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variable power battery pack

Discussion in 'Electronic Basics' started by John, Aug 8, 2003.

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  1. John

    John Guest

    Hi guys,

    I'm building a bicycle light and I've decided to go with the 7.2V battery
    pack (for RC) as you suggested. I'm using is 16W xenon bulb. How can I
    change the intensity? I assume I have to reduce the voltage? How could I get
    my pack to switch voltage to give me say 5W ,10W and 16W at the flip of a
    switch?

    These questions might have been asked before... but I'm new at this
    thing... so feel free to just point to the answer...

    Many thanks
     
  2. Joel Kolstad

    Joel Kolstad Guest

    Yes, but what you don't want to do is just drop the voltage through
    something that'll just turn it into heat like a resistor. Instead, what is
    often done is to send a square wave with an adjustable duty cycle to a light
    bulb. The lower the duty cycle, the lower the RMS value of the voltage the
    bulb sees and the less power the bulb will use. This is readily done using
    a 555 timer IC and a MOSFET as the switch. I'd suggest running the 555 at
    somewhere between 100Hz and some kilohertz, nominally -- lower values can
    start to allow you to _see_ the switching action (this is based on the
    thermal inertia of the filament -- how quickly it cools down), whereas
    higher values will make the switching action slightly less efficient
    (although until you get to megahertz, this is probably pretty negligible).

    Note that your 16W bulb is going to become a lot less efficient at 5W,
    because most of the filament's thermal energy will be emitted as infrared
    radiation that isn't going to help you, as a human, one iota. (You might
    make the comparison between a single 2 'C' bike light, which often use 2.4W
    bulbs, and you setup running at 2.4W -- you'll find that your light will be
    much redder and not as good. There isn't much of a way around this problem
    with blackbody radiation sources -- if you get ambitious someday, you can
    try high intensity discharge bulbs or LEDs which don't exhibit this
    behavior.)

    What I've described here is on open loop system in that, if the battery
    voltage drops a little, the bulb's power consumption will change a little as
    well. NiCads have a pretty flat discharge curve so this won't be a problem,
    but if you actually want _calibrated_ wattages regardless of battery input
    voltage, you'll need a rather fancier design with (most easily) something
    like a switching regulator controller IC.

    ---Joel Kolstad
     
  3. Which brings to mind the much simpler method. Just use multiple 2.4W
    bulbs. You can switch them on as needed. And you have the safety of
    never losing your only light source, since if one burns out, you still
    have the other(s).

    You could go with three 5W lamps, or some other combination. The nice
    thing is that the smaller prefocused flashlight lamps are avaiable at
    hardware stores and Rat Shack, too.

    Another plus that I noticed when I rode my bike is that a single light
    source causes dark shadows behind objects. With multiple light
    sources spaced a foot or two apart, this isn't so much of a problem.
    This is probably the reason why car headlights are not in the middle,
    but at the left and right corners of the car. And again here you have
    the issue of reliability, so if you lose one headlight you still have
    the other.

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  4. Make sure your bulb can handle the vibration and stress of being mounted on
    a bicycle. Bulbs that can are significantly more expensive.

    Adam
     
  5. John

    John Guest


    Do you mount you lights on your handlebars or on your helmet? Have you tried
    combining the two?

    Thanks
     
  6. John

    John Guest

    Yes, but what you don't want to do is just drop the voltage through
    Can you buy this ready made or do you have to make ti yourself? Also,
    wouldn't bypassing the wave generator be better when you want maximum power?
    I'm going with NiMh by the way.

    Thanks again!
     
  7. Ian Stirling

    Ian Stirling Guest

    No, the bulb will fail rapidly.
    If you can, to keep a reasonable bulb life, and brightness, you really want
    to keep the RMS voltage to within 5% or so.

    --
    http://inquisitor.i.am/ | mailto: | Ian Stirling.
    ---------------------------+-------------------------+--------------------------
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  8. John

    John Guest

    No, the bulb will fail rapidly.
    You mean 5% less than rated voltage? Why would the bulb die quicker with the
    rated DC? So when you underun your lights, they last longer?

    Thanks a million
    John
     
  9. John

    John Guest

    This is readily done using
    I know this is a really basic questions, but where could I find the
    schematics for this? I've found many for wave generators but I'm not sure
    how to include the MOSFET. Why is the IC more efficent than just a physical
    switch?

    Thanks again!
     
  10. It is not really necessary to include the MOSFET if it is the only thing you
    have difficulties with. In this case a (big) BJT transistor like the 2N3055
    (NPN) will also do. Be prepared however that it gets hot. In this case, use
    a 6 Volts lightbulb because 1) There are always losses in the transistor,
    somewhere around half a volt. 2) It will be hard to get an almost 100% duty
    cycle, so that some 0.7 Volts will get eaten up by the converter. Make sure
    that the signal is really square-wave, not sine neither sawtooth, this will
    increase the efficiency.

    To the second question: An IC is never as efficient as a physical switch,
    but the switch itself is not the cause of the losses. It is the resistors
    (or transistors) that are used to drop voltage that heat up and make the
    least efficient part of the circuitry apart from the light bulb. Using a
    switching circuit you can avoid the high voltage drop thus conserving
    battery power when it is run at a lowered wattage.
     
  11. Fred Bloggs

    Fred Bloggs Guest

    You can't pulse the H.I.D. xenon's like this. His only solution is to
    use multiple bulbs at lesser wattages and he may need multiple ballasts too.
     
  12. John

    John Guest

    I'll be using lights on the mountain so I've decided to use 2 lights for
    safety reaons. One will be on the helmet and I'll play with the second one
    to see where it works best. I'll be using H.I.D. Xenon bulbs. I'll use the
    555 wave generator to pulse the lights and have variable power. I'll include
    a bypass switch so I can use the lights on max power without having to feed
    the 555 circuit. I want the switches to be on the handlebars so I can change
    lighting as I ride.

    I want both lights to be on seperate battery packs to improve reliability
    and so I can take just 1 light on beer runs and for commuting. I'll be using
    rechargeable NiMh C or D cells for power. This way I can use regular
    batteries if I have no way to recharge the spent batteries.

    I can build this 'ideal' (at least for me) system for about the same price
    as a decent system. So I think I'll go for it....

    Any thoughts appreciated!

    Thanks
    John
     
  13. Tim Shoppa

    Tim Shoppa Guest

    No, he's saying that you want to be close to the rated voltage of the bulb,
    and the PWM regulator he tells you how to build should be designed to do so.
    5% accuracy is a good number to aim for with incadescent bulbs, you don't
    have to better than that.

    Running the bulb below rated voltage will lengthen life but will dramatically
    hurt efficiency. And the assumption is that if you're using batteries,
    efficiency is a prime goal.

    Running the bulb above the design voltage will greatly shorten the life.

    Tim.
     
  14. Fred Bloggs

    Fred Bloggs Guest

    If your lamps come with a ballast regulator then you cannot pulse
    battery power into it. These bulbs are very touchy- they MUST be run at
    rated power- too much by ~10% and they burn up/explode and too little
    causes electrode damage. The ballast is there to put power consumption
    right on the money.
     
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