save my optocoupler 4N25

Discussion in 'General Electronics Chat' started by vinodquilon, Jun 9, 2010.

  1. vinodquilon

    Thread Starter Member

    Dec 24, 2009
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    Pls find the attachment of a self-explanatory telephone ring detection circuit.

    The problem is that my 4N25 is not secure.
    'After' four or five times of operation, it destroyed itself.
    Then I replaced it with another, but same fate.

    I think the protection circuit of 4N25 has no better immunity against high ring volt.

    Any suggestions to rectify my problem ?
     
  2. Bychon

    Member

    Mar 12, 2010
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    I thought I just answered this. Oh...I did. It's on Electro-tech/projects.
     
  3. BMorse

    Senior Member

    Sep 26, 2009
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    You pretty much answered your own question without knowing :) .... you need to rectify your input from AC to DC....

    Check out my ring detect circuit that I am using in this project >> <snip>

    B. Morse
     
    Last edited: Jun 10, 2010
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    I don't think that explains why his 4N25 is getting fried.
     
  5. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    it's getting fried due to ring spikes
     
  6. BMorse

    Senior Member

    Sep 26, 2009
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    So if the 90+ volts (20hz) AC Ring signal coming from the TIP line is not frying the LED in the opto, then what is?? Maybe elaborating on what you think it could be can help the op more than mentioning what may not be causing his problems;)....

    B. Morse
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Why not use a Zener across the IR emitter's input, with an appropriate current limiting resistor for the IR emitter?

    That should pretty much take care of it.
     
  8. Ron H

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    Apr 14, 2005
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    I think that R1, D2, and the zeners should protect the LED in the 4N25. As to the cause, I don't have anything to add to what Bychon said in the Electrotech thread.
    You haven't explained why you think the ring voltage will damage his 4N25.
     
  9. BMorse

    Senior Member

    Sep 26, 2009
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    Even in that thread, another poster submitted a circuit for a zero crossing detector circuit, which is also rectified... go figure :rolleyes:....

    But atleast I gave a solution to what I think is the cause, I didn't know I needed to write a thesis on it so others could follow along ;).

    B. Morse
     
  10. Ron H

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    No thesis required, but I was hoping you could explain:
    How is the ring voltage going to overwhelm the 10k resistor and the antiparallel diode in our OP's circuit?
    How does the FW rectifier protect the optocoupler better than his circuit?
    I know I sound contentious, but I think the OP really wants to know why his circuit is killing his optocoupler. Personally, I suspect D2 is open.
    An alternate circuit will probably solve his problem, but it will not explain why.
     
  11. JimG

    Member

    Dec 7, 2009
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    ... or has its polarity reversed.

    Jim
     
  12. Ron H

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    Yep, that could do it, although the LED would never conduct, due to the low Vf of the diode. The OP said
    I thought this meant that the circuit worked briefly, but maybe not.
     
  13. vinodquilon

    Thread Starter Member

    Dec 24, 2009
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    Sorry for the late replay.

    Replay...

    Thanks for all.

    I set-up the same circuit again on a separate bread board with all new components except 4N25.
    The circuit was fine for the first few checks, again the problem arises. That is I cannot get +5V
    at pin4. This seems to be the problem is not with opto.

    CI tantalum, electrolytic is not avail.

    I cannot measure AC voltage across R1 with ordinary multimeter as intermittently occurring ring volts. I don't have any oscilloscopes.

    R1 10K 0.5W

    Deliberately I avoided isolation transformer as it only for electrical protection.
     
  14. vinodquilon

    Thread Starter Member

    Dec 24, 2009
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    How can I improve the current capability of D2 ?
     
  15. Bychon

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    Mar 12, 2010
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    See thread at Electrotech. There is no reason to increase the current ability of a diode that only passes .003 amps.
     
  16. SgtWookie

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    I threw together a model using LTSpice, and I didn't see anything unusual with a 120VAC 300Hz input. If I went up to 300v, the current started getting a bit high (around 26mA) through the 4N25's emitter - but that's still OK for pulse.

    I don't know what the ring signal is from the phone company. I thought it was around 90VAC, 20Hz, but really not certain about it.
     
  17. Bychon

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    Mar 12, 2010
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    Refering to post #13

    It is not important to have a tantalum capacitor in the C1 position. You're supposed to use a film capacitor.
     
  18. SgtWookie

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    Even then, the polarity across the cap never reverses, so he should be OK with that. The flip side of the coin there is tantalum caps that have been idle for a period of time tend to explode rather annoyingly when awakened from their long slumber...
     
  19. vinodquilon

    Thread Starter Member

    Dec 24, 2009
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    I have checked opto using multimeter in diode mode and its LED section outputs
    1257
    for transistor section
    6 to 5 outputs 715
    6 to 4 outputs 726

    I have checked zener also and outputs 850.
     
  20. Bychon

    Member

    Mar 12, 2010
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    I don't think it is possible to buy a 250 volt tantalum capacitor.
     
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