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A side note: please be forgiving of me because I'm just starting out in Electronics so I'm still confused about certain terms.

Why are optocouplers used?

Yes I know they link a circuit together since there is an LED inside and a photoresistor. I also read that it prevents high voltages. But why use an optocoupler over another component? For example, to reduce high voltages we can use step-down transformers but why, in a certain scenario, we would use an optocoupler over a step-down transformer?

In what scenarios are optocouplers appropriate?

Hope my question is clear enough for anyone who can help.

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  • \$\begingroup\$ When shit happens, like when things are fried, opto-coupler would keep things isolated while everything else has the possibility of bridging the low side to the high side. Like if the transformer over heats and insulation breaks, then your whole low side is gone. \$\endgroup\$ – user3528438 Oct 30 '17 at 5:13
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    \$\begingroup\$ @user3528438 can it be used in the toilet? \$\endgroup\$ – Harry Svensson Oct 30 '17 at 6:59
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Both transformers and opto-couplers can provide isolation between 'hot' and 'safe' regions of a circuit.

The difference is that opto-couplers are very small, cheap, and can work on simple DC, so can shift a logic signal from one side to the other with no fuss and no or few other components.

Transformers are big, expensive, and need AC to work, they cannot simply be inserted into a logic line. When you need the thing that a transformer does well, moving power from one side to the other, then you have to use a transformer. Otherwise, you use something smaller, cheaper and easier.

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    \$\begingroup\$ This doesn't really answer why an optocoupler is better than "digital isolators", analog.com/en/products/interface-isolation/isolation/…. These are smaller and probably cheaper than optocouplers. And they are also more reliable. \$\endgroup\$ – Lundin Oct 30 '17 at 7:41
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    \$\begingroup\$ I don't necessarily agree, Lundin. Standard optocouplers (LED + photo transistor) are cheaper than the high-speed pulse transformer based (AD) or capacitive-coupled (TI) "digital isolators". But the spectrum of products on the market is pretty broad, and I cannot exclude that you can find a traditional opto-coupler and some hip "digital isolator" where the optocoupler is more expensive of the two. The modern "digital isolators" have a clear edge in speed and propagation latency. The traditional opto-couplers are deficient in this respect. \$\endgroup\$ – frr Oct 30 '17 at 8:35
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The main reason for using an optocoupler is to realise a galvanic separation between circuits. The use of a transformer much more expensive and only possible in AC circuits.

For safety reasons: separating mains connected electronics from the low voltage electronics section.

Low cost: An optocoupler is very small and has a low cost.

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    \$\begingroup\$ They're also trusted. We've been using them for decades, and we know that they fail safe when they do fail. And AFAIK that's true regardless of the failure mode (mechanical, thermal, ...) \$\endgroup\$ – MSalters Oct 30 '17 at 11:33
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    \$\begingroup\$ Of course many items that require that separation will have both optocouplers and transformers. Transformers for power and optocouplers for signaling. \$\endgroup\$ – whatsisname Oct 30 '17 at 15:50
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Others have mentioned the isolation aspects of using an optocoupler. One benefit from this is that you can switch different voltage busses that are otherwise unrelated to each other.

Some examples:

  • On a circuit board, you can switch 12V signals using your 3.3V microcontroller output. Or the other way around.
  • In an industrial panel, you can run a power wire from an external system, switch it with an optocoupler, and return the voltage back to the external system. This could help if you don't know the other system's working voltage. It could also help if the external system doesn't share a common ground with yours.
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  • \$\begingroup\$ Good point about use as a level shifter, other options do exist if the isolation is not required and the galvanic ground isolation is also available with the transformer option. Use in a DC current loop is a nice case as you are not sure where the ground potential is because of series devices or remote reference. \$\endgroup\$ – KalleMP Oct 30 '17 at 23:25
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I don't know why the other answers didn't mention the term noise. Maybe I have a misconception about the opto-couplers. But from my experience in dealing with electronic circuits, one of the main reasons for using an opto-coupler is to separate the high-frequency part of the circuit -which does the processing jobs- from its low frequency and power supply part (and I know about the role of capacitors. That's not the point here). This separation/isolation is partly because high voltage results in higher amounts of noise which particularly affects A2Ds and measurement precision.

Again, I am talking from experience and don't have any source for my claims. Please correct me if I am wrong.

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    \$\begingroup\$ Yes indeed you can simply use an opto-coupler to form a current loop to pass a digital signal over considerable distances without having to worry about ground bias and many other noise issues. \$\endgroup\$ – Trevor_G Oct 30 '17 at 12:34
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    \$\begingroup\$ You can solve some noise problems with an opto-coupler, but I feel that the talk about low/high frequency and high voltage noise detracts from the actual issue. \$\endgroup\$ – pipe Oct 30 '17 at 14:26
  • \$\begingroup\$ @pipe Yeah I feel the same :) Sorry for poor wording, Trevor said it better \$\endgroup\$ – polfosol Oct 30 '17 at 14:30

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