Optocouplers and CTR

Thread Starter

dandy1

Joined Sep 30, 2017
178
I want to use an optocoupler for a signal on the output. Lets say there's 5mA on the input. Does this mean that with a current transfer ratio of 50% that the output is limited to 2.5mA or that I Should be driving a 2.5mA load thru the output?

So does it matter how much current (within specification) the input gets to drive a tiny current on the output?

Thanks
 

Ian0

Joined Aug 7, 2020
10,057
If you think of it like a transistor, so that the LED connection is the base, then the CTR is equivalent to the Hfe.
So a 50% CTR is like having a transistor with a gain of 0.5 ( which is a bit of a rubbish transistor).
Similar to HFE on transistors CTR varies all over the place, with current, with temperature annd there is a wide variation between devices; and it takes rather more current than you might expect to get the output transistor switched fully on.
 

Thread Starter

dandy1

Joined Sep 30, 2017
178
If you think of it like a transistor, so that the LED connection is the base, then the CTR is equivalent to the Hfe.
So a 50% CTR is like having a transistor with a gain of 0.5 ( which is a bit of a rubbish transistor).
Similar to HFE on transistors CTR varies all over the place, with current, with temperature annd there is a wide variation between devices; and it takes rather more current than you might expect to get the output transistor switched fully on.
Ok thank you Ian for your reply, I think you know where I'm going with this from my other post but I will try it out anyway. Cheers
 

Thread Starter

dandy1

Joined Sep 30, 2017
178
It depends on the circuit.

CTR degrades over time. Good designs will accommodate degradation, bad designs won't.
1Capture.JPG

Do you see any problems with this?

The signal has a mixed P-N-P pulse, so I'm blocking the positive with the diode and hoping to catch the negative.
 

Thread Starter

dandy1

Joined Sep 30, 2017
178
If you have a common ground, then why do you need an isolator, there's nothing to isolate?
I don't need an isolator it's just the way its configured its an easy option, It was a suggestion from another thread and i've just experimented. I dont know much about electronics so here I am
 

Ian0

Joined Aug 7, 2020
10,057
I don't need an isolator it's just the way its configured its an easy option, It was a suggestion from another thread and i've just experimented. I dont know much about electronics so here I am
If you have an opto-isolator with the diode cathode joined to the emitter, then you might just as well use an NPN transistor.
 
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