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LED indicator keeps failing

Discussion in 'Electronic Basics' started by [email protected], Feb 16, 2007.

  1. Guest

    We have a line that uses a bunch of tri color led indicators. They
    are listed to work with voltages of 12-24 VDC.

    They fail within hours to a few months. Since the system runs at
    24VDC, I figured putting a 820 ohm resistor to drop the voltage to
    about 15 vdc when illuminated would fix everything. Well it does not
    seem to be so.

    FWIW, the indicator is a P+F LED18-RYG-P.

    Any ideas on what might be killing these things or how to protect
    them. All the other components of the systems (plc's, motion systems,
    sensors) are working fine and not failing on us.

    Thanks,

    Wes
     
  2. John Fields

    John Fields Guest

    I'd suspect reverse spikes.
     
  3. Wes,

    Assuming 2.0v LEDs, with a resistor of 820 ohms you are putting around
    27mA through the LED. A lot of common LEDs are rated for 20mA constant
    so it sounds like you are overdriving the LEDs just a bit, which could
    account for the differences in how quickly they fail.

    You might try something a little closer to 1400 ohms to get ~15mA
    through the LED.

    I tried to find a data sheet for that part to find out the actual rating
    for the LEDs but couldn't find one easily. The numbers above are just
    safe(ish) guesses.

    Jason
     
  4. What is the failure mode? Have you put a line voltage monitor on the system
    to look for spikes?


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  5. Guest

    The led's already have a limiting resistor in the package. I added an
    additional one.

    Thanks,

    Wes
     
  6. Guest

    I don't have a line voltage monitor. I do have a fluke 12 that I put
    in min-max mode to monitor voltage. It didn't show any spikes but I
    doubt it is fast enough.

    WEs
     
  7. Guest

    I was thinking of adding a diode to protect against that. There are
    some power control relays that get switched in and out > 5000 times a
    day. I need to look to see if snubbing diodes were installed across
    the coils.

    Wes
     
  8. John Fields

    John Fields Guest

    ---
    Good idea. Try this too: (View in Courier)

    +--[<1N4002]--+
    | |
    1>--+---[GRN>]----+
    |
    |
    +--[<1N4002]--+
    | |
    2>--+---[WHT>]----+
    |
    |
    +--[<1N4002]--+
    | |
    4>--+---[YEL>]----+
    |
    |
    3>----------------+
     
  9. If you can't see/detect the fault causing condition, you have to guess at
    what it might be and try 'fixes' to deal with it. It's down to too much
    forward voltage / current or too much reverse voltage or too high a
    temperature.


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  10. Rich Grise

    Rich Grise Guest

    I don't think John was talking about line spike protection, but more
    switched-inductor spikes.

    Reverse-parallel diodes sound like a very good idea.

    Cheers!
    Rich
     
  11. Newark says those things are $92 per each, apiece!!!

    For that price you should get a personal visit from the manufacturer
    to find the problem.



    --
    Peter Bennett, VE7CEI
    peterbb4 (at) interchange.ubc.ca
    new newsgroup users info : http://vancouver-webpages.com/nnq
    GPS and NMEA info: http://vancouver-webpages.com/peter
    Vancouver Power Squadron: http://vancouver.powersquadron.ca
     
  12. Guest

    One would think. I recently sent an email inquiry to P+F concerning
    the problem. I thought someone engineering had checked but you never
    know. The Lean manufacturing stuff.... More like cut to the bone...

    There are about 10 of these things on the line. My adding limiting
    resistors to each extended their life a bit but has not solved the
    problem.

    I'm going to check my toolbox and see if I have the 1N4002's and put
    them in whenever the line goes down.

    I will also look for any contactors that might not have snubbers
    across the coils.


    I sure appreciate everyone's comments. I work maintenance and would
    like to fix it for good rather than swap in parts.

    Wes
     
  13. Guest


    Replying to myself. I took a look at all the control panels, the
    prints show diodes across the coils of all contactors with 24VDC
    coils. I didn't find a single one actually in the control panels.

    I think I have just had my first practical example of why those diodes
    are so important.

    Thanks all,

    Wes
     
  14. If an 820 ohm resistor drops 9 volts, then the current is about 11 mA.
    If an internal-resistor LED has 15 volts across it and is conducting 11
    mA, it will be dissipating about .165 watt - which sounds a little high to
    me for an induicator LED.

    Meanwhile, I would suspect spikes, maybe reverse ones.
    Which colors are failing? If red is holding up, what shade of green is
    the green?

    Most RYG indicators have older tech LEDs (the green is yellowish) but
    the superbright greens from the late 1990's and later that are pure green,
    whitish green or bluish green are often static sensitive.

    - Don Klipstein ()
     
  15. Guest

    These are 'high powered' led's. The green (really is green) go first,
    yellow second, reds seem to last. See recent post where I found all
    diodes that were supposed to be across contactors missing.

    http://wess.freeshell.org/usenet/sci.electronics.basics/scan0004.jpg
    (this is a datasheet from p+f)

    Thanks,

    Wes
     
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