I hate Radio Shack

From: Carlos Murillo <carlos_murillo_at_epm.net.co>
Date: Wed Jun 12 22:30:24 2002

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_at_nospammers.ieee.org
Received on Wed Jun 12 2002 - 22:30:24 BST