Re: Economizer circuit

[Lee Hart <leeahart@earthlink.net>]

From: Michael Bearden <techsan@pacbell.net>
> I bought a contactor (Kilovac) which says 'Requires economizer
> circuit". Is this the "snubber" which I have seen discussed,
> or a different animal altogether?

It's a different circuit. It means the coil is not rated for 12 volts
continuously. You need to apply 12 volts to make it pull in quickly, but
then reduce the coil voltage or the coil will overheat and fail. I don't
have specs for that Kilovac handy, but it is probably about 1/2 of
normal coil voltage.

The simplest circuit is to wire a light bulb in series with the coil.
When it is cold, a light bulb has about 1/10th its resistance when hot.
Pick a 12v bulb that puts about 4-6v across the coil.

Or, put a "slugger" circuit in series with the coil. This is a resistor
and capacitor in parallel. When you first apply power, the capacitor
looks like a short, and applies full power to the coil. Then the
capacitor charges, and the resistor limits coil current. Choose the
resistor to be about the same resistance as the coil resistance. Choose
the capacitor for about a 0.1 second time constant; C = 0.1 / R (where C
is in farads, and R is the coil and resistor in parallel's resistance).
Example: 12 ohm coil, 12 ohm resistor; is 6 ohms in parallel. C = 0.1 /
6 = 0.016 farads = 16,000 microfarads.

Or, hire a Microsoft engineer to program a dedicated microcontroller to
pulse width modulate coil current, correcting for battery voltage,
temperature, etc. Shouldn't cost more than $50-$100. :-)
-- 
Lee A. Hart                     Ring the bells that you can ring
4209 France Ave. N.             Forget the perfect offering
Robbinsdale, MN 55422 USA       There is a crack in everything
phone (612) 533-3226            That's how the light gets in
leeahart@earthlink.net                  Leonard Cohen