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    opto coupler CTR degradation

    Hi

    we are using this circuit for getting a feedback of AC mains to MCU. this is a mass produced circuit.
    the problem is that sometimes the opto-coupler CTR decreases so fast in couple of months and makes the circuit malfunction
    I have searched and know about CTR degradation with time and its speedup with high temperature but this degradation rate is so unusual

    ?anyone has experience with this
    do you have any comments about the circuit
    thanks in advanceClick image for larger version. 

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    Re: opto coupler CTR degradation

    Hi,

    Where do you have this schematic from?

    I doubt that this is a recommended circuit for generating AC voltage feedback. (At least I don´t recommend it)

    Klaus



    •   Alt17th November 2016, 10:12

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    Re: opto coupler CTR degradation

    this is a schematic that has been designed early , its a typical one
    why don't you recommend that Klaus ?



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    Re: opto coupler CTR degradation

    Hi,

    because the CTR has 1:1 influence on the feedback.

    **
    When you look at flyback optocoupler feedback circuits, then there usually is a zener that generates a threshold and thus the CTR has less (but still has minor) influence on the feedback.

    Klaus



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    Re: opto coupler CTR degradation

    This is just a AC mains monitor circuit, when ac mains is in circuit makes optocoupler turn on half in halfwave mode and it makes the output collector to get Low ,
    there is no feedback loop

    the problem is that the Current transfer Ratio of the opto coupler degrades with lifetime
    then the circuit doesn't work well ,



    •   Alt17th November 2016, 17:12

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    Re: opto coupler CTR degradation

    Hi,

    your text of post#1:
    we are using this circuit for getting a feedback of AC mains to MCU
    now:
    there is no feedback loop
    What is true now?

    Klaus



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    Re: opto coupler CTR degradation

    The peak current is only 2mA if there are no voltage spikes on the AC. Voltage spikes will also arc across a little resistor that has a 250V absolute maximum voltage rating causing very high current in the LED.
    I think you should rectify the AC, it feeds a series resistor with a capacitor to ground as a filter then another series resistor to feed the LED. Then the peak voltage of the AC is filtered and is shared by the two series resistors.


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    Re: opto coupler CTR degradation

    I agree with Audioguru that mains surges might be a problem if the resistor hasn't sufficient voltage rating. The risk to see actual flashover at a 0.25 W or larger resistor is limited, but I don't know about your operation environment. Adding a filter or overvoltage limiter is a good idea. I don't see a reason for fast degradation in normal operation. Of course the circuit should be dimensioned for expectable component aging and e.g. still work with 50 % of initial CTR.


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    Re: opto coupler CTR degradation

    Quote Originally Posted by KlausST View Post
    Hi,

    your text of post#1:


    now:


    What is true now?

    Klaus
    Sorry for that, maybe its because of my poor English

    - - - Updated - - -

    Thank you Audioguru and FvM !
    I think that is the answer for my question
    I observed some spikes on AC , and also my resistor is a small 1206 SMD ,

    just another question : can I solve this problem by using more resistors in series? for example 4*33k



    •   Alt17th November 2016, 20:09

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    Re: opto coupler CTR degradation

    Resistors in series will prevent a high voltage spike from arcing over one resistor. With series resistors a high voltage spike can still cause a higher current in the LED.
    The capacitor I added is needed to reduce the level of the spikes.


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    Re: opto coupler CTR degradation

    Yes, use several series resistors, some 1206 resistors are only rated to 50V. You need to have enough to make it at least 310V plus extra for safety margin. I would also consider dropping the total resistance to no more than 100K to allow more LED current to flow.

    A better solution is to use several resistors, choose values so the peak current is adequate for the LED, but wire the LED across a bridge of four diodes. If you connect the bridge at the LED side of the resistors, the voltage will be very low so you can use cheap silicon signal diodes (1N914 for example) but the transistor will then be made to conduct on both positive and negative AC cycles so you can also reduce the time constant of R8/C4 to make the circuit react faster to AC presence. For example drop C4 to 1uF.

    Brian.
    PLEASE - no friends requests or private emails, I simply don't have time to reply to them all.
    It's better to share your questions and answers on Edaboard so we can all benefit from each others experiences.


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