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Re: Silicon Controlled Rectifiers...
Original poster: David Speck <Dave@xxxxxxxxxxxxxxxx>
Chip,
In the DC domain, SCRs are just on / off devices. They are a
"controlled Rectifier" by definition -- either they are working as a
typical rectifier diode, or they are in a high impedance state, and
shut off. Once turned on, they stay conducting until the forward
current through them drops to some very small amount, and they they
stay in a blocking configuration until triggered again.
They can be used to modulate AC power, however, by turning them on a
different times. If you enabled a SCR only every other cycle, then
you would in effect be passing 50% power. When the AC voltage passes
through zero, the current through the SCR drops to zero, and the
device returns to a nonconducting state until triggered again.
You can even adjust the trigger time within a single 60 Hz cycle, to
give even finer control of the power. If you wait until, for
example, the very peak of a 60 Hz sine wave, and then trigger the SCR
into conduction, the load will see only 25% power. This is basically
how light dinners work -- they employ a variable time delay which
triggers the SCR (Actually, a Triac, or bidirectional SCR pair) at an
infinitely adjustable time during the ^) Hz cycle. (To be honest,
light dinners are trickier than that, but that's beyond the realm of
this discussion).
HTH,
Dave
Tesla list wrote:
Original poster: "Chip Ford" <chipford@xxxxxxxxxxxxxx>
I am just learning about this electronic stuff so, Can someone
please tell me if a SCR has states other then on and off. Can you
vary the amount of power passed thru them. I was thinking (probably
wrong again), that they either pass electrons or they don't. Thanks
in advance..
Chip