simple resistor/led advice

Discussion in 'The Projects Forum' started by circa27, Mar 11, 2011.

  1. circa27

    Thread Starter Member

    Feb 22, 2011
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    Im putting IR leds for a light source on my reverse camera on my car. so the voltage is 14.4 and the led forward voltage is 1.6 with 60 mA draw. total of ~90 leds ( i can add/subtract by a few if needed)

    since R=V/I could i just have chains of 9 leds (R= (14.4-(9*1.6) / 60mA = 0) without resistors?

    also, would it make a difference if i wired the chains all coming from the same wires in parallel ( like this ) or if i wired them in a series of parallel chains of 9?

    i just want the highest output possible and i would like some advice so they dont burn out too fast. power supply is not a problem

    thanks!
     
  2. Adjuster

    Well-Known Member

    Dec 26, 2010
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    No, this would be unsafe. The LEDs could easily burn out, because the current would be unpredictable, and would change dramatically with small changes in battery voltage.

    It would be much better to use shorter chains - perhaps 4 or 5 long - with resistors chosen so that the LED current would be safe up to 15V or so.
     
  3. circa27

    Thread Starter Member

    Feb 22, 2011
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    thanks, i never thought of that. well i have veryy limited space here, could i just use a 1 ohm resistor in front of each chain of 9?
     
  4. wayneh

    Expert

    Sep 9, 2010
    12,085
    3,024
    1 ohm won't drop enough voltage at 60mA. If you want a resistor that will shave off, say, 1 volt at 60mA, V=I•R gives you 1 = 0.06•R and R = 16.7ohms

    So something in the area of 15-20 ohms would be good. You may need to use just 8 LEDs and then recalculate. Power rating needs to be about double the anticipated heat, I^2•R or 0.06^2•20 = 0.07W. So a cheap 1/4W resistor would be fine.
     
  5. circa27

    Thread Starter Member

    Feb 22, 2011
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    thanks ill give it a try!
     
  6. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Eight LEDs in series still sounds a bit too big - if your 1.6V forward voltage is correct, that would give a chain voltage of 12.8V, which is still more than the minimum terminal voltage for a 12V lead/acid battery.

    By the way, are you sure that the LEDs are rated for 60mA continuous current? That's a couple of times what I would expect, but maybe you have genuinely high-power devices.
     
  7. Wendy

    Moderator

    Mar 24, 2008
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  8. Audioguru

    New Member

    Dec 20, 2007
    9,411
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    I think this discussion is also on another website.
    There I didn't think that 1.6V infrared LEDs were used because they are invisible. Probably 3.5V white LEDs were used for the camera and persons to see things. Then only two or three in series and in series with a current-limiting resistor could make a string and millions of strings can be powered from 14VDC.

    5mm ordinary LEDs will quickly burn out when their current is too high at 60mA. They are spec'd with only 20mA. Their max allowed current is usually 25mA or 30mA.
     
  9. circa27

    Thread Starter Member

    Feb 22, 2011
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    yeah i posted this on another forum i found, im not spamming dont get the wrong idea.

    heres an actual link to the lights i have so you know where i got my specs


    as for the voltage, its 14.4 as the lights wouldnt be on while the alternator isnt running. the lights are invisible for the most part to the human eye which is why its used to illuminate a camera area but im not getting into spectrum discussion.

    why would 8 be too big? i only ask because if it looks good on paper there must be a practical reason why it wouldnt work in a real situation, and i just dont know it yet. i have somewhat limited space and current draw is not a problem. no one can seem to agree on decent advice or explain their reasoning for why to do or not do a specified method
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    The IR LEDs have a narrow band of output frequency so the spec's say they are good for remote controls, not good for an IR light source.

    You can connect 8 in series which needs 12.0V to 12.8V. Then operate them brightly at 50mA if you can cool them enough. The current-limiting resistor is (14.4V - 12V)/50mA= 48 ohms. Use 47 ohms/0.25W. The total heat in the LEDs will be 0.62W.
     
  11. circa27

    Thread Starter Member

    Feb 22, 2011
    38
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    no. 940 is good for remote controls.
    anyways

    illl try that
     
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