A diode test is a simple but essential process in electronics troubleshooting. Whether you’re working with standard diodes, zener diodes, or LEDs, knowing how to test a diode ensures circuits function as intended. In this guide, you’ll learn how to check a diode using a multimeter, what the results mean, and how to handle different types of diodes like bridge rectifiers and LEDs.
Table of Contents
- How to Test a Diode with a Multimeter
- Zener Diode Test: Special Considerations
- Bridge Rectifier Test: How to Check Four Diodes in One Package
- LED Test Using a Multimeter
- Tips for Diode Test
- Did You Know About Diode Test?
- Frequently Asked Questions About Diode Test
- Diode Test Conclusion
How to Test a Diode with a Multimeter
To check a diode using a multimeter, use the diode tester mode if available. If not, you can still check the diode with a multimeter in resistance mode, although this depends on the particular multimeter.
An example of a multimeter dial set to the diode test mode is shown in the image below. As can be seen in the image, the diode symbol is selected for this diode test mode.
Steps to Check a Diode with a Multimeter
- Turn off power to the circuit before removing or testing any diode.
- Set your multimeter to the diode test mode (usually denoted by a diode symbol).
- Connect the multimeter probes (forward test / forward biased):
- Red (positive) to the anode
- Black (negative) to the cathode (normally marked by a band on the diode body on a silicon rectifier diode)
- Read the display:
- A typical forward voltage drop for silicon diodes is around the theoretical 0.6V–0.7V range. In reality the reading is often below 0.6V.
- For LEDs, it may be between 1.8V and 3.3V depending on the color.
- Reverse the probes (reverse test / reverse biased) to check reverse bias. The multimeter should show OL (open loop), indicating no current flow. Some multimeters show 1 on the diode test setting to indicate an open circuit.
The following images show the forward and reverse diode tests, as described above. In both images, the diode anode is on the left and cathode on the right. The cathode is marked by the band on the diode body. The diode in the images is a 1N4007, but this test works with any silicon rectifier or small signal diode.
Expected Multimeter Reading
The following image shows the expected reading for the forward diode test where the diode is forward biased by the multimeter in diode test mode. In this case the reading is 0.596V, which is close to the expected 0.6 to 0.7V range for a general purpose silicon diode. This means that the diode is OK and has passed the test. If the reading in this test still shows OL, then the diode is faulty and open-circuit.
A diode that has failed open-circuit is expected to show the OL reading for the forward and reverse tests.
A diode that has failed with a short-circuit is expected to show a diode test reading that is close to 0V for the forward and reverse tests.
Diode Test Examples with Images
The following images show a 1N5408 diode being tested using a multimeter. This is just an example using a specific diode part. In this case, the reading of 0.549V for the forward test shows that the diode is working in forward biased mode (left image below). The cathode band of the diode is to the right and the diode cathode is connected to the black lead of the multimeter
In the second image (right image below), the multimeter reading shows OL, which is correct for the reverse test. The cathode band of the diode is to the left in the second image, and the diode cathode is connected to the red lead of the multimeter.
Using Multimeter Resistance Mode to Test a Diode
If using resistance mode on a multimeter, a good diode is expected to show a low resistance in forward bias and high or infinite resistance in reverse bias. This method works well with analog multimeters because their internal battery can often supply enough voltage to forward-bias the diode and produce a measurable current.
However, digital multimeters do not always give reliable results in resistance mode. In many cases, you may see:
- OL (overload) or infinite resistance in both directions.
- No significant difference between forward and reverse readings.
- No reading at all, especially on auto-ranging meters or lower resistance ranges.
This is because most digital multimeters do not supply enough voltage in resistance mode to overcome the diode’s forward voltage threshold (typically around 0.6 to 0.7 volts for standard silicon diodes). If the voltage applied is less than the threshold, no current flows, and the meter cannot register a resistance value, even if the diode is functional.
In some cases, older or specialized digital meters may give usable results if their internal test voltage is higher—around 2V or more—but this is not guaranteed and varies by model.
For reliable results when testing diodes with a digital multimeter, it’s recommended to use the diode test mode, which is specifically designed to output a small voltage (usually around 2V) and measure the forward voltage drop directly.
Zener Diode Test: Special Considerations
A zener diode test differs slightly because zener diodes are designed to conduct in reverse breakdown. However, this requires more voltage than a standard multimeter can provide. Here’s how to check a zener diode:
- Use the diode tester to check forward bias—results are similar to regular diodes.
- For reverse bias testing, apply external voltage exceeding the zener’s breakdown voltage and measure voltage drop using a multimeter.
Bridge Rectifier Test: How to Check Four Diodes in One Package
A bridge rectifier consists of four diodes. Here’s how to perform a bridge rectifier test:
Bridge Rectifier Check Steps
A bridge rectifier contains four diodes arranged in a specific configuration. You can test it using a digital multimeter’s diode test mode by following these steps:
1. Identify the Terminals
- AC Inputs – usually marked as
~or unmarked (two of them) - DC Output Positive (+)
- DC Output Negative (−)
2. Test from Each AC Pin to DC Output Terminals
Use diode test mode. Let’s call the AC pins AC1 and AC2.
From AC1:
- Place the red probe on AC1, black probe on (+) → should read approx 0.6V–0.7V (forward-biased diode).
- Place the red probe on AC1, black probe on (−) → should read OL (reverse-biased diode).
- Reverse the probes:
- Black on AC1, red on (+) → OL
- Black on AC1, red on (−) → 0.6V–0.7V
Repeat the same steps for AC2:
- Red on AC2, black on (+) → 0.6V–0.7V
- Red on AC2, black on (−) → OL
- Reverse probes and repeat.
3. Check Between + and – Terminals
- Place red probe on (+), black probe on (−) → should read OL (no direct diode connection).
- Reverse probes → OL again.
Summary of Expected Readings
| Test Points | Expected Reading |
|---|---|
| AC to + (red on AC) | 0.6–0.7 V |
| AC to – (red on AC) | OL |
| + to AC (red on +) | OL |
| – to AC (red on –) | 0.6–0.7 V |
| + to – | OL |
| – to + | OL |
LED Test Using a Multimeter
To perform an LED test, use the diode tester on your multimeter:
- Connect red to anode and black to cathode.
- The LED may briefly light up, and a voltage drop (1.8–3.3V) will be displayed.
- Reverse the probes to check reverse bias; the display should show OL.
This method not only tests the LED but also helps identify its polarity.
Tips for Diode Test
- Always remove the diode from the circuit when testing for accurate results.
- Use the multimeter’s diode tester mode for the most reliable readings.
- For surface-mounted diodes, use fine probes or clips to access terminals.
- Avoid testing high-voltage diodes without the appropriate equipment.
- Label diode leads before removing them to avoid polarity confusion during reinstallation.
Did You Know About Diode Test?
- The diode test mode uses a small current to measure the forward voltage drop, making it more precise than resistance mode.
- Some digital multimeters can test LEDs and even display their forward voltage, which varies by LED color.
- Zener diodes were first introduced in the 1950s and are critical in voltage regulation circuits.
- The bridge rectifier was a significant advancement, allowing AC to DC conversion without a center-tapped transformer.
Frequently Asked Questions About Diode Test
How do I test a diode?
Use a multimeter in diode test mode. Connect the red probe to the anode and black to the cathode. A reading of 0.6V–0.7V indicates a good diode.
How do you check a diode in a circuit?
Preferably, remove the diode first. If testing in-circuit, be cautious of parallel paths that might affect readings. Use diode test mode for accuracy.
Can I check a diode with a multimeter in resistance mode?
Yes, but results may be less reliable. In resistance mode, a good diode shows low resistance in forward bias and high resistance in reverse bias.
How to measure a diode’s forward voltage?
Set the multimeter to diode mode. Connect probes correctly, and the display will show the forward voltage drop, typically around 0.6–0.7 volts for silicon diodes.
What’s the best way to perform a bridge rectifier check?
Use diode test mode and treat each pair of pins as a diode. Test all four diodes by checking between the AC inputs and output terminals in both directions.
Diode Test Conclusion
Performing a diode test using a multimeter is a foundational electronics skill. Whether you’re working with standard diodes, zener types, bridge rectifiers, or LEDs, accurate testing ensures your components are functioning correctly. Understanding how to test a diode helps diagnose faulty parts, prevent circuit failure, and build reliable electronic projects.
After learning how to test a diode, you may be interested in learning the similar transistor test for testing different types of transistors