Diode on relay coil

Thread Starter


Joined Oct 19, 2009

On the schematic, the diode put across the coil 24vdc is 1N4003. I don't have this diode, but can I subsitute it to 1N4005?




Joined Jul 17, 2007
You may wind up still getting spikes from the coil, as the 1N400x series of diodes don't have a specified turn-on time; so the turn-on time can vary widely from manufacturer to manufacturer, and batch to batch.

If your relay coil is low current (<~80mA) consider using a 1N4148 or 1N914 switching diode. These diodes have extremely fast turn-on and recovery times.

Another technique is to use a small capacitor; 100pF to ~330pF, in parallel with the diode. The small capacitor causes the voltage spike to build much more slowly, which "buys time" for a slow diode to turn on. You don't want to use a cap very much larger than I mentioned above, as if you do, you will have high peak currents through the transistor or MOSFET that is used to power the coil.


Joined Nov 20, 2008

SgtWookie has some good advise, if you are worry about relying on a devices unspecified characteristics.

However, it has been my experience that the 1N400x series of diodes work well for suppressing the back EMF pulse from most relays. Relay coils that have alot of turns on a steel core, means higher intra-winding capacitance and slow field colapse. This slows the EMF rise time down to where the 1N400x series diodes can easlily clamp the voltage to a volt or so.