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  1. #1

    Default basic driver question

    hi there

    I have a basic question that i just cant seem to find the answer to.

    I understant that if you run a P7 direct drive with 3 Nimhs, you get a peak voltage at the start of 4.2 v, which drives the led higher than its rated 2.8 amps and therefore it gets hot but rund brighter until the batteries drain to the normal voltage.

    but i want to current control the circuit, and this is where it gets interesting for me and my newbie maths...

    I also understand that, if you use a 2.8a current limited driver to run it, itll get its optimum 2.8amps until the batteries fall below the voltage required to power the driver to 2.8 amps, at which point it falls out of regulation.

    I was wondering what happens to 4 cells being used with a 2.8 amp driver. Do you get a longer burn time at 2.8 amps since it takes longer for the cells to fall to below regulation voltage? any way of calculating how much extra you get out of a set of cells?

    so, for 3 x 5000mAh cells, at 2.8A, im calculating almost 1hr 40mins. using the normal equation, for a 4.8v, 5000mAh pack, i also get 1hr 40 mins. What does the extra cell do, or am i missing something.

    the thing is, i have 12 cells available, 5Ah capacity. i can either rig 3 cell or 4 cell packs, but using these calculations, i pretty much 'waste' a cell in each 4 cell pack- since i cant see where the extra power goes.

    I hope that ll makes sense. Please help!

  2. #2
    Flashaholic* LukeA's Avatar
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    Default Re: basic driver question

    It depends on the driver. If you're using an AMC7135-based driver, the fourth cell is waste, the extra voltage will just serve to heat up the 7135 chips.

    A normal buck driver will give you more runtime off of a greater number of cells. Four cells will give roughly 4/3 the runtime of three cells.

    It really does depend on the driver.

  3. #3
    Flashaholic* Bimmerboy's Avatar
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    Default Re: basic driver question

    Quote Originally Posted by LukeA View Post
    If you're using an AMC7135-based driver, the fourth cell is waste, the extra voltage will just serve to heat up the 7135 chips.
    Is it a complete waste? I knew about the extra heat as these are linear type devices, but am I under the wrong impression that at least some of that extra energy goes to good use?
    I'm an excellent driver. Of course I don't have my underwear... I'm definitely not wearing my underwear

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    Flashaholic* LukeA's Avatar
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    Default Re: basic driver question

    Quote Originally Posted by Bimmerboy View Post
    Is it a complete waste? I knew about the extra heat as these are linear type devices, but am I under the wrong impression that at least some of that extra energy goes to good use?
    Sure, but you're making lots of heat with the driver. 4 cells will last a little longer, but not the whole amount you'd get from 4 cells and a normal driver.

  5. #5

    Default Re: basic driver question

    Quote Originally Posted by LukeA View Post
    Sure, but you're making lots of heat with the driver. 4 cells will last a little longer, but not the whole amount you'd get from 4 cells and a normal driver.
    Is there a 'normal' buck driver i could use for the P7, in place of the 8xAMC7135 i was planning? also, could i use 3 cells and get a boost driver to push the volts up, and lower the current draw on the cells? or do boost drivers not really work like that?

    Sorry for the newbie questions, my last experience with LED stuff was 3mm and a limiting resistor way back when.

    Still a little confused though- 3 x 5Ah cells in series and 4x5Ah cells in series both give a capacity of 5Ah, but at 3.6v compared to 4.8volts. So its this extra voltage that gets bled off as heat? how does affect the runtime, if both sets of cells are still giving 5Ah? or does the 5Ah from cell 4 not have a role, since its volts is being bled off? Hope that makes sense...

    cheers
    Last edited by neilp1; 07-02-2008 at 01:02 AM.

  6. #6
    Flashaholic* LukeA's Avatar
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    Default Re: basic driver question

    Quote Originally Posted by neilp1 View Post
    Is there a 'normal' buck driver i could use for the P7, in place of the 8xAMC7135 i was planning? also, could i use 3 cells and get a boost driver to push the volts up, and lower the current draw on the cells? or do boost drivers not really work like that?

    Sorry for the newbie questions, my last experience with LED stuff was 3mm and a limiting resistor way back when.

    Still a little confused though- 3 x 5Ah cells in series and 4x5Ah cells in series both give a capacity of 5Ah, but at 3.6v compared to 4.8volts. So its this extra voltage that gets bled off as heat? how does affect the runtime, if both sets of cells are still giving 5Ah? or does the 5Ah from cell 4 not have a role, since its volts is being bled off? Hope that makes sense...

    cheers
    Most drivers are meant for single-die LEDs, and put out around 1A. I think your best bet would be to get three 1A drivers and wire them in parallel.

    A boost driver increases current draw from cells to make up for low voltage.

    3 NiMH cells typically come off the charger with a voltage of 4.2V and 4 cells with 5.6V. Anything above the forward voltage of the P7 will be bled off as heat. With four cells, that's an awful lot of energy for AMC7135s to dissipate.

    Here's a 4-pack of 800mA drivers from DX. Three or all four will run your P7 off of 4 cells.

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    Flashaholic* Bimmerboy's Avatar
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    Default Re: basic driver question



    The batteries are still 5AH capacity, but there are two advantages to 4 X NiHM with a driver. You do get longer time in regulation, as you suspected, but the higher pack voltage will also translate to a lower current draw on each battery, effectively giving longer total runtime than 3 batts.

    Hope that helps!
    I'm an excellent driver. Of course I don't have my underwear... I'm definitely not wearing my underwear

  8. #8

    Default Re: basic driver question

    I think a basic driver question belongs in the flashlight electronics forum.

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