Initially I'm trying to make over current protection scheme. So if the R2 current (and therefore R2 + D1 voltage) will go higher than some certain level I should have a signal to break the circuit.

I'd make it with a comparator, but making a comparator with isolated output can lead to a relatively complicated schematic: in addition to comparator itself I will need isolator (optocoupler for example) and isolated DC-DC power source to feed the comparator: tall tale.

However if I don't need high precision, high speed and so on - it looks very attractive to made a whole comparator scheme using optocoupler only.

For the moment I made a schematic which basically works (comparing U(R2)+U(D1) voltages with some threshold), but pretty bad:

enter image description here

Blue line representing the forward current of the coupler LED.

Basically I need to make a signal if the R3 current will be higher than some threshold. The precision can be as bad as +100 -50%.

This schematic was built intuitively. I hope that I could make a better comparator of the optocoupler. Am I?

  • \$\begingroup\$ What are you comparing? I only see one input and one output. \$\endgroup\$
    – Daniel
    Apr 24 '16 at 18:43
  • \$\begingroup\$ @Daniel I'm trying to compare U(R2) + U(D1) with some threshold. I made needed comments to my question. \$\endgroup\$ Apr 24 '16 at 18:58
  • \$\begingroup\$ Can you sense across the 100 Ohm resistor? That seems like a more reasonable place to do it. \$\endgroup\$
    – Daniel
    Apr 24 '16 at 19:55
  • \$\begingroup\$ @Daniel 100 Ohm resistor is a variable here. As I need to sense the current and I don't know in advance the load resistance I should sense MOSFET channel voltage. \$\endgroup\$ Apr 24 '16 at 20:13
  • 1
    \$\begingroup\$ Please explain in the question (not in the comments) what the circuit is supposed to do. e.g., "I want an opto-isolated signal to indicate if an input voltage is above a certain (adjustable) threshold." That way someone can give you a good solution rather than try to fix a bad one. \$\endgroup\$
    – Transistor
    Apr 24 '16 at 21:00

Take something like below and add in current limit or protection circuit as necessary. Variations of this circuit are commonly found in isolated SMPS. The current limit box in the schematic can be a simple resistor if appropriate (when input range is not too great).

U1 can be replaced with a simple NPN transistor using the Vbe as the threshold. The accuracy would be lower, but from what you said it should be good enough.


simulate this circuit – Schematic created using CircuitLab


What about using an off-the-shelf comparator (or opamp wired as a comparator) to drive the optocoupler? Failing that, there are 'digital' optocouplers that are either fully on or fully off although I don't think you can set the thresholds on them.

  • \$\begingroup\$ I should add an isolated supply for the comparator. Of course I will do that if I will not be able to use a optocoupler. \$\endgroup\$ Apr 25 '16 at 7:43
  • \$\begingroup\$ The circuit rioraxe is suggesting is probably your best bet for a cheap and simple circuit (<$3). If you wanted more precision, you could replace U1 with a comparator. Failing that, I do know of a few optocouplers that have their own internal isolated power supplies so only one side of the optocoupler needs power, they cost quite a bit though. \$\endgroup\$
    – Sam
    Apr 25 '16 at 23:22

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