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How do you connect LED's in parallel?

colporteur

I have a need to connect a number of LED's in parallel. Can someone share their insight into why an additional resistor is required for the LED's of different colour in parallel to light?

 

The LED's in circuit A lights with no issues. In circuit B, only the Red LED lights. Circuit C lights both LED's but requires the extra resistor to achieve the success.

 

My formal electronics training occurred when vacuum tubes were still common in electronics. My understanding would be voltage drops. The Green LED requires a higher voltage drop to light than a Red LED. Circuit B doesn't light because the Red LED lights with it's voltage drop not being sufficient for Green to light.

 

I appreciate this is a pretty simplistic circuit. I find for myself exploring the basics refreshes the knowledge retained in my magnetic core memory:)

image

Parents

  • Nice question Seanimage,

     

    I have a need to connect a number of LED's in parallel. Can someone share their insight into why an additional resistor is required for the LED's of different colour in parallel to light?

    A - Yes, for the same forward voltage ( Vf )  they light up together fine.

     

    B  - While turned off, both LEDs are an "open switch". What happens is from the moment you turn on the power,

    1. the Red led will conduct first ( due to its lower Vf which has to do with its material nature ),
    2. then it will establish itself in the circuit forcing its Vf through it.
    3. Consequently, the RED Vf will be applied to the GREEN, and this will not be enough to light the GREEN.

     

    C - Meanwhile, the series resistor with RED led is adding flexibility to the circuit, this way

    1. you have the voltage necessary for the RED led and
    2. the resistor allows the voltage of the GREEN led to be achieved.
    3. Vgreen = Vresistor + Vred

     

     

    image

Reply

  • Nice question Seanimage,

     

    I have a need to connect a number of LED's in parallel. Can someone share their insight into why an additional resistor is required for the LED's of different colour in parallel to light?

    A - Yes, for the same forward voltage ( Vf )  they light up together fine.

     

    B  - While turned off, both LEDs are an "open switch". What happens is from the moment you turn on the power,

    1. the Red led will conduct first ( due to its lower Vf which has to do with its material nature ),
    2. then it will establish itself in the circuit forcing its Vf through it.
    3. Consequently, the RED Vf will be applied to the GREEN, and this will not be enough to light the GREEN.

     

    C - Meanwhile, the series resistor with RED led is adding flexibility to the circuit, this way

    1. you have the voltage necessary for the RED led and
    2. the resistor allows the voltage of the GREEN led to be achieved.
    3. Vgreen = Vresistor + Vred

     

     

    image

Children
  • in reply to jpnbino

    Thank you JB, you have lifted the veil that was my mystery.

    You explanation makes sense. Thank you for applying the knowledge to my ABC's. Once red is conducting the the Vf is constant. Green can't achieve the Vf it needs to conduct.

     

    I realize I should have made this a question. If that was the case you would have received the ANSWER token. Thanks again. Great table by the way!