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Re: AC/DC, No not 8 tracks (boring)

Subject: Re: AC/DC, No not 8 tracks (boring)
Date: Sat, 7 Sep 1996 01:13:36 -0400
>I thought that in the entire Radio Shack store, someone could tell me how
>many amps a 125 volt ac 10 amp switch would handle when run on 12 volts dc.

Well, now to make it boring...

If you double the voltage on a 125vac sw, the current rating is usually
halved; this has nothing to do with power ratings, but because as you
separate the contacts you get a arc going on (just like striking a arc w/ an
arc welder, for those of you who use one), and the higher voltage translates
into a longer (length and time-wise) arc.  Usually 125/250vac switches switch
the contacts apart faster (a snap action) that tries to minimize this.  I
would guess that a 10A 125vac switch is good for between 15-20A @ 12vdc, but
to be safe, it's better to take the current rating at face value & try not to
exceed it's rating.

On 12vdc, there isn't much of an arc, so the contacts don't melt from that,
but the contacts are designed for a certain max current, and if they get too
hot they tend to melt the rest of the switch.
I would say you generally can use a 120vac rated sw at 12VDC (although 120v
switches may allow more voltage drop across them-this might affect something
like lights that need every last bit of power or are dimmer)(as an example,
120v switch#1 drops 2V at 10Amps, at 120v 120-2v=118v.  Same thing at
12v-10A; 12v-2v=10V, a much more significant voltage (& power) loss.  True,
switches don't drop 2V (usually around .25-.5V at rated current for 120V
switches), but it still makes a difference.), but to not use 12VDC switches
at 120V-12V switches are usually built poorer, and most don't switch fast so
the contacts get burned a lot quicker than they should.

I said it was boring!  Also not as clear as it could be-it's after midnight &
I'm off to bed. <snore>

Take care, all!  Scott M Ryan

PS: if I didn't say so above, you can safely use the listed current rating as
long as you have equal or less voltage across the switch (when it's open)
 Inductive loads (ie fan motors) may degrade this rating slightly, but then
motors usually draw a huge current when you first turn them on, and it's the
current when turning it off that is related to the degree of sw arcing.

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