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Re: variac, dimmer switch?

Original poster: Sparktron01@xxxxxxxxxxx


IDK. You could try a _Fan_Speed_Control_ (i.e. designed for running an inductive load). Several folks on list have tried with smaller NST type
transformers and have had "some" success. Realize that fan speed
controllers are usually sized for loads around 500VA or less; max transformer
might be a 12 or 15kV x 30mA NST/OBIT type transformer. $10-15 USD
versus at least $25-50 for even a used small 2-5A Variac might justify
lashing up and trying ("pick your poison, take your chances").

Also see my thread concerning a DC Bridge + IGBT as a PWM controlled AC switch. There is definitely some work left (snubbers to accommodate free wheeling currents when turning off and on, LPF [low-pass filter] optimization based on system/load sizing and PWM carrier frequency, proper control
using a real PWM controller [TL494, 594, 598] with voltage and current FB

End result/goal is a compact controller that will have functionality of a variac
with these additional attributes...

- adjustable VAC out + automatic voltage regulation (independent of load to
  current limit setting)
- independent, high speed current regulation (i.e. integral current ballast)
- high power processing efficiency (85-90%+)
- soft start
- easy build (maintenance, repair, construction, upgrade)
- minimal AC waveform distortion (if average mode control used, if Ipeak
  current limiting is used, AC waveform will "flat top" to limit current)
- same PCB control card could be used (in theory) for a 0.1kVA to a 20kVA+
  controller, only changes would be in power path (rectifiers, IGBT's,
  snubbers, overcurrent protection, hardware controls).

Dave Sharpe, TCBOR/HEAS
Chesterfield, VA. USA

> Original poster: Devon Ferns <dferns@xxxxxxxxxxxx>
> Can you use a dimmer switch to control the input voltage to a NST or is
> that just asking for trouble?
> NST is probably going to be 30mA and 12kV that I can use for larger coils
> in the future, but the coil I'm building is probably considered really
> small. Based off of Derek Woodroffe's coil
> http://www.roffesoft.co.uk/tesla/it/ikletess.htm
> I'm asking if it's ok to use for this coil only. If so is it just in
> series with the input of the transformer or are there other things that are
> needed to be done?
> I know if I was to run a large coil and try and use one it'd not be a good
> idea but for such a small one will it work?
> Devon.