Variacs, autotransformers and light dimmers
 

Hi Folks

I've been playing with a variac (borrowed from work), an ancient 3-tap autotransformer which was once attached to (I think) a fan motor - must've been asynchronous - and I also have a few old light dimmers kicking about.

For voltage control on 240v kit (running old gear up to speed, for example) I've seen lamp limiters discussed - sounds good - and also variacs - OK if you have one.

The variac I've played with seems to control voltage even with no load (employs feedback?) whereas the variable 'L' of the autotransformer only drops voltage if you pull a current through it...OK if you measure L and know what current you plan to draw I guess...then it's v_drop=j(2*pi*50)*L*i...

But what about light dimmers? The ones I have lying about seem to be rated at up to 400W, which seems adequate - is it that people object to any wierd waveform they may create (HF harmonics defeat smoothing to some degree?) or is it that they are reputed to generate EM interference which perhaps corrupts the signal chain?

I know I could just experiment, but hey, what are web forums for...:)

cheers
Mark

Re: Variacs, autotransformers and light dimmers
 

Hello Mark, the variac is good, would recomend an isolating transformer, lamp limiter if you need one (I never used one) I would forget dimmer control to much mush. Make sure any mains derived supply (variac, Isolating Trans. etc) is rated, wired up and fused correctly for safety.

Lawrence.

Re: Variacs, autotransformers and light dimmers
 

Apart from the interference (switching noise) generated by light dimmers, the output voltage is still potentially 240V, just not for the full duration of the sine wave, this is how lamps appear to dim as the control is reduced. For this reason, they are not well suited for vintage radios.
Neil

Re: Variacs, autotransformers and light dimmers
 

gotcha - with the light dimmer it's an 'area under the curve' thing. Well, the old fan controller drops real (edit - I should say 'imaginary', but you know what I mean) volts and stays more-or-less sinusoidal, so I think it'll form part of my emerging test power supply.

Sorry for the flood of emails, by the way. I've been interested in this stuff for years and collecting old c**p for a long time, but only just found the forum. It's very satisfying to start doing something with it all...and sad to think of those odds and sods which I got frustrated and slung before finding you all (Valradio inverter and nixie tube counter both sting a bit...).

Re: Variacs, autotransformers and light dimmers
 

An autotransformer or variac (variable autotransformer) should still produce a reduced or increased voltage even on no load. They don't work by varying series inductance but by varying the number of turns of the "secondary" winding, so the transformer turns ratio determines the output voltage, not any load applied.

If yours does as you say, then either it's not a variac, just a variable inductor, or the neutral has fallen off the bottom of the winding!

Re: Variacs, autotransformers and light dimmers
 

I looked at Wikipedia variac article again - you're right, I think I wired mine as a variable inductor. Oops, I'll try again.:dunce:

Re: Variacs, autotransformers and light dimmers
 

I acquired a 2KVA variac when I worked at GEC in the 1980's; dozens of them were been thrown out - there was a mountain of them all being replaced by thyristor units :o. It has been my standard practice for many years to use the variac to bring the power up over a a few seconds on almost anything I have that has not been powered up for any length of time and I think it is a good thing to do, giving elderly capacitors a gentle wake up call.

I also have a constant voltage transformer, cannot remember the details of the type but originally bought to use with some sensitive scientific equipment as it also cleans up the mains of spikes. I gather that other uses for them was to keep photographic lights at a constant colour temperature and keeping ultra-purist HiFi enthusiasts happy that their 240V supply is nice and clean.

Re: Variacs, autotransformers and light dimmers
 

Claude-Lyons, perhaps? I've used their CVTs in the past, and very good they are too - even if a bit heavy! On account of the design, as a bonus, it was also an isolation transformer. I put it on a wooden frame with heavyweight castors so that I could move it around. When not in use, it made a wonderful door-stop!

Al.

Re: Variacs, autotransformers and light dimmers
 

No, I just checked out the CVT and it is a Cetronic ‘Reguvolt M100” Constant Voltage Transformer. Output is 240VAC (within +-3%) for fairly wide range of inputs. I am fairly sure these did not provide isolation. Obsolete but Cetronics still going. V Heavy!

Re: Variacs, autotransformers and light dimmers
 

I remember tracing out the circuit of that C-L CVT. The primary consisted of a split winding with one physically large capacitor between each half. The secondary was one winding with three similar capacitors wired in parallel and across that winding. I can't remember the values of those capacitors, but they were probably in the region of 1 → 8 µF each: big, metal-clad rectangular affairs - they certainly weren't cylindrical electrolytics! ;D

The more I write about this, the more comes slowly creeping back. IIRC, there were two types that were generally manufactured with slightly different characteristics to cater for different needs. Perhaps yours is one of the 'other types'.

Al.

Al.

Re: Variacs, autotransformers and light dimmers
 

Al: You're right. One style produced a kind of trapezoidal output waveform: not quite square, but flat-topped. Ideal for feeding bridge recitifiers. The other style produced something close to a pure sine-wave.

John

Re: Variacs, autotransformers and light dimmers
 

Thanks John: it's all come back to me now! It must have been in the mid 1970's when I met that CVT I refer to. (I didn't realize that my 'mental archiving' was that reliable! ;D) So now things are becoming much clearer as bits come out of the fog ~ that C-L one was sometimes referred to as a 'resonant transformer', which, at first, seemed a rather odd name to me ??? . . . 'till I traced out the circuit 8-). Hence, this was the type that produced an almost pure sine-wave - which for the application in question, was a necessary requirement. I now recall that this type required a lot more iron than the other type, and on the basis of what we now know about these two types, it now all makes sense.

Al.