Struggling with Optocouplers

Other Hardware General Electronics
1

Every time I think I have a handle on how optocouplers really work, it seems like I look at something new and lose it.

I am currently using a 4N25, arranged so IF is 12mA.
According to the datasheet CTR table, the CTR should range anywhere from 20-50% (2.4-6mA). However, Figure 2 shows a normalised saturated CTR of ~75% (9mA) which seems to match Figure 6. Since there was such an apparent discrepancy, I tested it experimentally by connecting a pot as the output load. When wound right down, I measured IC at about 23mA, so I was actually achieving a CTR of ~200%.

Could anyone please help me understand where I am going wrong with my calculation/experimentation?

2

the CTR should range anywhere from 20-50%

20% Minimum, 50% Typical, Maximum is unspecified (∞?)

The 200% you've measured is well within ∞.

3

dlloyd:
20% Minimum, 50% Typical, Maximum is unspecified (∞?)

The 200% you've measured is well within ∞.

Ha! Good point.

So, if I am designing a circuit to make multiples of and don't want to test every one, I still have to assume 20% as the worst case possibility?

4

Yes.