Led bulb not working

Discussion in 'General Electronics Chat' started by M.M.M. Arafath, Jan 1, 2014.

  1. M.M.M. Arafath

    Thread Starter New Member

    Jan 1, 2014
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    Hi,, Im new to this forum and electronics. i have a problem too... I connected two led bulbs (green and red) through usb cable to computer. but first led is only worked. 2nd one is not worked... 5v coming from the usb cable... y s that happend???
     
  2. adamclark

    Member

    Oct 4, 2013
    472
    6
    are the leds series or parallel? the leds are 3v not 5 so are you using a resistor?
     
  3. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    Did you make sure you oriented them correctly? They're diodes, so they will only work when placed in the correct direction.
     
  4. Gdrumm

    Distinguished Member

    Aug 29, 2008
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    The longer wire is the positive electrode and the shorter wire is the negative electrode.
     
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  5. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    That doesn't always help, especially if the LEDs are used or have trimmed leads. In most cases, the side with the flat edge is the cathode (negative), but if you really can't tell, look inside the LED. The side connected to what looks like a bowl is the cathode. Unless you want to discuss multicolor LEDs, then it's a whole different matter.
     
  6. Metalmann

    Active Member

    Dec 8, 2012
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    Some of my LEDs don't have a flat, so I check polarity with 2-3 volts.
     
  7. Metalmann

    Active Member

    Dec 8, 2012
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    5 Volts from the USB, may have blown one of the LEDs.
     
  8. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    Yes, this sometimes happens. I have a few 3mm LEDs that have the flat edge on the side (not above either of the pins), so I try to look at where the "bowl" is. You can test with a couple of volts though, and on some meters your diode test function will work as well. It will also tell you the forward voltage drop of the LED.

    Excellent point. Assuming you're getting exactly 5v from your USB port and your LED has a voltage drop of 3v, and an operating current of 25mA, you'll need a resistor with a value of (5-3)/0.025 = 80 ohms. I'd just choose 100 ohms to be safe. You definitely may have blown the LED if you didn't limit the current with a resistor.
     
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  9. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Not always! I buy LEDs from a number of different sources and some have the larger internal pin (the "bowl") as the anode.

    The best way to test is with a multimeter on diode test, they usually have enough voltage to show the LED forward conduction.

    I also have a 5v bench power supply with a 10mA current limit, it's great for testing and identifying LEDs in a hurry (because many are clear plastic now!).
     
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  10. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    Really? I have never seen an LED with the "bowl" as the anode. The "bowl" contains the die, and there's a tiny, extremely fine wire connecting to the die from the anode. I don't see how it could work the other way around, I thought the anode always had to be connected to the fine wire....?
     
  11. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Yeah I used to rely on that too. But with all the new colours and high-intensity process LEDs (that might have multiple or compound chips?) I got caught out a couple of times.

    Now I always test them before soldering junkbox LEDs. :)

    When I'm in the workshop later I'll try to remember to dig out some LEDs and take a photo.
     
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