Optoisolator?

Papabravo

Joined Feb 24, 2006
21,159
Can you tell me what an optoisolator is please?
It is an LED and a Photo-transistor. When the LED is illuminated the photo-transistor goes from cutoff to saturation. When the LED is dark the photo-transistor returns to cutoff. The photo-transistor can be configured as a common emitter switch or as an emitter follower depending on weather you want the output to be inverting or non-inverting. The chief reason for using one is to provide isolation between the input circuit and the output circuit.

Does that answer your question?
 

Nik

Joined May 20, 2006
55
"It is an LED and a Photo-transistor"

It may be an LED driving a photo-diode with one or two transistor stages to follow. These tend to be faster than photo-transitors. 6n136 & 6n139 are classic examples, the latter a de-facto standard for MIDI interfaces.

One gotcha is the Current Transfer Ratio (CTR) from LED to photo-diode is typically ~0.0015 (~0.15%) meaning it takes a lot of amplification. You can trade speed for bandwidth by using a resistor to draw down open base if available.

FWIW, Avago (TM) have a spice model for the 6n136. There's currently nothing available for 6n139.
 
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