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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