I have 208 VAC coil voltage relay that I want to operate with 240 VAC. Can I drop the voltage down with a capacitor? If so what value? Should the capacitor be rated for 370 VAC? I don't know the impedance of the coil. Can I find it with a meter?
If C1,C2 are capacitors,
Vin----- | = C1 | o-------Vout | = C2 | _ COM
Vout = Vin *C1/(C1+C2)
Can I do it with an 860mfd capacitor and a 140mfd capacitor? Should they be rated for 370 volts?
Can
be
Does the coil have a current rating?
If you use a voltage divider, you're going to be passing more current through the capacitors to COM than goes through the coil, which would be a waste of power.
With your 860 and 140 mfd capacitors in series, it will present an impedance of about 22 Ohms from Vin to COM assuming 60Hz AC. The current, without even the coil load, would be about 11 amps. Nasty.
If you can find the current rating for the coil you might find a suitable series impedance to use rather than a voltage divider.
The coil impedance is almost entirely inductive and the inductance increases when the relay is picked up and the armature completes the magnetic circuit.
Should you be unlucky enough to end up with an equivalent capacitance that sets resonance at line frequency, some very memorable things will happen. Bad things.
Track down the rated coil current and use a series resistor to drop the voltage.
Bryce
I do think you can put 240 directly in it with out much to worry about.
probably
depends on impedance
wouldn't hurt
stick known a resistor in series and measure the operating voltages. then do some complex math.
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