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Triac dimmers with hard-firing

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Marko

Sat Jun 16 2007, 05:30PM

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

Update: final version of the circuit

***
I am now really in need of a reliable pulse train-controlled dimmer, and at first glance the ST's circuit

looked like simplest solution at first.
But, I wanted to build it into a small handheld box, and that's where problems began.
Finding a good small triac with low holding current was impossible, and large ones like BTA12 needed current of >50mA to stay on. I needed to use really small dropper resistor wich dissipated huge amounts of power, even up to 20W or so to keep the auxiliary triac on. (I could have ended with 25W bulb in series!)

Obivously not something that can be held in hand.

Now I clearly needed a simpler, electronic solution but avoid use of large components like magnetics otherwise the dimmer won't be portable.

I designed a completely new circuit and tested it out... now I'm really curious what do you guys think! I could get some great advices before boxing it up!

EDIT: this schematic is now fixed, but it sucks. Don't build it, or build at your own risk, whatever you like. See the new circuit at top.

I used a simple dropper-capacitor power supply.
Line voltagte is sampled into zero-crossing detection circuit based around LM393 comparator, wich has nice open collector output. It's output are short (500us) negative pulses.

At a point I tought to use a diode bridge and optocoupler to generate reset pulses, but comparaotr idea just seemed better.

The 22nF capacitor charges slowly with each half cycle and is reset periodically by each zero cross of line voltage. Voltage on the capacitor is compared with control voltage and starts off the oscillator at depending phase angle. Another pot sets the firing pulse length. Frequency is set to about 5kHz and pulse length at about 15us.

I also planned to use the free comparator for some kind of overcurrent protection. The comparator would simply sense the voltage over a shunt, latch down the output control of tl494 and light a led in case of overcurrent, and would be reset by a switch or power cycle.

Voltage over the timing cap in comparison to mains voltage.

Output pulse train vs. cap voltage.

Exagerrated and 'real reset pulses with pullup and without timing cap connected.

Marko

Sun Jun 24 2007, 06:10PM

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

Duh.. am I really that boring?

I'm unconfident with the upper circuit. The zero crossing detector may have looked like good idea in the beginning but, it can *never* actually acheive perfectly symmetric waveform wich is bad for inductive loads. And I can practically never fully fix that.

I got the inspiration from here

where zero cross is apparently sensed directly by microcontroller pin. I intended to make something similar using a comparator.

In order for it to work without dissipating lots of power I needed to create a lot if high impedance nodes in front of the comparator wich needs to sense only few volts of difference.

The output sometimes flickers at random for +- few output pulses. I've not figured out what causes that, and it may not be important, but I don't like it at all.
I'm not going to be happy with the circuit as it is.

Only idea I have now, is to use an optocoupler with diode bridge to isolate the voltage level and generate (in this case perfect) reset pulses.
This looked like ''unneat'' and waste of resources (I again need one more IC if I want overcurrent protection).

I would really like to hear your opinions... if anyone cares.

uzzors2k

Sun Jun 24 2007, 07:16PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

You're not boring, it's just that nobody has any constructive criticism.

Why can't you achieve a perfectly symmetrical waveform with the current circuit? Maybe if you redesign the op-amp stage, you can still use it. If my understanding of these dimmer circuits is right; you want a pulse-train sent to the triac to hold it on and when the train leaves compared to the mains phase determines power. This might be incredibly dense, but...

Maybe if you put a bridge rectifier on mains, which feeds unfiltered mains into a resistor divider at a 100:1 ratio. This is sent into one comparator input. A second resistor divider is put on the regulated 15V, with a potentiometer. This output goes to the second comparator input. Turn the pot and the triggering voltage of the comparator is altered, and the pulse-train departs sooner or later compared to the mains phase. Capiche?

Like an AC to DC variable diac. Or have I completely misunderstood what you needed?

Marko

Sun Jun 24 2007, 07:53PM

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

Maybe if you put a bridge rectifier on mains, which feeds unfiltered mains into a resistor divider at a 100:1 ratio. This is sent into one comparator input. A second resistor divider is put on the regulated 15V, with a potentiometer. This output goes to the second comparator input. Turn the pot and the triggering voltage of the comparator is altered, and the pulse-train departs sooner or later compared to the mains phase. Capiche? Like an AC to DC variable diac. Or have I completely misunderstood what you needed?

Uzzors: all the trouble is because I *can't* do what you just described. A bridge rectifier connected the way you described will just create a short and I will always only be able to see only one half cycle.

I need some kind of isolation, transformer or optocoupler in order to do what you described.
Optocoupler is what I'm probably going to do after all, unless somebody has a more geniue idea.

Regarding the comparator circuit I used here, this is how it works:

Two references are set, with the negative one being somewhat lower than positive wich keeps the output high, or in this case high impedance.

As the mains voltage approaches zero it comes to a point where it pulls the positive reference to level lower than negative and output goes briefly low. But, as soon as the voltage crosses to negative the upper diode is reverse biased, returning the positive reference to it's normal value and further lowering the negative reference making the output go high again.

I can't get the output to be symmetric because I just have no way to sense negative voltage. I played with various resistor configurations but this circuit keeps confusing me, and I think things should be simpler than that.

I hoped somebody with lots of expeirence will show up at this point...

uzzors2k

Sun Jun 24 2007, 08:20PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Now I see the problem. I think sampling both sides of the waveform is going to be necessary for perfect symmetry, so I would try the bridge rectifier idea. You'll still have the most 1337 pulse-train controlled dimmer out there.

Steve Conner

Mon Jun 25 2007, 08:53AM

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

Hey Firkragg,

What's the problem with imperfect symmetry? I guess you're worried about generating a DC component that would saturate a transformer? I did notice that my dimmer circuits had slight uneven firing at low duty cycles, I could hear my isolation transformer buzzing in a way that suggested I was generating some DC.

But anyway, I think if you add some kind of trim pot to the comparator, you can trim it for lowest DC component. Your circuit as it is has an imbalance of about 7.5V built in. For suggestions on how to improve that, check out the "Single supply zero-crossing detector" in the LM339 datasheet.

Marko

Mon Jun 25 2007, 12:15PM

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

Hi steve

I know of lm339 datasheet circuit, but it simply clamps negative voltage to ground, so it's not what I need.

Also, there is an error in schematic, the 150k and 100k pullup resistors should be swapped. But circuit needs redesign anyway and I'l give it one last chance.

PS. I fixed the old schematic, but now I consider it obsolete and wouldn't recommend building it to anyone.

Marko

Wed Jun 27 2007, 04:49PM

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

Hi

I think I've now more or less perfected the circuit. Since oscillator is reset with every cycle output is very stable. Symmetry can also be set very precisely if that is needed.

I used LM339 in order to get free comparator for OCD, but LM393 will work as well.

Colin 99

Sun Jul 01 2007, 12:43AM

Registered Member #192 Joined: Fri Feb 17 2006, 03:08AM
Location: Canada
Posts: 44

I know this is a little late, but On Semiconductor has a design for your application, see link below:

Click on AND8011/D application note.

It's a bit simpler than you design, but I'm currious which one works better. Will they work with an inductive load??

Marko

Sun Jul 01 2007, 01:01PM

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

It's a bit simpler than you design, but I'm currious which one works better. Will they work with an inductive load??

It's more complex in design. Uses a large transformer for a power supply and isolation for triac drive, wich is what I wanted to avoid.

PS. after examination I realized that it doesn't generate pulse train at all, ne555 is in monostable mode. I don't know how much would the triac likecontinuous signal on it's gate.

So it definitely isn't as good.

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