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Some noob questions on wiring LEDs, current, etc.

  1. Jun 13, 2012 #1
    I'm making a wearable project that involves me wiring up about 16 LEDs into a piece of clothing. Unfortunately, it's been many a year since I took a physics course and I'm a little rusty on things. Here's my situation:

    The LEDs drop about 2V each, and are meant to be used with a current of around 20 milliamps. I want to use a 9v battery as a power supply if possible.

    I used a calculator online (can't link to it because of low post count!) and it showed that I can do this by wiring a circuit that has 4 branches connected in parallel, with each branch containing 4 LEDs wired in series and a 56 ohm resistor.

    Assuming this is correct and won't explode me or anything, I have a few (likely ridiculous) questions:

    1. I don't want to short out my battery, or turn myself into a fire hazard. What's the maximum amount of current I can safely be drawing?

    2. When I've got things connected in parallel to the battery, how do I calculate how much current is drawn by this whole circuit? Do I add together the amount of current that each branch would draw? (And, er, how do I calculate that? I know V=IR, but I don't know how to get the numbers to plug into the equation...?)

    3. I'm thinking my battery life might be pretty cruddy. To improve it should I reduce the number of LEDs hooked up to the battery? Is there a better power source to use that's still portable?

    Any assistance would be greatly appreciated. Thank you.
  2. jcsd
  3. Jun 13, 2012 #2

    Simon Bridge

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    You usually want to buffer a typical LED with a small resistor ... but you won't get anything hot enough to even feel warm at the currents you are contemplating. LEDs can usually be driven right off the battery without shorting it.

    When everything is in parallel - you use the 1/R = 1/R1 + 1/R2 + ... 1/R16 formula to get the total resistance and V=IR to get the total current drawn from the battery. I/16 will be the current through each LED.

    A typical 9V battery should last a fair length of time - I remember months of use for the LED displays I used to build as a kid. You may even get away with a stack of watch or camera batteries for a lower profile.

    The components you want to use are very cheap so you can experiment for the exact setup.
  4. Jun 13, 2012 #3


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    If you had 4 parallel strings which each had 4 LEDs and a 56 ohm resistor, you can work out the current as follows:

    There are 2 volts across each LED so there are 8 volts across 4 of them.
    This leaves 1 volt across the resistor, if the supply is exactly 9 volts.

    So the current in the resistor is (1 volt divided by 56 ohms) or 17.8 mA. This is Ohms Law.

    There are 4 strings of LEDs so the total current is (4 times 17.8 mA) or 71.4 mA.

    This is a lot of current for a 9 volt battery, though, and you might like to purchase a number of resistors of higher value than 56 ohms to find out how little current you can get away with.

    You could try 100 ohms and 220 ohms for a start.

    Leds vary greatly in efficiency, so you may be able to reduce the current in a high brightness LED more than in a less efficient one. High brightness LEDs are more expensive, but you tend to get what you pay for.
    You can compare brightnesses by the mcd (milli candela) figure given in the LED data sheets.
  5. Jun 13, 2012 #4
    That clears it up! Thank you so much! :)

    I've got a bunch of different resistors, so I'll definitely play around with that and see what works.
  6. Sep 6, 2012 #5
    Hi, i'm new here and have a similar question. Greatly appreciate anyone's input and help! here's the situation:
    I am building a LED hood for my aquarium. I intend to use 22 3 watt bulbs. I am wondering what sort of power supply i will need. It seems they will not need more than 80 watt power supply. I have one laying around that's 15V 3amp (does that mean it can only supply 45 watts?) . I'm not sure if 15V 3Amp is enough... i think not... for the amount of LED's i want to setup. I'm thinking of running 4 in series with a resistor (i have not calculated which one i will need yet). and so here's a little more info:

    series 1= (4) 3watt LED's ~3.6V each (WHITE)
    series 2= (4) 3watt LED's ~3.6V each (WHITE)
    series 3= (4) 3watt LED's ~3.6V each (WHITE)
    series 4= (4) 3watt LED's ~3.6V each (BLUEs)
    series 5= (6) 3watt LED's ~2.4V each (REDs)

    the website shows that each bulb is 700mAh (seems pretty strong, see attached JPG). Is that a minimum draw of current or a maximum possibly? i'm not sure what to make of that and so i need to know that in order to calculate the resistors i need, right?

    basically, i would like those bulbs and that many of them above my tank. Would my 15V 3Amp power supply work with that? if not, most likely, what watt/amp/volt psu can anyone suggest i buy that would be sufficient?

    I also intend to install 2 or 4 80 or 92mm fans for cooling.
    Thanks again in advance!

    Attached Files:

  7. Sep 7, 2012 #6


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    Hi rusk1y. Welcome to Physics Forums. It is better to start a new thread for your own question.

    A few preliminary questions:
    Is your 15V supply a regulated DC supply, and is the voltage adjustable? (It wouldn't happen to be settable as a constant current, by any chance?)
    Your LEDs are mounted on individual heatsinks, are they?
    700mA would be the maximum current. Are you sure you want that level of brightness? Is there a likelihood that you will later want to reduce the current to the LEDs because they are too bright, or maybe too bright at night? You probably wouldn't notice much difference in brightness between 700mA and 500mA. Might it be worth having variable brightness?
    How many LEDs of each colour do you have? Might you settle for less current (i.e., reduced brightness) for some colours than in others?

    Do you have a meter to measure current and voltage? Preferably a pair of meters.
    Last edited: Sep 7, 2012
  8. Sep 7, 2012 #7
    Sorry, NascentOxygen. I should have thought about starting new thread. my bad.

    the 15V 3Amp power supply comes directly off a toshiba laptop. i'm assuming that laptops require constant current too, this power supply should be?

    I have not purchased heat sinks yet but yes, they will be on individual heat sinks unless i find something that's about 30inches in length to use. (for now i just testing two LED's for short intervals).

    I am open to suggestions :) 500mAh sounds good too. It's going to be a aquarium hood for planted tank which will require a good amount of light. (more lumens, the better i hear).
    i have not purchased any LED's but i intend them to be all of the same source and type. generally, most will be 6500k whites with about 4 blue and 6-8 red's. All 3w. if i could run 4 blue as a night light that would be cool, but not a priority.

    That's the thing i want to learn more into about LED lighting. so it's the current that controls the brightness and the voltage is heat? or how would i calculate this? i can possibly draw it out and post a picture, but i would have to do that later.

    From my previous post, i am shooting to use 12 white bulbs, 4 blue, and 6 red. (give or take in respective proportion). My earlier post has the LED info attached. I want to learn or get some examples of how i can run the LEDs (how many in series, how many parallel threads, resistors needed and how that is calculated). I think i would basically need to get a stronger power supply that can support over 66 watts? correct?

    btw, i do have a meter for current and voltage, but it seems to be malfunctioning or i am not using it correctly :) (my guess is user error).
  9. Sep 7, 2012 #8


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    It should be usable, but it's anyone's guess exactly what voltage it gives. You'll have to measure it.
    Is that an aquarium containing fish and water plants? You will need to be mindful of the amount of heat in the LED assembly. Powering down to just 4 at night would minimize the night-time heating of the water surface, and plants floating on the surface.
    The voltage across a LED stays roughly fixed, and the more current it's given the brighter it glows.

    There are a number of ways the LEDs can be powered. If you needed to throw together a demo this very minute, you could connect 3 whites in series (3x3.6=10.8V) with a current-limiting resistor and connect across 15V DC. If we shoot for 500mA, then the resistor value will need to be (15-10.8)/0.500 = 8.4Ω. The nearest preferred value is 8.2Ω, and based on I².R go for a 5W resistor, or maybe 10W so it stays even cooler.

    But there are better ways.

    You will definitely need a working meter.
  10. Sep 7, 2012 #9


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    I'll suggest that you consider using a constant current driver. The FlexBlock module here should do the job, it's a high efficiency switching driver adjustable up to 700 mA: http://www.ledsupply.com/led-drivers.php

    One of them should be able to power all 12 whites, and a second one the other 10.
  11. Sep 7, 2012 #10
    I tested the 15V3Amp power supply. Reads a clean 14.99 Voltage... didn't spike on me and seems like a constant current to me :)
    Yes, the tank has fish and live plants. The light will not be on all day. It will only be used for 8 hours a day or so (probably at 4 hour intervals with 1-2 hours down to cool like i am with my fluorescent light now).
    In the equation so i understand this; the 10.8 is the voltage of the LED in series total; the .500 is the Desired amperage? With that being said, if i am using a couple parallel lines and in each line run however man series of LED's, do you calculate the current that it will draw (based on the resistor used of course) by adding the mA of each line in series (per led or per series?) to know if the power supply can supply the current? correct me if i'm wrong :)
    Can you explain this a little more to me please?
  12. Sep 8, 2012 #11
    The "DIY" section of the Aquarium Advice forum has quite a few existing threads on LED lighting.

    I've been toying with the idea of building a pseudo-real-time lighting system for mine. (Varying brightness to match time of day.)
  13. Sep 8, 2012 #12
    Thanks, i'm checking them out now :)
    But still open to info here. I have questions that need answering :)
  14. Sep 8, 2012 #13


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    You add the currents in each line to find the total.
    Once you decide on the current and resistor, you have to decide what power rating the resistor should have. Resistors that carry a lot of current get hotter, so you buy them in a large size with a large surface area to dissipate the heat. The power each dissipates is calculated as I².R

    This heat represents wasted electrical power. A switching driver doesn't waste much power, so it's more efficient.
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