How to Check Diode with Multimeter – Step by Step Guide

What is a Diode?

A diode is a semiconductor device that allows current to flow in only one direction. In forward bias, it conducts electricity; in reverse bias, it blocks current. Faulty diodes may become shorted (always conducting) or open (not conducting at all).

Why Check a Diode with a Multimeter?

Testing a diode helps determine if it is still functional. Common reasons include troubleshooting power supplies, diagnosing rectifiers, checking circuit boards, and preventing further damage from defective components.

Tools Needed

• Digital multimeter with diode test mode (preferred)

• Alternatively, a multimeter with resistance (ohm) mode

How to Check Diode with Multimeter (Step-by-Step)

1. Power Off the Circuit

Always disconnect power before testing. For best accuracy, remove the diode from the circuit.

2. Set the Multimeter to Diode Test Mode

Select the diode symbol on your multimeter. This mode applies a small voltage to measure forward voltage drop.

3. Forward Bias Test

• Red probe: anode

• Black probe: cathode (marked with a stripe)

• Good silicon diode: 0.5V–0.9V

• Good Schottky diode: 0.2V–0.4V

4. Reverse Bias Test

• Swap the probes

• A good diode should show OL (no conduction)

• If low both ways : shorted

• If no conduction both ways : open

5. Optional: Ohmmeter Mode

In resistance mode: forward bias = low resistance; reverse bias = very high resistance.

Common Results When Checking a Diode

Tips for Accurate Testing

• Remove at least one leg from the circuit for accurate measurement.

• Let overheated diodes cool before testing.

• Cross-check with the manufacturer’s datasheet.

• Take safety precautions in high-voltage circuits.

Tip: Always compare diode readings with a known good component if you are unsure.

FAQ

What is a normal forward voltage for a diode?

For silicon diodes: 0.5V–0.9V. For Schottky diodes: 0.2V–0.4V.

How do you know if a diode is bad?

If it shows 0V both ways, it is shorted. If it shows OL both ways, it is open.

Can I test a diode while it’s soldered on a board?

Yes, but other components may interfere. For best accuracy, test out of circuit.