Options for driving triac

There are also those nice opto-coupled triac triggers, like the MOC3021 and it's bretheren. However, I'm guessing a standard cmos output won't be able to generate enough current to trigger them (they require something like 10mA, as I recall). Way more expensive than the transistor, but good isolation.

Enough voltage and current comes out of the triac gate to potentially latch up the CMOS chip under some oddball conditions. I think the pennies for the transistor are very well spent, in fact for a real product I'd add more protection except for the cheapest applications.

Best regards, Spehro Pefhany

There aren't too many practical situations where the triac and control circuit are safely DC-coupled.

Driver matching and islolation are fairly easily obtained using pulse transformers, with little or no circuit mtbf degradation.

As triac conduction is most reliably maintained using pulse trains, a low power flyback configuration is an obvious choice for low-frequency control - pulse trains being gated by the controler.

RL

I've always liked to use an opto isolator. They are cheap and I get to sleep at night.

Dick Hagerman

Have you considered paralleling some CMOS gates to provide more drive current ? I've never done this myself but see no practical reason why it can't be done. I have a feeling the technique was mentioned in application notes of old.

Graham

Be careful with those MOC optos if you're looking for phase control - the 3030's, that I know of, have a zero-crossing detector built-in, so they switch at V=0.

But I was going to suggest optoisolators of some kind, yes.

Cheers! Rich

Another thing is that they trigger in both quadrants with a positive pulse. I seem to recall that some triacs take more current when triggering on the negative side with a positive pulse, or something like that.

The OP may not know that these are actually triacs themselves, which are turned on by the optical connection. Thus, the positive control pulse generates a negative pulse into the actual triac, which may make it trigger more reliably.

Also, the zero-crossing detector variants that Rich mentions are useful if you are using them for a switch, because it only triggers when the AC voltage is near zero (even if you power the LED before that), thus preventing a big transient current if you trigger it at the top of the waveform into lots of capacitance. If you are using it for phase control, use the ones that don't have zero-crossing detection.

Hi, I need to drive triacs from CMOS, the reliable gate current is somewhat above what the gates can source. There are two obvious options using bipolars:

OPTION 1. Emitter follower, base to gate output, collector to +Vcc, emitter via a suitable resistor to triac gate.

OPTION 2. Q1 (NPN) as a switch, with resistor divider from gate output to base, emitter to gnd, collector resistor divider driving a PNP current source from Vcc down to the triac. Current limiting resistor between PNP collector and triac gate. Esentially a complementary switch pair.

It intuitively feels to me that the second option may give better 'protection' to the sensitive CMOS, but in theory Option 1, which uses a lot less bits, should be OK. I do require a solution that doesn't significantly degrade MTBF (stated vaguely, I know).

Is there any objective reason why one would go option 2? TIA

Of course if your power supply is non-isolated, the isolation isn't all that valuable. Transistors don't age badly either. But, horses for courses, the isolation can be very useful, and is rather inexpensive since the AC line supplies the current for the triac gate.

Best regards, Spehro Pefhany