More current same voltage

Discussion in 'General Electronics Chat' started by cjdelphi, Jun 10, 2009.

  1. cjdelphi

    Thread Starter New Member

    Mar 26, 2009
    272
    2
    More current same voltage possible on an LED?

    3.0v might consume 50ma is it possible to force 100ma but keep 3.0v? (I presume impossible....according to ohms law)
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
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    Not at all impossible. A LED is a special case of a diode, with a PN junction. It requires a certain voltage to bias the junction into conduction, but essentially has no resistance after that. Many people burn up LED's with too much current, as they do not understand how LED's work. There will be some level of current that will overheat the PN junction and cause the device to fail.

    Here is our Ebook article on them - http://www.allaboutcircuits.com/vol_3/chpt_3/12.html
     
  3. Audioguru

    New Member

    Dec 20, 2007
    9,411
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    The datasheet for the LED has a graph of typical forward voltage vs current.
    If it is 3.0V at 10mA then it might be 3.2V at 30mA.

    Most ordinary LEDs burn out if their current is more than 30mA.
    The colour and temperature of the LED affects its voltage.
     
  4. leftyretro

    Active Member

    Nov 25, 2008
    394
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    Yes, forget about voltage when powering LED's, just limit or control current to the amount you desire that is still within the max specs of the LED. If you can PWM the drive then you can supply more current then the steady state value. Study the LED's spec sheet and learn the several ways to control or limit current, not voltage.

    Lefty
     
  5. David Bridgen

    Senior Member

    Feb 10, 2005
    278
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    You clearly don't understand l.e.ds in particular, or diodes in general. Nor do you seem to understand Audioguru's post.

    A diode is a non-linear device. The voltage developed across (not applied to) a diode is dependent on the current through it.

    Increase the current through it and the voltage across it will increase too, however inconvenient this may be for you.

    But, since the diode is non-linear, an increase in current by a certain percentage does not result in an a voltage increase of the same percentage.

    Take a look at this, it might help you ..... http://www.davidbridgen.com/leds.htm
     
  6. David Bridgen

    Senior Member

    Feb 10, 2005
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    0
    Yes, it's impossible.
     
  7. mik3

    Senior Member

    Feb 4, 2008
    4,846
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    It is impossible to force more current and keep the voltage across the diode exactly the same due to the bulk resistance of the diode and imperfections of the junction. However, if you force 100mA instead of 50mA the voltage across the diode will change slightly and won't double like a resistor due to the diode characteristics.
     
  8. Wendy

    Moderator

    Mar 24, 2008
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  9. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    I'm surprised nobody brought up thermal considerations yet.

    Assuming that you're supplying current to a PN junction (including LEDs) via a low-impedance regulated voltage source, you can increase the current through the diode without changing the voltage by increasing the temperature of the diode.

    However, just past the point where the power being dissipated in the LED is greater than the ability of the PN junction and it's surroundings to dissipate that heat by conduction, convection and/or radiation, the junction will enter a "thermal runaway" condition.
     
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