12 Jun
2002
12 Jun
'02
2:02 p.m.
From: ard(a)p850ug1.demon.co.uk
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
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. 
12 Jun
12 Jun
6:30 p.m.
At 04:02 PM 6/12/02 -0700, you wrote:
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
I'd say it is more like 200V at most; the transistors typically
used in electronic ignition circuits are rated at about 450-600V
Vcbo.
The real implications of this issue are on the "cleanliness" of the
12V system in a car. It is actually a pretty harsh environment,
and electronic devices must withstand these ignition spikes as
well as the failure of the battery (in which case, the full
alternator voltage, as much as 40V, will be present, rectified,
but not filtered).
I advised a BSc thesis a long time ago; the student used a
microcontroller to build an adaptive ignition system, but
proceeded (against my advice) to connect directly the drive
transistor to one of the microcontroller outputs, as opposed
to using an optocoupler. When
the transistor failed, the microcontroller (cmos) latched up
and the results were, well, flashing...
carlos.
--------------------------------------------------------------
Carlos E. Murillo-Sanchez carlos_murillo(a)nospammers.ieee.org
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carlos_murillo＠epm.net.co -
dwightk.elvey＠amd.com