of the red and black test leads is often reversed. This reversed polarity must be taken into account when performing the diode test. If you are using a digital multi-meter in the diode test mode, the normal polarity of the test leads is unchanged.
Before you can test a diode, you will also need to correctly determine the location of the diode’s anode and cathode terminals. The body of a diode typically has a thin band wrapped around it at one end, which indicates the location of the cathode terminal. The anode terminal will be located at the opposite end. Additionally, you will need to disconnect one of the diode’s terminals from the circuit board before you perform the actual testing procedure. It is also important to remember that you must first disconnect all electrical power supplied to the circuit board before you can safely test a diode.
Once the necessary preparations have been made, you will need to connect the positive test lead from the multi-meter to the cathode terminal of the diode. You can then connect the meter’s negative test lead to the diode’s anode terminal. After these connections are made, you should select either the diode or ohms function on the multi-meter and check for resistance. Once this step has been completed, you will need to reverse the order of the multi-meter’s positive and negative test leads and check for resistance once again. If the resistance reading from the first test is low and the second test reading is high; the diode is functioning properly. If the resistance reading from both tests is high, then the diode is not functional and should be replaced.