sine wave dimming

davt · Joined: 07 Oct 2003 Posts: 66 Location: England

Hi all Happy New Year!!

I wish to design a sine wave dimmer for both resistive and inductive loads 240v ac 50hz.
Has anyone out there had any such experience and could offer some advice and possibly some circuit considerations before I proceed to get my fingers burned!!
I have read a few articles which explain the basic principles of using a controller to PWM the mains supply using igbt's but need a bit more help to get started.
Many thanks.

Dave.

Pete Smith · Joined: 17 Sep 2003 Posts: 55 Location: Chester, UK

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

Does it REALLY need to be a sine wave or just AC?

A Triac will give a chopped AC signal which is fine for incandescent lights and most resistive loads but they are noisy. They are the usual way of doing lighting.

If you really need a SINE wave things get a lot harder. Basically you have two choices, generating a variable amplitude control signal and amplifting it, or some type of tapped transformer.

Tapped transformers are not very trendy these days but if you only need a few bits of resolution they are not bad. For 5 bits, 32 brightness levels, you would have five transformer secondaries controlled by SPDT relays. There are some 1kHz digitizers designed in the 1970's that did this to 22 bits for precision oceanographic work. Back then there weren't a whole lot of choices to get that kind of resolution, and some of those digitizers are still in use today!

Another path for a tapped transformer is a variac, either manually controlled or by servomotor. They have been used in theaters almost as long as they have had electric lights. You might find some good surplus deals.

Tell us more about your application and we can narrow the choices.
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davt · Joined: 07 Oct 2003 Posts: 66 Location: England

Hi all,

Thanks for your replies. Smile

I have been down the road of using triacs.

What I want to do is pulse width modulate the mains ( 240v 50hz )at about 40 khz and filter the output to give me back a sine wave.
I was wondering if anyone has taken this route to control power to a load?
Thanks

Dave

John P · Joined: 17 Sep 2003 Posts: 331

Well, speaking as someone who's never done it...

You could do it the way that switching power supplies work (sort of). Rectify and filter the incoming power line. Then feed the resulting DC supply via two power transistors into a center-tapped transformer primary, with the center tap connected to ground. The transformer secondary would be your output. Use of the split primary allows you to generate an output that's negative some of the time even if your available supply is always positive. Obviously, your power transistors must be rated for the voltages and currents that are involved, and they won't be inexpensive. But anything for science, right?

Edited a few minutes later to add: my suggestion makes the assumption that it's OK to have the power transistors and/or tranformer primary at some potential dependent on the incoming power line. It may be possible to have the driving electronics (microprocessor, etc) at a related potential, but in that case there won't be any usable "ground" that's at earth-ground potential, which is an awkward and possibly dangerous situation. You could get around this by using an isolation transformer at the input.

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

I have never done it that way, but it sounds good to me. Look into "Class D" audio amplifiers. There are some new chips that do very close to what you want, Texas Instruments TPA3001D1 for example.
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The search for better is endless. Instead simply find very good and get the job done.