24Vac relay

[tomy983]

Hi everyone, I need to drive a 24Vac relay coil. The coil will be energised only for short periods of time (at a press of a button).

Can I just use a resistor voltage divider? Is there anything better, excluding a transformer?  Should it be any different from a regular DC divider?

Thank you

[Ian.M]

Insufficient information supplied:
* Drive it from what?   
* Nominal coil current?
* DC coil resistance?

[tomy983]

I should drive it from a transformer. In 230Vac - out 110Vac. At the press of a bottom, it will be shorted.
Actually I made a mistake, the coil will be energised most of time.
The relay is a finder 40.31.8.024.0000
Operating range 19.2 to 26.4 Vac at 50Hz
Resistance 320 Ohm
Current at rated voltage 45mA at 50Hz

http://uk.farnell.com/finder/40-31-8-024-0000/relay-spdt-24vac-10a/dp/1169152

[Ian.M]

Ok.  I can model that relay coil as a 320R resistor in series with a 1.36H choke (as that combo will pass 45mA with 24V RMS @50Hz applied)

From the 220V supply, use a 0.68uF class X capacitor as a capacitive dropper, with a 5.8K resistor in parallel with the relay col to shunt 4mA to reduce the coil current to 45mA.  From 110V you could use 0.47uF || 0.68uF for the cap and leave out the resistor.

N.B the pushbutton *MUST* be mains rated.

[tomy983]

Ok.  I can model that relay coil as a 320R resistor in series with a 1.36H choke (as that combo will pass 45mA with 24V RMS @50Hz applied)

From the 220V supply, use a 0.68uF class X capacitor as a capacitive dropper, with a 5.8K resistor in parallel with the relay col to shunt 4mA to reduce the coil current to 45mA.  From 110V you could use 0.47uF || 0.68uF for the cap and leave out the resistor.

N.B the pushbutton *MUST* be mains rated.

Thank you Ian.
I will use the two capacitor (I assume they need to be those rectangular ones with the plastic case..)
The pushbutton is already there, it's a industrial wood processor..

Can I ask you for some keywords to have a look at the suggestion you gave me?

Just out of curiosity, is a resistor voltage divider not suitable in this case?

[Ian.M]

They *must* be sold as class X capacitors, rated for continuous operation across the AC mains supply.  DO NOT SUBSTITUTE with unrated caps.

You could use a resistive dropper, but even from 110V you'd need something  like a 2K 5W power resistor (and would have to do some analysis to confirm the exact value) and it would run quite hot.

N.B. please check the voltage across the relay coil before putting it into service.  If its over 26V, add a shunt resistor across the coil to bring it down a bit.  e.g.  6.8K (1/4W) would knock off about 1.9V.

[tomy983]

Thank you!

[Ian.M]

N.B. for safety you should fit a 1 Meg (1/4W) bleeder resistor across the class X caps as 1.15uF (0.68 || 0.47) charged to 150V, (if the switch opened at the peak of the 110V RMS waveform) would have quite a nasty bite.

[Zero999]

This is is a bodge. Why not simply use a relay with the correct coil voltage?

Another option is a 24V transformer which has the added benefit of a lower voltage for the push button.