Re: Variac / Cheap dimmers

From: 	Jim Lux[SMTP:jimlux-at-earthlink-dot-net]
Sent: 	Friday, January 09, 1998 9:56 AM
To: 	Tesla List
Subject: 	Re: Variac / Cheap dimmers

> For one thing, triacs are only used in _cheap_ dimmers.  In professional
> dimmers and high powered motor controls, back-to-back SCR's (sometimes
> in discrete packages, sometimes in pre-combined packages) are used.
> The reason is that a triac has four junctions through which current must
> pass, but an SCR only has three.  Believe it or not, this results in a
> well-designed SCR having ten times as much surge current capability as a
> triac with the same RMS current rating.

The better surge current is just one reason. A bigger reason, particularly
with inductive loads, is the dv/dt problem. The device (either triac or
scr) needs a little bit of time to turn off after the current goes to zero
before it can hold off the voltage. The time varies with the rise time of
the voltage. A triac, working with both half cycles, has to recover
essentially instantaneously, an SCR has a whole reverse biased half cycle
to recover. With an inductive load, the current lags the voltage, so when
the current goes to zero, and the device turns off, there is already a
growing voltage across the device (reverse in the case of an SCR, forward
in the case of the triac). Compare this with a resistive load: when the
current goes to zero, so does the voltage.

Hence, the popularity of back to back SCR's in higher power equipment.
>  Such cheap devices
> don't handle high frequency RF well, either, apparently...

You've hit the nail on the head."CHEAP" A consumer light dimmer or fan
speed control RETAILING for only $5, can't have more than about $0.50 worth
of parts in it, with the  single triac comprising a significant fraction.
Adding the three or four parts necessary to do it with SCR's would make the
dimmer cost $20.