Voltage specs for LED

Discussion in 'General Electronics Chat' started by SPQR, Feb 1, 2013.

  1. SPQR

    Thread Starter Member

    Nov 4, 2011
    379
    48
    Hello all - simple question.

    Recently I purchased THIS 7 segment LED display.
    The company stated that the forward voltage was 1.8V.
    Connected to output pins of an Arduino at 4.8V, the segments were all nice and bright.
    I didn't believe the manufacturer's 1.8V spec so I used my simplistic power supply and made this Voltage/Current curve.

    [​IMG]
    I'm happy to report that the manufacturer's specs are correct!:)
    1.8V at 10mA gives a nice bright LED.

    Questions:
    1 - Why would an LED be manufactured for such a low voltage when CMOS and TTL are 3.3 and 5V respectively?
    2 - Can I treat these just like any LED, use 5V and a 1k resistor to limit the current to 10mA?

    Again, I thank you in advance.
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,425
    3,359
    No LED should be connected to a voltage source without proper current limiting resistor or circuitry, which includes constant current sources and PWM circuits.
     
  3. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    The forward voltage of the LED diode is determined by it's chemical makeup in the silicon doping. That means it's colour, so a red LED will have a low forward voltage while a blue LED has a much higher forward voltage. They simply can't help that, it's the way the diode is.

    Yes, and as MrChips said it very important to use a resistor to limit the current to a safe value!

    Normally with a 7 segment display the common pin is driven hard (no resistor) and the 7 or 8 pins for the segments each have their own resistor, as each segment is an individual LED.

    There are some different ways of doing it, but as a general rule that is the best way.

    You should probably have a google around for "7 segment display driving" and look at all the ways this is done. :)
     
  4. SPQR

    Thread Starter Member

    Nov 4, 2011
    379
    48
    Excellent answers.

    I had gotten that feeling in a variety of posts over the last few months.
    It's good to see it written in perfect clarity.

    Interesting about the different voltage requirements as a function of color (doping).
    I've many different types of LED - perhaps I'll take some time and do more of the V/I curves.
    And I thank you for the electrical jargon - "driven hard" - it's good to know colloquialisms in any language.:)

    Again, thank you both for your answers.
     
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