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batteries and laser pointers and bulbs and brightnesses

Discussion in 'Electronic Basics' started by One Two, Dec 24, 2003.

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  1. One Two

    One Two Guest

    For fun, I am increasing the life of little laser pointers for cats.

    I noticed that 3 button batteries, (either alkaline or silver-oxide,
    357, or zinc-air hearing aid, 675) which produce 3x1.5 3x1.55 3x1.4
    volts respectively, are not nearly as bright as 2 x C cells producing
    only 2x1.6 volts initially.

    Why is this?

    The 3 button batteries are all over 4 volts although their capacities
    are around 105 165 540 mAH respectively.

    The C batteries are a little over 3 volts for 2 of them but their mAH
    are quite high relatively, 8350 mAH. Alkaline batteries for the C.

    There are discharge characteristics of batteries that I am woefully
    ignorant of, well, maybe not woefully. Got me curious.

    For a little test, I noticed that 2 C cells enable the laser pointer
    to draw 25 mA but 3 C cell will draw 27 mA. For those 2 extra mA
    coming from using 4.5 volts instead of 3 volts (nominally), the laser
    is noticeably brighter. I assume the nM of the laser is possibly being
    changed or just the power throughput or both. I read that as the laser
    drops from 660-670 nM to 650 nM the light gets brighter.

    My long-winded question is that I thought a device draws only the
    current it needs. But if there is more reserve capacity as with the C
    over the button batteries, the device draws more current or the
    batteries allow more current to be drawn or what?

    Would the true be same for D batteries over C also?
    I noticed that the lumens stay the same with Maglites using C or D
    batteries but the PBC or goes up slightly using D over C batteries.
    I can understand using more C batteries in series or D batteries in
    series would cause the brightness to go up. But it also goes up with
    the same number of batteries, like 6xD versus 6xC - unless the specs
    are just wrong when showing the Maglites and the different battery
    sizes and numbers for the same type of krypton bulb.
     
  2. CFoley1064

    CFoley1064 Guest

    I thought a device draws only the
    For some "illumination" on your question, measure the voltage across the 357
    zinc oxide batteries at no load and under 25 mA load, and compare that with the
    no load and 25 mA load with the C batteries. You'll find the output voltage of
    the small batteries is dropping pretty significantly under load, while the C
    batteries don't.

    All batteries have "internal resistance", which is a way of quantifying the
    voltage drop under load of batteries, which tends to increase fairly linearly
    as the load current increases. It actually has to do with the chemistry of the
    batteries, but that's a good shorthand way of describing it.

    Actually, real world voltage sources are modelled as an "ideal" voltage source
    (no internal resistance) in series with the source internal resistance.

    A 357 battery is made for loads of around 1 mA, with some capability for
    momentary current surges. A C battery is made for a load similar to a
    flashlight, typically around an amp. The C battery has much better voltage
    regulation, especially for such a "small" load as 25 mA. If you touch the 357,
    you'll probably notice it's getting a little warm, too.

    Short answer -- the small batteries aren't made for the load. Their output
    voltage is dropping, and that's why the load current is decreasing.

    Good luck.
    Chris
     
  3. This isn't odd, since the power expended by the 2 C cells is

    25mA x 3V = 75mW

    but the power expended by the 3 C cells is

    27mA x 4.5V = 121.5mW

    which is 1.6x the power output.
    A device will only draw the current it needs at its rated voltage because it
    has a certain resistance. If you vary the voltage, then the current draw
    will change.
    I just spent some time looking at the Duracell website. The 'nominal
    internal impedance' of C and D batteries are 0.126 ohm vs. 0.130 ohm,
    respectively. That would mean the voltage across the load for a new C cell
    will droop slightly less than that of a new D cell, given the same current
    draw.

    However, this difference is quite small. To all intents and purposes, then,
    the current delivered, and thus the candlepower of flashlights powered by
    them should be identical for two new batteries. However, the C cell will
    build up internal resistance faster than the D cell with the same load, and
    will thus dim more quickly.

    http://www.duracell.com/oem/primary/alkaline/alkaline_manganese_data.asp
     
  4. The small button cells have a relatively high internal resistance,
    which means that at that current, they probably are putting out only a
    fraction of their open circuit voltage. Hopefully the laser has some
    current limiting, because if not, they tend to have _very_short_
    lives. No biggie, tho, they're only ten dollars at Radio Snack, or at
    least that's what they used to sell them for. Just the laser module
    that is, cat. number 277-1011. The one I got uses 17.4 mA at 3V.
    BTW, even at normal currents, they still have a limited lifetime, the
    abovementioned says MTBF > 8000 hrs.



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  5. In the case of a LED or laser diode, that resistance is UNcertain,
    because the current is not linear with the voltage.

    In the case of a FET with its gate connected to source, above a
    certain voltage the current will _not_ change with voltage.


    [snip]
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    ###Got a Question about ELECTRONICS? Check HERE First:###
    http://users.pandora.be/educypedia/electronics/databank.htm
    My email address is whitelisted. *All* email sent to it
    goes directly to the trash unless you add NOSPAM in the
    Subject: line with other stuff. alondra101 <at> hotmail.com
    Don't be ripped off by the big book dealers. Go to the URL
    that will give you a choice and save you money(up to half).
    http://www.everybookstore.com You'll be glad you did!
    Just when you thought you had all this figured out, the gov't
    changed it: http://physics.nist.gov/cuu/Units/binary.html
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  6. The current-limited ones are more versatile too. My
    penlight-pointer has used the same pair of el-cheapo AAAa
    for three years or so. I'll finally post pics in the
    binaries group later today.

    Mark L. Fergerson
     
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