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Discussion in 'LEDs and Optoelectronics' started by scientifico, Sep 2, 2012.

  1. scientifico

    scientifico

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    0
    Mar 13, 2012
    Hello, for what I understood LEDs absorbe current in relation to the voltage given to them, so can I teorically connect a 2.1 Vf LED directly to a 2.1 voltage supply without any resistance?

    Thank you
     
  2. CocaCola

    CocaCola

    3,635
    4
    Apr 7, 2012
    Well there is no such thing as a 2.1V LED, instead there are LEDs that at 2.1V will pass a safe level of current as to not blow them up...

    If at 2.1V your particular LED passes a safe level of current then yes, technically you can skip any further resistance... But, it's not advisable in most cases... If you look at a graph of the LED forward voltage vs current you will see that there is a 'break away' voltage where the current passed is exponentially grater than the increase in voltage... Thus at 2.1V you might be AOK, but at 2.25V everything might go bad... The resistor will help eliminate such a harsh 'break away' point as the voltage increases and give you a more linear operating range...
     
  3. iknowvator

    iknowvator

    6
    0
    Sep 5, 2012
    Theoretically yes ! You can ( for a brief period of time as CocaCola rightly said) but providing 2.1 V to LED may damage it by passing excess forward current which may damage the diode junction of LED that is will be permanently "off". So it is better to have some resistor in between to make it "glow" forever.!
     
  4. CocaCola

    CocaCola

    3,635
    4
    Apr 7, 2012
    There is no 'brief period' about it you can done indefinitely as long as the system maintains balance and doesn't drift outside 'safe' perimeters...

    As I said it's a balancing act, think of it this way... The LED is a tightrope walker... Using a resistor is like giving the tightrope walker a balance bar as well as a safety net, are they needed? No, he can walk the rope all day long without either as long as he doesn't lose his balance... The same with LEDs, as long as balance is maintained the resistor is not 'needed' but it sure does make it 'safer' and less prone to failure due to a little hiccup...
     
  5. iknowvator

    iknowvator

    6
    0
    Sep 5, 2012
    We are talking about providing 2.1 V to 2.1 V LED without any register. It will be brief period for many LEDs !
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,175
    2,690
    Jan 21, 2010
    iknowvator, perhaps you need to read the sticky on LEDs.

    What CocaCola and others are discussing has to do with the choice of an operating point on the V/I curve, the characteristics of that curve, and the di/dv characteristics at that point.
     
  7. CocaCola

    CocaCola

    3,635
    4
    Apr 7, 2012
    I'm well aware of what we are talking about... As I said earlier there is no such thing as a 2.1V LED... And as I said earlier if the LED is passing a safe 'current' level while at 2.1V it will last indefinably without a resistor as long as the voltage is held in check and system balance is maintained... **Edit after browsing Steve's tutorial, as he stated there is thermal runaway and this has to be held in check as well, it's all part of the balance act as I stated, and as I also stated this is the reason it's not recommended/advisable to skimp on the resistor in most cases... This 'safe' point on the Volatage/Current curve is different for every LED...

    I have only glanced over Steve's LED tutorial so I can't comment in detail, but as he suggest you might want to read up about LEDs a little further and gain additional insight that I suspect he has included in the tutorial... There are WAY too many people authoring websites about LEDs and only telling (knowing?) half the story and preaching it as absolute gospel, when in fact it's only a half truth...
     
    Last edited: Sep 6, 2012
  8. iknowvator

    iknowvator

    6
    0
    Sep 5, 2012
    With due respect, we are trying the answer of "..can I teorically connect a 2.1 Vf LED directly to a 2.1 voltage supply without any resistance? " Just providing proper values in the equation I = (V - Vf) / R will give us the answer. Your sticky also points to the same answer.
     
  9. CocaCola

    CocaCola

    3,635
    4
    Apr 7, 2012
    With all due respect, please go back and re-read the sticky again, you are only grasping part of the picture... The equation you point out is how to 'destroy' the excess voltage with a resistor and lower the voltage to the desired Vf, in this case we are already at Vf and have no 'excess' voltage to destroy... Plug the numbers in and you will see the equation fails to result in a valid answer if V and Vf are the same... 0.02 = (2.1-2.1) / R ---> 0.02 = 0/R ---> 0.02 = 0 aka failed value for said equation to work... When reading the sticky this time focus on the impedance of the LED at a given voltage, yes LEDs do have their own "resistance" it's just not linear, and this plays a role... Look at a forward current vs voltage chart of any LED...

    Example: Lets call this a "3.5V white LED"

    Looking at that chart below you can see that at 3.5V, the LED will pass give or take about 19mA due to it's own impedance (resistance) at 3.5V and 'room' temperature...

    [​IMG]

    As long as everything remains in balance, aka the voltage doesn't drift and the temp doesn't rise or fall the LED will continue to pass just 19mA...
     

    Attached Files:

  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,175
    2,690
    Jan 21, 2010
    CocaCola, that graph looks so much better with a linear Y axis (at least in illustrating the particular point) :)
     
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