What is the purpose of a Zero-crossing circuit in a TRIAC optoisolator?

 

http://en.wikipedia.org/wiki/Zero_cross_circuit

 

Is this also for your heater project?

You could also use THIS but it's a non-zero-crossing type.

 

Yeah, this is for my heater project. The TRIAC/optotriac combo looks less expensive than the S108T01.

I am switching the heater driver with 1-5hz PWM. A lot of threads seem to say that PWM and TRIACs don't mix, but I think at this low frequency it will work. It's like switching the driver off and on every second, by hand, so why wouldn't it work?

I am still not certain of the Igt (MAX) question. Dad says Igt (MAX) is probably what it says and that the actual Igt used will be much lower. He hasn't read up on anything though and hasn't used TRIACs other than for a one time off/on switch. And reading up on it doesn't seem to be much help anyway!

I guess I will have to order some TRIACs and test them.

 

Boss says we don't have to worry about noise in the heater driver circuit, so a ZC optotriac over a regular one is not a requirement.

 

You're Dad's right. Igt Max is a limit where things burn up. You will not need to go to that limit.

Everything you said indicates that you are on the right track. You can trust your instincts on this one.

 

Is this also for your heater project?

You could also use THIS but it's a non-zero-crossing type.

praondevou, thank you for finding this. Boss says $1 more per unit isn't a big deal. Now I don't even have to know what the Igt of the TRIACs is!!! Will probably be going with this. Why on earth isn't this filed under TRIAC optoisolators on digikey...

 

You're Dad's right. Igt Max is a limit where things burn up. You will not need to go to that limit.

Everything you said indicates that you are on the right track. You can trust your instincts on this one.

Good to know for the future. Thanks!

 

With 1 to 5 Hz it's not a problem. With frequencies a bit higher a normal PWM wouldn't work, because it would have to be syncronized with mains frequency, triacs are not fully controllable and can only turn off under certain conditions. (apart from GTOs)

This is why you use phase angle control for triacs/thyristors.
Also, with your relatively small heater current it won't be necessary to use zero-crossing drivers.

 

Igt Max is a limit where things burn up.

According to the datasheet of the triac he had chosen the maximum peak gate current was about 1.6A (destructive), I think (this was in his other thread). Then there was a parameter Vgt max and Igt max. It was not clear at what gate current the device would begin to trigger. The app note from this manufacturer I posted mentions to use the Igt from the datasheet, it doesn't say anything about minimum values.

"Gate signal must exceed the IGT and VGT requirements of the Thyristor being used."
source: http://www.littelfuse.com/data/en/Application_Notes/AN1002.pdf

But there is only one parameter: Igt max.

Any explanation?

 

S108T01.

In this datasheet we have the information that is missing on the optoisolators with triac output: Minimum Trigger current is MAXIMUM 8mA.

Leads me to believe that the the minimum trigger current for the other triacs is the parameter Igt MAX.