what might be wrong with my red led

Discussion in 'General Electronics Chat' started by Yaşar Arabacı, Nov 12, 2014.

  1. Yaşar Arabacı

    Thread Starter Member

    Nov 11, 2014
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    I am pretty new to electronics and I am making my first circuits today. I have encountered a weird problem.

    I have connected 2 leds in parallel to each other and added a switch for each one. When I activate the switch for green led, green led light up. Same goes for red led. However, if I activate both switches only green led works.

    I switched red led with white led and I can activate both of the leds at the same time.

    I couldn't come up with any idea to explain this. Maybe red led have too little.resistance and getting all the current?
     
  2. ISB123

    Well-Known Member

    May 21, 2014
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    What is the voltage of power supply and show us the schematic?
     
  3. MrChips

    Moderator

    Oct 2, 2009
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    It is not advisable to wire LEDs in parallel because they have different turn-on voltages.
     
    adam555 and Gdrumm like this.
  4. crutschow

    Expert

    Mar 14, 2008
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    Do you have current limit resistor in series with each LED?
     
  5. Yaşar Arabacı

    Thread Starter Member

    Nov 11, 2014
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    I don't know the proper symbols for schematics but I made a sketch and also photographed my circuit.

    http://imgur.com/Od25fAi
    http://imgur.com/HogmAio

    To be honest I didn't know led lights were different than other lights. I just bought them to experiment with basic circuits. I am doing this as a hobby ;-)
     
  6. MrChips

    Moderator

    Oct 2, 2009
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    No problem. That's the best way to learn. Try a few things. If your results are unexpected come and ask for help on AAC.
    Get to know and understand Ohm's Law: I = V/R.
    Do the calculations and verify that Ohm's Law works for you.

    Know that an LED (and any diode) is non-linear, i.e. it does not obey Ohm's Law. That is another way of saying that the resistance is not constant. The current increases faster as you increase the voltage.
     
  7. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    The answer is very simple. You just see II Kirchhoff's law in action. You should never connect led in parallel especially if they have a different color.
    The LED with a lower forward voltage will be ON.
     
  8. Yaşar Arabacı

    Thread Starter Member

    Nov 11, 2014
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    Does that mean a LEDs colour have some kind of meaning? I thought colours were for the looks :)
     
  9. ISB123

    Well-Known Member

    May 21, 2014
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    Diodes are like wife's.
    The prettier she looks the more she asks for.
    [​IMG]
     
  10. MrChips

    Moderator

    Oct 2, 2009
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    Yes, the colour are there for looks. But the origin of the colour is directly related to the physics.
    When electrons go from the higher energy conduction band to the lower energy valence band across the P-N junction, energy is released as light.
    This energy is called the energy band gap and is dependent on the materials of the P-N junction. Red is emitted at lower energy than blue. That is why a blue LED needs a higher voltage.

    Reading this wikipedia article might shed some light on the subject:

    http://en.wikipedia.org/wiki/Light-emitting_diode
     
  11. wayneh

    Expert

    Sep 9, 2010
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    Do you mean, only the RED lights when both are on, and gets all the current? That would be more normal, since a red LED usually turns on at a lower voltage. If they are sharing a single resistor , the red LED comes on and hogs all the current, preventing the green from ever turning on. It may seem weird, but it's actually the expected result.

    You can fix it by giving each LED its own resistor. Then current will flow through both in parallel.

    Some red LEDs are clear and you cannot identify them until they turn on. I think some "red" LEDs may merely be red plastic with a white LED inside?
     
  12. ian field

    Distinguished Member

    Oct 27, 2012
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    It sometimes features in commercial designs - an E-cigarette charger I took apart has red & green charge indicator sharing a current limit resistor.

    A transistor senses when the battery is drawing current and connects the cathode of the red LED to ground to indicate charging.

    The green LED is permanently grounded, but because its Vf is higher than the red one, the red light cuts it off when activated.
     
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