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optocoupled output circuit

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www111

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Dear All,

I came across this schematic for opto-coupled output:
WP_20160414_07_26_04_Pro.jpg

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.
 

That is a darlington circuit, you could use a darlington-output opto and get rid of that external transistor. Not really sure what that diode is for. Sometimes a resistor put there to speed things up, as a darlington is relatively slow to turn off. Maybe it's there to sink leakage current from the input transistor.

It's not clear, but is your load simply a 270 ohm resistor?
 

I'm not 100% sure but the resistor is what the user have to calculate regard the voltage connected to V+ (3,3V to 24V) to regulate the maximum current throughout the transistors.

But I'm not sure for what voltage is the 270 Ohm resistor.
 

The diode is forward biased and acts as a non-linear resistor. Basically trying to improve the dynamic range- when the voltage is low the resistance is high and vice-versa. The external transistor, with collector and emitter connected to the supply rails, is badly configured as there are no load resistors given.
 
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What voltage , the 3.3V to 24V?
Can You explain the circuit step bay step please?

Best Regards.
 

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