drop AC voltage question

Tue Jan 05, 2010 6:22 pm   

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)

208/240 = 86/(86+14)

Can I do it with an 860mfd capacitor and a 140mfd capacitor? Should they be
rated for 370 volts?

Tue Jan 05, 2010 6:47 pm   

Jon wrote:

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.

Tue Jan 05, 2010 8:32 pm   

Jon wrote:


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

Wed Jan 06, 2010 3:12 am   

Jon wrote:

Wed Jan 06, 2010 2:13 pm   

On 2010-01-05, Jon <jon8338_at_peoplepc.com> wrote:

probably


depends on impedance


wouldn't hurt


stick known a resistor in series and measure the operating voltages.
then do some complex math.



--- news://freenews.netfront.net/ - complaints: news_at_netfront.net ---