Optoissolators

I'm kinda a noob with electronics, my question is what are the differences between all the optoisolators, phototransistor, darlington, etc. There are so many varieties.

Thanks

[quote author=Robert Fisher link=topic=54768.msg392096#msg392096 date=1299646058] I'm kinda a noob with electronics, my question is what are the differences between all the optoisolators, phototransistor, darlington, etc. There are so many varieties.

Thanks [/quote]

Too broad a question, can you frame a question that is answerable. Commenting on ALL possible is a little difficult. :D

Google is very helpful for general questions like this.

Ok is the phototransistor the normal or the type most people use? What's the Darlington type used for?

An optoisolator consists of a light emitting component (usually an LED), a light sensitive component, and perhaps some additional driver circuitry on the output side. The most common variable is the stuff on the output side.

If you have a "simple" phototransistor on the output, you will usually have an overall gain of less than one - you feed 20mA or so into the LED and the phototransistor goes from non-conducting to passing a couple of mA. Plenty to drive a logic gate or some analog circuitry, but not great for some other things. Various other characteristics of the output transistor may apply (in particular, the maximum voltage.)

If you add a second transistor, you get a "darlington" configuration that has much higher gain, but loses some of the ability to act as a linear device, and has other problems associated with darlingtons.

Some have logic circuitry in them so that the output will drive standard logic families (5V TTL) without additional external components.

Triac drivers are for driving triacs; you can find them with additional "zero-crossing" circuitry for solid-state-relay applications (on/off at low switching frequencies) or without, for dimmers and other stuff that wants to switch the output at places other than an AC zero-crossing (one other example: camera flash tiggers.) Triac drivers tend to be aimed at line-powered AC circuits with much higher voltages involved than the transistor-like outputs.

Those are the basic types...

phototransistor senses light and controls flow of current in its output stage. like the 2nd half of an optoisolater but exposed to the outside. darlington is two transistors sort of in parallel for increased current drive. optoisolator lets 2 circuits with different power supplies share signals while maintaining electrical isolation.

The two types of opto-isolators I've used are phototransistor and logic output.

Phototransistor: the light from the LED acts like the base of a transistor. There is a relationship between how much current is passed through the LED and how much current is allowed through the phototransistor. Good for situations where you need the functionality similar to a transistor, but also need the isolation. Most of these are just basic transistor-like switches. But some of them have a feedback loop for proportional analog output.

Logic output: the light from the LED simply causes the output to go high or low (depending on the specific isolator). Good for logic or simple switching of signals. Requires a power supply on the output side to power the logic circuit in the isolator.