MOCD208 opto isolator question

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SteveSFX

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Hello clever folk

I found an opto isolator circuit online, and have used it before in my circuits for inputting 12v to my projects (circuit attached).

This circuit (according to the thread where I found the circuit), was good for an input of 5v - 24v DC

However, it's not. It doesn't trigger the MOCD208 until 5.6v input. I need that to be 5v.
I changed the pull-down resistors to 10k (from 4k7) and that appears to have fixed the issue. But, its all a bit 'stab in the dark'. At 53 years old, I still have not figured out how you calculate all this stuff.

The MOCD208 has a forward voltage of 1.55v and a max current of 60mA. What will this circuit achieve with the 4k7 changed to a 10k? The input spec is surely still the same, it's just easier to reach the input threshold of the Teensy.

I have tried working it out, but my brain melted.

There is probably a much better way of achieving this input isolation, but this circuit is now part of my etch board, so I have to live with it. Maybe version 2 can have a better design.

The opto-isolator worked fine on my breadboard pre-build, so I didn't think any more about it.

Can anyone tell me the min/max voltage this circuit can accept (and ideally, how you worked that out), and maybe any suggestions for better value resistors?

Thank you
 

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  • Opto isolator MOCD208 - 5v to 24v.png
    Opto isolator MOCD208 - 5v to 24v.png
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Reduce R4. 4.7K is too high.

An old datasheet Google search turned up says this part wants 1mA through it's LED for the collector to be able to conduct at least 1.3mA.
 
OK Paul.
How do I calculate that new value? This is what I am struggling with. How to calculate this stuff.

My bad maths works out 1.3mA to be 2.65K at 5v. Entirely possible that is wrong however.

So, maybe a 1k? That is 3.45mA I think (and 22.4mA at 24v?)

The idea was to have an input that could accept 5v - 24v DC. In hindsight, maybe a diode on the input side as well to protect against reverse polarity? However, that lowers the voltage input even further.
 
Try 2.2K. That'll give about 1.6mA in the LED with 5V and about 10mA with 24V. Power in the resistor will be about 0.23W with 24V, so use at least a 1/4W rated resistor. 1/2W would be safer.
 
Try not to get the input & output currents mixed up in your mind. 1mA input (through the LED) is needed to be able to get 1.3mA (worst case min) to be able to flow through the output transistor.

So compute R4 so you always have more than 1mA when it's on. Then compute the R3 so the output current so you always have less than 1.3mA (4.7K with 3.3V power is fine).
 
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