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# Optocoupler CTR design

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#### CHAITHRA Y S

##### Newbie level 1
Hello,

In the opto coupler datasheet I can see two different If's. So which If should take to calculate CTR of an optocoupler??? . And one more how Vce is related with If??

With Regards,
Nidhi

Hi,

For the first row you see ctr range of 100 ... 12000
On the second row you see ctr range of 50 ... 12000

What you should take for calculation depends on your application and on what you want to calculate.

Should it be used as a (saturated) switch or should it operate in linear region?

If you need more help, then show us your schematic and give additional information.

Klaus

how Vce is related with If??
The question can be answered by looking at datasheet figure 14.

Hello CHAITHRA And S

Certainly in the data sheets of the Optocoupler FODM124, we can see two IF parameters.
But one of them is called:
IF(avg) continuos Forward Current.

While the other is called:
IF(Pk) Peak Forward Current.

So: what is guaranteed by FAIRCHILD is that the LED, in the Optocoupler, can supports a current of 20 mAmp. Continuously.
And it can also supports an IF of 1 Amp. For a period of time of 300 microseconds at a repetition of 300 pulses per second (PPS).

However, it is unwise to subject the LED, in the Optocoupler, to its maximum. 20 mAmp. Because the lifetime of this is reduced.

Depends on how you use this device, it's like you have to polarize.
In order to calculate the CTR (Current Transfer Ratio) You must analyze the figures 3 and 4, In the FAIRCHILD datasheet, as the device number that you intend to use.

What use are you going to give to this device (Optocoupler) ??
We might know ??

If you need reliable logic levels Vce(sat) =0.5 then drive at 1mA min. with CTR=100% min
If you dont and can accept Vce=1.5 and need to save current with If min=0.5 then CTR is only 50%

THis yellow IC version has extra current gain.
Of course more input mA is allowed too and gives extra margin for 50% aging of LED.

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