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davt
Joined: 07 Oct 2003 Posts: 66 Location: England
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sine wave dimming |
Posted: Tue Jan 06, 2004 9:45 am |
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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. |
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Pete Smith
Joined: 17 Sep 2003 Posts: 55 Location: Chester, UK
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Re: sine wave dimming |
Posted: Tue Jan 06, 2004 12:03 pm |
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davt wrote: | 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. |
Have a look at PICREF-4, a lamp dimmer using a 12???? PIC and a TRIAC.
From this point on, I will use the term "Brightness" to mean the brightness of a lamp, or the effective amount of power applied to a load.
The TRIAC they use can only switch 100W, but you should be able to get a more robust one. If necessary, you use the TRIAC they use to turn on a bigger one which does the switching - there should be plenty of resources on the net to help with that.
As for the theory behind it :
The TRIAC can turn on the mains supply. It can only turn the supply off when the supply drops to 0v.
You need to monitor the zero crossing of the mains supply. Traditionally this is done using an opto-isolator, and some resistors. The mains drives the LED in the opto-isolator, the PIC monitors the transistor output of the opto isolator. When the supply is in the positive phase, the transistor is on, so the output is low. When the supply is in the negative phase, the opposite is true.
You'll get a positive or negative edge whenever the mains is at the zero crossing point.
To get full brightness, you turn the TRIAC on permanently
To get full off, you never turn it on
To get a partial brightness, turn the TRIAC on, and then immediately off - it will stay locked on until the supply drops to zero.
Assuming a linear brightness change (which is not the case!), to get 50% brightness, you'd turn the TRIAC on half way through a power cycle. In your case (UK/EU?) this will be 5msec.
To simply do it, monitor the zero crossing pin, when it changes, wait a short while (how you do this is up to you ) then turn the TRIAC on for a little while, and then off again.
You'll find that the relationship between brightness and time is non linear, and will vary product to product.
Hope this helps,
Pete. |
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SherpaDoug
Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA
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Posted: Tue Jan 06, 2004 1:16 pm |
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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. _________________ The search for better is endless. Instead simply find very good and get the job done. |
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davt
Joined: 07 Oct 2003 Posts: 66 Location: England
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sine wave dimming |
Posted: Wed Jan 07, 2004 10:52 am |
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Hi all,
Thanks for your replies.
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 |
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John P
Joined: 17 Sep 2003 Posts: 331
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Posted: Wed Jan 07, 2004 12:07 pm |
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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. |
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SherpaDoug
Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA
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Posted: Wed Jan 07, 2004 12:22 pm |
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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. _________________ The search for better is endless. Instead simply find very good and get the job done. |
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