How to Identify and Test Diodes in a Circuit

Diodes are tiny components with a huge impact. They act like one-way gates for electrical current letting it flow in one direction while blocking it in the other. Without them, you'd risk reverse polarity damage, unfiltered voltage spikes, and a whole lot of circuit chaos.

Whether you're working on a DIY power supply, repairing an old board, or prototyping your next Arduino project, it's important to know how to identify and test diodes correctly. This guide will walk you through the process step-by-step and give you links to reliable diode products from Tomson Electronics.

What Is a Diode?

A diode is a semiconductor device that conducts current in one direction (forward) and blocks it in the opposite direction (reverse). It has two terminals:

• Anode (+) – current enters here
  

• Cathode (–) – current exits here
  

In circuit diagrams, the bar shows the direction in which current is blocked, and the arrow shows the direction in which current flows.

👉 Browse Diodes on Tomson Electronics

Types of Diodes You Might Encounter

Diodes come in a wide variety, each with a specific set of uses. Here is a brief summary of the most typical:

1. General Purpose Diodes (e.g., 1N4007)

Used for basic rectification in power supplies.

👉 Buy 1N4007 Diodes

2. Zener Diodes

Designed to allow current to flow in reverse above a specific voltage useful for voltage regulation.

👉 Shop Zener Diodes

3. Schottky Diodes

It is well-known for its quick switching speed and minimal forward voltage loss, making it perfect for high-frequency circuits.

👉 Explore Schottky Diodes

4. Light Emitting Diodes (LEDs)

Technically diodes that emit light when forward-biased.

👉 View All LEDs

5. Photodiodes & IR Diodes

Used for sensing light or infrared signals in automation and communication circuits.

👉 Shop IR Sensors and Photodiodes

How to Identify a Diode in a Circuit

Whether you're looking at a breadboard or a PCB, here’s how to spot a diode:

Visual Cues

• Glass or plastic body with a silver or black band on one side — this marks the cathode.
  

• Through-hole diodes often look like small cylinders.
  

• Surface-mount diodes (SMD) are rectangular with a bar marking the cathode end.
  

PCB Markings

• Look for labels like D1, D2, etc.
  

• In diode symbols, the anode is represented by a triangle pointing toward a line, whereas the cathode is represented by the line.

Color Code (for some vintage types)

• Rare, but some old-style diodes use a color band code like resistors.
  

How to Test a Diode Using a Multimeter

To make sure a diode is working properly, you can test it using the diode or continuity mode on a digital multimeter.

Step-by-Step (Out of Circuit)

1. Set your multimeter to Diode Test Mode.
   

2. Connect the anode to the black probe and the cathode to the red probe.

3. A voltage drop reading, usually between 0.6 and 0.7 volts for silicon diodes, is the anticipated result.

4. Reverse the probes.
   

5. Expected result: The meter should show “OL” or no reading meaning it's blocking current in reverse.
   

💡 If it shows a voltage drop in both directions or no reading at all, the diode is likely faulty.

Testing in Circuit

Testing diodes in-circuit can be tricky because other components might interfere. Still, you can:

• Identify the diode’s placement on the board.
  

• Test using the same method above.
  

• If unsure, desolder one end to isolate it.
  

👉 Need a Multimeter? Shop One Here

Common Diode Applications

Diodes are everywhere! Here are just a few ways they’re used in everyday electronics:

1. Rectifiers (AC to DC Conversion)

In power supplies, 4 diodes are arranged in a bridge rectifier to convert AC input into DC output.

👉 Get a Bridge Rectifier

2. Reverse Polarity Protection

A diode placed in series with a power input will block current if the polarity is wrong, saving your circuit from damage.

Best Diode: 1N5819 Schottky diode

3. Voltage Regulation (Zener Diodes)

Zeners are placed in reverse bias across a load to maintain a constant voltage level a simple and effective voltage regulator.

4. Signal Demodulation

In AM radio circuits, diodes help demodulate the signal by allowing only one side of the waveform to pass through.

5. Clipping and Clamping

Diodes can be used to limit voltage levels in analog circuits to prevent overload or shape waveforms.

Bonus: Diode Test Circuit

Want to build your own diode tester? It’s easy!

You’ll need:

• 1 LED

• 330Ω resistor

• A few general-purpose diodes

Wiring:

• Attach the resistor, the diode being tested, and the LED in series.

• If the LED lights up, the diode is conducting properly (forward-biased).
  

• Reverse the diode to check blocking behavior.
  

What Happens When a Diode Fails?

Diodes can fail in two main ways:

• Short Circuit: Current flows in both directions can cause damage downstream.
  

• Open Circuit: No current flows at all circuit won't function as expected.
  

Always test both directions with your multimeter to be sure.

Shop Trusted Diodes for Your Next Project

Having a selection of diodes in your parts box can save you hours when prototyping or troubleshooting. Whether you're building a power supply, adding protection, or designing a signal circuit, you’ll find what you need at Tomson Electronics.

👉 Browse Diodes Collection

Final Thoughts

Despite their small size and simple design, diodes are essential components of electronics. Once you understand how to identify and test them, you'll start noticing them in nearly every project from power bricks to microcontroller boards.

So the next time your project isn't behaving, grab your multimeter and test that diode. It could be the silent culprit or the unsung hero.

Need help choosing the right type? Just drop us a message or check out our blog for more guides!