>But how do you switch that SCR off?  Turns out, you switch
>a capacitor across the device which diverts the current
>around it for just a few micro(?)seconds and that's
>enough to turn it off.  Pretty impressive, I thought.

The earliest schemes just had another SCR that turned on into a dummy load
to dump the excess charge from the capacitor so turning off the flash tube.
This is wasteful as you always have to give the capacitor a full charge
before the next flash. Then came the sort of scheme you mentioned which
saves fully discharging the capacitor, hence making the batteries last
longer.

There are also commutating schemes using an inductor.

Another trick is to use a GTO SCR (Gate turn off SCR). These are a specially
processed chip where it is possible to turn off a conducting SCR by pulsing
the gate with a large enough voltage to stop the conduction. I believe these
get used in car ignition systems where it is desirable to have multiple
sparks over a longer period of time than the very short spark often used in
the early capacitor discharge systems.

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email listserv@mitvma.mit.edu with SET PICList DIGEST in the body