Posted 5/22/2020 20:33 (#8274092 - in reply to #8273872) Subject: RE: Electrical engineering question -- flyback diodes
If you're relay has a DC Coil, a diode will give you the best protection.
For AC coils you can't use a diode, so a resistor/capacitor network or a MOV (Metal Oxide Varistor) has to be used.
The reason they have a fly-back diode is for surge suppression when the relay coil is de-energized.
When the coil is energized, current flowing through the coil will create a magnetic field around the coil.
When the coil is de-energized, the magnetic field around coil will collapse, creating a voltage spike that can reach a couple hundred volts.(depending on the current draw of the coil)
If you're engaging the relay with a switch, the spike will arc across the contacts and will severely shorten its switching cyles.It also wreaks havoc on transistors used to engage the coil.
The Diode (mounted in reverse) will allow the voltage spike to travel back into the coil until it's totally dissipated and preventing it from reaching your switch or transistor.
Hope this helps.