The problem is they don't turn off unless you use the zero-crossing or a phase-crossing. I'm no expert on triacs but the two times I ended up redesigning circuits with them, they wouldn't turn off. Once someone had pulse width modulated a triac. The other time someone had it run into an end switch. The circuit would almost always work but when conditions were just right they would stay on and burn up. You have to make sure they are turned off when they want to be turned off.
Greetings, gentlemen,
As we all know, to get maximum efficiency out of a MOSFET, it needs to be driven hard and fast and spend as little time between fully on and fully off as possible. Does that same principle extend to Triacs? And if so, how can that best be implemented when one has to use AC to drive the gate?
CD.
Once a triac fires, the gate drive doesn't matter any more.
But they do need a hard, fast drive to fire properly, long enough to let the drain current rise above the latch limit.
AC gate drive will be slow-rise, which could just possibly damage the triac if the load current is high.
That should be on the data sheets.
I believe that is why a diac is typically used in the control circuit.
I thought diacs were sometimes used because triacs don't fire symetrically, but I could be wrong. So, as regards AC drive, is a higher frequency drive signal a safer bet? Also, the load is AC too, so the thing will turn off at every zero-crossing point, which makes life more complicated. :-(
Diacs are used with SCRs as well. The idea it the Diac lets very little current flow until the voltage is fairly high. Then it conducts suddenly, just what works best for the triac/scr.
I don't know why you are lamenting the turn off at each zero crossing. That's fundamental to how they work. Every half cycle you get to time the next turn on. What's hard about that?
Do you know anything about using triacs? There is plenty of reference material on the web. Data sheets are not a bad place to start.
How do you want to use it? Is it just a switch that you turn on like a light switch, or do you want to phase control the AC into your load?
And another question is, what kind of load? An inductive load might need a long gate pulse. Maybe even continuous.
Tell us about your project for maximum help.
No project in mind. No particular type of load either. Just a question that came to mind, nothing more.
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** See:
Diacs come to the fore if phase control of a load is needed.
...... Phil
Sorry, I don't understand what you are talking about, "they don't turn off unless you use the zero-crossing or a phase-crossing". When the current in the triac goes to zero, it turns off. If your gate signal is still above the threshold, it won't turn off, but that's because you are telling it to stay on.
Did you ever get your circuits working? What was wrong?
With an AC load, an AC gate drive is probably OK. The triac will turn on before the voltage gets too high, so the spreading damage mechanism won't happen much.
The bad case is when there's a lot of voltage across the device with a lot of instantaneous current behind it, and it gets a slow, wimpy trigger.
Post your proposed circuit and we can discuss it.
I still call the terminals anode, gate, cathode. I can never remember what's MT1 and MT2. Top and bottom would be OK too.
Assuming 50Hz operation use a much higher frequency like 100KHz to drive the gate.
Here's a link.
Did I fix it? I think I did. Sustaining work. Mfg is getting returns. They want me to fix the problem without changing anything so they can use existing stock. I can't reproduce the problem. Using app notes and books, I change the zero crossing circuit, the triac and add a snubber network. My circuit works but the old circuit works. We rework the boards and we don't get returns. I probably just got lucky.
The triac is one of those parts where folks get confused by fossilized analogies.
A triac is a bidirectional thyristor, but it does not behave like two SCRs in antiparallel. SCR triggering only works in one quadrant, for a start, so two antiparallel SCRs would be a two-quadrant device, whereas triacs trigger in all four quadrants.
The triggering also isn't that symmetrical, because the structures aren't--the second and fourth quadrant triggering mechanisms are fairly different, and the four-layer stacks aren't well separated, as they would be in two SCRs.
Triacs are also very slow even compared with SCRs, which are pretty poky devices by modern standards.
Someplace I have a paper that goes into the details of real triac operation--iirc it's a good read, but I'm not laying my hands on it at the moment.
(I haven't actually used a triac since I was a teenager, so this is just out of general interest.)
Cheers
Phil Hobbs
I haven't designed in a triac for maybe 15 years now. I used a couple of triacs to soft-start a giant power supply that had a 60 Hz power transformer. Switch in a giant resistor on the primary side, wait a second, then short it out. That was hard on resistors but prevented the occasional 1000 amp startup surge. And allowed a little C&K power switch on the front panel.
The customer insisted that we not use a switching power supply!
Real relays are usually cheaper than triacs, easier to drive, and don't need heat sinks.
Yes, but you can't fire them on and off anything like as fast and they don't have the same longevity AFAIK.
Plus a relay capable of switching 16A is going to be a hog, take up loads of space compared to a triac and cost a hell of a lot more, I would imagine.
It's easy to look up. Digikey has 15 amp relays starting around 60 cents.
The triac will need a heat sink, which will be big and cost more than the triac.
Relays don't need isolated gate drivers, TVS/MOV protection, or snubbers either.
Yeahbut I don't want to order a million of them to get that price.
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