Varying The Brightness of LEDs

Hardcoaler Jul 31, 2018

  1. Hardcoaler

    Hardcoaler TrainBoard Member

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    I'm working with 5mm LEDs in my control panel driven by a 12 VDC 3A "switching" power supply that delivers consistent output (unlike a wall wart transformer) and I'm using resistors.

    My Question: Can I burn out an LED with too little current/voltage?

    The circuit diagrams I'm using suggest a 2200 Ohm 1/4 Watt resistor on each LED and all is well (none have failed in many hours of testing and the resistor doesn't get hot), but the LEDs glow a bit too brightly for my taste. I'm uncertain of the LED specifications because I bought them from a surplus house, but a customer on their website posted that they draw 22.5 mA @ 2.2V, 17.3 mA @ 2.1V, 12.6 mA @ 2.0V and 7.6 mA @ 1.9V.

    If I add a second 2200 Ohm resistor in series, the brightness is nicely muted and is more comforting to my eyes. It's important to state that the LED remains glowing with suitable strength and isn't seriously dimmed.

    Is this okay or will I shorten the life of my LEDs doing this? Thanks again everyone.
     
  2. jdcolombo

    jdcolombo TrainBoard Member

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    You cannot burn out an LED by using too large a resistor. The worst that happens is that not enough voltage/current reaches the LED and it won't light up.

    What I would do is try different values of resistors until you hit on the one that provides the brightness that you want.

    John C.
     
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  3. Hardcoaler

    Hardcoaler TrainBoard Member

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    Thanks John! When I tried to research this on the Internet I got very confused.

    Some years ago I threw away a box full of miscellaneous semiconductors, including a fistful of resistors. Man, I regret doing that. :(
     
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  4. jdcolombo

    jdcolombo TrainBoard Member

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    Are these red or green LED's? Most red/green LED's hit full brightness at 20ma at 2v. Your 2.2k resistor is supplying about 5ma at 2v to these given the 12v supply. I don't know how much lower you can go and still light up the LED. That's why I'd add 470-ohms at a time and see what happens.

    John C.
     
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  5. wvgca

    wvgca TrainBoard Member

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    you can use a variable resistor or pot to adjust the brightness as well
     
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  6. Doug Gosha

    Doug Gosha TrainBoard Member

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    Yeah, put a pot in circuit and turn it until you get your desired brightness. Then measure the pot's resistance and substitute a fixed resister of the same value. Or leave the pot for future adjustment, if desired.

    Doug
     
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  7. wvgca

    wvgca TrainBoard Member

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    a 5k or 5000 ohm pot would be a good place to start .... and just replace with a fixed resistor that matches the desired brightness, as in ohms resistance
     
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  8. Doug Gosha

    Doug Gosha TrainBoard Member

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    Don't get it too bright, however, or it will suddenly get very dark.

    :D

    Doug
     
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  9. Hardcoaler

    Hardcoaler TrainBoard Member

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    Thanks everyone. I think I found a low-tech solution -- LED Diffusers like what's below. I remembered I had some in my stash. Using the Green and Yellow Diffusers with 2200 Ohm LEDs produce a really nice mellow light. Using the Red Diffuser with a 4400 Ohm LED works well too, though I may dial it down a tiny bit more with a small value resistor like jdcolumbo suggests. I wish I could find a pot to experiment with --- that's a great idea too. Anyway, I'm now making some headway with this. Thanks again all.

    LED Diffuser.jpg
     
    Last edited: Jul 31, 2018
  10. jdcolombo

    jdcolombo TrainBoard Member

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    A pot would be ideal for experimenting, provided that you also have a multimeter so that you can read the resistance of the pot when you get the brightness you want. If you don't have a multimeter, you probably should have one. They are useful for all sorts of things - not just model rr-ing. You can get a decent basic digital multimeter for under $25 (do a search on Amazon).

    John C.
     
  11. Hardcoaler

    Hardcoaler TrainBoard Member

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    These are the times when I really miss having a nearby Radio Shack store and being able to easily source what I need quickly and without shipping charges. Oh well. We still have a Radio Shack beachball in service though …..

    2012-07-30 Radio Shack Beachball 1 - for upload.jpg
     
  12. jdcolombo

    jdcolombo TrainBoard Member

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    Amen. Today I needed ONE miniature DPDT switch. Can't get it locally since RS is gone. Ordered 5 from All Electronics so that the NEXT time I need one, I won't be able to find the four extras :)

    John C.
     
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  13. Doug Gosha

    Doug Gosha TrainBoard Member

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    Yes, it's a real shame Radio Shack is gone. Years and years of a local parts source and then, blip! Gone!

    Doug
     
  14. Hardcoaler

    Hardcoaler TrainBoard Member

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    As I've started to get my new layout started, AllElectronics has become my preferred source for most everything. They have a good website that doesn't overwhelm with choices , plus they are rarely out of stock on what I need and their ship time is lightning quick. I've begun to do like you, ordering a few extras here and there figuring that I'll eventually need 'em. It grates against my parsimonious nature, but without a local source, it makes sense.
     
    Last edited: Aug 1, 2018

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