I have to build a device that can measure current among other things. For current measurement I need galvanic isolation. The complete device shall be supplied by USB. For communication I want to use a FTDI chip. For current measurment I want to use an ATtiny261V. I did some worst case current consumption calculation and came to the conclusion that the circuit never exeeds 2mA.
I don't want to use any other/additional power supply than the USB power supply. So I need to add a galvanic isolation circuit. The attached schematic explains my first idea with optocoupler. But the CNY74-2 optocoupler can't switch enough current to the ATtiny261V circuit. With a LED current of 20mA the opto-circuit can only pump 1.5mA to the ATtiny261V side.
The 2nd thought was to use a simple voltage doubler circuit on the input side of the galvanic isolation circuit, so the voltage drop accross the circuit can be higher. Then the circuit would be able to pump 2mA but no reserve left if the optos age.
The next thought was to discard the circuit and try a transformer - something like this one. It has a ratio of 1:3. But I don't have this transformer at hand and no experience with it.
if I use a low-drop/low-quiescent-current regulator on the ATtiny261V side, is it possible to gain stable 3.3V DC @ 2mA? I can pump up to 35mA into the transformer (to stay below the 100mA USB limit).
Or can you recommend better optocouplers or even a better circuit?
Any idea would be appreciated.