I found myself thinking about some 50, old (20+ years) optocouplers that I have (removed from working equipment, purchased somewhere or other, or I don’t remember where they came from), and began to test them.
They are all with transistors. I used this circuit to test them (based on https://leetsacademy.blogspot.com/2017/10/how-to-test-opto-coupler-find-bad-opto.html method 2).
I recorded the voltage between point A and B after varying RX between 220-1K ohms. In all cases, with the switch not pressed, the meter read 5.0v as per the power supply.
Here are the results (voltage between A and B while pressing the switch) for the 7 above as a sample.
In case the graph is difficult to follow, here is the spreadsheet with the voltages.
I want to make sure that I understand what I am seeing and my major question is: Does the low end of each curve mean that the transistor is saturated? The functions are relatively flat for 4 of these (4N25s, 4n33, TIL-113), over all resistors used for RX. The other 3 are notably different. So, I would rephrase the question to ask if low voltages (say, 0.25v or less) indicate that the transistor is operating in the saturation region?
Essentially, I use these as switches and want them to turn fully on, but the less current used for the LED, the better, since not all GPIO can provide 20-25 mA. So, am I right that the better choices are the ones with the lowest voltages at the highest value resistors and, hence the lowest current requirement.
I get that there are many characteristics for optocouplers and I am asking in a sense with a presumption of, “all other things equal or all other things good enough for what I want to use them for”.
The 4N33 and TIL-113 are relatively flat across the resistors, but have a higher voltage (~.6-.7v) – why is that and is it undesirable?
Thanks for the help.