Re: DC switches - where do ya get 'em?

[Lee Hart <leeahart@earthlink.net>]

Wilson, John A wrote:
> Where can one get high voltage DC switches? 120V DC and 10 amps...

Any switch with large contact spacings can switch DC (knife switches,
for example, would look really neat on your dash :-). 

Once upon a time, home 120v power could be AC *or* DC. Switches were
dual rated, 120v AC/DC at 15 amps, etc. So look for old switches
(probably still in use in your grandparent's home :-). The main
characteristic is that they made a loud "CLICK" when switched, and have
*huge* contact spacings when open (1/4" to 1/2").

A switch's DC voltage rating is about 1/4 of its AC voltage rating. A
115-120vac switch is thus rated at 28-30vdc; a 220-240vac switch at
60-65vdc. These ratings are commonly marked on switches, circuit
breakers, relays, etc.

You can wire switches in series to increase their voltage ratings, as
long as they have snap-acting contacts and *ALL* switch simultaneously
(i.e. are all physically part of the same switch or relay).

If each pole of a double-pole toggle switch is rated at 240vac 60vdc,
you can connect the two in series to switch 480vac 120vdc. Just be sure
the rating applies to each contact, and is not the rating for both
contacts in series.

RF antenna switches and relays have large contact spacings and high
voltage ratings. They can successfully switch DC.

You can put a "snubber" network across a contact to extend its life on
DC (or AC, for that matter). The DC snubber is a resistor and capacitor
in series, connected across the switch contact, plus a diode in parallel
with the resistor and oriented so the capacitor charges thru the diode
when the switch opens, and discharges through the resistor when the
switch closes. The capacitor value should have a 1-10 millisecond time
constant with the load resistance being switched. The resistor is chosen
to limit switch current when the switch turns on and the capacitor is
charged to the full DC supply voltage. The diode needs a current and
voltage rating above that of the load being switched.

As examples, C&H Sales 1-800-325-9465 lists:
- 1-pole 4-position rotary switch, 30amp 250vdc, #SW9456 $19.95.
- 2-pole 10amp 65vdc circuit breaker (use the two poles in series),
  #CB9101 $15.00. 
- 1-pole double-throw antenna relay, 15amp 5000vdc (yes, that's right!),
  12vdc coil, Kilovac #K41C234, stock #RL9703 $39.95.
-- 
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