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Circuit with piranha leds

Discussion in 'LEDs and Optoelectronics' started by RedQueenie, Oct 4, 2017.

  1. RedQueenie

    RedQueenie

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    Oct 4, 2017
    Hiya, I'm trying to make a small lightbox. I'm using a box photo frame and in the past have used a 9v battery with a switch and 5 LEDs. This has worked ok but I'd like to make it smaller. I was wondering how many piranha LEDs could I power with 2 CR2032 batteries placed on a circuit board? The reason for the circuit board is so I can place the LEDs more neatly than having to tape them in place. I'm a complete beginner and would appreciate any help :)
     
  2. bushtech

    bushtech

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    Sep 13, 2016
  3. RedQueenie

    RedQueenie

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    Oct 4, 2017
    Crikey just reading the first couple of paragraphs has caused my brain to smoke! I'll get some more coffee and try again. Do you know the short answer though? :)
     
  4. bushtech

    bushtech

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    Sep 13, 2016
    we're going to need the specs of your led
     
  5. bushtech

    bushtech

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    Sep 13, 2016
    Look, we've pretty much got the blind leading the blind here, but as long as you get your mind about this bit from the link I posted, you'll be fine: "LEDs are inherently unstable when operated from a voltage source. The answer is to operate them from a current source as the manufacturers intend."

    And don't shoot for the maximum current the led can handle. Drop it down a bit and the led will be a happy chappy.
     
  6. bushtech

    bushtech

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    Sep 13, 2016
    And if I've got something wrong will the wise men on this forum please advise
     
  7. RedQueenie

    RedQueenie

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    Oct 4, 2017
    Hiya thanks for trying to help. I naively thought if the battery volts and the number of lights needed to be lit were known the blanks ie the volts of each light could be filled in. I'll have a proper sit down later and read through the article you've provided and also have a look to see what LEDs are available. They don't need to be super bright just need to work for a while :)
     
  8. Audioguru

    Audioguru

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    Sep 24, 2016
    An LED is not a very hot simple length of tungsten wire (incandescent light bulb), instead it is a diode whose current must be limited by your circuit.

    CR2032 battery cells are small then can produce only a fairly low current for a very short time.
    AAA cells can provide much more current for a much longer time.
    AA cells can (as above).
     
  9. FuZZ1L0G1C

    FuZZ1L0G1C

    367
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    Mar 25, 2014
    As advised above, you should run LED's at a lower current than rated.
    In practice, you can drop the current quite a bit before it becomes apparent.
    2 series'ed CR2032 cells will output 6V so you could add the total forward voltage rating Vf of each LED until just below the 6V level.
    To drop the current you need a series resistor Rs with MINIMUM value to match the LED forward current If.
    Increasing the value of Rs lowers the current.
    Taking battery voltage as Vcc, you can calculate exact (minimum) Rs required per each leg (chain) of LED's.
    For Vd, add the total number of series'ed LED forward voltages.
    (Excel formula):
    Rs=,=(Vcc-Vd)/If or Rs=(Vcc-(Vf*Qty))/If
    I dunno what a "piranha" LED rating is, but here's an example:
    Each LED: Vf=2V If=20mA (0.02A).
    Vcc=6V Qty LED's=5
    2 LED's per leg=4V (this 4V allows for a slight voltage-drop as well).
    Rs.exact (ohms) = (6-(2*2))/0.02 = (6-4)/0.02 = 2/0.02 = 100R (minimum) so 110R per leg should be OK.
    1 remaining LED? Doodis. ;)
    Rs.exact (ohms) = (6-2)/0.02 = 4/0.02 = 200R.
    200 ohms non standard so use 220R.
    Using higher Rs gives leg-currents of 18.18mA, 18.18mA and 18.18mA (V.diff*Rs).
    18.1818181818*3=0.05454545454A
    Divide the Ah capacity of 1 cell by the current load to give total hours life until cells are drained.
    In reality, the diode junction will fail to trigger (lit LED) well before this.
    As a rough guide, divide hours by 2.
     
  10. Audioguru

    Audioguru

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    Sep 24, 2016
    The datasheet for the CR2032 battery shows that is designed to produce a very low current (0.19mA) for a long time (1300 hours). Then its capacity is 240mAh. If it is loaded with a normal or higher LED current then it craps out very early.
     
    FuZZ1L0G1C likes this.
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