Typical opto has several kV isolation voltage (which is not always needed), has relatively big cases (TLP291 in 4-SOIC is one of the smallest as I know) and typical price up to 1 USD in small amounts (not cheap in my case).

In my case I'd be satisfied if I'd be able to transmit a signal with as low as 350VDC (250Vrms) isolation. There are no safety reasons (I just need to transmit the signal from neutral to line).

At the receiving side the signal should be something similar to a switch (like collector-emitter of the opto): "on" or "off". This signal will drive MOSFET driver. The speed is pretty low (1 ms would be enough).

Capacitor can be cheap and small solution, but it will not work on DC.

So if I don't need such big isolation voltage is there any alternative to optoisolator?

  • \$\begingroup\$ What signal are you trying to convey? Is there a safety reason to use an "isolator"? \$\endgroup\$ – Andy aka May 23 '16 at 11:39
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    \$\begingroup\$ optoisolators. cheap? check. small? check. \$\endgroup\$ – PlasmaHH May 23 '16 at 11:41
  • \$\begingroup\$ maybe define "small"? \$\endgroup\$ – Wesley Lee May 23 '16 at 11:42
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    \$\begingroup\$ You can try using a trick like: modulate the control signal to 10 MHz or about; use high voltage small MLCC to isolate the gate from the signal source; use rectifier on the gate side to get DC from AC 10 MHz. \$\endgroup\$ – Master May 23 '16 at 16:48
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    \$\begingroup\$ The most difficult part in this design is: how to discharge input capacitance of MOSFET quickly when you want it to go to OFF state. \$\endgroup\$ – Master May 23 '16 at 16:49

There are no safety reasons (I just need to transmit the signal from neutral to line).

Just to be clear, no safety reasons should also mean that no user will be able to access any part of the system that is connected (directly or indirectly) to the "high voltage" system. It also means that the construction of the system is designed for fire safety as if it were all connected directly to the high voltage.

It means that the entire system will be designed as if all of it is at "high" voltage at all times, as far as fire and shock hazards are concerned.

I just need to transmit the signal from neutral to line

If this means you want to superimpose a data signal on to a mains wire pair, then AFAIK the usual way is to use a transformer. Since relatively low is required, it doesn't necessarily have to be very big or pricey.

Capacitor can be cheap and small solution, but it will not work on DC.

Finding a solution that works at DC will be very difficult, since mains frequencies are very close to DC compared to the message signal in most data communications situations.

Consider modulating your message signal to some higher frequency to enable separating out the message frequency band from the mains frequency.

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  • \$\begingroup\$ Thank you for the input! Now I have some thoughts to think of. Just have to note: this is not for PLC, this is for switching the mains off and on within smallest size and budget. \$\endgroup\$ – Roman Matveev May 24 '16 at 9:00
  • \$\begingroup\$ In that case, you can just use a relay or SSR, so that your switching device provides isolation between the switched circuit and the control circuit. In any case, if you don't use isolation you must treat the whole circuit as "hot" from a safety point of view. \$\endgroup\$ – The Photon May 24 '16 at 15:55
  • \$\begingroup\$ Relays are SO big, SSR needs high input current (typical 10 mA). Thanks! \$\endgroup\$ – Roman Matveev May 25 '16 at 6:23

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