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'[OT]: Solid state relay'
2001\08\02@172214 by Barry Gershenfeld

picon face
>>If it's a triac device it will only work on AC, as the triac will stay
>>conducting until the current crosses zero.
>
>While this is correct there might be situations where you could use the
>behavior to advantage on DC. Shorting the output with a momentary
>switch (or interrupting the current) will turn it off ( assuming dv/dt isn't
>too high). So, for a security alarm powered from a 12VDC battery, it might
>be just what you want.
>Spehro Pefhany

The triac is two SCR's back-to-back.  An SCR is what you'd
use if you had DC, since you'd be wasting the other "half"
of the triac.  Of course it's now been revealed that
they don't switch themselves off unless you cut the power
briefly (Mr Gates take note :)

So I was really intrigued to find out these are the things
that drive your camera's strobe flash.  The smart ones
adjust for the distance (they actually read the light
coming back) and will vary the duration accordingly.  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.

Barry

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2001\08\03@050957 by Alan B. Pearce

face picon face
>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.

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