Types of Opto-isolators

Other Hardware General Electronics
1

I'm currently looking to buy some opto-isolators, however, I see a lot of different types and I'm not sure what the best choice is. The types I've seen are Regular (no details), Darlington pair, TRIAC, and Zero Cross TRIAC.

The first two I think I understand

Regular Opto-isolator= LED inside turns isolated photo transistor on
Darlington Pair Opto-isolator= Same as regular but with higher gain (therefore higher current transfer ratio)

It's the second two which I'm not so sure about
TRIAC opto-isolator= Same as regular but is bidirectional for AC circuits
Zero Cross TRIAC optoisolator= Zero Cross detection? good for AC dimmers.

I don't understand how the ZCD TRIAC optoisolator works. Is a TRIAC opto-isolator always more advantageous than a regular optoisolator. For example can a MOC3062 (pictured)

be used as a regular DC optoisolator to turn an isolated LED on off with a PWM signal, or would it not work at all since there is no actual zero crossing?

2

A triac based opt isolator can only be used to switch AC. If you try to switch DC with it, you will find that you can switch it on but not off.

A zero crossing triac based opto isolator can also only be used to switch AC, and can't be used to switch inductive loads. If you try to switch DC with it, you will find that you can't even switch it on.

3

Thanks dc42. I think I understand the concept of a triac now. The isolated circuit would need to drop below threshold voltage on the main terminals of the triac otherwise it will remain on indefinitely. This happens automatically on an AC circuit at a given frequency but not on a DC circuit. Darlington pair opto-isolators it is then. Thanks again for your help.