Relay Flyback Diode Selection

Question

Problem: Relay Flyback Diode Selection. Solve this hands-on electronics task on EWskills.

Accepted Answer

1. Component Purpose

A relay coil is an inductor. When the driving transistor turns OFF, the coil’s collapsing magnetic field creates a sharp reverse voltage spike (back-EMF) that can be many times higher than the 12V supply. This spike can damage or destroy the transistor.

A flyback diode placed across the coil (cathode to +12V, anode to ground) provides a safe discharge path for this energy, clamping the spike to about 0.7V above the rail.

Required: General Purpose Rectifier · ≥1A · ≥40V · SMA · SMD · −40°C to +125°C · RoHS

2. How to Find It on DigiKey

Go to DigiKey.com → Discrete Semiconductor Products → Diodes --> Rectifiers --> Single Diode

Apply these filters:

Technology → Standard
Voltage - DC Reverse (Vr) → 40V or higher
Current - Average Rectified (Io) → 1A or higher
Supplier Device Package → SMA (DO-214AC)
Mounting Type → Surface Mount
Part Status → Active

Tip: On DigiKey, the SMA package is also listed as DO-214AC. Both names refer to the same physical package.

3. Key Specifications & What They Mean

Specification	Required	Why
Diode Type	General Purpose Rectifier	Standard rectifier; flyback pulse is brief.
Avg Current (Io)	≥1A	Coil is 80mA; 1A gives reliable margin.
Reverse Voltage (Vr)	≥40V	Flyback spikes exceed 12V supply easily.
Package	SMA	Standard SMD power diode package.
Temp Range	−40°C to +125°C	Industrial operating conditions.
RoHS	Yes	Compliance required.

4. Selecting a Safe, Production-Ready Part

The filtered list will show many matching parts. Before you pick one, check: Is it Active? Is stock available? Is the manufacturer reputable? Is a datasheet available?

Full checklist: How to Select a Safe, Production-Ready Component — EWskills Guide

5. Filtered DigiKey Link

➡ View Matching Parts — Rectifier, SMA, ≥1A, ≥40V

28. Relay Flyback Diode Selection