LED failure scenario

Discussion in 'General Electronics Chat' started by steveparrott, Dec 21, 2011.

  1. steveparrott

    Thread Starter Active Member

    Feb 14, 2006
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    I should know this, but... There have been several problematic cases where LED lamps are installed in outdoor low-voltage (12V) lighting systems.

    One LED lamp fails and all the other LED lamps on the same run fail to light. If you remove the bad LED then the others work fine.

    Unlike incandescent lamps that fail as an open circuit, LED's can fail and short the circuit.

    I'm assuming that the zero resistance across the bad lamp is causing a massive increase in amperage. But, doesn't the voltage stay the same? If the circuit breaker hasn't tripped and voltage is still being applied to the other lamps, why don't they work? Has the voltage dropped?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    If the LEDs are wired in parallel, the current will flow through the lowest resistance. Since the bad LED was shorted, all of the current went through it instead of sharing with the other LEDs.
     
  3. steveparrott

    Thread Starter Active Member

    Feb 14, 2006
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    Is this a case where the voltage across the leads of one of the good LED's could still read 12V? I've seen that with a nicked wire - the voltage reads 12V but the lamp does not light - almost like trying to power a 50W 12V lamp with a 12V watch battery?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    It's different. The LED shorts out the entire supply. You are referring to an open circuit, which is the opposite condition.
     
  5. steveparrott

    Thread Starter Active Member

    Feb 14, 2006
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    Ok, in one of the failed LED scenarios, the secondary 25A circuit breaker did not trip at the transformer despite the short. Instead, the lighting fixture superheated and started to smoke! Wouldn't you expect the circuit breaker to trip before that happened?
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    I do not know how your fixture is/was wired.

    I don't know if was UL approved.

    It sounds like adequate protection was not provided. However, you have only given the most vague information about these LEDs, so I don't know what you expect to hear. Outdoor 12v yard lighting should be pretty low power. I don't know why your fixture would have superheated.
     
  7. sheldons

    Active Member

    Oct 26, 2011
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    are these mains lights???heres a simple low voltage unit for led driving.....
     
    Last edited: Dec 23, 2011
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Sheldons,
    You should know that those types of circuits are against the Terms of Service, paragraph 6.

    Please remove the attachment.
     
  9. Wendy

    Moderator

    Mar 24, 2008
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    He is showing a 12VAC source, there is nothing wrong with that. The assumption is it is a transformer.
     
  10. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Depending on just how big a LED lamp we are talking about, the components inside it would more than likely overheat at currents a long way below level at which your 25A breaker would trip. The resistance of some of these parts (possibly even that of the wiring) might restrict the current to a level at which the breaker would stay on.

    The voltage at the LEDs would of course collapse because of excessive voltages dropped in the resistance of the overloaded parts.

    I would agree with SgtWookie that it sounds like there was not a lot of protection going on here: you might hope the lamp would have an fuse, or fusible resistor etc. suited to its rating to protect it. If repairing the system you might think about adding some in-line fuses.
     
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