LED as power indicator

Discussion in 'General Electronics Chat' started by Skubby SkinnyChubby, Jul 13, 2015.

  1. Skubby SkinnyChubby

    Thread Starter New Member

    Jun 19, 2015
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    1
    Hi,

    i want my LED to light up when the power supply is on, so question is i can design the circuit( image attached)
    supply voltage is 24V, and is there any problem doing this?


    thanks :)
     
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  2. DickCappels

    Moderator

    Aug 21, 2008
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    It should be fine if the current, or more directly, power is within the LED's specs. Also make sure you use a resistor rated at 1.3 times its power dissipation or more.
     
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  3. ScottWang

    Moderator

    Aug 23, 2012
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    If the led is rating at 2V/20mA, and considering the brightness and usage life, I will recommended that you should use it about 16 mA is better, so the resistor will be as 1K+360Ω or 1K+390Ω and the current will be as 16.2 mA or 15.8 mA.
     
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  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    For battery-powered equipment, where the additional battery drain of the LED indicator subtracts from the useful battery life, you might consider a self-flashing LED, which has lower total current demands. Also, I have operated Red "ultra-bright", high-efficiency LEDs on as little as 500uA (not near 20mA as Scott suggests), and they are still bright enough to be seen in direct sunlight as a power ON indicator.

    Do an experiment; the LED current only needs to be whatever it takes for you to see the indicator...
     
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  5. wayneh

    Expert

    Sep 9, 2010
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    I often set my indicator LEDs to just 5mA or so. It's plenty. Flashing would be even better.
     
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  6. ISB123

    Well-Known Member

    May 21, 2014
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    Get a blinker LED (LED Indication) and pick a resistor which is going to limit it to low current like 10mA.
     
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  7. ian field

    Distinguished Member

    Oct 27, 2012
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    You can also get ultra efficient LEDs that give a useable indication at only 2mA.
     
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  8. ScottWang

    Moderator

    Aug 23, 2012
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    What's your suggestion for the circuit and current?
     
  9. wayneh

    Expert

    Sep 9, 2010
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    Me? I would likely use the resistor current limiter the TS showed in #1, just using a higher ohms resistor to get the current in the 1-5mA range. If I had a flashing LED and I cared a LOT about current draw, I'd use that.

    If I really needed to limit power, I'd look at a 555 flasher with a duty cycle of, say, just 1%. (I assume a normal flashing LED has a higher duty cycle.) A 1ms flash every one second or so, that sort of thing. Of course it would have to be a low power 555.
     
  10. ScottWang

    Moderator

    Aug 23, 2012
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    As you suggested that as 555 and 1% duty cycle, how is the current (mA)?
     
  11. wayneh

    Expert

    Sep 9, 2010
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    This feels like a hijack of the TS's thread, but see here.
     
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  12. ScottWang

    Moderator

    Aug 23, 2012
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    I don't think that is hijack, you just given another idea and I'm trying to make it clearly.
     
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