Diodes/capacitors for relay coils

**ians** · 31-Aug-21, 01:34 PM

Thats why I logged on ... but got distracted by the other crap again.

Anyway ... a while back I had an issue with a dimmer switching off every time I switched of a relay ... I had an idea of what I thought it might be ... but then this morning when I got up a 3 am to learn more about node red and MQTT something interesting popped up.

Someone mentioned using a a diode to reduce the voltage spike when the rely switches off ... during discussion with other people a capacitor was mentioned ... which I tried but it didnt help ... what did help was when I connected more lights ... almost like the resistance went high enough for the voltage surge to dissipate into the circuit.

Any comments ... a diode or capacitor?

I also saw a circuit with a diode in line/series with a coil ... I will have to do more research into why you would do that.

**Justloadit** · 31-Aug-21, 02:56 PM

Diodes are to be used when switching DC circuits and coils. This is to reduce "Back EMF". Basically the diode creates a path when the power is removed from the coil, and the magnetic field collapses causing a current to flow in the reverse direction. Uncontrolled and depending on the inductance of the coil, could rise to hundreds of volts.

Capacitors are used in AC circuits with coils. The reason that it work, is that the capacitor is 90Degrees out phase with the supply, and the coil is also 90 degrees out of phase with the supply, but in the opposite direction. So theoretically, when power is removed from the coil, the capacitor absorbs the "back EMF" of the coil. The capacitor must be chosen to suit the coil. You can start around 470nF 275V X2 capacitor if working on mains circuit. Remember those capacitors you used to see in fluorescent light fittings, it does the same thing. However being an AC circuit, the capacitor also improves the power factor, as it compliments the coil during the phase shifting of the sine wave. Too small a value, and it will be too small to absorbs the back EMF, and too large a value costs money, with out adding any extra improvement.