Many LEDs on portable power

Discussion in 'General Electronics Chat' started by Medowin, Jun 23, 2010.

  1. Medowin

    Thread Starter New Member

    Jun 23, 2010
    16
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    Hello!

    I am a complete noob to electrical circuits and I am in need of some practical advice.

    I am currently interested in making a craft project that involves a prop sword illuminated with LEDs. The sword will have roughly 120 LEDs embedded into it and I'm not quite certain the best route to hooking them up. Here are the stats to the LEDs I'm using (all identical):

    Size (mm) : 5mm
    forward Voltage: 2.2v
    Lens Color : Water Clear
    Reverse Current (uA) : <=30
    Life Rating : 100,000 Hours
    Viewing Angle : 180 Degrees
    Absolute Maximum Ratings (Ta=25°C)
    Max Power Dissipation : 80mw
    Max Continuous Forward Current : 24mA
    Max Peak Forward Current : 75mA
    Reverse Voltage : 5~6V
    Lead Soldering Temperature : 240°C (<5Sec)
    Operating Temperature Range : -25°C ~ +85°C
    Preservative Temperature Range : -30°C ~ +100°C



    Here are my concerns:

    *Since the sword prop is held in hand, it needs a portable power source such as a typical 12v battery or something similar. It needs to be illuminated for roughly 12 hours. I don't mind changing out batteries from time to time but it needs to be somewhat practical. What would be an easy solution?

    *From what I've read on this forum running LEDs in parallel is "bad", but I don't see how else I could hook this many LEDs up in a handheld battery power source. Is it do-able to run them all in a single parallel, even if it's not optimal? Also, the LEDs will all be located on a single long column, so I'm not free to circuit things just anywhere or in any shape.

    *If it is indeed possible to run them all in a parallel, would it only require one resistor? If more than one should be added, in what quantity and in what locations should they be placed?

    *Is it unacceptable to solder a wire to the battery itself? I'm unsure of how else to hook them up to the battery, outside of just taping the wire to it with electrical tape. :p

    *Is it possible to string together numerous batteries for increased voltage? Such as putting together, say, 6 12v batteries to make a 72v power source? If so, are there any limitations to this?


    Any help would be greatly appreciated. If possible please keep it simple since I'm just a lowly beginner. Thanks! :)
     
    Last edited: Jun 24, 2010
  2. cjdelphi

    New Member

    Mar 26, 2009
    272
    2
    if it's a plastic sword and clearish, you'd only need about 3 - 4 of them led's running on a small lithium battery.... they're bright enough for something like this surely? what exactly do you have in mind?
     
  3. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
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    Ok, this may be a bit of a stretch. But what I would consider, is a clear sword, and just 2 or 4 leds at the base, lighting it up...ala lightsaber style.

    Also, about your series/parallel problem, you can run wires how ever you want, so even if you want to run 10 strings of 12 LEDs, you can. You run 2 wires from the battery pack to the location where the 12 LED string starts, light those 12, then run 2 more wires from the battery pack to the next string of 12 LEDs.
    You can have them all in a straight line..or whatever shape you want.

    If the clear blade wont do, you can also try EL (Electro-Luminescent) Wire. It looks like neon, and can run for many hours on 2 AA batteries. use 2 C or D batteries and you should be covered for the 12 hours.

    Take a look here: but do google around for good deals.

    http://www.elwire.com/

    [ed]
    cjdelphi,
    Ahh, great minds.
    [/ed]
     
    Last edited: Jun 23, 2010
  4. Medowin

    Thread Starter New Member

    Jun 23, 2010
    16
    0
    The sword is about 36" long and is in a katana-type shape. The LEDs will shine outward toward the opposing edge horizontally, so they have to stretch up the side and encompass the whole length. I'm using wide viewing angle LEDs but it still requires well over 100.

    It has to shine outward horizontally so it's a bit different than a lightsaber-esque deal. I've dealt a bit with EL wire before and unfortunately it's not nearly bright enough for this project. :(

    But you say that I can run it in 10 series of 12 LEDs and parallel them together? What kind of power would I need for that and what would the diagram look like?
     
  5. cjdelphi

    New Member

    Mar 26, 2009
    272
    2
    I have to question the amount of work involved vs cost, 2 high powered direct drive LED's one at each end of the sword might be a better option providing you are able to heat sink them.. otherwise 100 led's is probably the best way to go..

    hope you do really nice solder joints or they're going to break with stress over time with waving it around, space is your biggest enemy, 100 leds and all them wires might get in the way more of two more powerful leds.

    http://www.dealextreme.com/products.dx/category.917

    [​IMG]

    http://www1.dealextreme.com/productimages/sku_11022_1_small.jpg

    a couple of them for $5 each is roughly going to give you the same light as 66 LED's producing around 3 lumens each... more cost effective too, you could spend $10 on 2 Q4 emitters or a bit more on a Q5 or higher, you'll end up with 400 odd lumens at both ends like more brightness than 150 leds...
     
  6. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
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    This is what it could look like:
    This is 120 LEDs in an 8 x 15 layout. It is a parallel/series circuit.

    [​IMG]



    Here is the quick wizard I used:
    http://led.linear1.org/led.wiz

    I used 24v for your supply voltage
    3v for LED forward voltage
    20ma for current
    120 for # of LEDs
     
  7. Markd77

    Senior Member

    Sep 7, 2009
    2,803
    594
    Someone needs to have a word with the wizard. SgtWookie has pointed out and it makes sense that 1 ohm resistors are too low. There is only 0.024V across the resistors if my brain is working right and 24mA was used.
    That isn't enough to balance the currents.
    Probably best to tweak the values until he finds a solution of 100 ohms or more so the resistor drops at least a couple of volts.
     
  8. retched

    AAC Fanatic!

    Dec 5, 2009
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    Holy crap-shoop batman. I didn't even realize that the wizard used 1 ohmers.

    Jeez. Actually, I was more trying to show the OP how to series/parallel a circuit, but I am glad you caught that.

    But now I can see why.. At 24v supply, eight 3v LEDs IS 24v. So after a few moments under battery power, you wont have the forward voltage left to lite the thing.
     
  9. Medowin

    Thread Starter New Member

    Jun 23, 2010
    16
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    Hmm...

    So a 24 volt power source is best with 12 parallels each with a series of 8 LEDs? If 1 ohm resistors aren't best then what should I use?

    Also, can you make a 24v power source just by hooking up two 12v batteries together vertically?
     
  10. Wendy

    Moderator

    Mar 24, 2008
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  11. Markd77

    Senior Member

    Sep 7, 2009
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    Really anything over 9V should be fine. Wiring one of the + terminals to the - terminal on the other gives you 24V with 12V batteries.
    Look for batteries with high mAh ratings or use alkalines.
    You need to know the typical (and preferably min and max) forward voltage of the LEDs (Vf) and then use the wizard. If he gives you resistor values less than 100 ohms with 24V or 50 ohms with 12V then tweak the values a bit until he doesent.
     
  12. Medowin

    Thread Starter New Member

    Jun 23, 2010
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    Okay, let me get this straight since I've never soldered series into parallels before. Would a diagram of how it's wired look something like this?

    [​IMG]

    Basically, a series of 8 LEDs with 8 series total, strung together by connecting + and - leads, one resister per series, and having wire touch each positive then loop around to touch each negative. Powered by two 12v batteries touching each other + to - . Is this close to being correct?
     
  13. retched

    AAC Fanatic!

    Dec 5, 2009
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    Almost. The very last loop of wire on the bottom right shouldnt be there.

    You are connecting the + wire to the - wire. Thats called a dead short.

    It should be like this:

    [​IMG]
     
  14. Medowin

    Thread Starter New Member

    Jun 23, 2010
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    Oh, I see! I guess that does make more sense.


    Am I correct in that there should be just one resistor per series of 8 LEDs?
     
  15. retched

    AAC Fanatic!

    Dec 5, 2009
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    Yes, that is correct in this case.

    Have you found out the forward voltage for these LEDs?
     
  16. Medowin

    Thread Starter New Member

    Jun 23, 2010
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    Apparently the forward voltage for the LEDs are 2.2v.


    What kind of resistors should I use for them?
     
  17. retched

    AAC Fanatic!

    Dec 5, 2009
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    If you are using 24vDC for you supply

    and

    8 LEDs with 2.2v forward voltage in each string and running them at 20ma,

    8LEDs x 2.2v = 17.6v of forward voltage drop per string

    so

    17.6v - 24v supply = 6.4v

    then with 20ma of current, Using OHMS LAW, Resistance=Voltage*Amperage

    so

    6.4v x .020A = 128ohms

    So using 128 ohms of resistance will give you 20ma of current for the LEDs
    BUT
    128ohm is not a standard resistor value
    So using a link from SgtWookie to see standard values:
    http://www.logwell.com/tech/components/resistor_values.html

    We see from the 'E24' column that 130ohms IS a standard value..close enough.
     
  18. coldpenguin

    Active Member

    Apr 18, 2010
    165
    9
    What are you using the sword for (you mention it being a prop).

    Thinking outside of the box, if it were for the stage, then I would possibly consider UV reactive paint and a UV light. It might be easier in the long run (but I can't think of any stage acts which take 12 hours! [just some feel like it])
     
  19. Audioguru

    New Member

    Dec 20, 2007
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    896
    120 2.2V LEDs drawing 20mA need a battery that produces 5.3W. It will be hot and will need ventillation holes and a fan. For the thing to light for 12 hours then you will need a wagon to carry the huge battery (slightly exaggerated).
     
  20. retched

    AAC Fanatic!

    Dec 5, 2009
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    That is true.

    You might want to run the LEDs at 10ma.

    It will require some testing to determine led brightness -vs- batterylife.

    You may also need to pwm them in order to reduce the power consumption.
     
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