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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

  • Hi,

    this is just a basic calculation. The datasheet describes more details.

    image

     

    Best Regards

    Gerald

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  • in reply to geralds

    That is where the waters get muddy.

    image

     

    You would think the Green would work and the Red not when they are in series without the resistor. That is not the case. I can't work out the voltage divider to arrive at the bias voltage on the device.

  • in reply to neilk

    Thanks for the resource link.

     

    I wish I was sitting across the table at coffee break with a napkin to better generate an understanding.

  • in reply to colporteur

    The point is what Mr Kirchhoff once said.

    The sum of the incoming currents is the sum of the outgoing currents.

    For the thought with the LED wiring:

    An LED is a diode, that's it.

    In one direction it is a wire, in the other direction an "insulator".

     

    If a network (Kirchhoff) now divides the LED currents, and each LED has a different differential resistance,

    then a different current flows through the respective LED due to the exemplar-specific voltage drop,

    which can overload this relevant LED.

    So there will be more current flowing through a low voltage drop LED.

    -> An (the) other LED, which is still under the drop voltage (U_LED_forward) will not work until the voltage will go higher over this forward voltage of that LED.

     

    !! You can't see it optically (except with a measuring device), but the LED gets hot. !!

     

    You could also operate an LED without a protective resistor, but that only works when the "knee" changes,

    when the reverse voltage U_LED_forward is switched to the conductive state.

    And exactly THERE must the current be limited exactly.

    The diode, followed by the LED, is a current-operated component.

    There can also draw µA through the LED, but you cannot see this, but this current can heat the LED.

     

    There are also LEDs with 3V or more U_LED_forward.

     

    Therefore, for a given stable voltage, the series resistor is calculated taking into account the voltage drop from the

    diode and the temperatures (ta tc) so that the parameters are never exceeded during operation.

    So you can NOT excessively increase a voltage in the direction of current flow.

    The data sheet of the LED usually states from which current a defined suitable light output (100% desired) is achieved,

    and this is used as a parameter.

    Then there are tolerances, such as 100% + or - light output at temperatures, current limits, etc., without having to cool the diode or LED.

     

    !! So a separation the two, or more LEDs "parallel" is necessary.

    !! Each LED needs a separate resistor, if you use a parallel circuit.

     

    As Neil said, in serial, this is possible. But how high you want going with the supply voltage?

    If you have 12V, hm.... no  many LEDs you can use, and 1 resistor for the current flow is will be enough.

     

    Best Regards

    Gerald

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  • 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

  • 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!