I want to dim a 120Volt-60Watt light with a digital controller. I already have a circuit that use a Triac but I would prefer to use a power MOSFET.
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For the ones who want to know why I would prefer MOSFET, some reasons are summerised at
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like these: Reverse phase controlling has some advantages over traditional dimmers in many dimmer applications. The manufactuers of inverse phase dimmers adverstise their products to be more efficent and less noisy. Using proper controlling electronics it is possible to build a reverse phase dimmer without any magnetics or vibrations caused by them. Because turning on point is always exact at the zero phase there are no huge current spikes and EMI caused by turn on. Using power MOSFETs it is possible to make the turn-off rate relatively slot to achieve quite operations in terms of EMI and acoustical or incandescent lamp filament noise.
For the ones who want to know the basic functionning of a light dimmer with Triac:
The Triac is turned on with the impulse current on the gate and shuts off when the AC is zero. The MOSFET turns on when the AC is zero and shuts off with the impulse current on the gate.
Besides this basic functionning, my question is: Is there any other considerations I should take into account by replacing a Triac by a MOSFET in a circuit?
Do you have a dimmer circuit with MOSFET to suggest? Where in the net I could have more information about light dimmer circuit with MOSFET, IGBT...?
"We are sorry... ...but the URL you are trying to access could not be found or is not available any more."
Interesting site though -- I notice it links to an article I must have posted 10-15 years ago on dimming fluorescent lamps.
I built a MOSFET power controller -- effectively a dimmer except used for a heating load. My circuit actually chopped the middle out of each half-cycle by switching off when the full wave rectified sine wave voltage exceeded a threshold, which was adjusted so the resulting output voltage was 120V RMS.
The reason was that this was to drop the effective voltage so I could run a 120V heating appliance (electric frying pan) from a 240V source. I wanted to minimize the peak voltage in the element, to reduce the risk of insulation breakdown. Another side effect is that although the power factor was only 0.5, I was only drawing current in the portions of the sine wave where there's an excess available, and not at the peaks where there's an overload on the supply network due to switched mode PSU appliances, so power generating plant probably though I was the best type of load it could possibly have;-).
I used two MOSFETs opposite way round in series, utilising the reverse polarity diode in each to conduct when polarity was wrong way round for that FET. An alternative would be to use one FET and a full-wave rectifier which would be fine for 60W, but in my case it was a 10A load and the bridge rectifier would have been one extra PN junction heat load to dissipate which I wanted to avoid in a unit which was only a little bigger than a large wall-wart.
I don't know how well a triac would handle end of life lamp failure where an arc manages to establish in the bulb. OTOH, triacs are not known for surviving such events too well either -- they usually blow quite a bit faster than any fuse protecting them;-). In the case of a heating element, I didn't care about end of life failure, since if that happened, this circuit would become redunant anyway. Also, MOSFETs are static sensitive so you need some protection built into the circuit so it doesn't get zapped whilst being handled, whereas triacs are nowhere near as static sensitive and don't need any protection unless going into a particularly harsh environment.
The MOSFET should be suitable for the given application (vortage and current ratings). Triacs can pass AC power to both directions. MOSFETs work only on one direction, so you need to either use two MOSFETs (one for each AC current direction) or use one MOSFET plus a rectifier. On the control circuitry you need to take care of the following things:
Triacs are triggered with current usually at low voltage, while MOSFET is a voltage driven (make sure control circuit gives high enough voltage)
MOSFET needs drive signal all the time it needs to conduct, for a triac only pulse is enough. Some TRIAC circuits send contant drive from the start of conduction to the end of phase to triac (those suit for FET as well) while some other designs just send s brief pulse at one point of phase where triac is needed to start conducting (does not suit well for FET application).
Those are the first points that come to my mind on this.
CMOS gate implements reverse phase control - this circuit implements a "reverse" phase control, using only a single CMOS 4001 quad NOR gate, conduction begins at the zero crossing of the ac sine wave and the turn-off timing is adjusted based on dimmer setting
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Take a look at the links at
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--
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at
http://www.epanorama.net/
So why are reverse phase dimmers/mosfet/igbt so rare?
Avolites had a prototype on their stand at PLASA a while back but the production models went back to triacs.
Seem to remember there being a lot of wrangling about a major London theatre installation of reverse phase dimmers that didn`t work as intended, has the dust settled enough for anyone to say what the problem was?
The circuitry is a bit more complex, particularly given the ease with which a MOSFET/IGBT can be blootered. (Technical term for extreme failure.)
Fundamentally I think the issue is with the sheer expense of developing new RPC dimmers and the simple fact that most manufacturers are waiting for their rivals to bring one out so that they can copy it's circuitry.
It's also available without the epanorama.net header at
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Tomi, I'm curious, who owns and operates the epanorama website, with all your material on it? Who owns ELH Communications Ltd? Are you personally involved? Who receives the snipped-for-privacy@epanorama.net email?
Much of the epanorama.net material has the phrase, "Published by ELH Communications Ltd., all rights reserved." How is material selected for this designation, is it mostly your own stuff?
If your MOSFET failed, it likely would have failed shorted, which would then force up to 2x more current = 4x more power through the frying pan, creating a new meaning to the word fry. :>)
What kind of FET did you use? Did you consider IGBTs instead?
Somebody seems to have copied that document there... Lots of my material seems to been copied by different people around... Most copies without any permission from me. :-(
I am personally heavily involved. I am one of the founders and one of the owners of ELH Communications Ltd. I used to receive all the snipped-for-privacy@epanorama.net email. Now there is another person who handles those mails (mostly requests to add links). The relevant mails get forwarded to me if there are things I should handle in them.
It is mostly my own stuff.
--
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at
http://www.epanorama.net/
Yes, which is one reason why the circuit includes an FF10A fuse. The other reason being that if the control circuit failed to switch the MOSFETs off, the fuse should operate before the pan or MOSFETs overheat (the MOSFET being designed to handle double the current for long enough to blow the fuse).
Don't seem to be able to locate my design notes at the moment, but looking back at my purchase orders for 1998 when I designed and built this, it looks like I chose Motorola MTW24N40E TMOS power FETs (400V, 24A). (ISTR the problem choosing a high voltage/current MOSFET was finding one that anyone stocked and would supply in small numbers.)
No, mainly because they weren't around when I learned electronics so I don't know much about their characteristics or have any real life experience using them.
Well, your name, Tomi Engdahl, has a respected and extensive reputation, so why not highlight it, or at least reveal it where appropriate, on the epanorama site? Take advantage of the brand, so to speak, rather than hiding it while relying on creating a new brand. Or at a minimum, make it clear on the epanorama.net site that the official site for the classic Tomi Engdahl material is now epanorama.
What else does ELH Communications do? What does ELH and epanorama mean?
Win, Did you ever get anywhere with comparing the MOSFET models?
...Jim Thompson
-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |
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| 1962 | I love to cook with wine. Sometimes I even put it in the food.
You have to add the forward voltage drop of the integral reverse polarity bypass diode too, which probably takes it nearer 25W total, but it was easily dissipated with only a small fan running silently at low speed.
I'd have to work it out... Actual rating of the pan was 1050W @ 120V = 13.7 ohm. ISTR the phase angle at cutoff was 43º (a bastard to work out;-).
I assume you used the standard back-to-back series connection with both sources tied together and both gates tied together. In this preferred configuration, both FETs are on, with one conducting "backwards" shorting out its source-drain diode, and you end up with twice the power dissipation of a single FET. Of course, if the voltage drop across the FET rises to more than the diode drop, as in your case (see below), then these mechanisms work together sharing the load current.
OK, 17A^2 * 0.16 ohms = 46 watts peak for the forward FET. It's a bit painful to calculate the RMS power, say about 15-20 watts. The reverse FET, with its diode helping, would be a little lower.
The reason IGBTs are attractive for applications like this is that for a smaller part with similar or less silicon area to a FET, it enjoys a smaller voltage drop at high currents. Your MTW24N40E FET drops up to 2.7V at 17A, which a smaller IGBT can easily beat. For example, a TO-220 600V 12A Fairchild HGTP12N60C3D IGBT drops 1.7V at 17A. And Fairchild's HGTG30N60B3D, a 30A IGBT with a slightly smaller die size than your big FET (rated at 208W compared to the FET's 250W), drops only about 1.0 volt. That's a big improvement.
In response to "Do you have a dimmer circuit with MOSFET to suggest?" Tomi responded with "CMOS gate implement s reverse phase control - this circuit implements a "reverse" phase control, using only a single CMOS 4001 quad NOR gate, conduction begins at the zero crossing of the ac sine wave and the turn-off timing is adjusted based on dimmer setting
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I'm interested! I can get to the article but the circuit diagram (figure
1) is supposedly stored in a .PDF file...that will not come up. Has anybody got another source for this circuit? Or is it just me who can't get to the .PDF file?
Downloading the PDF did not work for me either :-( I has link to this ciruit, and posted the address whe I saw that circuit page to work... Did not test the download of all parts..
EDN magazine seems to change their archive every now and them... Result beuing that links tend to break down every now and them. It is now not the particular link on the t circuit, every PDF link on that page seems to be broken because of their new system.. One idea would be to contact the webmaster of that site to report of the problem and ask to correct it.
I might have that circuit printed on paper somewhere, maybe even electronic copy...
--
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at
http://www.epanorama.net/
Yep....every single link on the page seems to be busted. I sent an e-mail to the webmaster right after I posted here four days ago. I haven't gotten a response yet....but then again, it's Monday morning!
If you can reproduce the circuit, that would be a great help!
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