Many newer multimeters have a diode check function. If your multimeter has this function, follow the steps below with the meter function set to "Diode Check." Typically, you will hear a beep when you connect the leads to the diode in one direction, instead of seeing a resistance reading on the meter. When you connect the leads in the other direction, you will not hear a beep.
The directions that follow also apply to analogue and older meters.
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Things you need
- Multimeter with positive and negative leads
Check the user manual for your multimeter to determine which lead is positive and which is negative. Do not assume that the red lead is positive; depending on the manufacturer, it may not be.
Set your multimeter to the "Ohmmeter" function.
Identify the cathode (negative) side of your diode. The cathode is usually marked with an indentation, a line or a longer leg. Most electronics stores have the manufacturer's parts manuals, and can give you the information you need once you supply the part number. In some cases, you may need to consult the diode manufacturer directly.
Connect the multimeter's positive lead to the anode, and its negative lead to the cathode. (If your diode is still in the circuit, disconnect the power and one end of the diode before testing.)
As long as you have correctly identified the anode and cathode, this connection should forward bias the diode, resulting in low to zero resistance as measured on the multimeter.
Reverse the leads. Attach the negative lead to the anode, and the positive lead to the cathode. If the diode is working properly, and you have correctly identified the leads, this should reverse bias the diode, resulting in a high resistance measurement.
Tips and warnings
- The readings in this test are not accurate in any way. To accurately test the voltage drop across the diode, or the current through the diode, you must set up a test circuit.
- Even if you can't identify the cathode and anode, a properly working diode should give a relatively low resistance reading in one direction (forward biased) and a relatively high resistance reading in the other direction (reverse biased).
- Handle the diode carefully.
- Avoid high-voltage meters, which can damage the diode.
- Do not test the diode with the multimeter's function setting on "Voltmeter" or "Ammeter."
- Never test resistance (ohmmeter) on a circuit with the power connected.
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