LEDs not as awesome as presumed

  1. I bought some 'super bright' LEDs from sparkfun:

    https://www.sparkfun.com/products/8861

    I got the green one, the red one, and the blue one. Guys in the comments are saying, "Wow these things light up a whole room at 75mA!" So I bought them to make a night light.

    I checked out the datasheet. Max current is 75mA. Forward voltage is 3.4V. I plugged it directly into a current source (my power supply) to test it, and adjusted the voltage and current until the current was about 75mA. It was on... I guess kind of bright... but nothing like in the comments section of the site that I bought it at.

    Did I do something wrong? I tried setting the voltage at 3.2, 5, 7.1, then I adjusted the current up slowly until it was about 75 mA. I must be doing something wrong.

    Thanks
     
  2. jcsd
  3. First off, they are current devices. The spec means that a 'normal' LED will drop about 3.4 volts when passing the spec'd current. Limit the current. My guess is that when you 'adjusted' the voltage and then turned the current up until you got 75 mA, the voltage was the same in all cases unless you damaged the LED.
     
  4. I limited the current with a resistor, then later a current source. The current source will output whatever current I tell it to, no more and no less. Or do I misunderstand something?
     
  5. I will do this with another bulb:

    5VDC from power supply, unlimited current.

    22 ohm resistor: (5-3.4)/75e-3 = 21.33

    Ok?
     
    Last edited: Sep 19, 2013
  6. Your math looks right. I am certainly not saying you were not driving it correctly before. Just wanted to clarify.
     
  7. It just doesn't seem as bright as I expected. Maybe I just don't know any better. It seems dimmer than flashlights with similar candela ratings... is that all from the reflector in the flashlight?
     
  8. The lens in an LED flashlight makes a big difference. Also different LEDs have different lensing and different emission patterns. There is generally a plot of the emission pattern.

    I have a $4.00 cree flashlight that I bought from amazon (shipping included) and it is incredibly bright. But I think they drive a LOT more current.

    If it's cheap, I'd drive one until it burns out and see if it is bright enough at any current, then buy one rated for that current.

    BTW, 75ma is 75ma regardless of how you got it.
     
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  9. The majority of the light goes [itex]\pm 10^{\circ}[/itex] from the center top of the bulb. What if I put a diffusing cover on it? Would it light up the room like I want?
     
  10. Shine it up the wall and see how it lights the room. That will give you some idea, maybe.
     
  11. One LED at ~1/4 watt is never going to "light up the room".
     
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