Relays

Pat Alonzi

Hi everyone,

Happy Holidays!

I just realized that I have been driving a 24VAC Relay coil with 24VDC and its switching fine. Why is it still working and is it ok to use AC Coils with DC?

Thanks.
-Pat

allanlane5

Relay coils are magnetic, current driven devices. AC relays are designed to work with a 60 Hz alternating voltage (which results in a 60 Hz alternating current, too). If you drive them with DC, you MIGHT put too much current through them (since it's not alternating) and they might heat up a little more than they would with an AC voltage. Or it might work just fine.

Mike Green

There's a difference in heating with AC vs. DC. The AC coil is specified in terms of 24VAC RMS voltage. I'm not sure of the conversion factor for average power, but the 24VDC would have greater average power dissipation and the relay would run hotter than expected, probably not a lot.

Using a DC coil with an AC input is more problematic. There's usually a special 1-turn coil that's part of an AC relay that helps maintain the magnetic field during the "off" times of the AC current. This prevents "chatter" of the relay contacts when the AC current goest to zero. The DC coil is missing this.