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Harbor Freight 4 LED $7.00

Discussion in 'Electronic Components' started by Henry Kolesnik, May 21, 2005.

  1. I just got a 4 white LED flashlight that uses 3 AA cells from Harbor
    freight. I measured the current at 127 ma and and that leads me to beleive
    they are direct wired. Looking inside as best I can it looks like all 4 are
    in parallel and connect directly to the 3 cells in series. I think LEDs are
    supposed draw a lot less current if they have a dropping resistor. I'm
    guessing that a dropping resistor would reduce current draw substantially,
    increase led life and not reduce light output significantly. But I can see
    were I could easliy install a resistor. I also know just about zero on
    leds.
    tnx
     
  2. Eric Smith

    Eric Smith Guest

    LEDs have a more or less fixed forward voltage drop. If you supply more
    voltage than that to them, they draw more current (and produce more
    light). But beyond a certain amount of current, they will be damaged.

    I'd be surprised if the flashlight has no current limiting resistor.
    Light output is nearly a linear function of current. If you increase
    the resistance, you will decrease the current, and hence the
    light output. This will increase the LED life, but the life was
    probably good enough already.
     
  3. Fred Bloggs

    Fred Bloggs Guest

    Sounds like they are using the battery internal resistance to limit the
    current. Four LEDs at 30mA/ea gets your ~120mA with Vbatt pulled down to
    maybe 3.5V at that current. And it probably has a "good enough" graceful
    degradation with battery run down causing the current to taper off
    exponentially after a point-reducing battery discharge per use. It does
    require the LEDs to be matched to some degree.
     
  4. mc

    mc Guest

    White LEDs require about 4 to 4.5 volts. That is about what comes out of
    the AA cells. The internal resistance of the LEDs is probably sufficient.
    Newer LEDs have more internal resistance than older ones.
     
  5. Seems like everything is made for the flashlight that has the profile of
    the Mini-Maglite. It has two cells, and fits into all sorts of
    holsters, pockets, carriers, etc. So if you get it, or a similarly
    sized flashlight, then you're home free. Of course the Mini Maglite has
    a halogen bulb, so it sucks. I bought the MagmaLED and Opalec Newbeam
    conversion kits for mine, and I'm happy with them. I've seen the three
    cell flashlights and I would much prefer that mine not be that long,
    three cells is just too long.

    Dorcy sells some four LED, two cell flashlights, for under $16. I
    bought a couple and They work but the rubber gtrips fall off - no big
    thing.

    One other disappointing thing about three cells lights is that if they
    are direct connected, then you can't use Ni-MH rechargeables in them.
    Some flashlights with the V boost circuitry can run on Ni-MHs, possibly
    at reduced light output.
     
  6. It's fairly common that it doesn't.


    Best regards,
    Spehro Pefhany
     
  7. Si Ballenger

    Si Ballenger Guest

    I would assume that LEDs that are now made for flashlights have
    built in pulse circuitry to get the brightest output from the
    LED. They would include what is necessary to operate at the
    correct voltage.
     
  8. I agree. The ones that I've seen without a resistor are mostly the ones
    that use a CR2016 or CR2025 button cell, basically those keychain lights
    that you squeeze. The cells have enough internal resistance to limit
    the current.

    But alkaline AA cells are capable of delivering more than an amp. I
    don't know what the internal resistance is, but it's very low. The
    multiple LED Altoids box lights that I've built use 33 ohm resistors,
    and I've put as many as 8 LEDs on a set of 3 AA cells, and they're
    _bright_. More commonly I do 6 LEDs, each with a 33 ohm resistor, and I
    consider that a minimum for a decent source of light.

    By using 4 LEDs and no resistors, the light output is probably greater
    than my 6 LED lights, but from what I've seen over just a few hundred
    hours, the light output from these flashlights will drop off after only
    a few hundred hours or less when pushed to this high a current. But it
    may take more than a year and several sets of AA cells to put a hundred
    hours on a flashlight, and by that time, the warranty has expired.
    :-( This is a dirty little secret that no flashlight maker wants you to
    know.
     
  9. Your assumptions are wrong.
     
  10. Hank,

    Adding a resistor will not be that difficult. A thin piece of two sided PCB
    and some handiwork with a fret-saw and a fraise will do the trick. As AA is
    pretty small you may need to use two or more SMD resistors in parallel. The
    round piece of PCB should be place between a battery and its press spring.

    petrus bitbyter
     
  11. spring.

    My dictionary says a fraise is "an obstacle of pointed stakes driven
    into the ramparts of a fortification in a horizontal or inclined
    position".
    Which I don't think is what you had in mind. Maybe you could explain.
    Thanks.
     
  12. Don Pearce

    Don Pearce Guest

    He clearly means a strawberry.

    d

    Pearce Consulting
    http://www.pearce.uk.com
     
  13. Si Ballenger

    Si Ballenger Guest

    And your proof is...? ;-)
     
  14. mc

    mc Guest

    I would assume that LEDs that are now made for flashlights have
    I don't know about *his* proof, but I've never seen such things on the
    component market. They'd be handy if they existed. I've seen LEDs with
    built-in flashers and with built-in current regulators to run from much
    higher voltages.

    Also, if pulsing were going on, you'd see stroboscopic effects when you
    moved the flashlight quickly, wouldn't you?
     
  15. A google search comes up with "fraise toestel", but it's not clear to
    me what exactly it does. Cut a donut in the PCB?


    Best regards,
    Spehro Pefhany
     
  16. James Meyer

    James Meyer Guest

    If you think about it, the batteries do have an equivalent series
    resistance and the LEDs are not perfect voltage clamps either. There IS a
    series resistance even though there is no discrete resistor in the circuit.

    Jim
     
  17. Exactly. That's why they work. Maybe not as perfectly as some would
    like, but good enough to sell.

    OTOH, an LED backlit LCD graphics display I have uses some
    sophisticated white LED driver chip arrangement. The backlight snaps
    on as the voltage is ramped up, almost like a fluorescent.


    Best regards,
    Spehro Pefhany
     
  18. Rich Grise

    Rich Grise Guest

    Heck, I'm still stuck on 'fret-saw'. )-;

    Thanks,
    Rich
     
  19. Milling cutter. (Dutch -> Frees).
     
  20. OBones

    OBones Guest

    LOL, it is indeed one of the meaning of "fraise" in French. One of the
    other ones is "milling cutter", which I guess is a good version.
    Source is www.granddictionaire.com, an awesome resource.
     
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