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    Photocoupler Saturation?

    Hello everyone!

    There is a photocoupler (LTV-817) that I am using that is seeing a power glitch (~1 microsecond) and it seems to take a long time for the photocoupler to recover (~35 microseconds).

    Click image for larger version. 

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    The blue trace is the power glitch that I am seeing and the yellow trace is the "output" of the photocoupler.

    I was wondering if anyone had ideas on reducing the response time of the photocoupler (the yellow trace). At the output (yellow trace) what is happening? The part is saturated?

    Thanks for any ideas!

    Datasheet link:

    http://optoelectronics.liteon.com/up...0201610%20.pdf

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    Re: Photocoupler Saturation?

    Hi,

    Please show your complete input and output circuit of the photocoupler.
    We need to determine where exactly your scope probes are connected
    And we need to determine the photocoupler current (not the voltage) at input and output ... at normal conditions and at glitch...

    Klaus
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    Re: Photocoupler Saturation?

    Hello Klaus,

    This is the connection through the photocoupler:
    Click image for larger version. 

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    This is output of the photocoupler to our comparator:

    Click image for larger version. 

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    The blue trace in the previous post is the measurement across R421 (1O OHM) with a differential probe when the high voltage lines (HV_P and HV_N are shunted to ground.)

    The yellow trace is what is measured from the emitter of transistor across R422 to ground. (signal Over_I).

    The purple trace is the output the comparator U94 meaused on test point TP59.

    Thank you for any ideas.



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    Re: Photocoupler Saturation?

    Relaxation time during cutoff can be reduced by reducing Re to RL=100Ω and turn ON, VCE=2V, IC=2mA . Load capacitance affects the RC decay (pF) with emitter current shutting off, output impedance rises rapidly.
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    Re: Photocoupler Saturation?

    If lowering the load resistor value still does not give a fast enough response, you may have to use a faster opto such as a 6N136 (LTspice simulation below).

    Click image for larger version. 

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    How fast a response do you need?
    Zapper
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    Re: Photocoupler Saturation?

    If you read the datasheet of the used opto coupler, you realize that the shown recovery delay is in the expectable range. The fast attack time is in contrast suprizing, but it's only achieved by overdriving the optocoupler massively.


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    Re: Photocoupler Saturation?

    there is a fair chance your led will eventually die if subjected to unlimited current flow pulses ...

    opto's turn off slowly due to carriers stuck in the base region of the o/p xtor - faster opto or better ckt or both, needed ...


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    Re: Photocoupler Saturation?

    Hello FvM,

    What parameter are you looking at to know the recovery delay?

    Datasheet here:

    http://optoelectronics.liteon.com/up...0201610%20.pdf


    I am seeing about a 35 uS delay on the waveform on my scope.


    The datasheet shows a response time of 4 uS. Maybe I am missing something here.

    Thank you.



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    Re: Photocoupler Saturation?

    Crutschow,

    It would be nice to be as fast a possible. Closer to the 1 uS range instead of 35 uS? :)



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    Re: Photocoupler Saturation?

    If you are willing to invert the signal by looking at a signal from the collector instead of the emitter, you could use a cascode NPN transistor.

    But to reach 1 uS response time, perhaps you will require to connect the optocoupler in a photodiode mode.
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    Re: Photocoupler Saturation?

    Quote Originally Posted by mrblueblue1234 View Post
    It would be nice to be as fast a possible. Closer to the 1 uS range instead of 35 uS? :)
    For that I think you will have to go with a faster opto, such as the one in post #5.
    Zapper
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    Re: Photocoupler Saturation?

    HI,

    your circuit:
    I don´t like the idea to drive a LED without current limiting resistor.
    I assume this makes the LED current unpredictable. In worst case you kill the LED.
    When I see this correctly, then the pulse is about 6V .... this is way off specification, probably several Amperes of LED current.
    --> Try to add a series resistor.

    Klaus
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