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LED and Resistor Circuit WiringWhich resistors should i use?

  1. Feb 16, 2009 #1
    i have 20 ultrabright white LED's and i want to get a good brightness out of them. they will be in 2 circuits of 10 leds each. i think i need 1 resistor per led but im not sure which resistors to use. The LED's are 3.2 - 3.8V and 30mA. i plan to use 9v batteries in whatever combination is best for each circuit. Could someone tell me which resistors i should use? thanks
     
    Last edited: Feb 16, 2009
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  3. Feb 17, 2009 #2
    Basically im trying to make a circuit with 10 white LED's and any number of 9v or similar batteries. Iv heard i need 1 resistor per led but not sure whether to use 330ohm or 470ohm resistors (or different). The LED's are 3.2 - 3.8V and 30mA.
     
  4. Feb 17, 2009 #3

    Redbelly98

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    Are you thinking of the using the basic LED+resistor circuit?

    In your case it's 9V, not 3V, but otherwise:
    circuit_LED_basic.gif

    You can calculate the resistance in 3 steps:

    1. Assume the LED voltage is 3.5 V, and the power supply is 9V (not 3V). What's the voltage across the resistor?

    2. Assume the LED current is 30 mA. What's the current through the resistor?

    3. To have the voltage and current calculated in the first 2 questions, what's the resistance of the resistor?
     
  5. Feb 17, 2009 #4
    So the Voltage Across the resistor would be 5.5V, and the current through the resistor should be the same as the LED - 30mA. ohms law then says the resistance should be 5.5/0.030 = 183.3 ohms. so with 10x LEDs and a 9V battery i would have 10x 180ohm resistors, one between each LED all in series? or would that mean i need a power source greater than 35V? thanks
     
  6. Feb 17, 2009 #5

    mgb_phys

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    Firstly you probably don't want to use a 9v battery (PP3) if you can avoid it, they don;t store much energy for the price.
    You can arrange the LEDs in groups so that they are as close to the supply voltage as possible, which reduces the need for resistors and wasted energy.
    So with 3 x 1.2V rechargeable (NiCad/NiMh) batteries you would be able to drive an LED directly - so connecting all the LEDs in parallel would work without any resistors.
     
  7. Feb 17, 2009 #6
    I need to make a 'strip' with 10 leds about 15cm apart in a line. So if i use 3 x 1.2V rechargeable (NiCad/NiMh) batteries and wire all the LED's in parallel i shouldnt need resistors? im not sure what you mean about 'arrange the LEDs in groups so that they are as close to the supply voltage', do you mean by making them closer i will use less wire therefore less resistance? thanks
     
  8. Feb 17, 2009 #7

    Redbelly98

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    I would recommend against connected LED's directly to batteries. What happens as the NiMH voltage changes from 3x1.4V fully charged to 3x1.1V discharged? Better to use more batteries, perhaps 6, to get safely above the LED voltage. Then choose a suitable resistor.

    That's right. 170 ohms is a standard resistor size and would probably work, or you could try 200 ohms if you really don't want to exceed 30 mA.

    There are several options. mgb_phys is correct that a 9V battery is not the best power source for 10 LED's. At 30 mA, a 9V battery would run for 15-20 hours if running a single LED continuously, or 1.5 to 2 hours if running 10 LED's continuously.

    You could use 5 9V batteries, each running a series combination of 2 LED's and 1 resistor. The batteries would last 15 to 20 hours if the LED's are lit continuously.

    We probably have to know more about your application to give you the best information. Does it have to be battery-powered?
     
  9. Feb 17, 2009 #8
    ok it is for an instaltion i am doing involving a semi transparent tunnel 1.5m long which people will crawl through. i want to light the tunnel along the bottom edges either side (hence 2 strips of 10 led's) it dosent need to be extremely bright as it is meant to be somewhat scary. i thnk it would be much easier with batteries and a switch as using mains will complicate things and im not sure how available a mains socket will be.
     
  10. Feb 17, 2009 #9

    mgb_phys

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    NiMh drop from 1.4 to 1.2 volts in 2-5% of their capacity being used so I wouldn't worry too much about blowing the LEDs In a battery powered application I wouldn't want to waste energy in a current limiting resistor, although you could put a small limiting resistor just in case of a short if you wanted.
    With 3AAx2100MhA batteries and 10x30mA LEDs you would get around 6-7hours. You can also buy battery packs for remote control cars which contain multiple cells and have 5-10x as much capacity at this voltage.

    Another advantage of wiring in parallel is that of you have a bad joint you can easily see which LED is bad rather than having to check everything.

    You can buy LED strips which although less bright than a 3.6V ultrabirghts, have a led every inch or so stuck onto a tape. You just cut the strip to length and connect to the end.
     
  11. Feb 17, 2009 #10
    thanks, i have looked into led strips but they work out far more expensive than if i wire it myself.
    Not sure what you mean by 'I wouldn't want to waste energy in a current limiting resistor, although you could put a small limiting resistor just in case of a short if you wanted.' what sort of size resistor do you mean and where would i put it? would the resistor just be there to stop the led's blowing? are you saying that if i use 3 AA rechargeable 1.2v batteries and a small limting resistor, with the LED's in parallel the circuit should work fine for about 7 hours? this could be the solution i need. thanks
     
  12. Feb 17, 2009 #11

    mgb_phys

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    Suppose you used a 7.2V battery back and a current limiting resistor to drop it to 3.6V for the LEDs - then you are dissipating the same power in the resistor as in the LEDs = wasting half the energy.
    When the NiMh are initially charge they have a voltage of nearer 1.4V as Redbelly said, this means that your LEDs are being overpowered initially. Although the voltage drops pretty quickly to 1.2V as you take power out of them.

    You could put a 1 - 2R resistor in series with the battery to drop this to 1.2V, also in the event of a short circuit it would reduce the current drawn from the battery (although you would probably still damage the cells). If there is any reason to suspect you could have a short (moisture, rain etc) you would also fit a fuse, a simple in-line 1Amp fuse from a car parts store would do.
     
  13. Feb 17, 2009 #12

    Redbelly98

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    How do you propose to keep the LED current near 30 mA?

    By the way, here is another circuit ... more complicated than just using a single resistor, but you can run all LED's in series, provided the voltage supply is sufficient. In my experience, the supply voltage must be at least 0.8V more than the total voltages of all the LED's.

    LEDdrive_2-transistors-01.gif
    I've used this circuit to run 2 white LED's for 20 hours at 20 mA on a 9V battery, and have 2 home-made flashlights that my wife and I use regularly.
    For transistors, I use 2N3906 (real cheap).
    For resistor R1, you would need
    0.66V / 0.030A = 22 ohms, a standard size​
    The supply would have to be more than
    0.8V + 10*3.8V = 39V​
    to run 10 LED's, which can be done with five 9V batteries in series.
     
  14. Feb 17, 2009 #13

    mgb_phys

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    If you are only supplying them at Vfor I wouldn't have thought they would take more than their design Ifor.
    But I think you're right it would be safer and easier to use 4x batteries and a limiting resistor.

    I thought of using a LM317 as a current source, but it really only works if you have the LEDs in series and 40V is a bit inconvenient to generate with batteries (as well as getting close to LVD rules). If you use a current source with banks of LEDs in parallel you risk blowing all the others if a single bank fails.

    App note here http://www.onsemi.com/pub_link/Collateral/AND8109-D.PDF
     
  15. Feb 17, 2009 #14

    ok i think this will work so i need 3 1.2volt aa batteries and a 2ohm resistor put in just after the switch in series, with all the LED's in parrallel?
     
  16. Feb 17, 2009 #15

    mgb_phys

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    Or better, use 4x 1.2V batteries (they are sold in 4s and the holders will take 4)
    Connect 10LEDs in paralell and use the circuit Redbelly suggested in post #2.

    This does assume that the forward voltage for each LED is exactly the same, otherwise some will glow brighter than others - in practice mass produced LEDs form the same source are very similair, but you probably don't want to mix different manufacturers parts in a strip.

    Remember that you have 300mA flowing through the resistor so you want to use 1W watt resistor not a tiny 1/8W part.
     
  17. Feb 17, 2009 #16
    thats great thanks very much for the help guys!
     
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