+ Post New Thread
Results 1 to 12 of 12
  1. #1
    Newbie level 5
    Points: 124, Level: 1

    Join Date
    May 2019
    Posts
    10
    Helped
    0 / 0
    Points
    124
    Level
    1

    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. 

Name:	shunt_power.jpg 
Views:	12 
Size:	82.7 KB 
ID:	155978


    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

  2. #2
    Super Moderator
    Points: 77,915, Level: 68
    Achievements:
    7 years registered
    Awards:
    Most Frequent Poster 3rd Helpful Member

    Join Date
    Apr 2014
    Posts
    15,796
    Helped
    3593 / 3593
    Points
    77,915
    Level
    68

    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
    Please donīt contact me via PM, because there is no time to respond to them. No friend requests. Thank you.



    •   AltAdvertisement

        
       

  3. #3
    Newbie level 5
    Points: 124, Level: 1

    Join Date
    May 2019
    Posts
    10
    Helped
    0 / 0
    Points
    124
    Level
    1

    Re: Photocoupler Saturation?

    Hello Klaus,

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

Name:	opto.png 
Views:	14 
Size:	20.0 KB 
ID:	155979

    This is output of the photocoupler to our comparator:

    Click image for larger version. 

Name:	comparator.PNG 
Views:	10 
Size:	25.8 KB 
ID:	155980

    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.



  4. #4
    Advanced Member level 5
    Points: 34,834, Level: 45
    Achievements:
    7 years registered Created Blog entry Created Album pictures
    SunnySkyguy's Avatar
    Join Date
    Sep 2007
    Location
    Richmond Hill, ON, Canada
    Posts
    6,729
    Helped
    1679 / 1679
    Points
    34,834
    Level
    45
    Blog Entries
    7

    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.
    A best design is easily achieved with good test specs™
    A better question deserves a better answer. ™
    ... so include all your acceptance criteria ( values, % tolerance) and assumptions in your question or any design.

    ... Tony Stewart EE since 1975
    - slightly north of Toronto



  5. #5
    Advanced Member level 5
    Points: 19,100, Level: 33
    Achievements:
    7 years registered
    crutschow's Avatar
    Join Date
    Feb 2012
    Location
    L.A. USA Zulu -8
    Posts
    3,677
    Helped
    874 / 874
    Points
    19,100
    Level
    33

    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. 

Name:	Capture.PNG 
Views:	8 
Size:	28.1 KB 
ID:	155983

    How fast a response do you need?
    Zapper
    Curmudgeon Elektroniker



    •   AltAdvertisement

        
       

  6. #6
    Super Moderator
    Points: 260,308, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    45,462
    Helped
    13829 / 13829
    Points
    260,308
    Level
    100

    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.


    1 members found this post helpful.

  7. #7
    Advanced Member level 5
    Points: 11,225, Level: 25

    Join Date
    Aug 2015
    Posts
    1,824
    Helped
    675 / 675
    Points
    11,225
    Level
    25

    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 ...


    1 members found this post helpful.

  8. #8
    Newbie level 5
    Points: 124, Level: 1

    Join Date
    May 2019
    Posts
    10
    Helped
    0 / 0
    Points
    124
    Level
    1

    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.



  9. #9
    Newbie level 5
    Points: 124, Level: 1

    Join Date
    May 2019
    Posts
    10
    Helped
    0 / 0
    Points
    124
    Level
    1

    Re: Photocoupler Saturation?

    Crutschow,

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



  10. #10
    Advanced Member level 5
    Points: 14,971, Level: 29
    schmitt trigger's Avatar
    Join Date
    Apr 2013
    Posts
    2,488
    Helped
    805 / 805
    Points
    14,971
    Level
    29

    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.
    My batteries are recharged by "Helpful Post" ratings.
    If you feel that I've helped you, please indicate it as a Helpful Post


    1 members found this post helpful.

  11. #11
    Advanced Member level 5
    Points: 19,100, Level: 33
    Achievements:
    7 years registered
    crutschow's Avatar
    Join Date
    Feb 2012
    Location
    L.A. USA Zulu -8
    Posts
    3,677
    Helped
    874 / 874
    Points
    19,100
    Level
    33

    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
    Curmudgeon Elektroniker



    •   AltAdvertisement

        
       

  12. #12
    Super Moderator
    Points: 77,915, Level: 68
    Achievements:
    7 years registered
    Awards:
    Most Frequent Poster 3rd Helpful Member

    Join Date
    Apr 2014
    Posts
    15,796
    Helped
    3593 / 3593
    Points
    77,915
    Level
    68

    Re: Photocoupler Saturation?

    HI,

    your circuit:
    I donīt 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
    Please donīt contact me via PM, because there is no time to respond to them. No friend requests. Thank you.


    1 members found this post helpful.

--[[ ]]--