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Control of inductive loads by a triac - 'solid state variac'

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Marko

Tue Jun 27 2006, 09:49AM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

PS. fixed broken link.

I recently read ST microelectronic's app note about controol of inductive loads with triacs, without complex logic or special IC's.
(etc. known scopeboy's 'solid state variac')

Their regulator is basically a dimmer, with additional triac (pulse train generator) used for proper triggering of main triac. (there are waveforms shown)
In the end they show some real application schematics, if it works with an arc welder I guessed it could also work for stuff like NST's, MOT's, varying voltage to mains rectifiers and etc.

] 1151401778_89_FT0_triac__inductive_load_ssvariac.p df[/file]

Another (tested) schematic from different source

Bennem

Tue Jun 27 2006, 05:33PM

Registered Member #154 Joined: Sun Feb 12 2006, 04:28PM
Location: Westmidlands, UK
Posts: 260

I built a similar version of Steve Conners second solid state variac circuit, for my DRSSTC coils.
works great!

1151429594 154 FT12183 Soild State Variac

Marko

Fri Aug 04 2006, 10:35PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

I did some work on this thing recently;

Circuit works nice, altough it needs some furnishing between C1 and the pot in order to get desired regulation range.

Real test will be controlling an inductive load instead of bulb and scoping the waveforms on various power settings..

Waveform looks like this with a lightbulb, on some 20% setting. Noe that negative halfwave owns a bit higher amplitude than upper halfwave. Triac seems to be a bit faster on triggering there, and I'm not sure why yet. Effect corrects itself with higher settings.

Marko

Sat Aug 12 2006, 09:12PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

After assmembled circuit proved to be working I started playing with various loads.

This is secondary voltage of a small 40W, 36VAC transformer. It was important that there were no voltage spkes at any setting (with no load) wich would generally be bad for HV transformers.

I also wanted to see how does the circuit cope with a pseudo-capacitive load (rectified mains line). I hooked it up to a computer SMPS; it looks good when lightly loaded, altough a PFC choke in series with input would be a good idea. It could also help limiting inrush current.
I could also use a little more snubbing since waveform got a bit asymetric.

Finally I tried to regulate a small 15kV 20mA OBIT and it seems to be working good.

Not as close as 'smooth' like normal variac but it is usable as low-cost dimmer variant.
I could nicely regulate power (and spark length) without side-effects trough entire renge.

Steve Conner

Sat Aug 12 2006, 10:56PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

That's cool Firkragg! The ST circuit with the diac and two triacs seems to do exactly what my two solid-state variac circuits did, but it's much simpler

I once saw funny stepped waveforms like the ones you got when driving the SMPS. I think it's caused by the load current being too small to hold the triac on, so it keeps turning off and then getting retriggered by the next pulse from that hard firing network.

Marko

Wed Aug 16 2006, 11:57AM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

I found hat increasing snubber capacitor value across the triac helps making waveform symetric in most cases. (problem is that this increases leakage current a bit)

One another thing is instability of output; when 10k reisstor heats up (and it gets quite hot) setting 'increases' a little until resistor settles on it's working temperature.
And it's simply unpractical and un-elite to have a ig hot resistor in the circuit.

Improvment could be using a separate power supply (especially if more than one regulators are used) but a little problem would be finding a transformer with 68V secondary :/

I guess something like 40..48V would work nicely, just anything that would be over diac firing voltage.

Steve Conner

Wed Aug 16 2006, 01:50PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

What part numbers of triac and diac did you use in the circuit btw? I'd like to try building one here.

I think it would make a good power control for Terry Fritz's SISG system. My plan is that the "bits missing" from the voltage waveform caused by phase angle control will help the SISG to quench enough that I can get rid of the power resistors in Terry's current design. I thought of using my own solid-state variac but it seemed too complicated to fit well with the SISG idea.

Marko

Wed Aug 16 2006, 02:20PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

I used that little red diac from a energy saver bulb. It's a common DB3 diac from ST

Triacs I used were both BTA12-600 triacs because I had a lot of them at hand.

Trigger triac can be any small triac like TLC221 (as shown in AN) or similar.

Instead ov transzorb shown in AN I used two antiparalell zeners and they worked fine.
Most component values probably aren't very critical..

Part Scavenger

Sat Aug 19 2006, 01:45AM

Registered Member #79 Joined: Thu Feb 09 2006, 11:35AM
Location: Arkansas
Posts: 673

This might be a dumb question, but heck, I don't know the answer. Will the SS variac drive a doubler well? I'd like to use that on my coils, but I don't have 240 everywhere I go.

Marko

Sat Aug 19 2006, 01:56AM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Yes it will drive a doubler, altough peak voltage may differ a bit with setting.

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