# Is this a reasonable way to determine if an optocoupler transistor is operating in the saturated region?

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.

• +1Well researched and presented. Commented Sep 17, 2022 at 3:48
• You seem to have a couple of antique chips in that collection there--neat! Commented Sep 17, 2022 at 4:25
• The voltage between A & B depends upon the size of the input current, the opto's CTR (current transfer ratio), the hFE (beta) of the transistor, the size of R1 and when the transistor's saturated, VCE(sat).
– user173271
Commented Sep 17, 2022 at 4:31

Nice experiment.

Does the low end of each curve mean that the transistor is saturated? Yes when VCE is at its lowest value it is saturated.

1. IL1 and F are barely saturated. You could raise R1 to force them deeper into saturation.
2. MC2 is nicely saturated.

The others have a higher gain and so are well into saturation. Most likely the transistors have a higher hFE, but the diodes could also have a higher output.

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.

You are right.

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?

These two have Darlington connected transistors, so VCEmin will be ~0.7V at saturation. they will sink higher currents. Noise immunity for logic levels is not great but it will be fine for LEDs and Schmidt trigger inputs.

All this is easily verified in the datasheets. First hit on Google for the two that I looked for.