What exactly is an opto-isolator
Hello,
An optical isolator is an LED driven transistor. The LED is powered and shines on a photovoltaic cell of some sort, this photovoltaic applies a voltage to the base of a transistor. This charge then biases the transistor as required and allows current to flow to some degree on the other side. In this manner, one portion of a circuit can cause changes in the current of another part of the circuit with no direct electrical contact. This is used primarily to isolate one portion of circuitry so that current/voltage spikes in that one portion (say a large DC motor drive circuit) will not effect any other portions of the circuit; i.e. noise sensitive components which control them (PLC's, Microcontrollers, etc.). The entire purpose of isolation is to seperate the portions electrically so that noise in one node does not effect any other nodes adversely. Note though that when using opto's or any other form of isolation (relays, etc.) you can get different ground planes, and there can be a greater voltage difference between different portions of a circuit than expected (12 volts to ground on one side of the isolator may be more or less than 12 volts with respect to another side's ground);this is easily checked with a voltmeter if needed, and is essential when trouble shooting various problems (I personally burned out two $200 fiber optic amplifiers because of a ground plane difference I was unaware of).
Hope that helps,
Scott