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Thread: Calculate LED Watts

  1. #1
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    Calculate LED Watts

    I've got these SuperFlux LEDs from Lumileds. They have a forward voltage of 2.5V so I am stringing 4 of them in series. How do I calculate the total current and power?

    +12V ----|>------|>-----|>------|>-----GND

    Max current: 70ma
    Power dissipation: 221mW
    Forward voltage: 2.5V

    Thanks

  2. #2
    Top Ramen lgbr's Avatar
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    221 mW / 2.5V = 88.4 mA
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  3. #3
    FLAC Pudge's Avatar
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    But he has 4. So 353.6 mA. I think...
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  4. #4
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    Thanks. A few more questions

    a) Assuming a 10V power supply, wouldn't it be 0.175W per LED ( P = IV = 70mA * 2.5V)?
    d) When analyzing circuits can I treat LEDs as a resistor?
    b) Do LEDs have a series resistor in them?
    c) In this case equal to 35 ohm (R = V/I = 2.5V/0.07A)?
    d) These have a min and max flux rating. Is a higher flux achieved by running a higher forward voltage?

    Thanks again!

  5. #5
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    Quote Originally Posted by bambooracer
    I've got these SuperFlux LEDs from Lumileds. They have a forward voltage of 2.5V so I am stringing 4 of them in series. How do I calculate the total current and power?

    +12V ----|>------|>-----|>------|>-----GND

    Max current: 70ma
    Power dissipation: 221mW
    Forward voltage: 2.5V

    Thanks
    For practical purposes, a good, working LED has no resistance, it does however, produce a voltage drop.

    The circuit as drawn is only going to blow LEDs. Given the whole V = I * R thing, and that LEDs have almost no R, to calculate the current flowing in the LEDs is I = V / R. As R gets really small, I gets really big. Since the max. current for these things is small (70mA) you'll fry them before you see them light up.

    To fix this you need a resistor.

    12V is the source, and each LED drops 2.5V so 12 - 2.5 -2.5 -2.5 -2.5 = 2V. 2V is the remaining pressure you have to drop, and you want to draw less than 70mA doing it. Again V = I * R. This time you want R, so R = V / I = 2 / 0.070 = 28 Ohms. You wont find a 28 Ohm resistor, so get the next biggest size, probably a 30.

    Power is calculated for this circuit with P = V * I.

    A 28 Ohm resister would give you 70mA through each LED, but your using a different one, so you need to calculate the actual current. V = I * R (seeing a pattern yet?) I = V / R = 2/30 = 0.066 = 66mA.

    Power for each LED is V*I = 2.5 * 0.066 = 0.165W = 165mW. And there are four of them: 4 * 165mW = 660mW. Don't forget the energy being dissapated by the resister at 2V * 66mA = 132mW. So the whole thing is eating up 660 + 132 = 792mW.

  6. #6
    Constant Bitrate gubon13's Avatar
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    Dammit Sparc, good post. I wanted to sound all nerdy, but it looks like you beat me to it... =b
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