I hate Radio Shack

From: Dwight K. Elvey <dwightk.elvey_at_amd.com>
Date: Wed Jun 12 18:02:39 2002

>From: ard_at_p850ug1.demon.co.uk
>> >
>> > Just a sanity check, here. What would be the _DC_ rating of a
>> > momentary switch rated at 3A, 125VAC?
>> >
>> > Doc
>>
>> About 175 volts. Most 1 farad caps are rated at 5.5 volts.
>
>Never!. A DC arc is much harder to extinguish than an AC arc (the
>simplistic explanation is that the AC voltage drops to 0 twice per cycle,
>thus helping to extinguish the arc). The DC rating of a mechanical switch
>is not sqrt(2) times the AC rating.
>
>It's a lot less. I would not want to use that swtich _for resistive
>loads_ at more than about 24 volts. For significantly inductive loads it
>would be even less than that.
>
>When I was at university there was a physics experiment involving a large
>solenoid coil powered from a 24V 10A (or so) bench PSU. The switch was
>one of those old knife swtiches, which could therefor be operated slowly
>(no spring mechanism). I found I could easily generate and maintain
>copper arcs of about 3/4" between the switch contacts (No, that wasn't
>the point of the experiment, but it was fun...). Admittedly a quick
>break switch would make it harder, but then again the average small
>switch has pretty small contact gaps when open.
>
>-tony
>
>


Hi
 This is why there is a condenser on the points of a car.
This allows the points to open before the coil has a chance to
build up too much voltage on the primary. The size of the
capacitor is a compromise between getting the points open
and too much current when the points close. Many older car
manuals would tell you to look at the points to determine
when the capacitor was too large or too small by the amount
of material transfered from contact to contact by the arcing.
For those that haven't tested this by hand, the primary
will kick up to about 400-600 volts because of flyback.
It will make you jump a little.
Dwight
Received on Wed Jun 12 2002 - 18:02:39 BST

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