# Need to understand how optocoupler work

• posted

I am trying figure out how an optocoupler

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works.

Is optocoupler similar to a solid state relay?

What should happen if I apply current going thru pins 1->2? Should resistance drop to zero between 5 and 4?

My goal is to use LPT port to switch stuff on and off. I have a couple ways to achive this but I want try what works. I want to learn new stuff.

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• posted

works.

yes, but they operate at much lower output-side power than typical solid-state relays.

close to zero, for small currents.

depending on the stuff an optocoupler may be suitable.

Bye. Jasen

• posted

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Similar, but not the same.```
• posted

works.

Perhaps worth mentioing that you can also get optocouplers with triac, resisitve, fet and logic outputs.

Graham

• posted

works.

resisitve, fet

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Why confuse him with too much information?

He asked a specific question and got a specific answer. ```
• posted

Yes, it's similar. Usually a solid-state relay is an optocoupler and some other stuff packaged together to make it more convenient to use.

Resistance will decrease, but not all the way to zero.

Actually what will happen is that it will allow a certain amount of current to flow between 5 and 4. As long as you're under the limit, it'll act like a low resistance (about 20 ohms?). The limit depends on how much current you are feeding into 1->2. Figures 2 and 3 in the datasheet give all the gory details. For example, if you apply 5 mA through 1->2 ( If=5mA ) then it'll allow about 60 mA though pins 5 and 4 before it starts limiting the output current.

This makes sense. As long as the stuff you're switching doesn't require more than 50-100 mA of current, then you can treat the optocoupler as a switch. If you need more current than that, you'll need to add something else, like a transistor.

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Wim Lewis , Seattle, WA, USA. PGP keyID 27F772C1```
• posted

works.

resistance

resisitve, fet

That wasn't the intention but since he was talking of the output *resistance* I thought it valid to mention resistive output and fet types (and the others for good measure).

Graham

• posted

works.

resistance

resisitve, fet

thought it

measure).

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What a crock of shit.

All you were really interested in was, as usual, posting irrelevance for```

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