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What is the benifit of using more than one reistor vs one?

  1. Jun 18, 2010 #1
    I am building a LED array and most of these LED resistor calculators like this one http://led.linear1.org/led.wiz, seem to suggest that I need more than one resistor for the array. From my Calculations I need 1.2 Ohm resistor/s. So my question is given a 6 LED array, why use 6 resistors rather than just one. Also why does the software/website put the resistors on the cathode side of the LED's?
     
  2. jcsd
  3. Jun 18, 2010 #2
    Can you post a schematic of YOUR idea vs. THEIRs?
     
  4. Jun 18, 2010 #3

    berkeman

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    Staff: Mentor

    Unless the LEDs are well matched, their light output versus current will not be identical. So why do you think it would be a good design practice to use one resistor per not-so-well-matched LED?

    BTW, in some LED lighting applications, the LEDs are well matched, and there is only one current going through a series string of the LEDs...
     
  5. Jun 19, 2010 #4
    If you have several led's in parallel, one might get hot and begin to hog current. In that case it would get hotter, and the hogging would get worse. That's why high-current devices are needed - if we could safely leave diodes in parallel we would.
     
  6. Jun 19, 2010 #5
    Here is what I have already

    Nk25J.png

    Powered by a 4.8V battery pack, using 6 Cree XP-G LED's.
     
  7. Jun 19, 2010 #6

    vk6kro

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    Science Advisor

    The data sheet for these LEDs shows a possible turn-on voltage range of 3.3 to 3.75 volts.

    If you did have such a variety, the one with the lowest voltage would turn on first and hold the voltage output of the resistor to about that value. The voltage will rise as that LED will draw a lot of current and this would light up some of the other LEDs but to a much lower brightness. So, you could get a wide variation in brightness.

    If you did connect them all in parallel, and they happened to be perfectly matched, they would all get about 200 mA with that 1.2 ohm resistor. Quite safe, but probably not bright enough.

    If you want uniform brightness from your LEDs, you could try individual 1.8 ohm, 5 watt resistors with one of these in series with each LED. It doesn't matter which way around the LED and the resistor are, as long as they are in series.

    These LEDs are normally driven from a current regulator to get constant current for each LED, but this is difficult in your case because of the low battery voltage. So, the resistors are a bit of a compromise.

    Don't charge the battery with the LEDs connected.
     
  8. Jun 19, 2010 #7
    Well I want the the LED's to function as bright as possible, without damaging them. So would in series be better than in parallel like I have them now? Also how can I find out what type of resistor I need without using these online calculators?


    Thank you/
     
  9. Jun 20, 2010 #8
    Hi Llama77
    I think that vk6kro has anticipated your needs and designed the circuit which will give the best sustainable output for your led's.
    To learn to make the decisions he has made on your behalf is a matter of some effort, and the labour of the explanation that would required to justify his design is large.
    I concur with vk6kro's design and recommend that you adopt it.
     
  10. Jun 20, 2010 #9
    Resisters also have a wattage rating. So I would think that design considerations might include many resistors in place of one for wattage considerations as well as price. The larger wattage ones might be considerably more expensive than several lower wattage ones in series. I don't know for sure..but wattage rating is something to consider when selecting resistors not just their efficiency(by efficiency I mean within the 5% or 10% or so rated ohms.. the gold or silver stripe if you know what I mean) or Ohms rating.
     
  11. Jun 20, 2010 #10

    vk6kro

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    Science Advisor

    Well I want the the LED's to function as bright as possible, without damaging them. So would in series be better than in parallel like I have them now? Also how can I find out what type of resistor I need without using these online calculators?

    LEDs have a turn-on voltage which is a minimum voltage that the LED needs before it will turn on. If you stack LEDs in series, the voltages needed to turn them on add up.

    So, if you have two LEDs that both need 3.5 volts to turn them on, you will need 7 volts to turn them on in series.

    You only have 4.8 volts, so you can't put them in series or none of them will turn on.

    Calculation:
    As a first step, have your 4.8 volt battery and a 3.3 volt LED. The difference between these voltages will appear across the series resistor. So, it will have 1.5 volts across it. 4.8 - 3.3.

    For a current of 0.5 amps, this resistor should have a value of 1.5 volts / 0.5 amps or 3.0 ohms.
    At this current, the voltage across the LED will rise to 3.4 volts, so the resistor must have a value of 1.4 volts / 0.5 amps or 2.8 ohms.

    Your battery will rise to 1.35 volts per cell or 5.4 volts when fully charged, assuming NiCd or NiMH.
    So the resistor would then be (5.4 - 3.4) / 0.5 or 4 ohms.

    For a current of 1 amp, the calculation is similar and the voltage across the LED is now 3.5 volts:
    So the resistor would then be (5.4 - 3.5) / 1.0 or 1.9 ohms.
    1.8 ohms is near enough. Don't go any smaller if you can't find 1.8 ohms.

    5 watt resistors are not as cheap as 1/2 watt ones, but much cheaper than high powered LEDs. The power is actually about 2 watts, but it will run hot, so a bigger rating would be better.

    I would try one LED with a 3.3 ohm resistor, for a start.
     
  12. Jun 20, 2010 #11
    This answer puts me to shame; and shows the proper spirit.
     
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