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Every optocoupler has min and max current transfer ratio. I have read articles that said always design the circuit using the minimum current transfer ratio. However, I want to understand which parameter affect the transfer ratio. I did look at a datasheet of an optocoupler but couldn't find any information about it. Could someone please explain them to me?

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  • \$\begingroup\$ Digital-output optocouplers (e.g., H11L1, 6N137) usually do not specify a CTR. \$\endgroup\$ – CL. Jul 7 '17 at 8:17
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The transfer ratio will vary in manufacturing from lot to lot and from device to device. This will be due to variation in LED, receiver, insulator and gap distance in manufacturing.

The minimum current transfer ratio is guaranteed and if you design to this your circuit should work. Meanwhile you need to keep an eye on the max value if there is a risk of problems due to too much current on the output side.

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  • \$\begingroup\$ Adding to this answer: the transfer ratio will also be temperature dependent, and strongly so for phototransistor-type optocouplers. If linearity and predictability is important, photodiode-type optocouplers are more linear and have a much tighter transfer ratio tolerance at manufacture, but require a transimpedance amp on the output. There are also dual-photodiode devices available which have detectors on both sides, for LED linearization - for example, the Vishay IL300 is a classic part. \$\endgroup\$ – Peter Jul 6 '17 at 20:37
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    \$\begingroup\$ LED efficiency also decreases with age. \$\endgroup\$ – CL. Jul 7 '17 at 8:18

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