Need help with led project please.

Discussion in 'The Projects Forum' started by emachine88, May 14, 2013.

  1. emachine88

    Thread Starter New Member

    May 14, 2013
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    I have 6 led boards that have 9 leds on each board. They came out of flashlights powered by 3 triple A batteries, which would equal 4.5 volts each board. I am wanting to power them with a 12 Volt 1 amp dc power supply. What type of resistor do i need on each board, watt and ohm rating to get best life and performance and also what would be max amount of led boards i could run safe. Also could i make them dim able. Thank You for any and all information.
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,985
    745
    If the leds have resistors already then just put 12v onto the pcb, otherwise use a 1k ohm resistor in series with each led across the 12v supply. If you want to make them dimmable use a constant current source.
     
  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
    6,031
    3,802
    Can you figure out how the 9 LEDs on each board are wired? Are they all in parallel or in some series/ parallel configuration? Draw the circuit and post. Use a magnefying glass if needed. Also include resistors with values.
     
  4. emachine88

    Thread Starter New Member

    May 14, 2013
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    This is what they look like where the spring is, is the positive. from what i can tell the inner ring is positive and the outer ring is negative. Appreciate all you guys time and help.
     
  5. emachine88

    Thread Starter New Member

    May 14, 2013
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    On the resistor what size (watts) should it be. 1/2 watt 1k ohm ?
     
  6. #12

    Expert

    Nov 30, 2010
    16,283
    6,795
    I know this situation. Those flashlights depend on the internal resistance of the batteries to limit the current. Very slick design if you ask me. The problem is how to compensate for that. So I went to the shed and got my $3 POS flashlight with 9 LEDs in it and measured 3.0 volts at 25ma.
    12V-3V = 9V
    9V/.025A = 360 ohms
    .025A x 9V = .225 watt
    Use a 360 ohm, 1/2 watt resistor or put more than one set of LEDs in series and figure the resistor for the resulting voltage.
     
  7. emachine88

    Thread Starter New Member

    May 14, 2013
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    I was hoping to put at least 6 of these in a series. If i do that what size resistor would i need to put on each board. Just for curiosity sake. How many boards could the power supply run safely. Thanks
     
  8. #12

    Expert

    Nov 30, 2010
    16,283
    6,795
    Six in series requires 18 volts. You can't do that with 12 volts.
    If you put 3 boards in series, that's 9 volts for the LEDs. 3 volts for the resistor at .025 amps is 120 ohms at 1/4 watt.

    With a 1 amp power supply and 3 sets of LEDs for each .025 amps that's 120 light boards.

    Please study Ohm's Law and Watt's Law. They are only 3 letters long and they did all the math here.
     
  9. emachine88

    Thread Starter New Member

    May 14, 2013
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    I sure will. How did you come up with 120 light boards. If i only use three of the boards that will be 27 leds. 9 leds per board.

    I have another dc power supply that is 15 volts 5 amps. So with this one i could use 4 boards since there are 9 leds present in each board, the current for each board would be 20 x 9 = 180mA correct?
     
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