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
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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)