opto-isolator
I am using 6N137 opto-isolator for my circuit.
I have attached schematic for the same. I am giving Input and Enable from SMPS as shown and ground is same. VCC and EN is 3.3V from SMPS.
But while checking output voltage when Input and EN is High ( VCC is also High ie 3.3V ), I am getting Logic High at output (3.3V).
Ground for both is same.
Link for datasheet : http://www.vishay.com/docs/84732/6n137.pdf
Can anyone help me for above issue.
Thanks in advance.
In the case of the second circuit have you made sure that the GND2 connects to the return of the 5V output supply?
I ask this because if this connection is open it could explain why the coupler is unable to pull the output low. This particular case would be where the GND connection of your scope or meter is properly connected to the GND of the 5V power supply.
Assuming you're using a 5V-tolerant output in 3-state mode, and that the 5V is only present when 3.3V is, then the voltage should be okay.
Current will be about 7mA, which is almost twice the current at which the output voltage is guaranteed to be 400mV. Chances are very good that it will work okay, but a very conservative approach would be to buffer the output with something like a transistor.
Given the implied supply sequencing requirement, it would simplify your life and eliminate a worry if you just put the transistor in there (to drive the opto). An MMBT4401 with a few K base resistor would work fine. The 1K resistor is not necessary- but if you wanted to protect the MOSFET gate (against nasties on the 12V bus) by adding a zener it would be useful.
Your margin on CTR looks appropriate.
Here's a schematic showing Q1/R1 to buffer the output and an optional 2-BJT MOSFET gate driver. Note that the transistor inverts the logic (GPIO high = ON) compared to your original circuit.
simulate this circuit – Schematic created using CircuitLab
Outside the scope of this question, but you may need to add a driver circuit on the MOSFET side of the opto to make it switch faster. A couple BJTs would do it.
Best Answer
You should hew to the datasheet recommended minimum voltage of 4.5VDC if you want reliable operation, however it does not appear that is the proximate issue.
Your measurement of +5V at 2 and 0V at 3 indicates you have a wiring problem or you've damaged the LED (perhaps by accidentally connecting it without a resistor for even an instant). The LED forward voltage should be around 1.3-1.6V- and that is a good indication that it is actually emitting light. If it is much less or much more, that indicates a short somewhere (perhaps internally) or an open (again, perhaps internally). In your case it appears to be open.
As an aside, your 200 ohm resistor is a bit on the low side- I suggest keeping the nominal current more like 10mA. That is not what damaged the LED (if that's the problem) just better for long-term reliability.