28. Relay Flyback Diode Selection
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