3.3v supply to opto-isolator

S

SteveCS

Well-known member
Hi all

Having a few slight issues sending a signal to a 6n137 opto-isolator.

I 'think' this is the issue anyway. I am using a 3.3v high signal to drive it and I don't think it is switching on properly.

This same circuit (it is an assembled pcb) works fine at 5v, but I am having troubles with the lower voltage.

The original circuit I found online (can't find it now), fed the opto-isolator led with a 470r Resistor. I dropped that to 100r and found it worked much better (at 5v).

I don't exactly understand how to calculate the correct value to work on 3.3v. I would have thought 100r would be fine, but in frustration the other evening I tried a 47r and it still didn't work (but it work sporadically).

Can someone look at the 6n137 datasheet and explain to Mr Thick here how to calculate the correct resistor value please.

https://docs.rs-online.com/9a25/0900766b80de4953.pdf

Thank you
 
SteveCS said:
Hi all

Having a few slight issues sending a signal to a 6n137 opto-isolator.

I 'think' this is the issue anyway. I am using a 3.3v high signal to drive it and I don't think it is switching on properly.

This same circuit (it is an assembled pcb) works fine at 5v, but I am having troubles with the lower voltage.

The original circuit I found online (can't find it now), fed the opto-isolator led with a 470r Resistor. I dropped that to 100r and found it worked much better (at 5v).

I don't exactly understand how to calculate the correct value to work on 3.3v. I would have thought 100r would be fine, but in frustration the other evening I tried a 47r and it still didn't work (but it work sporadically).

Can someone look at the 6n137 datasheet and explain to Mr Thick here how to calculate the correct resistor value please.

https://docs.rs-online.com/9a25/0900766b80de4953.pdf

Thank you
Click to expand...

SteveCS:

Searching the forum for 6n137 finds several threads, including <this thread>, which appears to describe a successful implementation with the 6N137.

Hope that helps...

Mark J Culross
KD5RXT
 
The data sheet says it needs a current of 5mA to turn it on. The Teensy is rated at 4mA which could be your problem.
 
Ah... that is a pain.
Got an ESP32 here... wonder what the output current of those pins is.

40mA max by the looks. Maybe I should try that
 
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