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Overheating resistor??

Discussion in 'General Electronics Discussion' started by AGLite, Mar 2, 2012.

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  1. AGLite

    AGLite

    35
    0
    Feb 19, 2012
    Hi guys,

    I have wired six leds in parallel and calculated a resistor for them. It turns out the resistor is getting VERY hot. Like 90°C. It is NOT smoking but is still to hot to put in an enclosed space. I ordered alot of resistors for this, so I cant undo it. I have to figure out a way to fix this. Thanks.

    Additional Info- 9v, 6 5mm leds, and 56 ohm resistor.
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
  3. Harald Kapp

    Harald Kapp Moderator Moderator

    11,702
    2,717
    Nov 17, 2011
    Yes,
    1) place the LEDs (or at least some) in series.
    2) calculate the new resistor for the series connection which will be much smaller.
    3) If you're absolutely going to use the resistors you ordered, use them in parallel (http://www.1728.org/resistrs.htm) and possibly series connection. Don't worry if you can't match the mathematically computed resistance exactly. Allow for +-10% tolerance and you have a good chance of meeting your requirements.

    Harald
     
  4. AGLite

    AGLite

    35
    0
    Feb 19, 2012
    Thanks guys, the only problem is that I already made 4 18 led panels, hoping to wire one resistor to each line of six leds wired in parallel. so I'm kinda screwed. I need a simple plan to fix this stupid mistake.

    So i think if i get new resistors then unwire them in parallel and then wire rows of three in series then put a resistor for each row that would work?
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    As long as three LEDs in series has a combined Vf at least a couple of volts less than your supply voltage then this would be a far better way to go.

    You'll also be wasting far less power and distributing that small amount so ideally you won't even notice anything getting warm.
     
  6. AGLite

    AGLite

    35
    0
    Feb 19, 2012
    ok this is what ill do.

    The wizard you have on the led driving page said i need 1 ohm resistors? I cant find them anywhere?
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    1 ohm tells me that you have a solution that won't practically work.

    Tell me what the supply voltage is, the LED current, and the LED Vf and I'll work it out for you.
     
  8. AGLite

    AGLite

    35
    0
    Feb 19, 2012
    I need it calculated for two different voltages.

    9v, 3v drop, 20mA

    12v, 3v drop, 20mA
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    for 9V you have strings of 2 LEDs, with 3 volts dropped across the resistor, so at 20mA that's 150 ohms (1/4W is heaps)

    for 12V you can have 3 in series with, not surprisingly the same resistor, 150 ohms!
     
  10. AGLite

    AGLite

    35
    0
    Feb 19, 2012
    Is it possible for you to also calculate 5 volts for me? Thanks!!
     
  11. jackorocko

    jackorocko

    1,284
    1
    Apr 4, 2010
    it is easy, with a 3V drop LED then you can only have one LED in series for a 5V supply. 5V - 3V = 2V drop across the resistor. 2V/20mA = 100 Ohm resistor
     
    Last edited: Mar 10, 2012
  12. Joshie75

    Joshie75

    18
    0
    Feb 16, 2012
    You could have your 9v also power a PC fan to cool off the resistor. Or check out some nice water cooling systems. ;)
    *In all seriousness now*
    Here's a great website for ya AGLite;
    http://ledz.com/?p=zz.led.resistor.calculator
     
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