Non working LED

Discussion in 'General Electronics Chat' started by LDC3, Jul 22, 2013.

  1. LDC3

    Thread Starter Active Member

    Apr 27, 2013
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    I have a flashlight with 3 LEDs in it and now one of them fails to light. Would this be from low battery voltage? or has the LED burned out?
     
  2. Metalmann

    Active Member

    Dec 8, 2012
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    It could be both. How many volts are showing?
     
  3. LDC3

    Thread Starter Active Member

    Apr 27, 2013
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    The DMM is temporary unavailable, so I will need to use my battery checker at home. I'll check to see if I have some new batteries.
     
  4. bertus

    Administrator

    Apr 5, 2008
    15,638
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    Hello,

    Did you bump the flashlight?
    If so there could also be a broken contact on one led.

    Bertus
     
  5. LDC3

    Thread Starter Active Member

    Apr 27, 2013
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    What a piece of crap!

    It turns out that the flashlight was incorrectly wired. The leads from the batteries (3 AAA alkaline cells) are connected to either side of the diodes and the limiting resistor was ignored. I guess the LED has been overloaded and fried.
     
  6. wayneh

    Expert

    Sep 9, 2010
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    That's common, if not the rule. The batteries are the limiting resistance.
     
  7. #12

    Expert

    Nov 30, 2010
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    I have a couple of 19 LED flashlights with (3) AAA batteries. I did the math and found that the design was valid. The internal resistance of the batteries was correct for that set of LEDs with no limiting resistor added.

    I can't say if your flashlight is valid under these conditions, but it is POSSIBLE to do it properly with adding a resistor.
     
  8. LDC3

    Thread Starter Active Member

    Apr 27, 2013
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    The alkaline cells are capable of providing 200mA for several minutes. Without any resistor, I expect the current to be larger through the diodes (even if the voltage drop is 3.5V). The LEDs would probably be bright enough with 20mA. Most LEDs with a rating of 20mA will have a maximum current rating below 100mA, so this configuration is plain idiotic.
     
  9. wayneh

    Expert

    Sep 9, 2010
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    You need to adjust your expectations. Consider what the I-V curve looks like.
     
  10. RichardO

    Well-Known Member

    May 4, 2013
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    Often, LED flashlights come with "heavy duty" carbon/zinc batteries. I assumed that they were just using the carbon zinc batteries because they were cheap.

    In retrospect, I think that the batteries were chosen not just because they were less expensive than alkaline batteries but because -- I am guessing here -- they have a higher internal resistance than alkaline batteries.


    I also used to think it was idiotic to run the LED's well beyond the specified current. What if an LED lasts for 100,000 hours at the rated current and "only" for 1000 hours at the abusive high current? Well, that might well be dozens of sets of batteries! I doubt that most flashlights will last that long before breaking in some other way.
     
  11. LDC3

    Thread Starter Active Member

    Apr 27, 2013
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    Ahhh, I see, planned obsolescence. :rolleyes:
     
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