Common Diode Faults

The most common electrical failure modes of diodes are:

• Short circuit
• Open circuit
• Excessive reverse leakage
• Intermittent operation
• Excessive forward voltage
• Thermal damage
• Reverse-breakdown damage
• Physical or mechanical damage

1. Shorted Diode

A diode that has failed short-circuit conducts in both directions instead of blocking reverse current.

This is one of the most common failure modes in power circuits.

Possible Symptoms

• Blown fuse
• Excessive supply current
• Power supply shutting down
• Transformer overheating
• Low or missing DC output
• Other components becoming unusually hot

Testing

With the circuit powered off, use a multimeter in diode-test mode and test the diode in both directions.

If the meter shows a low-conduction reading in both directions, the diode may be shorted.

2. Open Diode

An open diode fails to conduct normally in the forward direction.

Possible Symptoms

• No rectified output
• Loss of a protection path
• Incorrect circuit voltage
• Failure of a signal path
• Incorrect bias conditions

Testing

A multimeter in diode-test mode will normally show an open or very high reading in both directions when an ordinary diode is open.

3. Excessive Reverse Leakage

A diode can deteriorate without becoming completely shorted. Its reverse leakage current may become much higher than specified.

This can be particularly problematic in high-impedance or precision circuits.

Possible Symptoms

• Incorrect bias voltage
• Unexpected current flow
• Unstable reference voltage
• Measurement errors
• Increased circuit power consumption

A standard multimeter may not be able to determine whether leakage is within the manufacturer's specification. A proper test requires comparison with the datasheet and appropriate test conditions.

4. Excessive Forward Voltage

A damaged or unsuitable diode may have a higher forward voltage than expected at a specified current.

This can increase conduction losses:

P ≈ VF × IF

Higher forward voltage can therefore result in increased heating and reduced circuit efficiency.

Before declaring a diode faulty, however, compare the measurement with the correct datasheet conditions because forward voltage depends on current and temperature.

5. Intermittent Diode

An intermittent diode may operate normally under some conditions but fail under vibration, temperature changes, or mechanical stress.

Possible causes include:

• Cracked semiconductor package
• Damaged internal connection
• Thermal stress
• Cracked solder joint
• Mechanical damage

Symptoms

• Equipment works intermittently
• Fault changes when the PCB is moved
• Failure appears after warming up
• Voltage changes unexpectedly

When troubleshooting, inspect both the diode and its solder joints.

6. Overheated Diode

Excessive temperature can damage a diode or reduce its operating lifetime.

Common causes include:

• Excessive forward current
• Insufficient cooling
• Incorrect diode selection
• High ambient temperature
• Excessive switching losses
• Insufficient PCB copper area

Symptoms

• Discolored package
• Burn marks
• Cracked body
• Unusual smell
• Intermittent operation
• Electrical failure

7. Reverse-Voltage Damage

If the reverse voltage across an ordinary diode exceeds its rated limit, breakdown can occur.

The resulting current can cause severe heating and permanent damage if it is not limited.

Possible causes include:

• Voltage spikes
• Incorrect component selection
• Transformer transients
• Inductive switching
• Incorrect circuit design

8. Reverse-Recovery Problems

A diode can be electrically functional at low frequency but unsuitable for a high-frequency switching application.

A diode with excessive reverse-recovery time can cause:

• Increased switching losses
• Excessive heating
• Voltage spikes
• Electromagnetic interference
• Reduced converter efficiency

This is not necessarily a failed diode. It may instead be an inappropriate diode type for the application.

9. Diode Installed Backwards

A correctly functioning diode can appear to be faulty if it has been installed with the wrong polarity.

Always check:

• Anode connection
• Cathode connection
• PCB polarity marking
• Schematic symbol
• Physical cathode marking

This is especially important when replacing diodes in bridge rectifiers, power supplies, protection circuits, and LED circuits.

10. Incorrect Replacement Diode

A replacement diode can have the correct physical size but still be electrically unsuitable.

Check the following parameters:

• Maximum reverse voltage
• Forward-current rating
• Forward voltage
• Reverse-recovery characteristics
• Power dissipation
• Junction temperature
• Package and thermal characteristics

This is particularly important in switching power supplies and high-current rectifier circuits.

11. Faulty Bridge Rectifier

A bridge rectifier contains four diodes. Failure of one or more internal diodes can produce several different symptoms.

Possible Symptoms

• Blown input fuse
• No DC output
• Excessive ripple
• Low DC output
• Transformer overheating
• Excessive input current

Individual diode paths can be checked with a multimeter according to the bridge rectifier's internal configuration.

12. Faulty Power-Supply Rectifier

A defective rectifier diode in a power supply can cause major problems.

For example, a shorted secondary rectifier can overload a transformer or switching converter, while an open rectifier can remove the expected DC output.

When troubleshooting, inspect:

• Rectifier diodes
• Filter capacitors
• Transformer windings
• Switching devices
• Fuses and protection components

13. Faulty Zener Diode

A Zener diode can fail short-circuit, open-circuit, or develop abnormal characteristics.

A failed Zener can cause:

• Incorrect reference voltage
• Excessive current through a resistor
• Power-supply output problems
• Protection-circuit failure

A simple diode test can identify some failures, but verifying the actual Zener voltage requires an appropriate test circuit.

14. Faulty Schottky Diode

Schottky diodes can fail through excessive current, excessive reverse voltage, overheating, or other electrical stress.

A common failure in power supplies is a shorted Schottky rectifier.

When testing a Schottky diode, remember that its normal forward voltage is often lower than that of a conventional silicon diode.

15. Faulty TVS Diode

A TVS diode can fail short-circuit after absorbing a severe transient.

A shorted TVS can cause:

• Power supply shutdown
• Blown fuse
• Excessive current
• Low supply voltage

A TVS that has suffered a severe transient may need replacement even when the physical damage is not obvious.

16. Faulty LED

LEDs can fail because of excessive current, excessive reverse voltage, overheating, or mechanical damage.

Possible Symptoms

• No light
• Very dim light
• Intermittent operation
• Short circuit
• Discolored or damaged package

An LED should always be operated within its specified current and voltage conditions.

17. Faulty Photodiode

A damaged photodiode may exhibit abnormal leakage, reduced sensitivity, or no detectable response to light.

Testing a photodiode generally requires a suitable measurement circuit because a simple multimeter diode test may not fully characterize its performance.

Diagnosing a Diode Fault

Use a systematic approach rather than replacing the diode immediately.

1. Identify the diode type.
2. Check the schematic and its function.
3. Inspect the component visually.
4. Check for overheating or physical damage.
5. Switch off and isolate the circuit.
6. Discharge capacitors where necessary.
7. Perform a forward diode test.
8. Perform a reverse diode test.
9. Test out of circuit if necessary.
10. Check surrounding components.
11. Compare the measurements with the datasheet.

Fault Symptoms and Possible Causes

These symptoms are not proof of a faulty diode. Other components can produce similar symptoms, so measurements should be used to confirm the diagnosis.

Why Diodes Fail

Common causes of diode failure include:

• Excessive forward current
• Excessive reverse voltage
• Overheating
• Voltage transients
• Inadequate thermal design
• Incorrect diode selection
• Mechanical stress
• Incorrect installation

Preventing Diode Failures

• Select adequate voltage and current ratings.
• Provide appropriate thermal management.
• Use suitable suppression components for inductive loads.
• Consider reverse-recovery characteristics in switching circuits.
• Observe correct diode polarity.
• Use appropriate PCB copper area for power diodes.
• Follow the manufacturer's recommended operating conditions.

When to Replace a Faulty Diode

Replace a diode when testing confirms an electrical fault or when the component has obvious physical damage.

The replacement should match or appropriately exceed the required electrical and thermal characteristics of the original device.

Do not automatically replace a diode with the highest-current or highest-voltage part available. Switching speed, forward voltage, leakage, capacitance, and thermal characteristics may also be important.

Important Safety Notes

• Never test resistance or diode operation on an energized circuit.
• Discharge power-supply capacitors before servicing.
• High-voltage equipment can remain dangerous after being switched off.
• Use appropriate test equipment and safe measurement procedures.
• Do not replace a failed diode without investigating why it failed.

Quick Diode Fault Checklist

• ✔ Check for a short circuit
• ✔ Check for an open circuit
• ✔ Check forward voltage
• ✔ Check reverse leakage where necessary
• ✔ Inspect for overheating
• ✔ Check diode polarity
• ✔ Check surrounding components
• ✔ Verify the replacement specifications
• ✔ Investigate the original cause of failure

Key Points

• Diodes commonly fail short, open, leaky, or intermittently.
• Excessive current and reverse voltage are major causes of failure.
• Overheating can permanently damage a diode.
• A diode can be electrically good but unsuitable for a high-frequency circuit.
• In-circuit measurements can be affected by surrounding components.
• Always investigate the cause of failure before installing a replacement.