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My very basic electronics education has taught me that parallel circuits are equivalent to separated circuits. To my surprise, when I was playing around with some electronics I found the following: enter image description here Essentially I connected two red and two blue LEDs in parallel. The red ones lit up, but the blue ones didn't. Only when I removed the red LEDs would the blue ones light.

Why is this?

DarkLightA
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  • @JYelton That addresses the superset case of diodes whereas the issue the OP is asking about is more obvious in that he is able to get two red LEDs to light. He should learn about the question you're trying to dupe it to, but they are two different questions: one's about "identical" diodes while this is about different diodes. – horta Sep 23 '14 at 20:51
  • The pitfalls of quantum mechanics! The "bluer" you get, or violet, the higher the energy of the photons and the higher voltage drop across the semiconductor junction. Your LEDs at the red end of the spectrum are basically short circuiting as far as the blue ones are concerned. – C. Towne Springer Sep 23 '14 at 21:50

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The red LED has a much lower voltage drop for a given current. In that way, the red ones light but the rest don't achieve their voltage necessary to light.

  • Red LEDs have a voltage drop of about 1.8V.
  • Blue LEDs have a voltage drop of about 3V.

You can see more colors and their corresponding voltage drop here in this table: http://en.wikipedia.org/wiki/Light-emitting_diode#Colors_and_materials

To solve this issue, you need a separate current limiting resistor for each led.

You could think about it as if you were putting two different zener diodes in parallel. If you have a 2 volt zener and a 5 volt zener, the 2 volt zener will reach it's voltage and prevent the 5 volt zener from ever passing any current.

horta
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Red and blue LEDs have different threshold voltages.

Red threshold voltages are lower, so the red LEDs are not "letting" the voltage get high enough for the blue LEDs to light.

To make your circuit work:

  • The LEDs could be in series (if you have a large enough supply voltage), or,
  • If in parallel, use a separate limiting resistor for each LED.
payne
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The red ones have a lower forward voltage than the blue ones.
I.e. as soon as the voltage across the diodes is more than the "red" forward voltage the red diodes start to conduct and use up all the current.

Even if you put two diodes of the same type (e.g. two red ones) in parallel it is not a good idea because there may be a small difference of their forward voltage causing the currents going through the diodes being quite unballanced.

If you want to make sure the current passing through all diodes is exactly the same you have to connect them in series (of course this works only if your supply voltage is large enough, i.e. larger than the sum of all forward voltages; and some voltage reserve must be left to drop at the resistor)

Curd
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