Before Testing a Diode

Before measuring a diode, make sure the circuit is safe and the component is not being affected by other components connected in parallel or series.

• Switch off the equipment.
• Disconnect the power source.
• Discharge capacitors where necessary.
• Identify the diode terminals.
• Use the diode-test function when available.

For the most reliable test, remove at least one diode terminal from the circuit, or remove the diode completely, when other circuit components could affect the measurement.

Using the Multimeter Diode-Test Mode

Most digital multimeters have a dedicated diode-test function. In this mode, the meter applies a small test current and displays the voltage measured across the diode.

For a conventional silicon diode, a forward reading is often somewhere around 0.6–0.7 V, depending on the device and test current.

The exact reading is not a universal fixed value.

Step 1: Identify the Cathode

On many through-hole diodes, the cathode is identified by a stripe or band around the body.

        Cathode
           |
           |
───────────|>|───────────
           |
         Anode

The physical marking varies between component types and packages, so check the datasheet when necessary.

Step 2: Forward-Bias Test

Place the multimeter probes across the diode in the forward direction.

• Red probe → Anode
• Black probe → Cathode

A normal silicon diode should normally show a forward-voltage reading.

These values are approximate and should not be treated as exact pass/fail limits.

Step 3: Reverse-Bias Test

Reverse the probes:

• Red probe → Cathode
• Black probe → Anode

A conventional diode should normally show an open-circuit indication or a very high measured voltage in reverse bias.

If the meter shows a low voltage in both directions, the diode may be shorted.

Normal Diode Test

This is the basic test for an ordinary PN-junction diode.

Testing for a Shorted Diode

A shorted diode conducts in both directions.

Test the diode in both directions using diode-test mode.

If the meter shows a low forward-type voltage in both directions, the diode may have failed short-circuit.

A shorted diode can cause:

• Excessive current
• Blown fuses
• Power-supply failure
• Incorrect circuit voltage
• Overheating of other components

Testing for an Open Diode

An open diode does not conduct normally in the forward direction.

If the multimeter indicates an open circuit in both directions, the diode may be open.

Possible causes include:

• Excessive current
• Excessive reverse voltage
• Overheating
• Mechanical damage

Testing for a Leaky Diode

A diode may fail without becoming completely shorted or open. Excessive reverse leakage can also indicate damage.

Basic multimeter testing may not always be sufficient to determine whether the leakage is within the manufacturer's specification.

For precision testing, compare the measured leakage with the value specified in the component's datasheet.

Testing a Diode In-Circuit

Testing a diode while it remains installed on a PCB can sometimes produce misleading results.

Other components can create alternative current paths that affect the multimeter reading.

If the result is uncertain:

1. Switch off the equipment.
2. Discharge capacitors.
3. Lift one diode terminal from the PCB.
4. Repeat the diode test.

Testing the isolated component provides a more reliable basic assessment.

Testing a Schottky Diode

Schottky diodes normally have a lower forward voltage than conventional silicon PN diodes.

A typical multimeter may therefore display a lower forward-voltage reading.

The reverse leakage of a Schottky diode can be higher than that of a comparable silicon PN diode, especially at elevated temperature.

Do not automatically classify a Schottky diode as faulty simply because its forward reading is lower than approximately 0.6–0.7 V.

Testing a Zener Diode

A standard multimeter diode test can check the forward direction of a Zener diode, but it normally cannot determine its actual Zener breakdown voltage.

To test the Zener voltage accurately, a suitable test circuit with a current- limiting resistor and an appropriate supply voltage is required.

The measured breakdown voltage should then be compared with the Zener's specified value and test current.

Testing an LED

Many multimeters can illuminate a low-power LED when the meter is placed in diode-test mode.

• Red probe → LED anode
• Black probe → LED cathode

A forward-voltage reading should normally appear, and the LED may emit a small amount of light.

Some high-power or specialized LEDs may not illuminate with a standard multimeter because the meter's test current is insufficient.

Testing a Bridge Rectifier

A four-terminal bridge rectifier contains four diodes internally.

Many bridge rectifiers have terminals marked:

• +
• −
• ~
• ~

Each internal diode can be checked using the diode-test function by following the manufacturer's internal circuit or datasheet.

A short or open reading where normal diode conduction is expected can indicate a faulty bridge rectifier.

Testing a TVS Diode

A basic diode test can sometimes identify a shorted TVS diode, but it cannot fully verify the device's transient-suppression characteristics.

A proper TVS test requires appropriate equipment and conditions to verify parameters such as breakdown voltage and clamping behavior.

For many repair situations, the first useful checks are:

• Look for a short circuit.
• Compare with a known-good device when appropriate.
• Check the manufacturer's specifications.

Resistance Mode vs Diode-Test Mode

Diode-test mode is generally preferred because the meter is designed to provide a suitable test current and display the forward voltage directly.

Common Diode Test Results

These results are general diagnostic guidelines. Specialized diodes require interpretation based on their design and datasheet.

Diode Testing in Power Supplies

Diodes in power supplies are commonly tested when diagnosing problems such as blown fuses, excessive ripple, low DC output, or complete loss of output.

Important components to check include:

• Bridge rectifier
• Secondary rectifier diodes
• Fast-recovery diodes
• Schottky diodes
• Protection diodes
• TVS diodes

A shorted rectifier diode can place a heavy load on the transformer or switching circuit and may cause protective devices to operate.

Testing Diodes in Amplifiers

Diodes are commonly found in amplifier bias, protection, rectification, and signal circuits.

When troubleshooting an amplifier, a faulty diode may cause:

• Incorrect bias voltage
• DC offset
• Protection-circuit activation
• Power-supply problems
• Signal distortion

Compare diode measurements with the schematic and, when available, with measurements from a known-good channel.

Professional Troubleshooting Procedure

1. Identify the diode and its function.
2. Locate the anode and cathode.
3. Turn off and isolate the equipment.
4. Discharge capacitors where necessary.
5. Perform a visual inspection.
6. Test the diode in forward direction.
7. Test the diode in reverse direction.
8. Compare the readings with expected values.
9. Lift one terminal if in-circuit measurements are uncertain.
10. Check surrounding components if the diode tests normally.

When to Replace a Diode

A diode should generally be replaced when testing confirms an electrical fault or when the component has suffered visible physical damage.

• Short circuit
• Open circuit
• Excessive leakage
• Cracked package
• Burn marks
• Overheating damage

The replacement should have appropriate voltage, current, switching, thermal, and physical characteristics for the circuit.

Important Safety Notes

• Never perform resistance or diode testing on a powered circuit.
• Discharge capacitors before working on power supplies.
• High-voltage circuits can remain dangerous even after power is removed.
• Use appropriate test equipment and measurement procedures.
• Do not rely solely on a multimeter when diagnosing specialized diodes.

Quick Diode Testing Checklist

• ✔ Switch off the equipment
• ✔ Discharge capacitors
• ✔ Identify anode and cathode
• ✔ Use diode-test mode
• ✔ Check forward direction
• ✔ Check reverse direction
• ✔ Look for short or open readings
• ✔ Test out of circuit when necessary
• ✔ Compare specialized components with their datasheet

Key Points

• A normal PN diode conducts in one direction and blocks in the other.
• Diode-test mode is generally the easiest way to perform a basic test.
• A low reading in both directions can indicate a shorted diode.
• An open reading in both directions can indicate an open diode.
• In-circuit measurements can be affected by surrounding components.
• Zener, TVS, Schottky, LED, and other specialized diodes require appropriate interpretation.
• Always compare critical measurements with the manufacturer's datasheet.