EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: pulse on December 25, 2015, 01:32:49 pm
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Hi!
I want to make a dimmer circuit with triac, but first, I want to know, how certain things work in that circuit.
Triac usually has diac in series with its gate. Diac starts conducting at let's say 30V... So, until the capacitor charges to 30V, there is no conduction and triac stays off, which means that if conduction angle, which is set with potentiometer is too small to charge capacitor fast enough, up to 30V, triac will simply not turn on, right? So, if I understand this correctly, that circuit would have some problems: bulb wouldn't start glowing at very low voltage, but would instantly turn on at some point, when turning potentiometer. Also, as mentioned before, diac starts conducting at 30V, but my triac's datasheet says that the gate voltage is 5V max. So, how would that not damage my triac? Also, why can't I control the triac with simply limiting current to its gate - or voltage?
Thank you very much and please excuse me for my english mistakes...
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Anybody, please?
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- Yes, in the usual dimmer circuit the conduction angle has a lower limit, so there is a "dead zone" at the lower end. Not really an issue, at least with 230 V.
- Triacs are current controlled devices. The gate of a triac basically looks like two silicon diodes in anti-parallel. I.e. it clamps to +0.7 V / -0.7 V (relative to MT1), with very little current flow below the threshold.
- When the diac fires the voltage across collapses to a few Volts (so a Diac isn't just a Z-Diode für AC), until the current drops to ~zero (the holding current) through it. This dumps the charged capacitor in the front of the diac into the triac gate, triggering it.
(https://upload.wikimedia.org/wikipedia/commons/thumb/2/22/Phase_control_schematics.svg/800px-Phase_control_schematics.svg.png)
Dimmers in the EU also contain a fuse and a series inductor to meet safety and EMI requirements, respectively. Usually a snubber across the triac is present, too. Often P is a combined pot / switch unit ; when the switch is open no current at all will flow.
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Thank you very much for reply!
1. So, now when you confirmed, that there is a "dead zone" at the lower end - are there any diac's with lower breakover voltage? Or could I use two zener diodes in parallel (each turned 180 deg., respectively to each other)?
2. So, what if I'd simply control the triac's gate current with low voltage power supply? Wouldn't that also have dimming effect for the lamp?
3. So, even if my triac's peak gate voltage is only 5V, it wouldn't hurt it, while receiving the 30V pulse from diac, because of limited current and therefore voltage drop through its gate?
I'm not very familiar with the standards, but I've seen the use of choke in some dimmer circuits (which was parallel to the load) and I've also seen the snubber, being used in a lot of circuits, so that's probably worth considering, while making a dimmer :)
Thanks again!
Best regards ;)
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1. So, now when you confirmed, that there is a "dead zone" at the lower end - are there any diac's with lower breakover voltage? Or could I use two zener diodes in parallel (each turned 180 deg., respectively to each other)?
No, diacs and Z-diodes operate very differently. A Z-diode limits the voltage across it to the Z-voltage, while a diac latches when the breakdown voltage is exceeded to a much lower voltage.
Two Z-diodes anti-parallel don't work anyway, since they also have a normal PN junction and if forward biased behave like normal Si diodes.
2. Yes
3. No, the capacitor might be charged to 30 V but the voltage across the gate of the triac won't exceed 1 V or so even at full peak gate current.
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1. So, now when you confirmed, that there is a "dead zone" at the lower end - are there any diac's with lower breakover voltage? Or could I use two zener diodes in parallel (each turned 180 deg., respectively to each other)?
No, diacs and Z-diodes operate very differently. A Z-diode limits the voltage across it to the Z-voltage, while a diac latches when the breakdown voltage is exceeded to a much lower voltage.
Two Z-diodes anti-parallel don't work anyway, since they also have a normal PN junction and if forward biased behave like normal Si diodes.
2. Yes
3. No, the capacitor might be charged to 30 V but the voltage across the gate of the triac won't exceed 1 V or so even at full peak gate current.
Ok, that explains everything, thanks!
I have only one question left...
If I'd use low voltage gate control, does that mean that triac would operate in "linear mode" (I mean like in linear power supply, which has linear voltage control)? I'm guessing triac also has some resistance (or impedance) curve vs gate current? So, why is it not shown in triac datasheet?
Thanks! :)
Best regards!
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The triac is either off or fully on, there is no region where it is partly on aside from briefly during turn on and turn off as it transitions from one state to the other.
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There's a "charge reset" circuit that can be added to lower the minimum dimming level.
http://techno-fandom.org/~hobbit/lighting/dimmer.html (http://techno-fandom.org/~hobbit/lighting/dimmer.html)