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Use an LED as a snubber diode? — Parallax Forums

Use an LED as a snubber diode?

homosapienhomosapien Posts: 147
edited 2014-05-22 11:44 in General Discussion
I have some small relays (5v, 500 ohm coil) that will need a snubber diode to prevent inductive flyback. Would like to be able to use a SMD diode for speed of assembly (hot-plate soldering), but I don't have any on hand. But then I realized I do have a ton of cheap Chinese 0608 LEDs on hand. Any problem using those as the snubber diode? I do not have any real specs on the LEDs, only that they work well as LEDs with the normal in-line resistor...


Thanks,
Nate

Comments

  • ercoerco Posts: 20,256
    edited 2014-05-20 08:35
    Great question, my guess is probably, depending on the LED, but I'll defer to the hardware gurus here. You need to absorb a high-voltage spike at low current. But as soon as the LED starts to conduct any current at ~2V, the voltage levels out and dissipates the energy. I used a piezo disk as a tap sensor input to a BS1, and used an LED to limit the input voltage. There was a tiny flash on the LED with each tap, which you will probably also see when your relay turns off. Obviously you need to get the polarity right here!

    I'm guessing these are reed relays? 500 ohms is pretty high for a 5V relay. I like low-current relays as they can be driven directly by a uC pin.
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2014-05-20 08:45
    Ummm, LEDs emit light by running the diode forward with a threshold of about 2 or more volts (depends on color) right?

    Not sure that they are really rated for high volts in the forward state, not high watts. Not exactly the ideal snubber diode.

    http://www.daycounter.com/Calculators/Snubbers/Snubber-Design-Calculator.phtml

    Over the years I am sure I have used the wrong diodes - either too slow or not robust enough. But I have never tried an LED.
  • jmgjmg Posts: 15,173
    edited 2014-05-20 17:23
    homosapien wrote: »
    I have some small relays (5v, 500 ohm coil) that will need a snubber diode to prevent inductive flyback. ... Any problem using those as the snubber diode?

    LEDs have a reverse breakdown spec, if you cannot find it in data, you could measure a couple.
    5V is likely ok, but 12V I would avoid.
    With a 10mA peak and short duration, forward current is no problem to any LED .
    You should be fine.
  • kwinnkwinn Posts: 8,697
    edited 2014-05-20 21:21
    Best way to find out is at the bottom of this post.
    homosapien wrote: »
    I have some small relays (5v, 500 ohm coil) that will need a snubber diode to prevent inductive flyback. Would like to be able to use a SMD diode for speed of assembly (hot-plate soldering), but I don't have any on hand. But then I realized I do have a ton of cheap Chinese 0608 LEDs on hand. Any problem using those as the snubber diode? I do not have any real specs on the LEDs, only that they work well as LEDs with the normal in-line resistor...


    Thanks,
    Nate
  • Mark_TMark_T Posts: 1,981
    edited 2014-05-22 03:25
    Most LED datasheets have reverse breakdown listed as -5V for some reason. It might be laziness
    on the manufacturer's part as I don't believe every colour is the same, but it limits their usefulness
    as general purpose diodes.
  • ercoerco Posts: 20,256
    edited 2014-05-22 09:01
    Mark_T wrote: »
    Most LED datasheets have reverse breakdown listed as -5V for some reason. It might be laziness
    on the manufacturer's part as I don't believe every colour is the same, but it limits their usefulness
    as general purpose diodes.

    +1, others are chatting about the same issue.

    http://www.eng-tips.com/viewthread.cfm?qid=175789

    http://www.edaboard.com/thread300568.html
  • homosapienhomosapien Posts: 147
    edited 2014-05-22 11:44
    +1, others are chatting about the same issue.

    Interesting that the consensus seems to be... there is no consensus.

    I hooked one of the 0603 LEDs up in serial w/ a 470 ohm resistor and then to my bench supply in reverse polarity. Ran the voltage up to 30VDC, no current showing on the ammeter.

    Currently have the LED in series w/ 6.8k resistor and 30VAC transformer to see how it will handle that, since I also wanted to use the 0603 LEDs as an indicator for the switched power (which is 30VAC). If there is no change in performance after an overnight I will assume the reverse breakdown voltage is over ~42V.
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