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LED's in Series Vs LED's in Parallel

  1. May 8, 2013 #1
    This should be a simple question yet I am struggling to see the benefit of using one over the other.
    I was building an LED and LDR based sensor head for a line following robot, and I needed to choose a method of connecting 3 LED's inside of it. I figured that the 3 options are:

    1) All in series with a resistor

    2) All in parallel, with the parallel bit in series with 1 resistor

    3) All in parallel, each with their own resistor

    4 important points:
    .The LED's are rated at 3V, 20mA, 60mW. This is what I am aiming for.
    .The LED's should all be as bright as possible, and more importantly they all need to be the same brightness (so that the sensor head is accurate).
    .The 5V is set, this cannot be changed, however the resistors can be any value at all.
    .I'm guessing the LED's are 150 Ohms as 3V/20mA = 150 Ohms, but the resistances of each may slightly vary due to manufacturing inaccuracies.



    The voltage and current through the resistors can be controlled equally well in each case as the value for the resistor in each case can be changed.

    I was told that option 3 is the best option, and option 1 is completely useless, but I don't see why. Is it something to do with the fact that a diode does not have a linear I-V characteristics?
    0VGk7F4.FiPeOdnS12jRdw.jpg
    So the relationship between current and voltage is linear like with a resistor. I have always assumed that this doesn't matter for an LED, as the main reason there is a voltage across this is because energy has been converted to light, (like a resistor does as heat). This is unlike a diode whose "resistance" changes as you change the voltage across is.

    Please help!

    Thanks!
     
    Last edited: May 8, 2013
  2. jcsd
  3. May 8, 2013 #2

    phinds

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    You are saying that each LED requires 3 volts, yes? If you put 3 of them in series, how many volts would you need?
     
  4. May 8, 2013 #3

    davenn

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    Do you see the problem with this scheme ??

    look at your supply voltage 5V, Vs you voltage drop for each LED 3V and the resistor


    thats why parallel LEDs are better if there's going to be a lot of them


    Dave
     
  5. May 8, 2013 #4

    davenn

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    hey phinds SNAP LOL

    you typed less ;)

    Dave
     
  6. May 8, 2013 #5
    Im trying to understand this:

    Why would you ever put lights you wished to be of equal "brightness" in series?
     
  7. May 8, 2013 #6

    Integral

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    A instrument I build has LEDs in series. But this is to guarantee that either both are working or neither.
     
  8. May 8, 2013 #7

    davenn

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    Ohhh its no problem 3 LEDs in series and with a current limiting resistor work well
    all the same brightness.
    But as the OP will discover he will need more voltage ;)


    Dave
     
  9. May 8, 2013 #8
    Ah yes, it would be impossible to have 3 Volts across each in series, as this would add up to more than 5 Volts. I see now, thanks for answering.

    Although I'm still not sure why each having their own resistor is better than just having one in series with the all the parallel resistors? What is wrong with having one resistor that splits the voltage between itself and the parallel part as 2V and 3V?

    If they are all of equal resistance (or at least very close) then the voltage across all of them will be the same, and as they are in series the current through all of them will be the same, so they will all be equal brightness, no?
     
  10. May 8, 2013 #9

    berkeman

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    With newer "matched" LEDs, you can put them in series. You do however, need enough voltage headroom to make it work, as pointed out by other posters.

    The disadvantage is that matched LEDs tend to be a bit more expensive. The advantage is that you can run them from a higher voltage, and lose less power in the series current-setting resistor for each series string.

    Series LED configurations are common in LED lighting applications, where you may have several strings of series connected LEDs (each with its own current setting resistor or low-side current source transistor) placed in parallel.
     
  11. May 8, 2013 #10
    Yeah, I have to much WCAD (who cares attention deficit) to work the simple math stated by the OP. I'm going to guess equal "brightness" is on the less than probable result of the hypothesis
     
  12. May 8, 2013 #11
    Guess what I'm really trying to say is in the situation where the "brightness" is so paramount to the problem itself that I would never consider series with a 5V supply. We can assume your supply is VDC? Yes, I know you can also go AC with LED. Just curious...... what happens to the equation as your supply begins to deplete it's ampacity???????
     
  13. May 8, 2013 #12

    berkeman

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    Actually, the best way to do this for good battery life would be to do a boost DC-DC circuit to drive the 3 LEDs in series, and use current-mode control (with a small low-side sensing resistor) to keep them operating at 20mA... :smile:
     
  14. May 8, 2013 #13
    Yeah the supply is from a 5V DC battery (a pretty big rechargeable one). This battery is also powering a motor drive circuit (in parallel with the LED circuit).
    As the source depletes the voltage across it will decrease, and therefore the voltage across everything in the circuit will deplete, and hence the current through everything will deplete, so they will become dimmer.

    I've never heard of a boost DC-DC circuit, but my robot is running about of space to mount new circuits anyway so it probably couldn't be done. But out of curiosity, how do they work?
     
  15. May 8, 2013 #14

    berkeman

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    http://en.wikipedia.org/wiki/Boost_converter

    They are one of the topologies of DC-DC converters. Normally you use voltage feedback from the output voltage supply to control the duty cycle of the switching action. But in LED applications, you put a small-value resistor at the bottom of the string of LEDs, and use that sense voltage to regulate the current out of the boost DC-DC to the desired value (20mA in your case).
     
  16. May 8, 2013 #15

    davenn

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    google DC - DC Boost Converters.
    And for your interest also look at DC - DC Buck Converters, for dropping and regulating voltage
    They are much more efficient than linear regulators like the LM78xx series.

    Dave
     
  17. May 8, 2013 #16

    davenn

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    also for the obvious reason .... to get 3 times the brightness out ;)

    I have a commercial product I produce that has 3 sets of 3 LEDs in series
    all hi-intensity 5mm LEDs, 3 sets of red, of green and of blue


    Dave
     
  18. May 8, 2013 #17
    I will do thanks, I should have done that before asking really haha.

    Just one last question though:

    I was told option 3 is better than option 2, do you know why this is? To me it just seems like a waste of resistors.
     
  19. May 8, 2013 #18

    davenn

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    well yes more resistors ... but the cost is irrelevent, they are so cheap.
    but if you are just going to use 1 resistor supplying ALL the parallel branches, it will be a higher wattage to handle the extra current. and if it fails then your whole display dies where as if you have individual resistors in each branch and one failed then you would only that one branch


    Dave
     
  20. May 8, 2013 #19
    Let's do this for fun please????

    Debate this topic from the true stand point of modern engineering. No, I don't mean academic theory vs real world experience.

    I mean, the cheaper the better!!!!

    Now pretend you own the market share and like you are trying to gain market share.

    Accounting balance sheets lol
     
  21. May 8, 2013 #20
    Dave, why not the1970's christmas light argument on the series option? Which is more efficient? Do the math, I don't want to. Which loses the most due to heat? Which ever one is mathmatically using less wattage is the option I pick. Would you consider that a load resistor? :devil:
     
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