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# Build an A/C adapter?

T

#### Top Spin

Jan 1, 1970
0
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
one?

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?

Thanks

D

#### DaveC

Jan 1, 1970
0
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
one?

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
require?

Good luck,

T

#### Top Spin

Jan 1, 1970
0
Easier & safer to just find one already available.

Absolutely. I just wasn't sure one with the exact specs would be
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.

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
load?

I haven't taken the thing apart yet. I don't know how many batteries
there are or how they are wired.
Put a switch between the adapter and the machine.

My plan was to put in a switch that would switch the motor between the
battery and the A/C adapter.
Is this a ball "launcher"?

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:

http://tennis101.com/portable_tennis_ball_machines/md_4_plus.htm
How many and what size batteries does this
require?

Dunno. I thought I'd wait until I found out if it was even possible
before taking it apart.

Thanks

D

#### Don Bruder

Jan 1, 1970
0
Top Spin said:
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
one?

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?

Thanks

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

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
with..

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)

D

#### DaveC

Jan 1, 1970
0
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?

H

#### happyhobit

Jan 1, 1970
0
Top Spin said:
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
one?

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?

Thanks

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
battery.

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.

R

#### Ross Mac

Jan 1, 1970
0
Top Spin said:
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
one?

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?

Thanks

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
project....Ross

H

#### happyhobit

Jan 1, 1970
0
Top Spin said:
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
one?

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?

Thanks

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.

T

#### Top Spin

Jan 1, 1970
0
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?

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

T

#### Top Spin

Jan 1, 1970
0
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.

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.

T

#### Top Spin

Jan 1, 1970
0
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?

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 adapter. 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? Thanks R #### Ross Mac Jan 1, 1970 0 Top Spin said: 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
adapter.

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?

Thanks

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

R

#### Ross Mac

Jan 1, 1970
0
Top Spin said:
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 adapter. 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? Thanks Sounds like the way to go here....You would probably be hard pressed to build something cheaper..... T #### Top Spin Jan 1, 1970 0 Sounds like the way to go here....You would probably be hard pressed to build something cheaper..... 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! H #### happyhobit Jan 1, 1970 0 Top Spin said: 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
adapter.

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.
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.

R

#### Ross Mac

Jan 1, 1970
0
Top Spin said:
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!

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 specifics....best of luck on your project....Ross

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