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I am thinking of using the above opto : Datasheet=http://uk.rs-online.com/web/p/products/8076591/
What I am not clear about is that how much current should I program the led for to get 35mA & 70mA respectively as collector current! I was expecting a CTR graph to get an idea but it's not there so I need someone with experience to guide me on this please. Thanks
On page 3 it says that the minimum current transfer ratio at 5V CE is 500% so to get 35mA collector current you'd run 35/5=7mA thru the LED.
I'd imagine that this is a rather "loose" specification and it's not going to be linear so if you need 35mA exactly you're going to have to tweak it, or even implement a feedback loop.
/Henrik.
Thanks. I am actually switching a 24v relay. Minimum current is 30mA, 35 or above to be safe. Would it be wise to assume:
1) 500%is at 5v. At 24v will be 6 or 7 times?
2) if the above assumption is wrong purely on switching voltage basis, then passing 5ma in led should make it true-ish? As 5 times is for 1ma!
For a switching type application you want the transistor to be fully saturated. If you're not worried about power consumption, which I guess you're not since you're using a relay, just run 15-20mA thru the LED and measure the voltage across collector/emitter, in saturation it should be around 1V or less. If it is you can decrease the LED current, if it isn't you need to increase it.
/Henrik.
Try to learn from the Optocoupler H11G1 design guide. That may help you.
Why would you use an opto to drive a relay? Just curious...
No way to get precise current from the output transistor ... with such optos
BUT ... it exists LINEAR optocouplers
just have a look to HCNR 200/201 or HCPL 7520 ... i.e.
or you can try to make your own clone to those couplers ...
or simpler ... use two outputs with two classic optos wired as current generators !
Alain
doing it the old way ...
http://schematicrise.blogspot.fr/201...m-circuit.html
Alain