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LEDs in parallel

Discussion in 'Electronic Basics' started by Adam Funk, Jan 24, 2012.

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  1. Adam Funk

    Adam Funk Guest

    I understand the points about calculating the series resistance for an
    LED and the battery voltage, and about using a small resistor just to
    be safe even if the battery voltage is "about right" for the LED.

    The general advice "do not connect LEDs in parallel" refers to this
    sort of circuit:

    R1 /---|<---\
    ground------\/\/\/\/--| |-----+V

    where even if D1 and D2 are the same model, slight variations could
    cause one to burn out.

    Of course, the following circuit is OK, provided R1 is correct for V
    and D1, and R2 is correct for V and D2. And it can be extended for
    additional Rs and Ds.

    R1 D1
    ground-----| |-----+V
    R2 D2

    What about the following variation on the first circuit? Both LEDs
    are the same model, R1 + R2 is calculated for V and D1 and R2 is quite

    R2 D1
    R1 /----\/\/\/-----|<----\
    ground---\/\/\/--| |-----+V
    R2 D1
  2. Chiron

    Chiron Guest

    This circuit is more complicated than the others, without offering any
    benefit. You've already got two R2's to limit the current through the
    diodes. R1 serves no additional purpose. If you want to lower the
    current through the whole circuit, simply raise the values of the R2

    So you should ask yourself what advantage this circuit offers over the
    other. How is it better, or what can it do that the others can't?

    In this case you seem to believe that having low values for R2 is an
    advantage. Why would it be? You need to limit the current through the
    diodes, and because of the way that semiconductors work, a resistor in
    parallel would not reliably do this. The reason for this is that a diode
    that heats up begins to offer lower resistance, which allows more current
    to flow through it, which causes it to heat up. You can wind up with
    thermal runaway.

    With diodes in parallel, you can wind up with most of the current flowing
    through one diode. Since the resistor is valued for twice the desired
    current per diode (since it's in parallel), the diode can easily be
    stressed into failure.

    With individual resistors, the current is limited for each diode,
    avoiding this problem. Adding yet another resistor in parallel would
    offer no advantage that I can see.
  3. Adam Funk

    Adam Funk Guest

    That's what I thought after I posted it. I guess the only advantage
    is if you're short of higher valued resistors!
  4. Jamie

    Jamie Guest

    the last two will work.. and the problem is not some much burn out, it
    is uneven forward current. One will dominate the other, which ever one
    has the lowest forward voltage.

    LED's work with current, not voltage, and have a forward starting
    point of voltage before they even start to exert current, which is the
    reason for the R, a simple way to get sufficient voltage but limiting
    the current.

    Trying to regulate the voltage with out over doing the current rating
    on the LED is very hard to do, unless you have a current source that is
    regulated or simply linearly limited like an R.

  5. Kaz Kylheku

    Kaz Kylheku Guest

    LEDs "don't work with voltage" but have a "starting point of voltage".

    Brilliantly worded! :)

  6. LEDs are current drive devices so you will always need to limit the current
    somehow. You will sometimes meet LEDs driven directly from a battery but
    then the internal resistance of that battery accounts for the current
    limiting. Sometimes you see LEDs directly in parallel which may do for LEDs
    from the same batch as long as they are not driven to their limits. It is
    nevertheless considered bad practice.

    Usually you will not use three resistors if you can do with two of them. If
    the LEDs differ widely they may also influence each other. Nevertheless
    there may be some use for this circuit. For instance you may want to spread
    the dissipated heat.

    petrus bitbyter
  7. Jamie

    Jamie Guest

    Yes it was, and right to the point.

  8. Adam Funk

    Adam Funk Guest

    Doesn't that flashlight design cause inconsistent behavior when you
    switch between alkaline & rechargeable batteries?
  9. Guest

    Some people still think an exponential has a "knee".
  10. Adam Funk

    Adam Funk Guest

    Ha ha, thanks!
  11. Adam Funk

    Adam Funk Guest

  12. Guest

    John, I told you there are people who believe there is a "knee" in an
    exponential function. ;-)
    What do you think constant dI/DV means?

  13. Guest

    Your eye is "fairly constant" over that range. Measure it with a light meter.
  14. josephkk

    josephkk Guest

    Switch famous to infamous and the description of yourself is all too apt.

  15. DarkMatter

    DarkMatter Guest

    Is that what you call the crap you post?
  16. Numer0 Un0

    Numer0 Un0 Guest

    Hey, idiot. YOU are UNqualified to make a valid assessment about
    anyone else.

    You got that, boy?
  17. Guest

    Week? He's been on the rag for years.
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