Resistor and Circuit help

Discussion in 'The Projects Forum' started by Moose621, Nov 19, 2010.

  1. Moose621

    Thread Starter New Member

    Nov 19, 2010
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    Ok, so I'm pretty clueless when it comes to wiring/circuits and whatnot. I'm trying to make an LED hula hoop and I got all the supplies except for the resistors. I've used the resistor calculator but I don't know if I'm using a series or parallel circuit and how many LEDs to put that I am using since i'm using all different colors with different voltages. I'll give you some background of my supplies:

    3.7V Li-ion 18650 battery 2200mah
    22ga speaker wire
    LED's in the hoop:
    *4 red - 2V
    *4 orange - 2.2V
    *4 yellow - 2.2V
    *4 green - 3.2V
    *4 blue - 3.2V
    *4 UV - 3.2V
    *4 pink - 3.5V

    -These LEDs are all 5mm and the current should be 25mA. (I'm not sure if that's the requred current but that was he example given in the chart off the ebay store I bought them from). Here is a link to the page that shows the technical details of each color LED this seller has: http://cgi.ebay.com/100-x-5mm-round-Red-LED-superbright-Lamp-Bulb-light-/120498209017?pt=LH_DefaultDomain_0&hash=item1c0e40c0f9 (just for reference if anyone wants it - scroll about 1/3 of the way down to see the table).

    I have done HEAVY research on how to make a hula hoop but nobody that has given tutorials on how to has explained the basics on how to make a circuit. I've seen some use resistors, some not use them. I am assuming I am using a series circuit, because the wire goes from one LED to the next until it all wraps back around the battery. But, how do I figure out the resistors?

    Thank you so much for any and all help :)
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,153
    3,058
    If your only power is the 3.7v battery, then you'll have to wire them all in parallel, which has an advantage anyway, see below. In series, you might be able to light 2 at once but no more than that. They need about 1.6v to begin showing any light at all, and ~2v to get bright. And not much more than that to burn out. The advantage of series wiring - if you have the voltage to supply it - is that controlling current thru the loop controls ALL the LEDs at once. And I'd aim for half the spec first, and only go up carefully, to avoid roasting your LEDs.

    The advantages of parallel wiring - besides being the only choice for low voltage - are that 1) all lights will stay lit even if one blows, and 2) the resistor can be tweaked for each LED. Unfortunately you need a resistor for each LED, but they're cheap.

    The resistors you'll need will be in the range of 100Ω or so. Start at 300 and move down. Resistors of 470Ω are often supplied "free" along with LEDs, but that's for 12v systems.

    Are you hoping to light all 28 LEDs at once? Even at 10mA each, that's 280mA and the battery won't last real long before the lights go dim.
     
    Last edited: Nov 19, 2010
  3. iONic

    AAC Fanatic!

    Nov 16, 2007
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    Just a note of caution about the LED's you have bought or are considering buying. Although the price is attractive they will may not be as nice as you might hope for. I have purchased some on ebay before just like this. After about 6 - 8 months the leads begin to corrode, so be carfull.
     
  4. Moose621

    Thread Starter New Member

    Nov 19, 2010
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    0
    Thank you very much for the information, both of you. I hope these LEDs don't corrode....for future reference, what are some good sources to buy LEDs from?

    And yes I am hoping to light all 28 at once. I bought an LED hula hoop a few months back with the same exact battery..32 LEDs and half are slow/fast blinking RGBs. It lasts probably 2-3 hours on a charge and starts to get dim. I would be happy with getting that much use out of one charge. However, I am planning to sell them once I get the hang of it so I would prefer a range of say, 4-8 hours. But I'm figuring on learning as I go. As long as I get a good foundation and have a successful first attempt, I'll be happy.

    Two more questions....
    (1) Does a parallel circuit use much more wire than a series? I have a 75' roll of 2-conductor wire. A tutorial I found uses this same wire and about 12' or less of it for one hoop. Am I looking at using more with a parallel?
    (2) With the specs I gave, what would be a good range of mA to use? I am assuming that, as you said wayneh, using 280 mA total will add up until it hits the 2200mAh of my battery and then it's dead?

    I am so sorry for my lack of knowledge! Thank you so so much for the help!
     
  5. wayneh

    Expert

    Sep 9, 2010
    12,153
    3,058
    I just found the brightest and cheapest I could find on e-bay, and so far I'm perfectly happy. But mine are all in dry applications. I've had several landscape lights fail due to corroded LED leads. The OEM ones, not my e-bay LEDs.
    You may want to play with the current and brightness you pick. If you use a bigger value resistor, the LEDs won't be as bright but you'll get a bit more life. On the other hand, I think the efficiency will go down overall, since more of the battery's energy is wasted in the resistor.
    Not really. Just imagine a loop of 2 conductors going all the way around, with LEDs attached every ~13°, each with its resistor
    Unfortunately the LEDs will be far too dim long before the full capacity is drained, and you don't really want to drain it fully anyway. So you really only get maybe half of the rating.

    You might want to consider something a little more sophisticated in the circuitry, to simplify assembly. I have a few 50-LED strings of Christmas lights that are powered by a 4v battery that is charged by a solar panel. There is no resistor with each LED, because the circuitry in this thing somehow regulates it all, probably by a high frequency PWM that allows the string to stay lit even as the battery fades. If you had such a circuit, you wouldn't need all those resistors and you'd get more life. Maybe a version 2.0.
     
  6. Moose621

    Thread Starter New Member

    Nov 19, 2010
    3
    0
    Ok, I would rather sacrifice the life for brightness. I was worried that the value of the resistor needs to be exactly as calculated or I would ruin everything, so it's good to know I can experiment a little bit.

    And THANK YOU for explaining the parallel vs. series!! I didn't realize it was that simple, and that was how I was intending to make it anyways based off my tutorial, they just didn't refer to it as such. I'm glad the option I familiarized myself with will work out best.

    I will definitely be looking into this circuitry you speak of for Christmas lights. I would much rather not have to worry about resistors, that's for sure.


    I think I'm finally ready to start my project! Thanks again for clearing up my questions :)[/QUOTE]
     
  7. Audioguru

    New Member

    Dec 20, 2007
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    I bet the cheap LEDs have a very narrow beam angle like in the photo, not 40 degrees.
    The beam is brighter when it is focussed into a narrow beam.
    Then they will not be visible most of the time.
     
  8. iONic

    AAC Fanatic!

    Nov 16, 2007
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    500mA/18mA = ~28 LED in parallel and no resistors required. (via ledsupply.com) Micropuck operating on (2) AA NiMH 2500mAH
    Wide angle LED's will be better.
    Wiring in parallel in a hoop will require alnost 2x the wire.
    With resistors and the 3.7V battery you will dissipate 1/5 of the total power to the resistors.
     
    Last edited: Nov 21, 2010
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