A Test Bench Power Supply
the time, however, and wouldn’t have
done it any other way.
The four supplies for these meters
take up very little room and the total
cost for all the components required
was just over $5. These supplies need
only half wave rectification and little
filtering since load requirements are
on the order of 5 mA each.
I built this unit as small as
possible, but you may want to open
up the overall dimensions slightly. The
power supply’s overall dimensions are
9” wide x 3-1/2” high x 6” deep. If you
alter the design, lay out all your parts
first to get specific measurements for
cabinet clearances and dimensions
needed before starting construction.
When I make up an enclosure for
circuits requiring a fair amount of heat
dissipation, I usually build it as follows
(see Figure 3).
The bottom and back are made of
one piece of 1/8” aluminum sheet. A
90 degree bend is formed at the intersection of the bottom and back. In my
case, this was a sheet 9” x 9-1/2” with
the 90 degree bend 3-1/2” from the
long end. This part of the enclosure
will also double as the heatsink,
eliminating a lot of expense and clutter. Pop rivet three 1/2” aluminum
angle brackets to the sides and front
of this base, flush with their edges. (In
my unit, these were two 6” lengths
and one 3-3/4” length.) These will be
the attachment points for the front
panel and top cover.
Next comes the front panel. This
was a piece of 1/16” aluminum cut to
3-1/2” x 9”. Lay out mounting holes at
the bottom and transfer these to the
front angle bracket on the bottom
plate. Drill and tap these for the front
panel mounting screws.
Next, the metal cover is formed.
Mine was 16” x 6-1/2” with two 90
degree bends, 3-1/2” from the long
ends, to form the sides. The 6-1/2”
dimension gives the cover a little
overhang on the front and back of the
enclosure. Drill two
mounting holes at the ■ FIGURE 4
bottom of each ■ FIGURE 3
side. Align the
cover to the base and transfer
those holes to the side angle
brackets. Drill and tap these
for the cover mounting screws.
Some fitting may be
necessary as you are constructing this. Disassemble
and then later paint it the
color of your choice, but do
not paint the 1/8” bottom
sheet. Add four feet (can be
self-adhesive) and the basic
enclosure is done. Strong,
functional, attractive, and
simple, to boot!
At this point, the major
components can be installed.
Make sure you have allowed
ample room and clearance in
regards to overall dimensions for the
components you have selected. I
installed the main power transformer
(T1) in the rear center of the bottom
plate. This is a logical mounting point
and gives the unit good balance. On
either side of T1 are two circuit boards
spaced and stacked on standoffs, and
measuring 2-3/8” wide x 3-1/2” deep.
(That will be four boards total.) To the
left side of T1 is the five volt panel
supply transformer T2, a low profile
type mounted to the bottom board.
Standoff mounted, the upper board is
one of the 0-20 volt (A) boards.
To the right side of T1 are the five-volt supply regulators along with T3,
T4, and is the bottom board. As with-the left side, the mounted standoff is
the other 0-20 volt (B) board. The rear
panel receives two LM317K ICs
mounted directly behind their respective regulator boards. The 7805 (IC5)
chip is mounted to the bottom plate
near the front left hand corner of its
regulator board. F1, S1, and the line
cord input are mounted through the
back panel wherever it’s convenient.
The front panel components should
be laid out in a nice symmetrical order
and then machined. Watch for clearance here to interior components!
For front panel labeling, I have
been experimenting with various
escutcheons. Recently, I have been
using some of the CAD programs
from PC and front panel manufacturers. Any CAD program will work as
long as it gives you a reference point,
precise layout cursors, and the ability
March 2007 45