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Thread: Resistor values

  1. #1

    Default Resistor values

    Such good stuff that it needs to be archived here for easy searches.

    From: DaveH, 8-22-00
    <BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>For parallel hookup the ohm calculation works out this way,
    (voltage source - LED voltage )/ LED current. White LED's are usually rated at 20 ma, but some products their driven at 50 ma, so lets set the LED current at 30 ma.
    So substituting in, (6 - 1.7)/.03 = 143 ohms more or less. You can probably get 150 ohm resisters at radio shack in a pack of 6 for $1.<HR></BLOCKQUOTE>

    From: Doug P. 7-17-01
    <BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>((Voltage supplied)-(Voltage required)) / Current = Ohms resistance
    (Vs-Vr) / I = Ohms

    so to apply to the above problem

    if Vs = 4.5
    and Vr = 3.6 (single LED or Paralled LEDs)
    and I = .020 Amps

    then you would need a (4.5 - 3.6)/.020 resistor = 45 ohms if you play "by the rules" and keep it in the factory specs for Nichias (if that's what you're using)

    The LED will dim when the battery gets weak no matter what you do. The resistor just "uses up" the extra voltage that would destroy the LED. Essentially the resistor (in the example above) becomes a little tiny .9V heater that you run whenever the LED is on, wasting the .9V as heat but protecting the LED. If you want, you could build a switch into the circuit that jumps the current around the resistor when the LED starts to get dim.<HR></BLOCKQUOTE>

    Thanks to both!

  2. #2
    Flashaholic*
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    Default Re: Resistor values

    I just wanted to say thanks for archiving such useful data. I only wish that there was a reference section on the message boards for places such as formulas run-times, etc. It is very difficult to search through several different message boards to find a formula that you need for a project. Too bad nobody has written a book on LED mods yet. That way someone could compile it all into one handy reference. [img]images/icons/smile.gif[/img]

  3. #3
    Flashaholic* Mr Ted Bear's Avatar
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    Default Re: Resistor values

    Hi all

    What happens if I have a 12.6 volt source? Can I string several leds in series and NOT use a resistor?


    Jeff [img]images/icons/confused.gif[/img]

  4. #4

    Default Re: Resistor values

    <BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Mr Ted Bear:
    Hi all

    What happens if I have a 12.6 volt source? Can I string several leds in series and NOT use a resistor?


    Jeff [img]images/icons/confused.gif[/img]
    <HR></BLOCKQUOTE>

    No need to be confused...

    For LED safety I'd always use a resistor even if it is a real small one.

    All these questions can be answered using ohms law. Voltage = current x resistance. And the sum of the voltages around the curcuit has to be equal to the source voltage.

    So, if you want to use 3 LEDs in series powered by a 12.6V source you first need to decide how much current you want. If you decided you want 20mA then you look up what the voltage across your LED would be at that current (or find out by experimentation). Then add up the voltage drops for each of the 3 LEDs (maybe 3.5V x 3 = 10.5V). Note that your LED voltage may vary and this number may not work for your LED.

    Take your source voltage and subtract the voltage drops across the LEDs (12.6 - 10.5 = 2.1V).

    Now use ohms law. (V/I = R)
    2.1 volts / 0.02 amps = 105 ohms

    Find a close standard resistor value (like 100 ohms) and then test out the circuit.

    Hope that helps.

  5. #5
    Flashaholic* Silviron's Avatar
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    Default Re: Resistor values

    Originally posted on the LED forum- Mr. Gadget recommended I post a copy here too:
    <HR>
    There are people on this forum that are WAY better at math and electronics than I am, but so many people have asked how to calculate resistors for LEDs that I am posting this.

    (If any of the experts find errors, feel free to smack me upside the head and I'll correct them.)

    FORMULA FOR DETERMINING RESISTOR VALUE FOR VOLTAGE DROP WITH LEDS:



    Examples:


    To run 1 Nichia on three 1.5 V batteries it would be:
    4.5 - 3.6 / .02 = 45 ohms That would be the value to run the Nichia at 3.6V at 20 mA as the manufacturer recommends.

    If you want to run 2 Nichias in parallel, at the recommended current it would be a total of 40 mA, so: 4.5-3.6 /.04 = 22.5 ohms

    Since easily acquired resistors (like say from Radio Shack) are rarely available in the values you calculate, the easiest (and safest) thing to do is to use the next higher rating. 47 Ohms is a common value, and very close to the 45 you need for one Nichia on 4.5V.

    If you want to run three SERIES {3.6V X 3 = 10.8V} connected Nichias on 12 V you would need a 60 ohm resistor: {12 -10.8/.02 = 60)


    <HR>

    If you can't find a resistor in the neighborhood of what you need, you can "construct" one using easily found resistors in parallel or series using the following formulae:


    <IMG SRC=http://www.az123.com/LED/resseries.jpg>

    For instance if you need a 20 ohm resistor and can only find some 10 ohm values, just put two in series with each other.


    <IMG SRC=http://www.az123.com/LED/resparall.jpg>


    If you need a 70 ohm resistor and only have 220 ohm resistors available, three of them in parallel will give you 73.333 ohms -close enough.

    For working with "regular" LEDS, including Nichias a 1/4 watt resistor is usually enough (although if you are using 20 or more LEDS, I'd move up to at least 1/2 watt sizes for single or series resistors). If using resistors in parallel their wattage is increased (2 parallel 1/4 watt resistors will handle at least 1/2 watt.) so you don't need to use 1/2 watters in this case.

    If you want to run a single Nichia at 20 mA on 6 Volts you need a 120 ohm resistor:
    (6.0 - 3.6 / .02 = 120)

    If you want to overdrive 2 SERIES Nichias to 30 mA on a 9V battery you would need a 30 ohm resistor (9V - 7.2V /.06) = 30 ohms. (this calculation isn't exact but close enough, I think). Since 30 ohm resistors are hard to find, you can use the more common 33 ohm resistor or use three 100 ohm resistors in parallel.

    If you are using LS modules (Luxeon Star), you will vary the numbers accordingly: they are supposed to run on 3.4 Volts @ 350 mA rather than the 3.6V @ 20 mA for Nichias.

    (I'd also recommend using at least 1/2 watt resistors with an LS module - others may disagree, but I say better safe than sorry.)

    So, to run an LS on 4.5V you would want a 3.15 ohm resistor- (4.5 - 3.4 / .35 = 3.1428..) Since resistors in this range are hard to find, use three 10 ohm resistors in parallel which will give you 3.333 Ohms - close enough.
    { 1 / (1/10 + 1/10 + 1/10) = 3.333}

    To run an LS on 3 NiCd or NiMH rechargable batteries which are nominally 1.2V each you would need a .57 ohm resistor- { 3.6 - 3.4 /.35 = .571 }

    As usual these are hard to find- I'd recommend a stepping up to a 1 ohm resistor anyway for LED safety's sake. Most Radio Shacks don't carry 1 ohm resistors in anything less than 10 watt units and those are too big physically for most conversions. So, you can use six 10 ohm resistors in parallel. {1 / (1/10 + 1/10 + 1/10 + 1/10 + 1/10 + 1/10) = 1.0166).


    <HR>

    Because most easily found resistors have what they call a 5% (sometimes 10%) tolerance you might get an actual value of anywhere from 90 ohms to 110 ohms out of a 100 ohm resistor. So it is always a good idea to go to a higher value in selecting a resistor- better to have a slightly dimmer than optimum light rather than burning out the LED. This is somewhat less critical on lights that will be used only for a couple of minutes at a time with fresh batteries, but for lights that will burn for longer periods at a time, it is very important.

    Also, most fresh batteries will have a higher than nominal voltage, and you should take this into account. A fresh Alkaline may have slightly over 1.6 volts, a fresh Lithium AA can actually have 1.8 V and a freshly charged NiCd or NiMH can have 1.45V initially. Although this over-voltage usually disappears quickly with heavier current draw things like incandescent bulbs, digital cameras etc., When dealing with sensitive solid state components (including LEDs) at a low current draw, this initial over voltage CAN be a disaster. Again, this is less likely on lights that are just used for a moment at a time, but you should be aware of it.

    Just yesterday, I accidentally plugged a colored LED that was supposed to run on 2.1 to 2.4 V into a freshly charged NiCd 3.6 V battery pack (that actually had 4.04 V on it)---- It only took about 2 seconds to completely destroy the LED. It didn't explode as is possible, but it did turn brown, burn my fingers, and stink to high heavens before it burnt out.

    Some multiple LED arrays may require series/parallel arrangements to run most efficiently on the available voltage. The best way to do this is to calculate the resistor value for each series chain and put that value of resistor on each series chain, rather than just putting a single resistor on the whole array.

    HOPE THIS HELPS (and that I didn't make too many errors.)

  6. #6
    Flashaholic* Quickbeam's Avatar
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    Default Re: Resistor values

    I put this together to assist those who have math angst. Just a quick 4.5v and 6.0v resistance calculation graph for Nichia Whites (usually run on 3.6v @ 20mA).

    So, if you had 3 nichia whites you wanted to drive at 40mA each (120mA total) running on 4.5V, just follow the 120mA line up to where it meets the blue line and trace over to the resistance axis. You would need about a 8 ohm resistor. Or for a single LED on 35mA, 4.5V, you need a 25 ohm resistor!

    If the image doesn't show because of Tripod's [img]images/icons/mad.gif[/img] agreements they have with their f#$%&*g advertisers not to allow hotlinking, (see post below) go HERE

    And while your at it, check out AdSubtract so you never need to see their ads again.

    (EDIT) fixed - pic now hosted on az123 - Thanks Silviron!

  7. #7
    Flashaholic* Chris M.'s Avatar
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    Default Re: Resistor values

    Doug- your picture won`t show.

    The reason is that Tripod don`t allow outside Hotlinking. I know, cos I can`t hotlink images form my site any more, I had to find another host. To post images here, try Excite`s Webshots (excite.com, and a link to Webshots should be at thew top of the page somewhere), or if your ISP gives free web space, use that, it should work- cos Tripod just won`t allow it. They don`t even seem allow a click-on link to the jpg image`s address. You`ll have to create a HTML page on Tripod to sit the image on if you want to show them here.

    It`s a pain, but it`s free, so it`s not all bad is it?

    [img]images/icons/rolleyes.gif[/img]

  8. #8
    Flashaholic* Silviron's Avatar
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    Default Re: Resistor values

    <BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Chris M.:
    Doug- your picture won`t show.

    The reason is that Tripod don`t allow outside Hotlinking. ...

    It`s a pain, but it`s free, so it`s not all bad is it?

    [img]images/icons/rolleyes.gif[/img]
    <HR></BLOCKQUOTE>

    So many people have had similar problems, that I have decided to offer some server space for Candlepower forums members to post pictures--- It won't have automatic upload features like some of the other picture sites- (you don't really want them rooting around in your hard drive anyway, do you?) You will have to use FTP ----

    So, if you want some space, for Candlepower Forums related purpose only, email me ( rlt@az123.com ) and I'll email you back with the Web address of your directory and password. If you don't have a FTP program, let me know, and I'll send a FTP program as an attachment that is all set up for your directory..

  9. #9

    Default Re: Resistor values

    Any one have a circut to run one of the Luxeon Star into a minmag (2xAA)? I am looking to retro fit a minimag frame and put drive one of the luxeon Stars I can get at wholesale. But I want to run it on only 2 cells so the resister will not work. I need to get a step up circut to get the voltage to drive the LED.

  10. #10
    Flashaholic*
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    Default Re: Resistor values

    <BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Weylan:
    Any one have a circut to run one of the Luxeon Star into a minmag (2xAA)? I am looking to retro fit a minimag frame and put drive one of the luxeon Stars I can get at wholesale. But I want to run it on only 2 cells so the resister will not work. I need to get a step up circut to get the voltage to drive the LED.<HR></BLOCKQUOTE>

    Welcome Weylan,
    You don't actually need a special circuit if that's all you're trying to do...
    My 2-AAA DB white LS light works just fine on only two batts. [img]images/icons/smile.gif[/img]

  11. #11
    Flashaholic*
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    Default Re: Resistor values

    <BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Mike:
    No need to be confused...

    For LED safety I'd always use a resistor even if it is a real small one.
    <HR></BLOCKQUOTE>

    A white (and of course blue) Nichia LED has already a resistor of some 20 Ohm 'built in'. You can add this in your calculations and maybe you don't need any additional one.

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