BEFORE Wiring The Power Supply Transformer
The power supply board was intentionally
designed to allow a wide variety of transformers to be used. As such, no
transformer will be a perfect mechanical fit. While the mechanical footprint
will vary, any transformer that can provide 35V at 220ma will suffice (that's
The transformer specified in the parts
list is a good, economical, torroidal variety and if chosen you will need
to attach wires to the legs as shown below. (Again, our Power Supply Board
does not provide holes to accept the legs for this type of transformer.)
Look at the top of the transformer and
note the voltage markings. The two 115V windings are the PRIMARY and will
be connected to the AC mains via fuse and power switch. On the SECONDARY
side are two 22V windings that will be connected to the rectifier via J1
on the power supply board.
NOTE: Since there are two windings,
this transformer can be rewired for 220-volt operation. Fuse and switch
connections are external to the power supply board and will be mounted
to the chassis. YOU MUST USE A FUSE for safety and economics reasons. We
all make mistakes. Mis-wiring on the Secondary (low voltage) side of the
transformer could cause irreversible damage.
For 120 volt operation the two primary
windings will be connected in parallel. The secondary windings are connected
in series with the center tap going to ground. Begin by taking 3 different
colors of 24-guage stranded wire. A good choice of "standardized" colors
would be Black (hot), white (neutral) and green (chassis / ground).
The type of power cord, switch and fuse
are left up to you. You may want to put a chassis mount fuse holder and
IEC type removable AC connector on the back. The power switch should be
mounted on the front panel for easy access.
Assuming the above, a white wire will connect
the neutral side of the IEC AC connector to the transformer primary. Safety
ground from the IEC AC connector should be attached to a star point on
the chassis. See the drawing below for details on wiring a fuse and switch.
A black wire will connect from the hot
leg of the IEC AC connector to the fuse and then from other fuse terminal
to the front panel power switch. (Each is in "series."). The other side
of the front panel power switch will be attached to the transformer primary
(but don't do that yet).
ONE WIRE TRICK
The black wire from the power cable / IEC
socket will feed the fuse and then the power switch.
Cut a piece of black wire, about 18" long,
to go from there to the power transformer. Strip and tin one end of the
black wire, loop it over the appropriate leg on the transformer as pictured.
Make sure you are on the high voltage PRIMARY side of the transformer (but
donít solder it yet). See Figure-1.
TWO WIRE OPTION: Primary (line
After cutting through the circumference
of the insulation, cut horizontally across each vertical cut and then peel
away the insulation. If you canít get the insulation to cut away cleanly
without breaking the wire strands underneath, skip this step and using
two wires instead. Keeping the wire in one piece is a neat way of making
this connection but has nothing to do with function.
Now bend the exposed section of wire around
the transformer leg and solder each end in neatly. Trim any excess wire
that may be sticking out so it does not come in contact with the adjacent
legs of the transformer. Continue with a 6" piece of white wire in the
same fashion as the black wire and solder to the remaining two primary
legs as shown.
SECONDARY (low voltage)
Wire the secondary side of the transformer
by attaching a 6" piece of green wire to the two legs as shown below. Make
the long piece extend out the left side of the transformer as you look
at it from the bottom. The green wire is the center tap and will be attached
to ground near the rectifier via J1. Pictured Below.
To differentiate the mains voltage from
secondary voltage, itís a good idea to use at least one different color
wire on the secondary. I used red, and white for the AC connections to
the rectifier D-1 via J-1. Cut 4" of red and white wire. Solder them to
the outside legs. Put red on the right and white on the left as shown below.
Cut the secondary leads to fit into J-1ís holes and solder them in the
order as pictured. From this view, the red wire goes on the left
outer hole, the two inner holes are ground (for the green center tap wire
ó either hole is correct) and the white wire goes to the outer right hole.