LED Headlamp

Discussion in 'The Projects Forum' started by Major Tom MIB, Feb 18, 2011.

  1. Major Tom MIB

    Thread Starter Member

    Nov 28, 2010
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    RE: Show us how YOU use white LED's

    Before I redesign the wheel the wheel again, I thought I should check first...

    I'll be upgrading the headlamp to use two Luxeon Rebel Tri-Stars or Indus Cree XP-G
    Stars
    , but with a boost driver so I don't have to double the battery pack cell count. Problem is I haven't found any 700ma or 1000ma constant current LED boost drivers, except for an Eval Kit which looks like it can run three tri-stars at 700 (1000ma after modification for the Rebels.)

    The eval board is physically larger than the BuckPuck I'm currently using and it'll require a protective housing and possibly modification.

    So, before I start making the enclosure and making other changes,does anyone know of any BoostPuck type products that support higher currents?

    Thank you.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    This thread was closed in error, due to a misreading on my part. Sorry for the inconvenience. One more cup of coffee might have helped.
     
    Last edited: Feb 18, 2011
  3. John P

    AAC Fanatic!

    Oct 14, 2008
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    OK, having protested against the Mistaken Moderator, I suppose I have to answer.

    This seems very simple. A buck converter is just an oscillator driving a transistor, which sinks current to ground from the power supply via an inductor. When the transistor turns off, the inductor forces the current to flow to a higher voltage than the power supply.

    I did some calculations based on a 10KHz operating frequency and a 0.71 duty cycle (that's determined by the ratio of your battery voltage to the voltage drop in your LED). I think something like a 100uH inductor would be right. Jameco has these very cheap. Go to http://www.jameco.com and insert 286601 in the search box.

    The transistor should be an N-channel FET with the lowest on-state resistance you can get, but you need one that can turn on using your battery voltage--or you could put in a charge pump to generate a higher voltage to run the FET gate. That would very likely save you power (and keep your head cooler) versus running the gate at lower voltage and not getting the FET completely turned on.
     
  4. Major Tom MIB

    Thread Starter Member

    Nov 28, 2010
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    My bad, I cited the sources and failed to provide a summary...

    My current headlamp configuration uses a buck converter powered by 12x AA NiMH 2700mAh batteries, NiZn cells, or a 12v 8 ah SLA driving the Luxen Rebel Tri-Star ~9v forward voltage drop with a 1000ma constant current (operates ~3.5 hours at full power--never bothered to attach a dimmer pot, not really bright enough for my task). The active heat-sink has a 12v fan with a LM7812 regulator so I could vary the battery pack voltage up-to the buck converter limit of 30v.

    I'd like to double (or triple) the number of Tri-Stars without a significant change in the battery pack, better yet, I like to reduce the cell count to make it more suitable for head mounting. I'm using a belt pack now--the long wire is annoying.

    For this I need a boost converter of 700-1000ma (based on which Tri-Star I select, but the commercial boost converters I found are only 350ma, so I bought that eval kit. Thirty watts, up to 40v, and at 350, 750ma, or with modifications 1000ma .

    The 3 Rebels should run about ~19 watts @ 700ma or ~28 watts @ 1000ma. I'm not sure transmission loss will be a problem (the belt power line is long and my solder joints are 'messy.')

    I'll have to add a diffuser to protect anyone 'looking' at the headlamp--can't be blinding people at work, then I'll have to do their jobs too :) I seen some scary numbers like 80% and less effective transmissivity, so I'm thinking 3 Tri-Stars is the way to go.

    A future add-on for the headlamp...auto dimming based on reflected light. There's a hot spot in the Lambertian pattern right about where I 'read.' There are no optics over the LEDs, so it's 120° lighting--actually 'feels' like standard over head lighting--which is actually very nice :)

    [Sidebar] What's with the 260A rating on say this MOSFET--the 5 source legs in the picture look more like fusible links.

    [Edit] Correction: One, not five source legs--changed reference MOSFET without changing my comments.
     
    Last edited: Feb 19, 2011
  5. John P

    AAC Fanatic!

    Oct 14, 2008
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    My bad too--I talked about a buck circuit when I really meant a boost. The battery voltage is less than the voltage drop in the LEDs, right? Then it's boost.

    Dunno about that FET. It's in a TO-220 package, just 3 leads. 260 amps sounds like wild fantasy, but your headlamp doesn't need anything like that. For high current with low gate voltage, it looks reasonable.
     
  6. Major Tom MIB

    Thread Starter Member

    Nov 28, 2010
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    Direct mount to heat sink CeramCool® with metalization 6 x 1 W Rebel LED

    Use SEPIC, single-ended primary-inductor converter, to avoid driver re-selection when changing battery pack and/or LED.
     
  7. John P

    AAC Fanatic!

    Oct 14, 2008
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    Well, if you must. The SEPIC circuit, I mean. It'll be simpler and more reliable if you can use a setup where the voltage source is always more, or always less, than the voltage drop across the load. As Einstein (allegedly) said, "Everything should be as simple as possible, but no simpler."
     
  8. Major Tom MIB

    Thread Starter Member

    Nov 28, 2010
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    Typo? Should be 386601, no need to reply unless I'm wrong :)

    [Edit] What value should I use for the unspecified parallel resistance and capacitance? (I'll figure it out eventually, but it can't hurt to ask :) )

    Code ( (Unknown Language)):
    1. * C:\Program Files\LTC\LTspiceIV\JameCo_Inductor.asc
    2. L1 NC_01 NC_02 100µ Ipk=7 Rser=0.037 Rpar=0 Cpar=0 mfg="JAMECO VALUEPRO" pn="2312-H-VP"
    3. .backanno
    4. .end
    5.  
     
    Last edited: Feb 23, 2011
  9. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
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    Some super-bright headlamps, such as those used in caving/spelunking, use a rather heavy head to dissipate the heat from the the LEDs, and on the back of the band, the batteries and power supply are housed to balance the weight (to keep neck from getting sore holding up 6 LEDs and the heatsink).

    Just another thing to think about.
     
  10. Major Tom MIB

    Thread Starter Member

    Nov 28, 2010
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    New approach--a more off the shelf solution using a brooder lamp, 12v 25w CFL, and a AGM 12v 8ah battery suspended from above or the side(s) using either a clip, clamp, or strap.

    I only had the chance to test it once so far. A single lamp clamped on the side near the top. It didn't provide as much light as the stock LED headlamp, but it provided an even lighting without the 'hot spot.' I suspect that two of these lamps suspended from above will provide sufficient lighting.

    The battery voltage under load lasts at least 2 hours before dropping to 11.5v (50% discharge capacity?) which should manage to make the battery last 400 cycles (UB1280.)

    Lamp breakage might become a problem--I've managed to break one of the four CFLs I bought. The ballast from the CFL does appear to drive a T8 32w 4' tube, so it's possible to fabricate/buy a light weight reflective housing to protect and contain the fluorescent tube.

    The bad news for this project is that I no longer perform this task daily, so I might not need to make any improvements and it might not be worth it to bring in the brooder lamps--just the LED headlamp with primary batteries.

    The good news for the project is that some other people might be interested in using some lighting system, so I'll most likely finish off a few more details to make it effective.

    Thanks for the comments.
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    How much current does your load draw from the battery? Somewhere around 2A?

    If you're letting it discharge down to 11.5v, that's actually considered 100% discharged. Your battery life will be short. Either double the AH rating of your battery, or cut the run time in half.
     
  12. Major Tom MIB

    Thread Starter Member

    Nov 28, 2010
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    Starts out at 1.25A when cold and climbs up as it warms.

    Thought that was 10.5v for 100% discharge? I thought I verified it against the specs (page 2.)
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    I'll assert that if you discharge it that deeply, you'll wind up buying batteries a LOT more often.

    You don't have to believe me if you don't want to.

    Batteries tend to keep their voltages up until they're nearly discharged. The run-time you'll get between 11.4v and 10.5v will likely be just a few minutes.

    As your battery discharges, the current draw to keep up the voltage necessary to power the lamp will increase due to increasing internal resistance; so the battery will discharge more quickly and dissipate more power internally which creates heat and increased chemical activity. This shortens battery life considerably.
     
  14. Major Tom MIB

    Thread Starter Member

    Nov 28, 2010
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    That was my understanding too, so at what voltage under load should I disconnect the load to maximize battery life cycles?

    Since the 8ah batteries only cost ~$20, it just might be worth sacrificing life cycles for weight. It'll take a few uses to determine what weight is acceptable for mobility.

    In the meantime I'll keep scouting out 12-24v ballast and light weight reflective 4' tube housings as possible future solutions.
     
  15. SgtWookie

    Expert

    Jul 17, 2007
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    Rather than trying to measure the voltage, just do it by time.

    Make sure the battery is charged fully before you start using it.
     
  16. Major Tom MIB

    Thread Starter Member

    Nov 28, 2010
    31
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    Sounds like a good plan--thanks.
     
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