Opto Isolation issues

I'm trying to figure out why different opto's for a midi isolation bridge(4-20mA current loop with isolation) doesn't work for most of the op amps I've tried.

The one that works, but not well is the TLP630. The rise time is aweful but enough so that I can sharping using two mosfet to get it to work.

The others,

H11G3, 4N33 - Darlingtons CNY17, MOC8205

The darlingtons are total crap. I may be using them wrong though but the rise time is about 1/20th of the others. I'm not sure what to do with the base but I have tried various things as seen in other circuits including

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Usually, such as connecting the base to V, complete ruins the signal and draws a significant amount of current.

The other opto's don't work well either.

I'm not sure if this is normal behavior and they are just extremely slow or if it the CTR and all the ones I have are too low(although this doesn't explain why the TLP630 works).

Any Ideas what I'm doing wrong?

Reply to
Jon Slaughter
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Miller is a bitch. So don't let him get into your circuit.

High speed optos (e.g. 6N136) use a photodiode effectively cascoded into a transistor. You can add base-emitter resistance to reduce CTR, improve noise immunity and reduce fall time.

Even a 4N35 will do ~2us edges if you use the right combination of drive current, B-E resistance and load current. Caveat: if CTR drops, it won't turn on as fast, or at all.

Anything with "phototransistor" or "darlington" in it (let alone both!) isn't good for anything above ~100kHz.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms
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Reply to
Tim Williams

On a sunny day (Wed, 17 Feb 2010 16:12:55 -0600) it happened "Jon Slaughter" wrote in :

If you use that diagram with a CNY it wont work. The CNY17 has different connections!

Leave the base not connected. To get 5 V over the 280 Ohms, you need 17 mA, That means the CNY17 needs about *170 mA* LED current (1/10 transfer ratio). However it has an absolute max of 60 mA LED current. So no way will it work in that circuit.

For 2 x 220 drive at the LED, say about 500 Ohm, you get 3.5 V in 500 Ohm = 7 mA. The transistor will then do .7 mA. To get 5 V drop at .7 mA you need a collector resistor of 7 kOhm! That would mean a cable mismatch, and cable capacitance will attenuate high frequencies.

Darlingtons may work, or you could add a transistor after the CNY17 to make a darlingtom. But the speed would not be great with that opto.

Reply to
Jan Panteltje

Ok, I think I got it. The opto is simply a bjt with an optically driven base instead of electrically(I know it's obvious but just now sinking in that it's this simple). All the standard rules of bjt's apply with the obvious adjustment of the base current by the CTR. Obviously the opto's bjt's may not be the same spec as discrete, etc but all the info should be in the datasheets.

Reply to
Jon Slaughter

You wish. ;-)

Although if you're lucky, you might find a plot of CTR vs. Rbe. Not sure I've ever see t_r / t_f vs. Rbe.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms
Reply to
Tim Williams

Hence the "should" ;)

Reply to
Jon Slaughter

Another idea is to use one of the Analog Devices iCouplers.

Joe

Reply to
Joe G (Home)

But MIDI is 31.25kHz so??? I recall back in the '80's not being able to get the Sharp opto in the MIDI spec and using generic OTS TI parts which worked just fine. What's the big deal?

Reply to
Bob

Still a false assumption. The very assumption that is reasonable to=20 fully characterize every possibly useful parameter over many=20 production runs is so beyond cost acceptable. They are in business=20 to make a profit, not to do and publish the results research that=20 will their competitors more than it could possibly profit their=20 customers.

Reply to
JosephKK

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