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Electronics => Beginners => Topic started by: paul23 on July 02, 2013, 08:27:05 pm

Title: Opto-Coupler Calculations
Post by: paul23 on July 02, 2013, 08:27:05 pm
I need to connect to MCU´s togther through an opto-coupler.  Basically one sends pulses to the other, speed is not important as it will be worse case a couple of pulses per second, but generally much less than that.

I will be using a Vishay VO615A-8X017T part, datasheet here if needed. (

Basically the configuration is simple, MCU > Resistor > Anode - Cathode > GND on the input side and the collector emitter tied to ground, with the emitter collector going to the other MCU with a 10K pull-up resistor.  The reason for 10K is that I already have a few 10K resistors on the board doing other things, so I need less values on the board.  Is that OK to do? 

So if I have understood everything correct, then there is 500uA on the collector (5v through 10K), so I need to supply at least that amount to the LED to get the Transistor to switch on fully.  From the datasheet, the CTR is 130 - 260% so if i give the LED the same, 500uA,  there should be plenty to switch the transistor on.  The LED side is 3.3V, and the LED Vf is 1,05 at 500uA, so I would need 4,5k (4,7K is a better value) resistor.

Is that right?

Title: Re: Opto-Coupler Calculations
Post by: edavid on July 02, 2013, 08:42:09 pm
Input side: tie anode to Vcc, cathode to output pin through resistor.  You get a little more drive that way.

Output side: you have this wrong (typo?) - should be emitter to ground, collector to Vcc through resistor.  10K is OK if you are driving a Schmitt trigger input.

Your LED resistor calculation is basically OK, but derate CTR to no more than half the minimum spec to allow for aging etc., so I'd make it 2K.
Title: Re: Opto-Coupler Calculations
Post by: paul23 on July 02, 2013, 09:25:30 pm
Yeah, that was a typo, I was rushing.

I hadn´t thought about aging, that is a good point.  Im not sure what the second MCU will be yet, probably an ATMEGA so it will be a schmitt trigger input.

Thanks for the other suggestions.