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Thread: LEDs at risk in Carling switches?

  1. #1
    Expert Brissieboy's Avatar
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    LEDs at risk in Carling switches?

    A lot of installations using Carling type switches require/use a relay instead of directly switching the load.
    The way most of these switches are configured almost always puts the internal on/off indicator LED directly across the load - like this:
    switch1.JPG
    This is fine for non-inductive loads, but when switching a relay there is a significant back EMF produced when the relay is de-energised.
    This will put a large reverse voltage across the LED.
    A LED's maximum reverse voltage is very difficult to obtain with any degree of certainty, but with the older types it is considerably greater than the more modern 'super-bright' ones which are generally speced very low (as low as 5V according to a lot of information). But they will all be far less than the peak back EMF which could be in the vicinity of 1,000 - 1,500V.
    Does anyone know if any protection is provided for the LEDs within the switch (apart from the current limiting resistor)? I have not been able find any relevant information on the web.
    Has anyone had LEDs fail when switching relays? Have you ever dismantled one of these switches?
    It is a relatively simple matter to resolve the issue with a suitable diode across the relay like this below, but I'd like to know if it is really necessary.
    switch2.JPG

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    I am he, fear me the evil twin's Avatar
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    Quote Originally Posted by Brissieboy View Post
    snip... Has anyone had LEDs fail when switching relays? Have you ever dismantled one of these switches?
    Yes, and, Yes...

    I only use relays that have the 'onboard' reverse voltage (or flyback) diode across the coils these days as almost every lamp is now LED.

    and... pulled one apart yoinks ago to see what there was to see but I just check 'em away these days.
    Dolphins are so smart that within a few weeks of captivity, they can train people to stand on the very edge of the pool and throw them fish.

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    Not a lot of relays actualyy have the diode built in, its easy enough though to grab a 1N400* from Jaycar or similar.
    Its basic 101 to ensure you have a diode across any coil.

    Cheers,
    Paul
    Last edited by pspeirs; 19th June 2019 at 12:37 AM.

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    Patrol Freak BillsGU's Avatar
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    As said above ^^^ - only make sure when you insert the diode across the relay you place it in reverse bias (as shown in your diagram). You will know if you get it wrong because it will a) Blow a fuse, b) The diode will go black and smelly, or c) The diode will vaporise!
    Last edited by BillsGU; 19th June 2019 at 12:10 PM.

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