Running a 24v relay from mains voltage?

[John Fields]

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
resistor)?

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

 

[DaveC]

I just posted something for you on abse.

Thanks, John. One more option.

 

[Jochem Welvaadt]

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
survive.
Oscillator frequency is not critical, I used 50KHz.

Hope this helps.

Kind regards,
Jochem.