optocoupled output circuit

Thread Starter


Joined Apr 19, 2013
Dear All,

I came across this schematic for opto-coupled output: Attachment

We have FPGA out 1.8V and resistor 100Ohm to regulate the current for diode from MOCD207-M.

I'm not sure how the other side works.

1. How works this circuit?
2. What is the purpose of the diode CDSU4148-HF?
3. What will happen if we put a series resistor with the diode?
4. What will happen if we remove the diode?
5. Does some one have experience with opto-coupled darlington output and can the darlington substitute the proposed circuit?

Best Regards.



Joined Nov 30, 2010
It looks like a high frequency switcher.
To keep the transistor from saturating, which slows it down.
It won't work as well.
The transistor will saturate and slow down.
I have included a datasheet for a photodarlington.