How to figure proper driver and battery for a given LED?- Mtbr.com
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  1. #1
    shining bright
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    How to figure proper driver and battery for a given LED?

    Hey folks,

    Some of you may remember me coming in to this forum a few years ago and building a little light. I have it as my avatar picture now and here is another pic-



    I have a dilemma and I was hoping some of you may be able to help me with this. I built this light with help from many of you here, as well as some people on candlepower forums. I'm not sure what happened, but I got sidetracked with this and have wanted to pick back up and build more lights for some time. I've been very busy with life and work and have not had a chance until now.

    I had compiled a lot of notes and info on how to do things, and I can't find any of it. I have checked PM's and threads on here and candlepower and I don't have any of it.

    anyway, one of the most important pieces of information I had was how to calculate drivers and battery specs. somebody explained this to me beautifully, and I have lost that info.

    for example, the first light I built was made with a cree XPG 3-up star. I can't remember the specs on this. The driver was a buckpuck model #03023-D-E 1000p..so a 1000mA with a pot. The battery ended up being a 14.4v lithium Ion with protection, and it was 2600 mAh - if I remember correctly. I can't remember how I came up with this, and how to figure out how long it would last.

    There was a bunch of math involved.

    If anybody could help me understand this again, I would be greatful.

    I have another idea to build, but I'm not sure how to pick my components because I can't remember how to figure everything. I want to build a very small light with possibly one cell, for instance a 18650 or smaller. I want the light to be pretty bright, and have enough power to run for 30 minutes or so. I'd also like for it to have modes, the two I am most interested in are strobe and steady.

    I'm not sure if one 18650 is enough to power a decent bright LED for 30 minutes or so. I guess what I want is as small as possible, as bright as possible, and able to run for 30 minutes or so.

    also, this particular light will be like a flashlight, the battery will be self contained.

    For what it's worth, I asked pretty much this same question on CPF and I've had no replies. Hoping you guys can help.

  2. #2
    jl
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    I don't have all the mathmatics figures out but you can get one of these, it is super cheap for the amount of light it produces. You'll need to provide the 18650 cell.

    $23.76 Convoy S4 Cree XM-L T5-5C 828-Lumen 2-Group 3/5-Mode LED Flashlight - 1*18650 / 8*AMC7135 2800mA / clip at FastTech - Worldwide Free Shipping

    You could also take the driver and LED components specified and use them to build a light with an external battery pack.

    Emmitter
    $6.90 Cree XM-L T5-5C1 / 4000K Neutral White LED Emitter with 16mm Base - T5-5C1 / 4000K at FastTech - Worldwide Free Shipping

    Driver
    $3.05 8* AMC7135 2-Group 2~5 Modes LED Flashlight Driver (Nanjg 105c) - 3.0V ~ 4.5V / 2800mA constant current / 17mm at FastTech - Worldwide Free Shipping

    Final Package


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  3. #3
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    Some quick figures. @ 1A a white cree led will need between 3 & 3.15V.
    Each 18650 cell delivers between 4.2 & 3V, although commonly referred to as 3.7V - hence your 4s1p pack being 14.8V. The actual voltage is between 16.8 & 12V. Your 3 XP-G leds need 9.45V. A buck driver needs ~1V, hence the setup works just fine for 3 leds driven by 4 cells.

    1 led changes things a bit - there is not enough overhead to use a buck driver. To use 1 led & 1 18650 cell, your best option is a linear driver like an $25 lflex, a $10 mobydrv/q or a $3 nanjg 105C, which is the one JL linked to

    Many people including myself don't like the fast strobe on the nanjg 105c, hence the mobydrv which is a modified version & gives different strobe options. Dr Jones' website seems to be offline so I have linked to the cached version. I haven't used one but the lflex, like all taskled drivers, gets plenty of good reviews here.

    How long will 1 cell give you? Using a nanjg 105c & 1 Panasonic 3100 mAh 18650 cell, I get ~1 hour on high (2.8A) so it will meet your time nicely.

    Each of the drivers I have linked to are capable of running an XM-L @ 2.8-3A. If you are using a different led you will need to set the current to 1-1.5A &will get more than double the run time

  4. #4
    shining bright
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    Thanks for the replies folks. I called and talked to LEDSupply yesterday and they gave me a little more info.

    I actually gave a little bit of wrong info in the idea that I had. When I said 18650, I meant CR123A. The idea here is to get as small as possible. LEDSupply explained how most LEDs need at least 2 volts before they start coming into power - at least a light with any power anyway. The particular XML-2 of interest has a Vf of 2.85 @ 700mA.

    I have a Surefire 6P LED flashlight that is supposed to be 80 lumens. It is not near as bright as what I want to use this particular idea light for, so obviously I need more output.

    After talking with them a while, we pretty much decided that the XML-2 is a good choice. The specs on it are 318 lm @ 700mA. With a max drive current of 3000mA, I should be good as far as it being a strong performer no matter what is thrown at it.

    Now, to get the battery discussion out of the way. Obviously, an AA, a C, or any other common single short cylindrical battery is not going to be enough, because most are only 1.5 volts or so. However, looking at a CR123A in my surefire, I see that it is 3 volts. I just measured one of the CR123As that has been in my surefire since I don't remember, and it is still showing 2.906 volts on a DMM. This could possibly work, but I think the problem will be with the driver.

    So then the question is what driver. After looking at LEDsupply's page, the only problem I see on drivers deals with input voltage. on their page, starting at 700mA, nothing has an input voltage of under 5 volts. Looking at the options you guys detailed, the LFlex has what looks like a minimum of 2.7 volts..the nanjg 105C has 2.9-3 volts, and I can't tell on the mobydrv/q.

    obviously, trying to use a CR123A I'm right on the lower end of the spectrum as far as input voltage. I'm not sure if I should try to make this work (or if it even will), or if I should focus on something like a 18650.

    I realize that XML-2 is going to suck a lot of power, especially if driven at 1000mA or so.

  5. #5
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    CR123A cells will drop rapidly under any heavy load to <2.5V. Of course with a white LED you won't be pulling much current for long before it all just goes direct drive with diminishing output, it'll run a long time, but not much light. The only realistic way to use a single CR123 with a white LED is with a boost driver. But again, you won't get much runtime at any higher output levels (700mA or higher). CR123 a good for lights you don't often use but want excellent shelf life.

    So, don't bother, or at least use a li-ion 123 form factor cell so you have at least ~3.7V to play with for most of the runtime.

    Better yet a larger li-ion to give you more runtime and ability to run higher current for longer.

    cheers,
    george.

  6. #6
    jl
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    Just for clarification the battery pack in the picture is a 4P1S. I was looking for the smallest light with the longest run time possible.
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  7. #7
    shining bright
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    Quote Originally Posted by georges80 View Post
    CR123A cells will drop rapidly under any heavy load to <2.5V. Of course with a white LED you won't be pulling much current for long before it all just goes direct drive with diminishing output, it'll run a long time, but not much light. The only realistic way to use a single CR123 with a white LED is with a boost driver. But again, you won't get much runtime at any higher output levels (700mA or higher). CR123 a good for lights you don't often use but want excellent shelf life.

    So, don't bother, or at least use a li-ion 123 form factor cell so you have at least ~3.7V to play with for most of the runtime.

    Better yet a larger li-ion to give you more runtime and ability to run higher current for longer.

    cheers,
    george.
    Point taken and understood. I guess an 18650 is only logical, and about as small as I'm going to get. I will work on a design like that. Question is, which driver do I want to use. So at this point, the design will be the XML2, and a 18650 battery.

  8. #8
    jl
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    Quote Originally Posted by cncwhiz View Post
    Point taken and understood. I guess an 18650 is only logical, and about as small as I'm going to get. I will work on a design like that. Question is, which driver do I want to use. So at this point, the design will be the XML2, and a 18650 battery.
    Here's a few to investigate. I purchased the nanjg and the lithium driver from fasttech, but others are available. Here was my list when I was doing my investigations for a 'small but bright' light.

    Fasttech
    $3.05 8* AMC7135 2-Group 2~5 Modes LED Flashlight Driver (Nanjg 105c) - 3.0V ~ 4.5V / 2800mA constant current / 17mm at FastTech - Worldwide Free Shipping

    $4.63 1* Lithium 2-Group 3/5-Mode 2.6A LED Flashlight Driver (LD-25) - 3V~4.2V / low voltage warning / reverse polarity protection / 17mm at FastTech - Worldwide Free Shipping

    $5.59 1-2* Lithium 2-Group 3/5-Mode 2.8A LED Flashlight Driver (LD-29) - 3V~8.4V / low voltage warning / reverse polarity protection / 20.5mm at FastTech - Worldwide Free Shipping

    Custom Lites
    8X7135 / 2.8 amp ( selectable mode ) XML driver

    lck-led
    17mm CC 2600mA LED Driver 2 Group Mode, 3-4.2v, 2.6A, XM-L

    Shining Beam
    3-Mode Regulated Circuit Board for XM-L, XM-L2, MC-E, SST-50 and SSC P7 (2.1A Max.)

    Light Malls
    $4.5 | AMC7135*8 2800mAh 5-Mode Circuit Board For cree xml Free shipping Worldwide Free Shipping!!!

    17MM 3~6V 5A 4-Mode Memory LED Circuit Board With Protection For Cree XM-L/Cree XM-L2,SSC-p7(Moonlight Mode) - LED Emitters - DIY & Accessories Worldwide Free Shipping!!!

    A new one I found Mtn Electronics
    QLITE REV.A 7135*8 3.04A LED DRIVER
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  9. #9
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    In my opinion the best driver for 1 LED and 1 Li-ion battery is the Taskled Lflex: LFlex Product Information

  10. #10
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    Don't feel stuck on the 18650 if you really need smaller size... there are readily available 18350 batteries (35mm long, not 65mm long). I don't have the info at my fingertips, but if you're running at 1A, you'll go dark pretty quickly on such a small battery... maybe 30 minutes, but maybe not.
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  11. #11
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    Easily done. For an 18650 and a white power LED you just need a linear driver - either a cheap one like the thing the Mobydrv is based on, or maybe a Taskled L-flex. Pretty straightforward I believe.
    Savvas.

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