Specific LED Question

Discussion in 'The Projects Forum' started by srgould41, Dec 28, 2015.

  1. srgould41

    Thread Starter New Member

    Dec 28, 2015
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    Hi all, first post here.

    My project, I have an old vehicle that I am slowly adding to. My current project is to add a gear position indicator. I am building a narrow panel with 7 individual cells with each having an LED's behind a covering mask. One LED for each gear position and the mask will be an acetate with the correct letter/number masked out for that specific Gear. The LED's will be switched via micro switches that will correspond to the selected gear. My plan has been to supply a common + lead with the required resistance for 14VDC (typical automotive working voltage). The ground side will be switched by the micro switches. Clear enough?

    I went shopping yesterday and found what I believe are the perfect LED's.
    LINROSE B4302H4-12V with integral resister
    695 mcd (should be bright enough and maybe too bright depending on the heat output)
    14Vf @ 10mA

    I was expecting to need a resister(s) but this one seems to be set for an automotive voltage. Can some much more learned person tell me about these? Can I truly hook them up without any ballast resistor?

    Thank you much!

    Steve
     
  2. dl324

    Distinguished Member

    Mar 30, 2015
    3,250
    626
    Welcome to AAC!

    LEDs with integral resistors have been around for over 4 decades. The only disadvantage is that you can't adjust the LED current, and therefore intensity, in an automotive application.

    If you're not sure whether they contain a resistor, you can connect them to a 10mA current source and measure the voltage drop. Or examine them to see if they have a resistor.
     
  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    Those likely have an internal resistor of (14-2)V/0.01A = 1200Ω

    You will not be happy with the brightness at night (they will be blindingly bright) . I would try to tap into the existing car's dimming circuit and feed them from there.

    Over the years, I have added a lot of LEDs to indicate various things in my aircraft (Low voltage, run-on starter, flap position, over-temperature, low fuel, etc). I find that if it takes 10mA to make a LED visible in sunlight, it will require less than 2mA at night...
     
  4. srgould41

    Thread Starter New Member

    Dec 28, 2015
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    It sounds like I might not want to mount the display to the roll cage dash cross bar in front of my eyes. I was actually thinking it might be best to have it on the floor near the shifter. At night it would not be so glaring if it were down there.

    I purchased a set of Red LED's with 2Vf @ 20mA. I can try each and see what the difference is. I calculated 600 ohms at 14 volts or 500 ohms at 12v. I am a little worried about the voltage differences. I have seen 14.5 down to 12 depending on what the load is. If I were to use 600 ohms would the LED's be visible at 12v? Or if I used 500 ohms would they burn out at 14.5v?

     
  5. dl324

    Distinguished Member

    Mar 30, 2015
    3,250
    626
    Modern LEDs are visible at very low currents. Read the datasheet to find the maximum DC operating current. For most LEDs, that will be higher than the 25mA you're concerned about. At that current, LEDs are rated for 50,000+ hours of service. LEDs normally dim at end-of-life.
     
  6. srgould41

    Thread Starter New Member

    Dec 28, 2015
    3
    0
    Thank you for all your responses. I have enough to go forward with.

    Steve
     
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