How to wire 4 torch LEDs on one circuit without 12 x AAA batteries

Discussion in 'The Projects Forum' started by Joelonthegc, Aug 10, 2015.

  1. Joelonthegc

    Thread Starter New Member

    Aug 10, 2015
    2
    0
    Hi All,

    Firstly, thanks for having me here. Second, I am completely inept at this stuff, intelligent enought to follow instructions and grasp the basics, so apologies in advance for potentially needing to ask follow up questions.

    I am trying to hook up 4x UV LED torch heads into a computer I have built. The torches were purchased cheaply online so I can cut the heads off and use as miniature "spot lights". They are much brighter than current applications offered for PC modifications otherwise I would have gone an easier route. Also FYI I have left one of the torches on for an hour in a confined space and it produced almost no heat so should be good on that front.

    Details:

    Each torch runs off 3xAAA batteries in series so 4.5v
    Obviously running off 12x batteries and replacing them constantly not an option so;
    Either hook them up to my PCs 1000w power supply or more likely;
    Find a suitable transformer to power them (this I have no idea about)

    So I am guessing I have to hook the LED heads up in Parralel, which should still mean 4.5v but much higher current draw... I will stop there as I really don't know where to start.

    I need advice on:
    How to wire them up
    Best power source and;
    How to choose the right power source
    Anything you think I have missed in my thinking this "thingrig" up

    Thank you for reading and more so if you reply.

    Joel
     
  2. TheButtonThief

    Active Member

    Feb 26, 2011
    219
    38
    Do you know the current consumption of each LED?

    Your best bet would be to use the PC power supply. It should have outputs at 12V (the yellow wires), 5V (red wires) and 3.3V (orange). Find a spare floppy drive or hard drive/optical drive connector and lob off the plastic connector, strip back the red (5V) wire and solder on (remembering to use heat-shrink) your circuit of parallel LED's each with their own current limiting resistor in series with the LED.

    Your 4 current limiting resistors are calculated using Ohms law, since I don't yet know the current consumed by each LED, I can only assume something standard like 20mA

    Supply voltage - Forward LED voltage (5 - 4.5 = .5)

    R = V/I

    R = 0.5V/0.02A

    R = 25 Ohms
     
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  3. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
    1,440
    368
    I'd power them from the 12V supply via a 390 or 430Ω resistor per LED. 5V is very close to the forward voltage which means that, with the necessary low value resistors, small changes in either forward voltage or supply voltage could cause quite large changes in current. e.g. for a given change in voltage, the current variance is 15.6x greater for a 25Ω resistor than for a 390Ω resistor.
     
    Last edited: Aug 10, 2015
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  4. TheButtonThief

    Active Member

    Feb 26, 2011
    219
    38
    Aye, suppose that's more sensible. I guess I just wanted to keep things simple for him =P
     
  5. #12

    Expert

    Nov 30, 2010
    16,346
    6,831
    One resistor each from 12 volts seems just as simple as one resistor each from 5 volts, but the stability is greatly improved. The more voltage you waste in the resistor the less thermal variations in the LED or voltage variations in the power supply matter.
     
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  6. TheButtonThief

    Active Member

    Feb 26, 2011
    219
    38
    Understood ;)
     
  7. Joelonthegc

    Thread Starter New Member

    Aug 10, 2015
    2
    0
    Hi all,

    Thank you so much for taking the time to think about this and reply, much appreciate.

    Ok so I think I am grasping what you are suggesting. And the wiring would be Parralel like this image?

    Again thanks for your time. I have to say the Internet is an amazing place! Standing on the shoulders of experts using forums like yours to do these sorts of things still amazes me!

    Joel
     
  8. #12

    Expert

    Nov 30, 2010
    16,346
    6,831
    1) You got it.
    2) Yes, we do this every day. :)
     
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