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Amperage of 200 LEDs?

Discussion in 'LEDs and Optoelectronics' started by carebare47, Nov 21, 2010.

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  1. carebare47

    carebare47

    66
    1
    Oct 21, 2010
    Hello,

    I am currently doing a project which I think may use up to 200 LEDs. If they run at 25mA each, does this mean that I will need a 5 amp (12v) transformer? I thought that 5A sounded a bit excessive...

    Cheers,

    Tom
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    If the LEDs are placed in parallel (each with their own resistor), then you will need a 5A power supply (depending on the colour 3 to 5 volts or higher)

    If you put them all in series then you will require a power supply that can provide 25mA (albeit at between 350 and 600 volts (depending on the LED forward voltage)

    You may want something in between :)

    I will presume the LED forward voltage is 3.4V (white or blue).

    You can place three in series with a single resistor and repeat this 67 times. Each string requires 25mA thus the current required is 1.7A

    This site will do all the hard work for you.

    edit: you'll find that a higher voltage power supply will have benefits. Repeat the calculations for a 24V power supply and see what I mean. (these circuits do assume that your supply is reasonably well regulated)

    edit2: But beware of solutions that have very low value resistors (8.2 ohms is very low)
     
    Last edited: Nov 21, 2010
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    This might help you.

    You inspired me.
     
  4. carebare47

    carebare47

    66
    1
    Oct 21, 2010
    The 200 LEDs are driven by three transistors and one fixed 12v source, so I think I am going to have to break it down into:
    Blue: (two transistors alternating 96 (48x2) LEDs @ 1Hz ) Vf: 3.2, I: 25mA. 2x(3x16) arrays with 2x(16) 100ohm resistors
    Red: (50 flashing at about 2Hz from a transistor) Vf: 2, I: 25mA. 5x10 array with 82ohm resistors (is 82ohms too low?)
    Green: (50 from a fixed 12v source) Vf: 3.2, I: 25mA. 3x16 array with 16 100ohm resistors
    Thing is, these LEDs will be occupying a 8" by 8" square (ish) arranged into various patterns and such. This is going to be a lot more work then I bargained for ^_^

    Thanks for the help =)

    Tom
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Determine what the likely Vce(sat) will be for those transistors at the current required, then take this into account as well.

    What is your 12V supply? Is it a well regulated power supply (say a smps), or is it something that is unregulated and only nominally 12V?

    Those resistor values would be the smallest I would choose, and I would only consider them if the power supply was well regulated. I have not calculated the values required, I have assumed that you've done the math right, and they seem to be in the ballpark.

    Using surface mount components would save you drilling 500 or so holes, but matrix board and point to point wiring would also work. You need to ensure that if you take a more free-form approach (like the latter) that you wire it up correctly.

    Building something of this scale requires you to consider how you will power it up the first time. I would recommend you get a 1K, 100 ohm, 10 ohm, and 1 ohm resistor (of appropriate wattage) and first connect it up with the 1k resistor in series, do some reasonableness checks, before using the 100, 10, and 1 ohm resistors. If nothing untoward shows up (some LEDs surprisingly bright, current consumption more than expected, some LEDs not lit, etc.) then you can feel reasonably safe.

    Remember that connecting a LED backwards can kill it in very short order. Be very careful with orientation and try to spot strings of LEDs that are not lit. If you're lucky, the reduced voltage, and the forward drop across the LEDs oriented correctly in a string may protect the one that's backwards.
     
  6. carebare47

    carebare47

    66
    1
    Oct 21, 2010
    My +12v supply is at the moment (during prototyping) the yellow 12v cable from an old computer PSU. This will ultimately be a present for someone, and thus I was hoping to buy a 12v 2A power supply (something like this: http://www.dabs.com/products/generi...l?utm_source=google&utm_medium=product+search).




    The design is currently built on a protoboard using only a couple of LEDs (two off each transistor and one fixed) whilst I am waiting for the LEDs to arrive. I was then hoping to transfer the design to strip board and then find some kind of cover to hide the electronics but show the LEDs.

    Thanks again,

    Tom
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Both of those should qualify as well regulated for this purpose.

    The only possible concern is that even at the same current, the different coloured LEDs may appear to be significantly different in brightness. Test one string of each colour and if this is the case, reduce the brightness of the brighter colours (by reducing the current) until they are acceptably balanced.

    Apparent differences in brightness can be caused by efficiency of the LEDs, the angle over which the LED projects its light, and the different sensitivity of the eye to different colours.
     
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