ken via EV wrote:
AC switches are built lighter duty, to save on cost, so using the ac power
plug for swithcing current is a better way.
It's not really that the switch is cheap; it's just that it was only designed
for AC. An AC switch can open relatively slowly, because the duration of the arc
is limited by the time to the next zero-crossing of the AC line. Bill had a good
explanation of the problem.
Bit AC/DC rated switches are still fairly common. 120 volts *DC* was a standard
in the US for a long time (in the days of Edison). You can still find old
switches in basements and garages left over from the time. They have a loud,
distinctive "click" when switched.
Pulling the plug to turn something off on DC is also a poor way to do it. You
still get substantial arcing, which will damage or destroy the plug and outlet.
If you really must use an AC-rated switch on DC, one band-aid is to add a
resistor-capacitor "snubber" across the contact. See
<http://www.sunrise-ev.com/LeesEVs.htm#snubbers>. A snubber substantially
reduces the amount of arcing.
Even though you can run these devices on DC, the switches on these
appliances are not suitable for DC, only AC. You risk a fire. You can close
the switch just fine on, say, your hairdryer, but when you attempt to turn
it "off", the DC will often simply arc across the switch, and current will
continue to flow. This arc will start a fire.
AC turns off and switches polarity 120 times per second, which
extinguishes the arc when switches and fuses open. DC is steady with not
breaks or reversals.
You get a small warning when this happens. The switch will make a small
"hiss" and the appliance only partially shuts off. Turn the switch back
on and pull the plug. There will be an arc at the plug and you may avert a
fire. The switch is typically toast, however.
If we knew what we were doing, it wouldn't be called research, would it?
-- Albert Einstein
Lee Hart, 814 8th Ave N, Sartell MN 56377, www.sunrise-ev.com
Read EVAngel's EV News at http://evdl.org/evln/
Please discuss EV drag racing at NEDRA (http://groups.yahoo.com/group/NEDRA)