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  1. #1
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    problem with powering LEDs by two different power sources

    I am having a problem with powering LEDs by two different power sources. Basically this is just a test setup for what I intend to do with my overhead panel, roughly following Ian Sisson's ideas.

    Here is a schematic I made that shows as good as possible the way I connected pieces of stripboard with either LEDs or DIODES mounted on them, through wires.



    WHAT IS IT SUPPOSED TO DO
    There are two circuits, No. 1 and 2. Circuit 1 is lighting all LEDs and can be used when switching a test switch on the overhead panel, that lights all annunciators.
    Circuit 2 is used in normal operation. Switch 1, when on, will light LEDs 1. Switch 2, when on, will light LEDs 2, Switch 3, when on, will light LEDs 3 and 4. LEDs 5 is not used in this circuit.

    WHAT DOES IT ACTUALLY DO
    Circuit 2 does work.
    Circuit 1 doesn't, (obviously because otherwise I wouldn't be writing this post )
    When I provide power to circuit 2 all it does in this setup is lighting LEDs 5. When I disconnect the ground wire (brown) from LEDs5 it lights LEDs 1,2,3 & 4, but not LEDs5.

    So there should be something wrong in the way I connect the ground wire. Can someone point me in the right direction with this problem? I would also appreciate any tips as in how to make the circuits more efficient.

    Some other info. I use a 5V computer power supply. 5v is routed through a breakout board with 220 Ohm resistors to reduce the voltage that is send through the LEDs.

    Thanks

    Jeroen Bos
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  2. #2
    500+ This must be a daytime job ian@737ng.co.uk's Avatar
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    Re: problem with powering LEDs by two different power sources

    hello jeroen........
    i see a series/parallel conflict on the LED's 5 board.
    led's are tricky little critters being pole sensitive. connecting LED's this way will lead to problems.
    also, remember diodes give you a voltage drop of .7v across it. in circuit 1, you have 2 diodes in
    line reducing the voltage after the resistor of 1.4v.
    i had a similar issue when i was wiring this way and this is how i cured it. keeping each LED
    seperate is going to save you so much grief
    sorry it's a bit 'rough round the edges', but i threw it together before having to shoot out.
    hope it helps you out captain......
    regards from wales ......

    ian

    Last edited by ian@737ng.co.uk; 07-13-2010 at 02:12 AM. Reason: worded wrongly
    Mr. Ian. P. Sissons is hereby recognised as an Honorary Flight Sim Captain following his passing in February 2016. This is in recognition for his commitment to Flight Simulation.

    www.mycockpit.org Featured Builder August 2008 www.737ng.co.uk
    FS9/PROSIM737/CPFLIGHT/Lots of BU0836X's and a Beer Fridge

  3. #3
    75+ Posting Member Leo Bodnar's Avatar
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    Re: problem with powering LEDs by two different power sources

    Jeroen,
    Do you use LEDs of different colour? Different colours have different voltage drop across them. If you connect them in parallel like you do the one with lowest drop wins and consumes all the current leaving the others high and dry. E.g. red wins over blue, etc.
    Ideally you would need to use individual current limiting resistor in series with each LED. You may get away with using one resistor per each group of the same colour/type/production batch.

  4. #4
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    Re: problem with powering LEDs by two different power sources

    @ Ian & Leo
    Thanks both for your help. I am going to try your solutions tonight.

    @ Leo
    Yes, I was actually using different colours. LEDs 1 - 4 are blue and 5 are yellow. Panel I am working on is the wing and engine anti-ice of the 737

    @ Ian
    If I understand it correctly it, using your schematic everytime I want LEDs powered is going to work. It seems that you're suggesting to have a diode and resistor in series to each LED. That's a lot of soldering Is it also possible to have a diode and resistor in series with a pair of LEDs parallel? (Meaning that each annunciator on the panel consists of 2 LEDs)
    Last question: I am currently using a 220Ohm resistor. Am I correct in saying that together with the diode in series I can better use a 330 Ohm resistor? I remeber reading that in one of your pdfs about the bus system.

    Jeroen

  5. #5
    500+ This must be a daytime job ian@737ng.co.uk's Avatar
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    Re: problem with powering LEDs by two different power sources

    hi jeroen.......
    i originally always used 330ohm resistors to protect the LED as normal practise.
    but some of the 'cheapo' LED's were not as bright as i had hoped. so by dogged
    determination and experimentation, i now use as my standard a 220ohm resistor
    if there is no diode in-line or a 180ohm resistor if there is a diode in line because
    of the voltage drop across it.
    ok, i admit using a diode and a resistor on each LED is more work, but being a 'belt
    and braces' sort of chap, i would much prefer to over do it than have to keep dropping
    my overhead out to change popped led's. in two years, i have never had to change
    an led in my new overhead. you are working with stripboard and diodes, so it's just
    a few seconds more to solder in the resistor.
    sorry, i didn't realise you were using 2 led's per annunciator. i have several 2 led
    lights on my overhead......... and this is how i have wired them

    hope that helps you out........
    have fun experimenting
    rgds from the welsh borders

    ian
    Last edited by ian@737ng.co.uk; 07-13-2010 at 11:32 AM. Reason: spelling, must get a new keyboard :o))
    Mr. Ian. P. Sissons is hereby recognised as an Honorary Flight Sim Captain following his passing in February 2016. This is in recognition for his commitment to Flight Simulation.

    www.mycockpit.org Featured Builder August 2008 www.737ng.co.uk
    FS9/PROSIM737/CPFLIGHT/Lots of BU0836X's and a Beer Fridge

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  7. #6
    75+ Posting Member Leo Bodnar's Avatar
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    Re: problem with powering LEDs by two different power sources

    If you really don't like extra soldering then you can try using one resistor per few parallel LEDs but they should be of the same colour and preferably from the same model and batch.
    If it works - fine but if does not, you know why (see Ian's explanation.) In such case remember that the current is split between the LEDs so resistor should be adjusted accordingly.

    I have found an interesting link that lists almost all currently available LEDs including intensity and voltage drop: http://www.densitron.com/displays/LED_Color_Chart.aspx

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