LEDs

Discussion in 'General Electronics Chat' started by duxbuz, Apr 6, 2014.

  1. duxbuz

    Thread Starter Member

    Feb 23, 2014
    133
    0
    Hi

    I am after getting some LEDs to light in a 5v circuit. There are different attributes e.g forward voltage and forward current.

    My LEDs I already have don't light up in 5v circuit seemingly.

    What should I be buying for My circuit

    Thanks
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,652
    2,348
    Hello,

    What are the specifications of your leds?
    What is the forward voltage at what current?

    Bertus
     
  3. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    Did you have a resistor in series with the LEDs?
     
  4. duxbuz

    Thread Starter Member

    Feb 23, 2014
    133
    0
    The LEDs have no spec or should I say I don't know the spec.

    I need new ones so I can at least know what I have bought.

    The ones I have actually just light up at 5v

    What do I look to buy for 5v circuit

    Forward voltage 2v?
     
  5. Tealc

    Member

    Jun 30, 2011
    140
    10
    The simplest solution is to buy LEDs in the colour of your choice that are described as 5v LEDs, otherwise you need to add a resistor in series. If you do neither any standard led will blow nearly instantly.
     
  6. duxbuz

    Thread Starter Member

    Feb 23, 2014
    133
    0
    So forward voltage is the attribute?
     
  7. duxbuz

    Thread Starter Member

    Feb 23, 2014
    133
    0
    Is there a way to work out what my existing LEDs forward voltage is
     
  8. spinnaker

    AAC Fanatic!

    Oct 29, 2009
    4,887
    1,019

    It will be different based on the manufacturer but you can take a rough guess by using the color.

    http://www.oksolar.com/led/led_color_chart.htm

    But the best thing to have is your datasheet.

    For 5V your most LEDs should light. There are three issues that could cause the LED not to light.

    1. The LED is connected in reverse.

    2. You have exceeded the max current of the LED and it is now fried.

    3. The LEDs were bad out of the box (unlikely).

    While forward voltage is important to get the LED to light. Probably the most important rating is the max current of the LED. The purpose of the series resistor is to reduce the amount of current flowing through the LED. More advanced techniques would use a buck current regulator to maintain current more efficiently.
     
  9. Metalmann

    Active Member

    Dec 8, 2012
    700
    223

    The best way I've found, is to gradually/slowly....turn up your bench power supply.
    Never fails.
     
  10. #12

    Expert

    Nov 30, 2010
    16,355
    6,852
    Put a 220 ohm resistor in series with the LED, connect that pair to the 5 volts, and measure the voltage across the LED. You will probably only have about 10 milliamps flowing, but it will get you a voltage number to calculate the real size of resistor it needs.
     
    duxbuz likes this.
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