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  1. #1
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    The Big Fatman topic

    The Fatman is a step-up driver from Taskled. In the technical pages at the website from taskled some things are already explained, like how to use it with external potentiometer for dimming and the input current limit. I will skip the parts that are explained by Taskled and give some schematics with calculations to let your Fatman run your leds on more then output level.

    As first some general facts about the dimming of the Fatman. The Fatman has an onboard trimpot to set the output current. I designed all schematics below so that it can stay at the PCB and it shouldn't be desoldered. The connections for the dimming are electrical parallel to the trimpot. This is important for the lifetime of your light, when you'd make an error or a wire comes loose the current through the leds can't become higher then the current that you'd set with the trimpot.

    When you choose to use the Fatman with an external logarithmic pot, it doesn't matter how you connect the pot to the 2 connections on the Fatman. As long as you connect one of the connections to the Wiper from the potentiometer. More about this potentiometer see the website of Taskled. The following schematics are all the same in operation, with switches it works the same:


    But there are a lot more options then using a potentiometer for creating multiple output levels when you use switch(es). I'll start with the most easy circuit that has the least output levels and extra components.

    This schematic makes use from the onboard resistor that is in series with the trimpot. When the trimpot is turned to 0 Ohm or is short-circuited this resistor makes that the output current becomes 30 mA. So this schematic creates 2 levels: the first level is set by you with the trimpot and the second level is set by the onboard resistor (so 30 mA). This set-up would do the trick when you only want to dim your light when you're in the city. This extension is easier to solder then the leds to the Fatman, no calculations have to be done, all what have to be done is solder to wires to a switch and it works.


    The following schematic is the same as the first one except the resistor. This resistor makes it possible to set the current for both output levels. With the trimpot you can set one level for example to 1A and the other to 300 mA with the resistor. Below the schematics I will show how to calculate the resistor.


    The next schematic gives the same options as schematic 1, but then with an other type of switch. But this schematic isn't preferable, the trimpot needs to be set higher. When the wire with the resistor brakes the output will raise, not an ideal situation:


    The same as above only then for schematic 2, it will work but when you can choose a different schematic (and swich):


    Now it is becoming more interesting. There is still only one switched used, so not too much space for your tiny housing. The switch has 3 stands. 1e, the output is set to 30 mA by the onboard resistor. 2e the highest light output, the current is set only by the trimpot. 3e The output is somewhere between 30 mA and the maximum, it is set by the combination Trimpot and resistor.


    The following is again the same as above, but with the difference for the lowest output level. This isn't fixed to 30 mA, but it can be choosen with the additional resistor.


    When you'd like to have more then 3 output levels without using a (large) potentiometer you'd need to use 2 switches. The 4 stands:
    1: 30 mA, set by the onboard resistor.
    2: the output is set by the onboard trimpot
    3: doesn't really exist, when you put the first switch to this stand the other switch is also in use.
    1(3): the output is set by the combination of the trimpot and resistor Rexternal7a
    2: the output is again set by the onboard trimpot (so the same as 2)
    3(4): the output is set by the combination of the trimpot and resistor Rexternal7b
    By adding an extra resistor between the 2 switches, you can create a 5e stand, but the calculation won't become easier.


    An other possibility with 2 switches is to use them is series instead of parallel. But I am afraid you'll need a good memory when you'd want to select the right stand.


    And the last schematic for the ones that can't choose between a potentiometer and switches


    When there is more then one switch used the complexity rapidly raises and so do the possible options. There are a lot more circuits possible, but it has no use to discuss them all. My personal favorite is schematic 5 and when it is really needed schematic 7.

    Tomorrow I will explain/give the calculations to calculate the external resistors

  2. #2
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    Schematic Resister calculations?

    Super-fast good article. I want to connect my DIY set-up per your shematic 5. What is the rating of the resister in series with the POT? I would then calculate Rext5 per the parallel resistance formula [(r1+r2)/r1*r2], correct?

    Thanks

    DFH

  3. #3
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    Quote Originally Posted by dirkfh
    Super-fast good article. I want to connect my DIY set-up per your shematic 5. What is the rating of the resister in series with the POT? I would then calculate Rext5 per the parallel resistance formula [(r1+r2)/r1*r2], correct?

    Thanks

    DFH

    Yes, your switched resistor will add to the on-board resistor (trim pot) in parallel. So determine what value of resistor you would need based on what your overall end resistance should be.

    For my Fatman setup, I set the trim to 10.1k (for 750mA output - btw, that trim-pot is sensitive!). I wanted to reduce the output to 400mA by switching in a resistor. The final resistance for 400mA output is 5.22k from George's site. The value needed to get close to that was 10k-ohm in parallel to the pot.

    1/((1/10k)+(1/10.1k))=R where R is the final resistance..

    R=5.025e+3

    http://www.1728.com/resistrs.htm



    This is a bit off topic, but it fits in the "Big Fatman" thread...

    I've yet to run my light (3x Cree Q5) long enough to drain the battery to the point where the Fatman can't get enough input current. I've been wary to let this happen from fears of the ever increasing current demands of the Fatman as input voltage drops. My question is, what should I expect if I were run my light to that point? Will the battery (5, 4/3A cells) melt into a puddle? If the results aren't as catastrophic as that, will the light dim before going completely out?

    Thanks!

    op
    www.msmtb.org - Mississippi Mountain Biking

  4. #4
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    I was a bit busy so the late answer. The calculation above is right, here is the direct calculation:
    First use the formula on the website from Taskled to calculate the totall resistance needed for your drive current: Resistance_in_ohms = ( 13.87 * Current_in_mA ) - 322.7 = R_total
    Then we calculate the external resistor value (the calcation 2 posts above will also do the job):
    R_extern = 1 / (1/R_total - 1/R_trimpot)

    When your cells are empty they are not going to cache fire or something, so it isn't dangerous. But you will damage your cells when you use them till they have no capacity left. The light will probably go out quite fast (in a few seconds).

  5. #5
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    Double posts...
    Last edited by super-fast; 11-21-2007 at 03:37 AM.

  6. #6
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    Double posts...
    Last edited by super-fast; 11-21-2007 at 03:38 AM.

  7. #7
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    5x 4/3A so you must be on NiMh cells. You can safely run to flat unlike Li batteries. As done by RC boys with nimh cells.

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    Quote Originally Posted by heatstroke
    5x 4/3A so you must be on NiMh cells. You can safely run to flat unlike Li batteries. As done by RC boys with nimh cells.
    You can, but you'll kill one or more of your cells as the weakest one will get voltage reversal. If you want to get decent life from a battery pack it's best not to discharge below 1V per cell for NiMH.

  9. #9
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    Since this is the "Big Fatman topic," here's a question for y'all.

    If I'm running my Fatman driver (powering 3xCree Q5s) at 1000mA, do I need to bond it to a heatsink? I'm guessing that's what the small circle on the back side is for, but didn't know for sure and wasn't sure how much heat the Fatman would have to deal with. I'm running it from a 8.4V NiMh cell, and will have a switch to dim to 350-500mA (haven't decided how dim I want it yet).

    Thanks!

    Another thing, with this talk of damaging batteries, what's the "shutdown" pin on the Fatman for? I don't see any mention of its purpose or function on the documentation on the website.

  10. #10
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    Quote Originally Posted by dnlwthrn
    Since this is the "Big Fatman topic," here's a question for y'all.

    If I'm running my Fatman driver (powering 3xCree Q5s) at 1000mA, do I need to bond it to a heatsink? I'm guessing that's what the small circle on the back side is for, but didn't know for sure and wasn't sure how much heat the Fatman would have to deal with. I'm running it from a 8.4V NiMh cell, and will have a switch to dim to 350-500mA (haven't decided how dim I want it yet).

    Thanks!
    I can't answer your heatsink question. I'm running my 3x Cree setup at 750mA and haven't had any problems yet, but I've got no experience with 1A..

    However, I can recommend you go with 350mA (or even 300mA) for the "low" level. My 750mA setup is dimmed to 400mA, and though there is a definite difference it sure doesn't appear to be "half power". I'm sure this is due to our logarithmic eyes needing 4x the power loss to appear half as bright.

    op
    www.msmtb.org - Mississippi Mountain Biking

  11. #11
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    Good point. I'm looking at possibly just putting a pot on it instead of a switch at this point. However, radioshack.com was acting up so I can't check my local store to see if they have a sufficient pot in stock. I also have to run out and get connectors today. I want this thing finished so I can use it tomorrow...

  12. #12
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    Got a response from George at TaskLED (talk about good CS!):

    Quote Originally Posted by george@TaskLED
    3 Cree = 3.7V x 3 = 11.1V, running at 1A that's 11.1Watts. Fatman running at 90% efficiency means you have 1.1W of heat being dissipated in the driver board. So, yes, heatsinking would be a good idea. The silver circle is another VIN+ connection. Most folk will build up a thin layer of Arctic Alumina epoxy on the bottom of the PCB and then mount a heatsink tab to the body of the light from there. Make sure there is no conductive electrical path through the epoxy to the heatsink you are using - i.e. careful with any protruding wires that you have soldered into the holes of Fatman.

  13. #13
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    Just doublechecking my math - if I am going to run 2 Cree Q5's with my fatman @ 1A, and want a 300mA dimming capability on a switch, I will need a trimpot value of 13.5Kohm, and an external resistor value of about 5.3Kohm between the POT+ and POT-Gnd, with a switch in there, right?

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    Quote Originally Posted by adrenalnjunky
    Just doublechecking my math - if I am going to run 2 Cree Q5's with my fatman @ 1A, and want a 300mA dimming capability on a switch, I will need a trimpot value of 13.5Kohm, and an external resistor value of about 5.3Kohm between the POT+ and POT-Gnd, with a switch in there, right?
    Yes, your numbers are right. What battery are you planning on using? I'd stick with 6V to make sure your input voltage is less than the Vf of the 2 Q5s.


    Brian
    www.msmtb.org - Mississippi Mountain Biking

  15. #15
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    I just bought a brand new 6V 5.0Ah Nimh pack for my old 6V halogen, I was planning on using that pack, or, going to a Triple Cree setup, and using 7.2 or 8.4V Nimh, or 7.4V LiIon

  16. #16
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    6 or 7.2?

    Quote Originally Posted by ohpossum
    Yes, your numbers are right. What battery are you planning on using? I'd stick with 6V to make sure your input voltage is less than the Vf of the 2 Q5s.


    Brian
    Would a 7.2V battery work in that setup or would it be better to go with a 6V?
    See the trails, be one with FOO-MTB.
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  17. #17
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    Quote Originally Posted by notaknob
    Would a 7.2V battery work in that setup or would it be better to go with a 6V?
    Vf for 2 Crees would be 7.2V (2x3.7V), so a 7.2V input would put the Fatman out of regulation and into direct drive until the input voltage drops to less than Vf, which probably wouldn't take long because the LEDs would be going all out. Not really the most ideal situation..

    Personally, I'd go with 6V for 2 Crees and 7.2 for 3.
    www.msmtb.org - Mississippi Mountain Biking

  18. #18
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    So, has anyone figured out the purpose of that "shutdown" pin??

  19. #19
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    The pin isn't really usefull for most of us. It works better to disconnect one of the power supply lines to the fatman to shut it down. When you use the shutdown pin the fatman will still consume a little bit of current, the amount is so small you won't notice it after a few days, but after a few mounts you will do.
    When you'd use a microcontroller to control the fatman, this pin is handy (without you'd need an extra power mosfet).

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