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Will this work?

Discussion in 'Electronic Basics' started by beekay, May 20, 2006.

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  1. beekay

    beekay Guest


    I'm an occasional reader of this forum, and I have a question. I want to use
    a Radio Shack 13.8VDC 15-Amp Power Supply Model: 22-508, or something
    similar, to power some devices in my backyard. I want to power some outdoor
    lights that I plan to make from car backup lights (they are cheap - $3.50
    each at AutoZone) as well as a water pump also from a car (such as
    windshield washer pump or gas pump) for a small waterfall. In other words,
    everything made from car equipment, designed to work off a car battery, but
    running off a power supply that should be like a car battery. Will this
    work? If so, how many lights could I operate from it?


    Bruce Kimball from Louisville
  2. K `Sleep

    K `Sleep Guest

    There is no reason why this shouldn't work. The power supply is 15 amps,
    which should be plenty for this purpose.
  3. I would look for another pump, the windshield pump will fail quite fast if
    run continuous. Look for what is called a bilge pump at a boating shop.
    They run on 12 volt and will last longer than the car windshield pump.
    But yes on the power supply running it all. I assume you wanted 12 volt for
    safety reasons outside.
  4. beekay

    beekay Guest

    Actually, in addition to the safety, I want to do it because it seems
    cheaper and I may have more control over the items. I'm trying to do this on
    a low budget. I wonder if an electric gas pump would also work with water -
    they must run pretty continuously.
  5. Tim Auton

    Tim Auton Guest

    It should work fine. Here's the basic electronics stuff you need to
    know to work out if your chosen components are suitable, and how many
    lights you can hook up:

    The number of lights you can run depends on how much power each one
    needs. It's possible your lights will only display a rating for power
    (Watts), so you need to convert that to current (Amps) so you know how
    many you can use before you reach the 15A limit of the power supply.
    For that the formula you need is:

    Current = Power / Voltage

    Once you know the current, just total up the current for all the
    devices (pump + lamps) and make sure it's under the maximum your
    supply can provide.

    If you want to put several amps through a reasonable length of wire at
    a low voltage the resistance of the wire itself becomes be a concern.
    In order to work out how big a concern it is, you need to pick a
    number for the acceptable voltage drop, let's say 2 Volts (you could
    get away with a 3V drop, or even more - your lights won't be as bright
    and you pump won't be as fast, but they should still work). Let's say
    you have 20m of wire, and you stuff draws 10 Amps. Ohm's law is:

    Voltage = Current * Resistance
    rearranging for our example, we get
    Resistance = Voltage Drop / Current
    Resistance = 2 / 10
    Resistance = 0.2 ohms

    So we need to find the thickness of wire which has less than or equal
    to 0.2 ohms for 20m, or 0.01 ohms/metre, or 10 ohm/km. A quick search
    on the 'net for 'wire gauge resistance' finds a table which shows that
    copper wire 1.45mm in diameter (15 AWG) is suitable. Less current
    and/or shorter wires and/or greater allowable voltage drop means you
    can use thinner wire than in this example.

    If I were an electronics teacher, I'd now ask you to use all the above
    to work out why long-distance power lines are run at really high
    voltages. But I'm not, so I won't.

  6. They will not generate much flow. You say you want it for a waterfall type
    setting, so you will need it to create substantial flow depending on the
    size of falls you want. Still to get that from even a bilge pump may be
    pushing it some.
    The windshield pump would get more flow than a gas pump, but like I said it
    will burn out fast. Your only problem I see with what you want to do is the
    water pump. The lights may draw .5 amp at least so if you don't overload
    the supply to much, maybe keep it down to 10 amp for that 15 amp supply.
    How much is that supply going to cost you? 13.8 volt at 15 amp. It can't
    be cheap.
  7. beekay

    beekay Guest

    Radio Shack sells the 15 amp for $80 (180 watts) and a 25 amp for $100 (300
    watts). There may be cheaper routes, such as building my own, but these seem
    competitive with the transformers that come with outdoor lighting kits, and
    I like the idea of being able to use car parts for it. The waterfall part is
    being built around a creek that runs through my yard, and I want to pump a
    (possibly) small amount of water up and have it come down over some rocks or
    other surface I will embed in the banks. It doesn't have to be a huge flow
    of water, just enough to be visible and make some nice watery sounds. It
    could be that the car pumps won't be a practical choice - maybe they draw
    too much current. Maybe the idea is too exotic to actually work well.
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