Recipe
Choose and fit a flyback diode
Use when: a relay, motor, solenoid or other coil is switched on and off
Goal: a diode whose ratings cover the coil, fitted the right way round
Taught in Diodes
Quick recipe
-
1
Use the datasheet's coil current, or the supply voltage divided by the coil's resistance.
Example
A 5 V relay with a 70 Ω coil: 5 / 70 \approx 71 mA.
Why: at switch-off the diode suddenly carries this whole current.
-
2
The diode must block it all the time the coil is on.
Example
5 V.
Why: the diode is reverse-biased across the coil whenever the coil is on.
-
3
Check the current rating and the reverse-voltage rating on the datasheet.
Example
71 mA and 5 V: a 1N4148 (150 mA, 75 V) has room on both.
- coil current well under 150 mA — a 1N4148 will do.
- coil current up to about an amp — a 1 A part such as the 1N4007 or 1N5819.
Why: a diode pushed past either rating fails, often as a short circuit.
-
4
The cathode (band) goes to the end of the coil that connects to +.
Why: facing that way it carries nothing while the coil is on, and conducts only at switch-off.
-
5
Use short wires between diode and coil.
Why: the loop the current circulates in should be small, so the spike stays local.
Common mistakes
- Using whatever diode is in the drawer without checking its current rating.
- Fitting it band toward ground, which shorts the supply when the coil turns on.
- Fitting it on the board end of a long cable instead of at the coil.