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Running a 24v relay from mains voltage?


John Fields

Jan 1, 1970
I need to power a relay from 120 vac such that its action is delayed for a
few seconds after power-on. I realize that this means getting DC voltage (via
diode) and an R-C combination.

Ideally, a relay with DC-rated coil would be most appropriate, but I've got a
nice new relay with properly-rated contacts sitting on the shelf with a coil
rated at 24 vac, 330 ohms, 71 mA.

How complex would it be to run this relay with mains voltage? Is it as simple
as a diode followed by a resistor? Would this resistor be 120/0.071/2 = ~500
ohms and 2.5 watts?

And a fundamental question: can a relay with an AC coil function properly &
reliably on DC?

Since I need to delay the relay's turn-on, I'll be adding a capacitor,
anyway, whether I use this relay or a true DC-rated type. What should the
value of the capacitor be to give me a 1 or 2 second delay with the 500 ohm
resistor (or, if I've miscalculated that value, with the proper value

Any observations, suggestions, corrections (I'm getting used to these!!) are

Jochem Welvaadt

Jan 1, 1970
Hello Dave,

I just installed my newsreader, so I do not know if anyone reads this, but
here goes:

I had a problem like that once, I solved it with a rectifier for the mains
and a 4093 as a gated oscillator to drive a 600V switsch mosfet.
Very small circuit, not a lot of heat generation.

In your case, you can use one of the spare ports on the 4093 to generate the
time delay again with small parts.
You can use a resistor/zener to generate the low voltage rail for the 4093.
Don't forget to add a clamp diode, otherwise the mosfet will probably not
Oscillator frequency is not critical, I used 50KHz.

Hope this helps.

Kind regards,