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Re: Fan Controller and Variac differences



Original poster: "Jason by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jasonp-at-btinternet-dot-com>

David,

> Dan, Jason
> Fan controller is basically a light dimmer (phase controlled triac) that
> has been modified to run on an inductive load by addition of an RC
> "snubber" network around the triac, and in some units a small RF choke.
> If you look at waveform on an oscilloscope, the power is controlled by
> the phase delay of triggering the device on both sides of the AC waveform.
> A fan controller (or light dimmer) can be used to adjust the voltage on
> HV transformers, but the AC wave is distorted and there are core
> saturation effects that can occur.  Bidirectional SCR's used as high power
> triacs have been used successfully in commercial broadcast transmitters
and
> induction heating for three phase voltage control of HV power supplies
> since mid to late 1960's.
I know how a dimmer works, but if you used a dimmer on an inductive load
wouldn't the feedback blow the SCRs? The possible ring-up would in my
opinion completely fry the SCR.

> A variac is an acronym for VARIable AC Transformer, which is a fully
> adjustable transformer that allows stepless voltage output control with NO
> waveform distortion.  Originally was invented in 1930's by employees of
> General Radio (now GenRad), and other manufacturers used
> contructional modifications to get around GR patent issues.  Key players
now
> in variac buiness include Staco, Superior Electric, and Technipower (now
> owner of Variac trademark).  The no waveform distortion is a compelling
> advantage of this technology, as well as being mechanically and
electrically
> robust and a relatively simple and mature device.  But the additional
> insertion loss in power supplies negatively effects the regulation (how
> "stiff" the power supply is to load changes).  These effects were seen
> on a large 4kW LVDC regulated power supply that I built for an R&D
> experimental effort.  So bottom line, both technologies are similar in
> application
> but different in operation and theory.
I am also aware of how a variac works :) Variable autotransformer, and the
brush varies the number of windings and therefore the voltage. I have never
come across so called fan controllers before and was wondering if they were
a variac or a triac circuit.

Out of interest, I blew a 500W dimmer. Rather than buy a new one, does
anyone know how to hook up two SCR's backl to back?

Thanks
Jason