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Build an A/C adapter?

Discussion in 'Electronic Basics' started by Top Spin, Jan 19, 2004.

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  1. Top Spin

    Top Spin Guest

    I have one of those portable, battery-powered tennis ball machines. It
    doesn't come with an A/C adapter. How difficult would it be to make

    The motor has to be running on DC current from the battery. It seems
    to me that if I could determine the voltage and amperage coming out of
    the battery, I should be able to find an A/C adapter that puts out
    that same power. I would then need to wire it in, maybe with a switch.

    Is this possible? Difficult? Am I missing anything?

  2. DaveC

    DaveC Guest

    Easier & safer to just find one already available. Most devices have a label
    describing the voltage and current required. If not, measure the battery
    voltage and current and go to Radio Shack and get one of their universal
    adapters. Put a switch between the adapter and the machine.

    Is this a ball "launcher"? How many and what size batteries does this

    Good luck,
  3. Top Spin

    Top Spin Guest

    Absolutely. I just wasn't sure one with the exact specs would be
    OK, thanks. Is that all I have to do? If I get an accurate reading on
    voltage and current, am I good to go?

    What's the best way to measure the voltage and current? Do I charge
    the battery fully and put the tester across the battery leads? Does
    battery output vary as it discharges? Does it vary according to the

    I haven't taken the thing apart yet. I don't know how many batteries
    there are or how they are wired.
    My plan was to put in a switch that would switch the motor between the
    battery and the A/C adapter.
    Yep, it shoots balls by pinching them between two rollers.

    It's a Tennis Tutor Model 4 Plus. I couldn't find a link to the
    manufacturer, but here's one to one retailer:
    Dunno. I thought I'd wait until I found out if it was even possible
    before taking it apart.

  4. Don Bruder

    Don Bruder Guest

    (Warning: ASCII-art ahead. View with a fixed-width font like Monaco or

    Possible? Yes. Difficult? Maybe.

    What you're going to need to know two critical things:
    (1) What voltage does this unit expect for input?
    (2) How much amperage (current) does it require?

    For #1, it's pretty simple - Find the "+" and "-" output connection on
    the battery/battery-pack, and slap a meter (set to a high range, if
    possible - if at first you don't get a reading, back down the ranges
    until you get something usable) across the terminals. You don't mention
    whether it's using rechargables or throwaways, but I would "ass-u-me"
    rechargables. If that's the case, check the voltage after a fresh
    charge. If not, check it with fresh batteries/battery pack.

    For #2, find the same connections, then run a jumper wire from the "-"
    connection of the (fresh or freshly recharged) battery/battery pack to
    the "-" connection of the device, then put your meter (Again, set on as
    high a scale as it has - you can fry your meter if it doesn't have
    enough range, or you start out too low) in the circuit by using the
    leads - one lead (the red/hot/positive lead would be best here) on
    battery "+" conenction, other lead on power-input "+" connection on the
    machine. Fire up the device, and read the meter. Like the voltage
    measurement, if you can't get a reading, try backing down a notch on the
    range scale. Keep backing down the range until you get a usable reading.
    What you read is the *ABSOLUTE MINIMUM* amperage the unit needs. I
    personally wouldn't try to feed it from a supply having less than 1.5
    times that number.

    You need a power supply that's rated for whatever voltage you found for
    item 1, +/- a couple volts (For a DC motor, voltage isn't *REALLY*
    critical, but if the voltage is too high, it can either cook off the
    motor, or spin it so fast it basically explodes. Too low, and it won't
    operate as intended - Instead of a "cannon-shot", you might end up
    getting something more like a spitball shot by someone in the middle of
    an asthma attack) and *AT LEAST* the amperage you measured. Preferably,
    whatever you measured plus some "elbow room".

    So, let's assume for the moment that you've figured out that the machine
    wants 12 volts at 1 amp. (Yes, Virginia, I pulled those number out of
    thin air.)

    If that's the case, you want a power supply that puts out somewhere
    between about 11 and 14 volts, and is capable of supplying a minimum of
    1 amp. For something like this application, I'd prefer one that can
    supply 1.5 or even 2 amps, since I expect that there will be momentary
    heavy draws each time a ball is shot out of the machine (due to the
    increased load on the motor that each ball being run through the rollers
    will cause) and at startup (when the motor is trying to get up to speed
    from a dead stop - sort of an "extreme" case ofwht happens when pumping
    out a ball)

    In some situations, amperage rating of the adapter is critical. In this
    one, I doubt you're going to need to worry about it beyond being certain
    that you've got *AT LEAST* as much amperage available as the beast
    wants, plus some (25-50% is a good starting point, but more may be
    needed) in reserve. If your supply can handle feeding out more than
    what's required, that's fine, and should do no significant harm unless
    it's ridiculously oversized, but the extra is basically going to be
    wasted. If it can't supply enough, the adapter is all but guaranteed to
    cook itself trying to supply more than its components are able to cope

    You may be able to buy an adapter to fit your needs "off-the-shelf",
    especially if the thing wants a fairly "standard" voltage/amperage
    combination. If the motor wants some crazy voltage/current combination
    (22 volts at 9 amps, or something similarly off-the-wall), you may be
    stuck building from scratch, modifying something you can find, or having
    to have something custom-built for you.

    As far as wiring it in, that's pretty easy: negative to the negative
    input, positive to the positive input.

    If the "switch" you're talking about is intended to flip between
    battery/AC, rather than just being an "on/off" concept, it's a LITTLE
    more complex, but no big deal. Instead of putting in a SPST switch on
    one side or the other, you're going to want a DPDT switch.

    For simple "on/off" operation, wire it like so:

    SPST switch
    1 2
    Supply+ --------------O O------------ Load+

    Supply- -------------------------------------- Load-

    As should be intuitively obvious, when "on", Terminals 1 and 2 of the
    switch are connected, and when "off", they aren't.

    An alternative, using a DPST switch:

    DPST switch
    1 2
    Supply+ --------------O O------------ Load+

    3 4
    Supply- --------------O O------------ Load-

    For "on", terminals 1 and 2 are connected, as are terminals 3 and 4. For
    "off", none are connected.

    For Battery/AC switchover, you want to use a DPDT switch, and wire
    things like so:

    DPDT switch
    1 3 5
    From adapter+ O--------O O-+ O------------O From battery+
    2 4 | 6
    From adapter- O--------O O | O------------O From battery-
    | |
    | |
    | +---------------O To load+
    +-----------------O To load-

    When in the "Adapter" position, terminal 1 is connected to terminal 3,
    and terminal 2 is connected to terminal 4. When in the "Battery"
    position, terminals 3 and 5, and terminals 4 and 6 are connected. No
    matter which position the switch is in, the "Load" lines get juice with
    the correct polarity.

    (Note: In all three diagrams, my terminal numbering is COMPLETELY
    arbitrary, and may not have even the slightest resemblance to how the
    switch you actually end up using is numbered, if its numbered at all.
    Adjust accordingly.)

    Whichever way you wire it, be certain to use switches rated for at least
    the voltage/current you figure out your ball-chucker needs, or you can
    expect them to burn up quickly. Again, a "safety margin" is a good idea,
    so if you need 12 volts at 1 amp, it wouldn't be an even slightly bad
    idea to put in a switch rated for 15, 20, even 50 volts, and current of
    1.5, 2, or even 5+ amps. Design it "over-rated" from the start, and you
    won't be having to replace things as they burn up. (which is certain to
    happen sooner or later if you use under-rated components)
  5. DaveC

    DaveC Guest

    This thing uses a lead-acid rechargeable battery. These batteries
    provide huge currents, meaning that this "launcher" requires large
    currents. If you were presuming some kind of small "wall wart" kind of AC
    adapter, that's totally out of the question. The power supply to run this
    launcher will be sizable, and probably not easy to find/construct, IMNSHO.

    Does the manufacturer have an option for AC operation?
  6. happyhobit

    happyhobit Guest

    Solution #1
    What are the specks on the battery charger? (Voltage and Current) With a 12
    hour charge time and a 6 hour run time you'll need about 2.5 times the
    current rating of the charger. (Ballpark.guess) If the charger output is
    below 12 volts it's a 6 volt battery. If it's over 14 volts it's a 12 volt

    Solution #2
    The battery (Lead-acid sealed) is probably 6 or 12 volt. (2 volts per cell)
    It should be marked as to voltage and current (Amp hours). Divide amp-hours
    by 6 to guess current draw. If the battery's not marked then look it up on
    the internet.
  7. Ross Mac

    Ross Mac Guest

    Have you tried to contact the manufacturer?...They may have an off the shelf
    solution that would be easier/cheaper than reinventing the wheel. If you
    measure the current I would suggest you use an amp clamp with a hold feature
    so you will know the maximum current draw. You would stand to fry your power
    supply if you don't pay attention to this value...good luck with your
  8. happyhobit

    happyhobit Guest

    Why do you want an A/C adapter? Do you plan to remove the battery?

    I'm trying to understand why five or six hours of run time isn't sufficient.
  9. Top Spin

    Top Spin Guest

    I am having trouble finding the manufacturer. I have sent email
    messages to a couple of retailers. We'll see.
  10. Top Spin

    Top Spin Guest

    For just myself, 6 hours of continuous tennis drills is almost enough
    for a decent workout! ;-)

    I am thinking about donating it to my club where it might be used by
    several people in the same day. Being able to leave it courtside
    (where they have A/C power) would be preferable to having to remember
    to plug it back in for recharging after each use.
  11. Top Spin

    Top Spin Guest

    I got a reply from one retailer. There is an A/C adapter for this
    machine. It goes for $140 and plugs into the same jack as the A/C
    charger. The retailer said that the battery must be at least 75%
    charged for it to work, but then it will run all day off of the A/C

    I am curious how this works. It sounds like this is just a
    bigger/faster battery charger. The machine is still running off the
    battery while it is being quick-charged.

    I would imagine that this would affect the battery life. No? It's
    supposed to have 1,000 charges (I think) under normal use. How much
    more quickly will the battery die using this device?

    Would it still be worth it to explore making a true A/C adapter that
    could bypass the battery altogether?

  12. Ross Mac

    Ross Mac Guest

    Sounds like the way to got here....You would probably be hard pressed to
    build something cheaper.....
  13. Ross Mac

    Ross Mac Guest

    Sounds like the way to go here....You would probably be hard pressed to
    build something cheaper.....
  14. Top Spin

    Top Spin Guest

    I'm sure you are right, but if it burns up batteries quickly, it might
    be cheaper in the long run to build something that would bypass them
    altogether. Besides, it's a project!
  15. happyhobit

    happyhobit Guest

    Lead-acid batteries are charged until the voltage/cell equals 2.2 to 2.4
    volts @ 70 degrees F. 2.2 volts/cell if the battery is on continuos charge
    and 2.4 volts/cell if the charger will be removed when it reaches this value.
    Ask the retailer what voltage / current the $140 charger is rated for. Look
    at the ratings on your charger. The ball's in your court.
  16. Ross Mac

    Ross Mac Guest

    Give tech support a call and they should be able to answer your specific
    questions. It would be hard, in this forum, without all the data to give
    accurate answers. That is unless you are lucky enough to find someone who
    has the of luck on your project....Ross
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