Optocoupler - sanity check please

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Svejk

Senior Member
I'm a little confused with use of a SFH618-A2 optocouplers so please bear with me.

What the 47R does in the circuit ilustrated in the picture? Would it limit the current for the voltage that I'm monitoring [10V]? That would give me about 213 mA and I'm not confortable with that. Should be read 470R? Or should I use 1k to give me the 10 mA?
 

Attachments

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Andrew Cowan

Senior Member
LED Forward Voltage ~1.1v
Maximum current 50mA

In that circuit, they don't say what the voltage being applied to the emitter.

For 10v, the minimum resistance you can use is (10-1.1)/0.05 = 178 ohms (I'd recommend about 220 ohms minimum).

Andrew
 
D

Dippy

Moderator
No, yes, no...

You have only provided part of the Data Sheet, though I must admit it doesn't look clear.

I'd ignore the 47R value and work it out for yourself.
But you MUST have a series R of an appropriate value or limit the average power by some means.

BUT, whatever the scenario, ALWAYS do the calcs. for yourself

IF= (Vsupply - LEDVf) / R

or if the LED side has a semi-conductor switch e.g. a tranny , then

If = (Vsupply - LEDvf - TrannyVdrop) / R

When doing these calculations for R , ALWAYS remember to include any series elements which have a voltage drop.

Hopefully you can transpose?


The actual Ic current you need will depend on the CTR and the current Ic you need on the 'output' side. You don't need to run the LED flat out in many, many apps.
Test, try and calculate.
 
S

Svejk

Senior Member
Thank you Andrew and Dippy.

I've used 680R in the actual circuit, that gives me ~13mA switching current [it just monitors the line supply, if power goes off it saves data to eeprom].

What puzzled me is that using 47R for 12 hours didn't smoke the optocoupler so I may have missed something, hence the question.
 
techElder

techElder

Well-known member
The charts you attached are describing the switching characteristics of the component.

That implies that the input is a very low duty cycle pulse for which they allow a relatively high current to insure saturation of the transistor part of the component.

You wouldn't typically operate that component continuously with those values.
 
D

Dippy

Moderator
It's called luck mate.
It won't last forever :)
In future, Data Sheet + calcs could save the day.


Read, read, data , data , check , calculate and test .. keep inferences and guesses to a minimum.


But, unlike many, at least you checked - and hopefully will have learnt something ??


I never take any notice of implied conditions or parameter and I try never to infer too much.
If a Data Sheet doesn't clearly define the conditions for the parametric test I tend to do one of the following.
1) Do my own tests.
2) Email the manufacturer.
3) Talk to a colleague with a pressurised cerebellum.


Many of us over-40 old hacks (especially we band of genuine professional designers) have a 'feel' for components like this ... usually ... :eek:
 
J

John West

Senior Member
While the opto spec (as noted) is incomplete, neither chart shown indicates an I(forward) of more than 20mA (a typical value for such devices,) as a reference level for any spec.

I'd suggest you keep your I/R calculations within that current limit to ensure the Magic Smoke stays where it belongs.
 
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Dippy

Moderator
John, he has ; see post #4.

I'd have thought he could get away with a lot lower but if it works that's fine.
 
J

John West

Senior Member
Yes, I saw he had selected a reasonably acceptable value, but I wanted to point out that there were indications in the doc that 20mA was indeed a reasonable limit, and that the resistor value specified in the doc would need to be used in conjunction with a low enough input voltage to ensure that current limit was not exceeded.

In short, the opto's current limit was the spec to pay attention to, not the value of resistor shown in the example. That value is merely incidental and depends on the input voltage one is working with. As you already know, optos are all about current, and the resistance values are just selected to keep within the appropriate current limits.

That was the point I was trying to make.
 
D

Dippy

Moderator
Ah yes, I see. Like post#3.

I'm just being cheeky.
It's wise to reitititerate.
Almost inculcatory - look it up Tex ;)
 
S

Svejk

Senior Member
Thanks John for underlining the current limit of the opto, thats what I used anyway.

I just didn't see why the very low resistance in the datasheet.
 
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