Wierd Opto Behavior

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This is from a 6N138. I used a PC900 configured exactly the same way(but obvious changed in pin configuration) which gave much better results.

Whats causing this behavior? The rail voltage is 5V.

Reply to
Jon Slaughter
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Methinks we need more info..a circuit perhaps?

Reply to
Robert Baer

That's what I thought too.

Reply to
pimpom

Basic circuit. As I said, the PC900 works just fine. There is no load and obviously the drive is not the issue. Hence on the output it's obvious(just a resistor in the standard configuration). i.e., the circuit is the obvious one... the minimal one to get any useful output from the device.

i.e., it's what you call a "test circuit"... which, IMO, is what you should assume when no other information is given.

Reply to
Jon Slaughter

Three "obvious"'s when nothing is obvious.

When no context is given, what we assume is that the problem is underspecified.

John

Reply to
John Larkin

Or perhaps your simply trying to overanalyze the issue? It's not complicated. You can easily gather basic information from the original post to remove most doubt. First, it's for digital logic by the pic. Second, the PC900 works in the same circuit. Third, it's most likely not some complex circuit(just by shear probability). This is enough information, if you have seen this behavior before, to know what it is caused by. If you haven't seen it before then obviously you will want more information because you'll not have a clue what is causing it.

In any case I'd imagine that if I showed the circuit(which I won't because it is too obvious) you still wouldn't know what is causing it and would then want to know what kinda scope I'm using, what resistor types, tell me to remove the capacitors(which there are none but that won't stop you), A photo of the actual circuit since surely I must have wired up something wrong, etc...

If I give you too much information it will simply give you more opportunities to be wrong. The key here is that the PC900 works fine in the exact same circuit. This alone is enough to deduce the behavior(since the

6N138 is similar specced and generally used as a replacement).

The point is that if you didn't get an idea of the problem in the first post your not going to get it without knowing way more information than I'm willing to sit down and type in.

Don't be ashamed of not knowing though. It's not that big a deal. I'm sure you could figure it out if you had the problem but I'm not willing to be asked what color socks I was wearing when I took the pic. AGAIN, either you have seen such behavior before and have come clue what is causing it(one may think initially it is capacitance but then it should most likely cause the same problem with the PC900) or you don't have a clue.

If you want to know what the circuit probably looks like then do a google image search for "Opto circuit" and pick one at random(at random) and you'll probably have a 19/20 chance of getting the circuit I'm using.

Reply to
Jon Slaughter

Circuit?

Since, in subsequent replies, you appear to be reluctant to provide one, savor my suspicion: are you using the common ("emitter") pin for voltage? Is there any common mode voltage (LED to output side), and if so, how much? Is the base pin shielded, or are you doing something bizarre with it?

I've tried using 6N136's at 1000V/us common mode (300V step) and signal quality fails. Fairchild I think rates theirs to 10kV/us but only for a 10V step, useless.

Tim

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Reply to
Tim Williams

...

The problem is obvious, the solution likely depends on context. ...

In that case you already have your answer too.

Grant.

Reply to
Grant

No, nothing bizarre... your standard basic isolated newbie digital comm stuff that you can find when searching.

What I see in the image is we have a capacitor rise time to about 3/4Vcc then a short time later we get another capacitive effect with a much slower rise time to Vcc.

I've never seen something like this before and only could understand it if it was intentionally done.

Think of it as a problem on an exam: "How can a PC900 give almost ideal rise time characteristics but a 6N138 give the wierd behavior in the image"?

What would be your answer? If you say "Common mode voltage" then that would be wrong would it not? Since the PC900 would experience the same issue(more or less). You say "It has something to do with the base pin shielding"... well then why doesn't the effect show up with the PC900? etc...

Now, I am going on the assumption that the 6N138 is a direct replacement for the PC900... I have read this from about 10 different sources showing the

6N138 used with the same results. This is the issue: I dropped in the 6N138 and didn't get the same results with contradicts what those 10 sources have said. It has nothing to do with the circuit and giving the circuit will only get more dead end answers. Again, I'm not saying the circuit may be excerbating the problem but it is not the root cause of the problem. It may be simply that all opto's behave this way more or less and the circuit I'm using magnifies the problem. But if someone knew this answer they would say that instead of asking for the circuit.

It seems that many of you guys don't want to make the assumption that I'm using a simple and common circuit but don't mind the fact of making the assumption that I wired of the circuit wrong. If your not going to allow one assumption then don't allow the others as both are just as important for finding the real answer. (Tim, I'm not talking about you here since you obviously gave some potential answers)

Reply to
Jon Slaughter

If you don't post a schematic or equivalent, we have no idea what you're trying to do. You are being a PITA on purpose.

John

Reply to
John Larkin

Did those 10 different sources explicitly say that the 6N138 is a direct replacement for a PC900? Judging from a quick glance at the datasheets, the PC900 is a Schmitt device while the 6N138 is a linear device. That's certainly not direct replacement. The two may function equally well and may indeed be interchangeable *in some applications*, and those 10 sources may be talking about specific applications in which the 6N138 can replace the PC900. But I certainly would not say without qualification that one is a direct replacement for the other.

Under some drive and load conditions, it's quite natural for the PC900 to exhibit snappier switching because its internal Schmitt trigger works on the threshold principle. OTOH, the 6N138 has a roughly linear transfer characteristic.

I didn't have time to go through the 16-page datasheet of each device in detail, but this much is clear: The two are not identical devices and they will definitely exhibit differences in behaviour under certain conditions. And most prominent among those conditions are the associated circuits.

Reply to
pimpom

Ok, I was able to drastically reduce the behavior so it is close to ideal. The problem seems to be because the base of the 2nd transistor in the darlington pair is storing charge and cannot turn off quick enough. Adding a resistor from the 2nd base(pin 7) to ground helped a lot. Strange all those people claiming that the 6N138 was a direct replacement without mentioning this(maybe it is suppose to be obvious since it is a darlington opto) ;/

Now I guess I know what slow base recovery looks like in a darlinton. Can someone give a good explanation why is going on as I can't seem to find anything online about it? (I just remember somewhere reading that one could add resistors to improve the speed of darlingtons)

At least I think this is whats going on...

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Reply to
Jon Slaughter

This application note may be useful.

" Optocoupler Applications"

From California Eastern Laboratories

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Reply to
Hammy

Cool, thanks!

Reply to
Jon Slaughter

So it _was_ the circuit after all?

Do you mind providing the URL for at least some of those sources?

As I said in my other post which you haven't responded to, the PC900 and the 6N138 are very different devices. The PC900 is a Schmitt trigger device which is optimised for fast, clean switching and will not do well at all in a linear circuit. The

6N138 is a linear type and its switching behaviour will depend on the circuit which you stubbornly, even rudely, refused to provide. The two are *not* direct replacements for each other except under certain specific conditions.
Reply to
pimpom

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