Re: triacs

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "davep by way of Terry Fritz <twftesla-at-qwest-dot-net>" <davep-at-quik-dot-com>

Tesla list wrote:
> 
> Original poster: "Jan Florian Wagner by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <jwagner-at-cc.hut.


 > > A fellow at uni gave me half a dozen 30 A triacs for free today. Is
> > there anyway I can use these in parallel or whatever to replace a
> > variac? I have never seen anyone use a triac for voltage regulation for
> > a tesla coil before.

	cf any of the archived discussions of light dimmers.
	
> A variac can be thought of as an inductive voltage divider, exactly the
> same way like a potentiometer.

	I wouldn't think of it that way.  There is some similarity,
	many differences.  The the losses inherent in a potentiometer
	are essentially absent, improving efficiency and cooling
	problem.  The variac is a tapped, variable transformer, so
	power in is roughly power out, the voltage reduction being
	accompanied by a current increase, via the mag flux in the core.

> The output tap gives you some fraction of the input voltage, i.e. you
> can smoothly control the output voltage.

> With triacs you can only control the output power (for the case of AC
> output, which you need to drive the HV transformer prmary), which is done
> by "chopping up the mains voltage into small bits".
> See:
> http://www.st-dot-com/stonline/books/pdf/docs/3566.pdf
> 
> Triacs work just nice nowadays even with inductive loads like
> transformers (NST, MOT, whatever, dunno about pole-pigs though ;o), but I
> think those 30A ones might be rather low-voltage ones and not suitable for
> 2..3*mains voltage(?).
> 
>  - Jan
	best
	dwp