Power
Supply DIY Project
Safety Precautions : First
thing to be concerned about if you are going to take on building a power supply
as a project is safety. Remember that you will be working with 115 volt AC
power from the wall within this device and it misused, that amount of power can
be deadly. This project should also not be the first time you ever used a
soldering iron ... and good basic building techniques must be followed to build
a reliable & safe unit. Pay close attention also to the specs for
components in the parts list. Incorrect or misassembled parts can cause
serious problems, up to and including an explosion ! Do not operate one of
these widgets with the cord plugged in and the box open and likewise to not
operate in or around water. Using a three wire power cord with the ground
(green wire) attached to the housing provides added insurance and safety against
potential problems and is recommended.
Construction Tips : There
are a number of soldered connections in this supply. To make good solder
joints use a good grade of 60/40 rosin core "electrical" solder.
Make sure that you never use acid core solder or acid flux in any electrical
connection. The acid is highly corrosive and eats copper wires for lunch
.... not what you want here. You will need to secure wires and components
within the enclosure. To do this you will need pop rivets (preferred) to
secure the transformer to the housing, double stick servo tape to hold down the
capacitor(s), plastic wire ties to hold wires in place and a sturdy screw and
nut (preferably with a metal lock washer) to hold the full
wave bridge to the aluminum case (required for heat sinking). In addition, all high voltage (115 volt AC side)
must be fully insulated using heat shrink tubing and potentially a blob of
silicone glue/caulk on the back of the on/off switch. Insulating the low
voltage side connections (after the transformer) is not required, but certainly
recommended. If the 2 wire polarized plug is used, make sure the hot side
of the current (black wire inside the 115 volt electrical box) is the wire that
goes through the on-off switch (improved safety if wired that way !). Make
sure the box is fully enclosed and buttoned up before plugging in the power
cord.
Parts List (suggested hardware for 5
Amp "12 volt" supply)
T1 - Transformer : 12 volt secondary, 115
volt primary, 3 amp rating
C1 - capacitor : 15,000 microfarad
polarized electrolytic 25 volt rating (or larger)
S1 - switch : miniature/subminiature
toggle switch single pole, double or single throw, 3 amp
D1 - full wave rectifier bridge : 25 amp
50 PIV (peak inverse volts) aluminum case preferred
Fuse : 2 to 3 amp rating (enclosed unit
recommended)
Circuit Breaker : 5 amp rating (fuse
holder and 5 amp fuse can be used)
Enclosure : Aluminum 2 part case (do not
use plastic)
Power cord : 2 wire (polarized) or 3 wire (3 wire recommended)
Rubber grommets (2) : to isolate incoming
/ outgoing wiring from enclosure
Rubber feet (3/4) : stick on recommended
(optional)
Banana Plugs - red/black (optional)
Wire : 14 gage minimum recommended
Comments on da parts : The
fuse used is on the high voltage side and as such should be enclosed or
otherwise insulated (Professor Motor can provide an enclosed glass fuse
holder {$1} or you can also use a 2 amp circuit breaker {$2.50 Professor Motor
part number PMTR-2035} or other such device , open frame glass fuse holders are
not recommended. The transformer choice may seem odd, but transformers are
rated in RMS terms (root mean square), the resulting current capability is
actually 1.4 times the RMS rating, or in this case 1.4 x 3 amps = 5.2
amps. In a similar way you will find that the finished supply will
actually produce measured voltage output no load (without anything attached to the
output) of approximately in the 16 to 16.5 volt range. If the 3 wire power
cord is used, pop rivet a soldered-on eyelet connector (attached to the green
wire !) to the aluminum case from inside that housing.