led threshold

Discussion in 'The Projects Forum' started by sasek, Apr 28, 2010.

  1. sasek

    Thread Starter Member

    Jan 3, 2010
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    hai all,

    i just want to know how much min current/voltage to start light up the LED???? :):):):):):)
     
  2. retched

    AAC Fanatic!

    Dec 5, 2009
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    well, they are current dependent devices, so as long as the voltage is over the Vf of the LED, and there is required current, it will work.

    Usually you can start to see a red LED light at around 5mA. and standard operating current is 20ma. AVERAGES
     
  3. Markd77

    Senior Member

    Sep 7, 2009
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  4. Wendy

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    Mar 24, 2008
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  5. MMcLaren

    Well-Known Member

    Feb 14, 2010
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    May I ask why you're asking, please? Are you trying to reduce current or affect perceived brightness?

    Kind regards, Mike McLaren
     
  6. sasek

    Thread Starter Member

    Jan 3, 2010
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    no..........i want create dimmer using optocoupler.......then the starting is not very good
     
  7. Wendy

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    Mar 24, 2008
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    Why an optocoupler? Is this an LED bulb in a socket?

    LEDs typically respond best to PWM, which is mentioned in the link I gave you.
     
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  8. sasek

    Thread Starter Member

    Jan 3, 2010
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    yesss bill i have try using pwm..........n the LED response is not very good
     
  9. retched

    AAC Fanatic!

    Dec 5, 2009
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    Then your PWM was not done properly. PWM is by far, the best way.

    Which circuit did you use for your PWM?
     
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  10. kingdano

    Member

    Apr 14, 2010
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    PWMs are definitely the best way to do an LED lighting application.

    and a great way to do so is use of a 555 timer circuit.


    bill can tell you a lot about the 555 timer
     
  11. sasek

    Thread Starter Member

    Jan 3, 2010
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    form now, pwm application is like DAC........pwm variable is 0-255 means it have 8bit DAC. hence each variable difference give 19mv, this value is to high for LED. Now my pwm output connected with optocoupler where we know that optocoupler input is connected to LED. Right now i want the LED voltage go smoothly equal to >4mV for each pwm variable.
     
  12. retched

    AAC Fanatic!

    Dec 5, 2009
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    use a resisitor in series between the pwm output pin and the LED. That will drop the current to the LED.
     
  13. Wendy

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    Mar 24, 2008
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    As retched says, a resistor is always needed for an LED. A PWM circuit will allow you to very intensity linearly, whereas a variable resistor won't.
     
  14. sasek

    Thread Starter Member

    Jan 3, 2010
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    ok! you means i need put resistor in series with LED to have smooth control rite
     
  15. Wendy

    Moderator

    Mar 24, 2008
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    An LED becomes a short once the Vf is exceeded. It is similar to how a zener diode works. The resistor keeps the current under control, and sets the max current.

    This is why it is referred to as a current controlled device, as opposed to a voltage controlled device.
     
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