HIGH-POWER
45V/8A VARIABLE
LINEAR SUPPLY
Over the last two issues,
We described how our
45V/8A Linear Bench
Supply works and how
to assemble its main PCB
control module. Now it’s
time to finish it off. That
involves cutting some holes
in the case, mounting the
components inside, attaching
the front panel controls, wiring it
up and the final calibration/testing.
W
e chose to put the Bench
Supply in a Jaycar HB5556 instrument case because it’s just
big enough to fit everything without making it too large or heavy; it’s reasonably
priced and easy to get, easy to work and
it has plenty of ventilation for the required cooling air.
The following instructions assume
you are using that case. If using a different case, make sure that all the parts
will fit inside and that nothing will foul
anything else; if it’s substantially larger,
you should be fine.
You also need to ensure that it has adequate ventilation, especially in the top
and bottom panels around where the
heatsink will be mounted. Ambient air
is sucked in through holes at the rear of
the case, blown over the heatsink and
exits through holes above and below the
heatsink fins. Your case will need to have
a similar arrangement.
It also needs to be made of steel or aluminium, not only for strength but also
so that all of its panels can be earthed
for safety.
Any case that meets these requirements can be used, but you will have to
vary the instructions regarding where
to mount the components inside the
case and on the front and rear panels,
and adjust the cutout and wiring placements to suit.
34
So without further ado, let’s get to finishing off the Bench Supply.
Preparing the case
Several holes need to be drilled and cut
into the metal instrument case. The front
panel hosts the panel meter, control potentiometers, output binding posts, overcurrent LED and load switch, while the
mains socket and fan cooling holes are
on the rear panel. All six panels also have
earth screws to ensure safety.
The bottom part of the case also needs
to be drilled to mount the transformer,
PCB and heatsink. The top and bottom
panels are vented; the case is oriented
with the vents at the rear, as this is where
the fans and heatsink are mounted.
It may help you to start by putting the
case together, so you understand how
all the parts fit, then mark where holes
will be drilled in each panel while it
is in place.
Fig.8 shows the hole locations and
sizes for the front and rear panels. We
recommended in the article last month
that you use the blank PCB and heatsink
spacer to mark out the required hole locations in the base, as well as the hole
for the transformer mounting bolt. Check
now that these are in the right places.
The case is made of aluminium, so
it is not hard to work. No holes need
to be made in the case sides, but their
Part 3
by Tim Blythman
internal ribs must be trimmed to allow
all the components to fit. We recommend
test-fitting all the parts before doing any
drilling or cutting, to make sure it will
all go together properly later. This is
especially true if you are making any
variations from our design.
Rear panel preparation
Even though the panels are not flat, they
can be held in a vice by placing them
between some scraps of timber. This
will also help to prevent damage to the
enamel finish.
We opened up the large holes in the
panels using a 3mm drill bit on a drill
press, making numerous closely-spaced
holes inside the outline. The holes were
then joined with a hacksaw, after which
the edges were brought to dimension and
finished with a file. You may also find a
nibbler useful, if you have one that can
handle 1mm thick aluminium.
For the hacksaw cuts, we removed
the blade from the hacksaw, threaded it
through the pilot hole, reattach the blade
to the hacksaw and then made the panel
interior cuts.
We suggest that you use a similar
technique to make the cutout for the IEC
socket. Mark its outline on the rear panel
and then drill a series of small holes inside the perimeter. Keep the holes well
inside the markings.
Practical Electronics | December | 2020
Fig.8: these are the cutting and drilling diagrams for the front / rear panels. Note they are 60% of life size, so to copy and use as a
template you will need to enlarge them by 166.7%; or download them as PDFs from the December 2020 page of the PE website.
Drill a larger hole (large enough for a
hacksaw blade or other small metal saw)
inside. Then use a hacksaw blade to cut
towards the corners from the large hole
in the centre.
Take care that the sheet metal does not
bend and break on the forward stroke.
Once the cuts have reached the corners,
the triangular shapes may be flexed along
the drill holes, to break them off.
Use a file to carefully bring the edges
of the cut to their correct dimensions.
Keep the mains socket nearby to test fit,
as you do not want to take away too much
metal. This could cause the receptacle to
be not held securely by its tabs.
Try fitting the socket at an angle to
test the height and width independently.
Once the dimensions are correct, gently
run a file across any sharp edges of the
opening to remove any burrs.
Now is a good time to drill a 3mm hole
in the rear panel for the earth connection.
The location is not critical, but placing it
near the receptacle minimises the earth
wire length. Sand the inside of the panel
until you have an area of exposed bare
metal 1cm in diameter around the hole.
The aim is to make a good metal-tometal connection with the eyelet lug at
the end of the earth wire.
You will also need to drill eight 3mm
holes to mount the fans. Test fit the fans
to check their locations as there is not
much room around the fan guards, and
they need a small amount of clearance to
allow the filters to be clipped on and off.
You may need to space the fan mounts
so that they aren’t hard against each other.
Two large holes are required so air can
be drawn in by the fans. We traced out
a circle using the inside of the fans as a
There isn’t much mounted on the rear panel; just the switched, fused IEC mains input socket and the two cooling fans.
The small screw head visible to the right of the mains socket is the main earth point inside (see photo on page 39).
Practical Electronics | December | 2020
35
The front panel of the Bench Supply has two knobs to set voltage and current and a switch to connect or disconnect the load,
along with the three output terminals. The red LED above the current knob indicates when thermal limiting is occurring.
The LCD screen shows the actual and set voltages, actual current and current limit, plus the heatsink temperature.
template, but any circular object around
80mm across will be fine (or copy/print
Fig.8 to use as a template). Check that the
fan guards completely cover your marked
hole before cutting it out.
Use a similar technique to the IEC receptacle to open out the holes. Drill a series of small holes and then open up the
panel with a hacksaw blade and finish
by filing down the rough edges.
You can now fit the mains socket. Orient it so that the lead plugs in below the
switch, allowing access to the switch
from above. Now is also a good time to
insert the fuse.
While the 6A fuse chosen may seem
excessive for a 500W transformer, this is
the recommended rating for that transformer. Lower-rated fuses will blow
due to inrush current when the unit is
switched on.
If you want to use a lower-value fuse,
it will need to be a slow-blow type.
Front panel preparation
The front panel is treated similarly to the
back. Assuming you are using our Fiveway Panel Meter, check that your LCD
screen’s dimensions match our template
and then transfer this to the front panel.
We have designed an acrylic bezel that
suits the LCD on the Five-way Panel
Meter, which hides any small inaccuracies in cutting the front panel around
the meter.
You can place the bezel over the LCD
to see if it matches the dimensions and
if so, use it as a template to mark out the
front panel. Otherwise, use the LCD dimensions or Fig.8 as your guide.
If you have separate panel meters,
check their specifications for recommended cutout dimensions, and plan
how they should be laid out, leaving
room for the binding posts, switch and
the potentiometers.
Cut out the opening for the panel
meter(s) using the same technique as for
36
the mains socket. Don’t forget you need
to drill the four 3mm mounting holes.
You also need to drill two or three
holes for the binding posts; three are
required if you want an earth post,
which can come in handy from time
to time. Otherwise, the supply outputs
are ‘floating’. Check the diameter of
the holes required for your posts and
drill them with equal spacings. Ours
were 9mm.
Start these holes by using a punch to
locate the centre of the hole and then by
drilling with a smaller size to create a
pilot hole. Finish with the recommended size drill bit to complete the hole.
Similarly, drill a hole below the panel meter for the output on/off (load)
switch. Typical panel-mount toggle
switches require a 6.5mm hole, but
again, it’s best to start with a smaller
pilot hole and then enlarge it to the final size before deburring.
Below the banana socket holes, add a
3mm hole for the front panel earth. As
for the rear panel, sand the inside to remove enamel for about 1cm around it.
The two potentiometers require two
holes each to mount; one for the shaft
and a second to hold the locating lug so
that the pot won’t rotate. Drill the two
holes using the usual technique.
Drill a hole for LED1 as well, taking
into account the bezel diameter.
If you wish to add our front panel artwork, you should do so now. You can
download it as a PDF from the December
2020 page of the PE website; then print
it out and laminate it. Note that the front
panel is wider than an A4 piece of paper
is long, so it will look better printed on
A3 so that no joins are needed.
Mounting the front
panel components
Solder a 20cm length of black 10A-rated wire to the black binding post, and a
20cm length of red 10A-rated wire to one
terminal of the output switch. A second
5cm length of red wire is then soldered
between the other switch terminal and
the red binding post.
Insulate the solder joints with heatshrink tubing. Strip back the last 5mm of
both free wires for connecting to CON1
on the main PCB.
If adding an earth binding post, attach
a short length of 10A green/yellow wire
stripped from mains flex or a mains cord,
and crimp or solder an eyelet (ring) lug to
the other end. It will attach to the front
panel earth screw later.
The binding posts and output switch
can now be secured using the supplied
nuts and washers. Orient the switch so
that it makes the connection from the
red binding post to CON1 + on the PCB
when it’s down (the standard position
for ‘on’ in Australia and New Zealand).
Thread the potentiometer shafts
through the panel from the back and locate the lugs into the smaller holes to
stop the potentiometers from rotating.
Secure at the front with mounting nuts
and fit the knobs.
We used spline shaft potentiometers,
which allow the knobs to be attached
at almost any angle. If you have D-shaft
potentiometers, you may need to rotate
the front part of the knob later so that
the pointer sweeps over an appropriate
range (these can usually be prised off
with a knife).
Now mount the rest of the front panel
hardware. Fit the LCD bezel by threading
a 12mm M3 machine screw through each
corner, then feed the screws through the
holes in the front panel. Secure with M3
nuts at the back of the panel.
If your LCD has mounting holes which
are too small to fit an M3 screw, these can
be carefully enlarged with a 3mm drill
bit, ideally in a drill press. Avoid inhaling
the fibreglass dust which results.
The Five-way Panel Meter LCD can
then be threaded over the back of the
Practical Electronics | December | 2020
machine screws and held in place by
four more nuts. Attach the IDC cable to
the header, ensuring the marked pin 1 on
the cable lines up with that on the PCB.
Finish by pushing the LED with bezel
through the hole you drilled for it earlier.
Transformer and main PCB
If you haven’t already marked out and
drilled the required holes in the bottom of the case, use the populated PCB,
heatsink spacer and transformer to determine where the holes need to go. All
of these need to be drilled to 3mm and
deburred, except for the transformermounting bolt hole which will need to
be larger. Measure the diameter of the
supplied bolt; around 8mm should do.
Before drilling those holes, it’s a good
idea to slot the front and rear panels
into the case to make sure that the internal components will not foul anything mounted on either panel. Test fit
the transformer and PCB according to
the markings, to ensure that everything
fits as expected, then drill the holes.
You may need to remove the side
panels as they are likely to conflict with
the PCB and transformer mounting positions. You can test fit these later to
confirm how they need to be trimmed.
We needed to trim away some of the internal parts of both side panels on our
prototypes, as the side panels protrude
slightly into the case near their fastening holes and screws.
Check that there are no collisions between the PCB, transformer and front
and rear panel hardware. Keep in mind
that the fans and their spacers will sit
between the heatsink and the rear panel. You might also like to check that the
transformer’s leads reach the mains plug
receptacle and the bridge rectifier tabs
on the PCB.
If everything appears correct, then
drill the holes in the base. The smaller holes for the PCB and heatsink that
sit in the vented region of the base can
be tricky to drill, but if they end up
slightly out of the marked positions,
that should not be a big problem. In
the worst case, you will just have to
enlarge these holes slightly.
Also drill a 3mm hole for the mains
earth in the base. Place it near the mains
receptacle, but clear of the vented region. As with the other earth holes,
sand the area around it to expose the
underlying metal.
The transformer is quite heavy so
take care not to drop it while working
with it. Feed the bolt through the bottom of the case, then place one of the
rubber gaskets over its shaft on the inside. Lower the transformer into place,
rotating it so that the wires are close to
where they need to connect.
The second rubber gasket goes on
top of the transformer, followed by
the dished metal plate with its convex side facing down. Slide the small
washer in place, thread the nut onto
the bolt and tighten it up to a reasonable degree, so the transformer is held
securely in place. Do not overtighten
it or you could damage the transformer windings.
Remove the two 9mm tapped spacers
from the PCB that are nearest to the
heatsink. Alternatively, if you haven’t
already fitted them, fit the two spacers
furthest from the heatsink but leave the
other two off.
Getting the PCB into position in the
case can be tricky due to the weight of
the transformer. We found that it was
possible to balance the case on its edge
by using the weight of the transformer to
hold it upright.
Start by feeding one M3 × 10mm nylon
machine screw through the base of the
case and into the heatsink, making sure
to thread it through the acrylic spacer.
Then fit the other three nylon machine
screws to hold the heatsink in place.
This should also hold the PCB in place,
for now. Metal screws cannot be used on
the heatsink as this would connect the
live heatsink to earth.
Use two machine screws to secure the
front of the PCB to the bottom of the case.
Now is a good time to attach the feet
to the case. We used taller feet than
those included with the enclosure, as
those were so short that the transformer
mounting bolt head was still touching
the bench with them in place. Taller feet
The main requirement
for the SPST ‘LOAD’
switch (mounted under
the display) is that it
must be capable of
handling the whole
output current – up to
8A DC. Practically, this
means you’ll need a
10A DC switch – don’t
be tempted to use one
only rated for 10A AC
– it’s not enough!
Practical Electronics | December | 2020
also provide more space for cooling air to
escape via the underside vents.
Rear panel and fan mounting
The fans can now be fitted. They are
mounted to the rear panel on spacers.
Ideally, they should be as close as possible to the heatsink, but not touching.
Take one fan and thread four 32mm
machine screws through the corner
holes. Fasten them to the fan using the
15mm-long M3 tapped spacers. These
will sit against the rear panel, so if there
is room to bring the fans closer to the
heatsink, nuts or washers can be placed
under the spacers.
Just make sure that the fans don’t
touch the heatsink fins.
Now separate the fan filters/guards
into two pieces and place the fans on the
inside of the rear panel and the guards
on the outside. Attach the fans using
9mm-long M3 machine screws through
the guards and rear panel, and into the
tapped spacers attached to the fans.
Clip the fan filters back into place on
the guard frames.
With the PCB and transformer in
place, you can mark and cut the required
cutouts in the side panels, to clear the
internal components.
You can see how much material we
had to remove in our photos. There is a
fair degree of overlap between side, top
and bottom panels, so slight inaccuracies
in cutting the side panels will be hidden.
Firmly hold the side panel in a vice
using timber off-cuts to protect the finish. Make the marked cuts with a hacksaw. If the panel vibrates as you saw,
try clamping it closer to where the cut
is being made.
Check that the panels now clear the
transformer, PCB and heatsink. Once
everything fits together correctly, dress
any sharp edges of the side panels
with a file.
The side and top panels will also
need to be earthed. This can be done
via the remaining sections of the
mounting tabs. These are already slotted, so you don’t need to drill any holes.
Just remove the enamel from a small
area on one of these tabs, where the
earth eyelet will be attached later (see
photo on page 38).
Use an area near the back of the side
panels, as the earths will all connect
back to the rear panel.
For the top panel, choose a location opposite the earthing location on
the bottom panel, which is otherwise
clear of components. Drill a 3mm hole
and sand the inside of the panel as for
the others.
Making the final connections
The leads to the fans, LED, panel meters and thermistor can be plugged into
37
their respective board connections. The
leads for the banana sockets screw into
terminal block CON1.
Ensure that they are connected with
the correct polarity, ie, red wire to the
‘+’ terminal.
Mains wiring
The transformer needs to have spade
crimp lugs fitted to mate up with the
IEC plug receptacle and bridge rectifier. The transformer we used has two
115V AC primary windings, which are
intended to be connected in parallel for
110-120V AC mains and in series for
220-240V AC mains.
The secondary windings are 40V AC
each, and in this application, they need
to be wired in parallel.
Also, the integral DPST switch in the
IEC input socket is not joined internally
to mains live or to the fuse. Instead, it
has separate spade lugs to make connections. So we will need two short
leads, one brown and one blue, to make
these connections.
Ensure there’s no chance that a mains
cord can be plugged in while you are
working on the mains side of the circuit.
Cut a 100mm length of brown wire
and another 100mm length of blue wire,
stripped from 10A-rated mains flex or a
spare 10A mains cord. Strip both ends
of both wires and securely crimp spade
lugs onto them. Insulate the exposed
metal using heatshrink tubing.
Once you’ve made up those two
wires, plug them into the rear of the
IEC socket, with one going from the
fused live terminal to one pole of the
switch and the other going from the
incoming neutral lug to the other pole
of the switch.
Do not connect them both to the same
switch pole!
Now is also a good time to insulate
the exposed metal strip on the back of
the IEC socket using neutral-cure silicone sealant, to make working on the
inside of the Bench Supply a bit safer.
To wire the transformer primaries in
series, solder the grey wire to the purple wire and cover the joint using two
layers of heatshrink tubing. Remember
to slip the tubing over the wires before
soldering them.
If you are using a different transformer than the one we specified, check the
manufacturer’s instructions for wiring
it up to a 230V AC supply.
Next, fit spade connectors to the
transformer’s brown and blue (primary) wires and insulate them with
heatshrink tubing. Push these onto the
two remaining switch terminals on the
mains socket, so that the wires going to
the two switch poles match (ie, brown/
brown and blue/blue).
It’s essential that you now use multiple cable ties to tie all the mains
wiring around the IEC input socket
together, so that if any of the wires
come loose, they won’t flap around
the case and potentially make contact
with the heatsink, PCB or any other
non-mains conductors.
The two 80mm fans we used were
specifically chosen for their high flow
rate. They’re Digikey P122256 24V
models, available from digikey.com
If you substitute other fans they
may not have the essential cooling
properties of these ones.
You will also need to fit a Presspahn
insulating barrier alongside the heatsink and PCB, so that if a mains wire
does somehow come loose, it cannot
come in contact with those parts.
Cut the sheet of Presspahn to 105 ×
208mm and score it 20mm in from
one long edge, making a 208 × 20mm
foldable section.
Now fold that part by 90°, place it
in the case alongside the heatsink and
drill two holes in the base, through the
bottom of the case, close to each end.
Attach it to the case using 6mm M3
machine screws and nuts.
The photo opposite shows what it
will look like when you’ve finished.
This piece will come close to touching the lid when it’s attached forming
an insulation barrier between the heatsink/PCB and the mains wiring.
You will need to use side cutter to
make two cuts along the top edge and
fold it down, for the transformer secondary wires to pass through. Again,
see the photo for an idea of how this
was done on our prototype.
It’s not immediately obvious here, but each of the mains spade connectors on the
IEC (input) socket (upper left of pic) is covered with a clear shroud. Also note
each of the removable case panels has its own earth wire attached, connecting
back to the main earth point on the rear panel (alongside the IEC socket).
Earth wires
The next step is to make and fit the
panel earths. Five green/yellow wires
are required with eyelet connectors
crimped to each end. These will go
from the rear panel earth screw to the
other panels. A sixth wire is needed,
with a spade lug at one end (to suit
the mains socket) and an eyelet at the
other, to go to the rear panel star earth
point. None of the crimp connections
need to be insulated.
Cut the earth leads to length, giving
enough slack so that you can pull the
panels apart later, and so that they can
avoid any components which might be
in the way. The lead for the top panel
38
Practical Electronics | December | 2020
should have more slack than the others,
as it will need to allow the top panel to
be detached and moved out of the way
while still being connected to earth.
Once the wires have been made up,
plug the spade terminal onto the earth
terminal of the mains socket. Thread
a 12mm M3 machine screw through
the rear panel hole, then place a star
washer over the screw shaft, followed
by the six earth wire eyelets.
Secure with an M3 hex nut and tighten well. Then add another nut on top,
doing it up moderately tight, to act as
a locknut.
Now terminate the other end of the
five remaining earth leads to the five
other panels similarly. The screw heads
should be on the outside of the case,
with the eyelet connected to each panel
through the star washer, with the screw
held in place by a nut done up tightly.
The front earth binding post (if fitted)
should have its eyelet placed on top of
the front panel earthing eyelet.
The final connections to be made are
from the transformer secondaries to the
bridge rectifier (BR1) on the heatsink.
To parallel the secondaries, solder or
crimp the orange and black wires into
a spade together and insulate it with
heatshrink tubing. Do the same with
the yellow and red wires, into a second spade lug.
Again, if you are using a different
transformer, you should check this configuration as it may be different.
Plug the two spades on the AC lugs
on the bridge rectifier. Check that everything else has now been connected
Final assembly
The back, front and sides of this case
can be tricky to assemble. You might
find it easier to join the front, back
and sides together as a unit and then
slot this onto the bottom panel. Screw
two of the panel screws into the sides,
securing them (and thus the front and
rear panels) to the bottom.
Check that these screws do not foul
the transformer or PCB as you do this.
They are much longer than necessary,
so can be trimmed, if it comes to that.
You can test fit the case lid as well.
It should slot onto the remainder of the
case, with the last two screws used to
secure it. But leave it loose for now, as
we will need access to the PCB for the
final tests and calibration.
Now is a good time to tidy up the wiring. Use cable ties to secure the wires
into neat bundles (you should have already tied the mains wiring together).
The slotted ribs on the side panels
are great places for attaching the cable ties, holding the wire bundles out
of the way. This is also a good chance
to run your eye over everything and
Practical Electronics | December | 2020
A
B
A close-up of the rear of the Bench Supply showing (A) the main earthing point
and (B) the Presspahn insulation forming a barrier between the high and lowvoltage sections. Don’t leave these out: they’re for your safety!
make sure you can’t spot any wiring
or construction problems.
Final testing
Ensure nothing is connected to the
supply outputs and that the front
panel knobs are fully wound down
to their minimums.
Connect mains power and switch the
unit on via the rear panel switch, keeping yourself well clear of all the internals. It’s best to leave the wall socket
switch off, ensure the IEC input socket
switch is on, then stay clear of the unit
while switching it on at the wall.
The front panel meters should light
up and should all have readings close
to zero; if they do not, power off and
check for problems. The temperature
reading on the Five-way Panel Meter
should be around ambient.
If the temperature is above 20°C,
then the fans may start up. Connect a
multimeter on its volts range to the output terminals, with the output switch
on (down). The reading should be 0V.
If not, shut down and check for faults.
If all is well, turn up the current limit pot to slightly above zero, maybe to
around one-tenth of its range. At the
zero position, the output is completely inhibited.
Slowly advance the voltage pot;
you should see the voltages on the
meter rise. If this is the case, then we
can calibrate the voltage display. Dial
up the voltage until you get 50V DC
across the output terminals. If it does
not reach 50V at its maximum, adjust
VR1 to allow this.
Now adjust VR5 and VR6 until their
respective meters (set voltage and actual voltage) are both showing 50V. This
will probably be at around a third of
their range from the minimum position.
So far, all the work is being done
by REG3. We will now test that the
Bench Supply will hand off to the
current-boosting transistors at higher
currents. Dial the voltage pot down
to the minimum and connect a 1kΩ
resistor (1/2W is fine) across the
output binding posts.
Now dial the voltage up to 20V; this
will be just below the power limit of a
1/2W 1kΩ resistor. Check the voltage
across the 68Ω resistor near REG3. It
should give a reading of around 0.6V,
the base-emitter switch-on voltage of
transistor Q3.
If the reading is above 1V, then REG3
is passing all the current, and the transistors are not taking the load. Power
off the unit, give it a minute for the
capacitors to discharge and check for
problems around the heatsink-mounted transistors.
Assuming all is well, dial the voltage and current down and remove the
1kΩ resistor.
We can now calibrate the current
meters. You can connect an ammeter
(or multimeter at 10A setting) directly
across the outputs, although this will
involve running the PSU at maximum
dissipation. It is a good idea to connect a high-power series load resistor
if you have one.
We want the Bench Supply to be delivering 8A to provide the best calibration. Dial up the voltage slowly; if you
only have an ammeter connected across
the outputs, you should not see a voltage reading much higher than 1V (depending on lead and load resistance).
If it goes much higher, that suggests
that there is a problem with the current
limiting. The voltage will be higher if
you have a series resistor connected.
As you advance the current-limit
pot, assuming the set voltage does not
match the actual voltage, that means
that current limiting is occurring. The
fans should start running if they are
not already.
Continue winding it up until the meter shows 8A. If it does not reach 8A,
then adjust VR2 to fine-tune the maximum current limit.
Now adjust VR7 and VR8 until the
Five-way Panel Meter (or your individual panel meters) show 8A for both the
39
Secure the top panel in place with the
two supplied screws. The High Power
Linear Bench Supply is now complete.
The underside of the Power Supply case, showing the locations of the holes
required for the transformer (the big black bolt), the heatsink (nylon screws
on/near ventilation holes) and the PCB mounting pillars (right side of pic) The
single screw on the left side is for the case earth. All holes are 3mm with the
exception of the transformer mounting (we used an 8mm bolt).
set current and actual current. These
pots will need to be wound to around
2/3 of their range from the minimum.
At this stage, the Bench Supply will
be dissipating close to 400W, so the
temperature will be steadily rising
and the fans will be working harder as
it does. You can use a contactless (IR)
thermometer to check the heatsink
temperature, which should be close to
what’s shown on the Panel Meter.
If you leave the current set to 8A,
you can test the thermal limiting. When
the temperature reading gets to around
80°C, the limiting LED should come on,
and the current will drop. You may also
hear the fans run a bit harder too. This
is not a ‘boost’ mode, just the effect of
the sagging DC voltage disappearing as
the load is reduced.
If the temperature keeps rising past
80°C with no change in the output
current, then shut the Bench Supply
down and check for faults in that part
of the circuit.
If it does enter limiting, then the
Bench Supply is working as designed.
Dial the current and voltage down to
their minimums and let the fans run
for a moment so that the heatsink cools
down, then turn it off and disconnect
your test load.
Finishing up
Now that everything is functional, it’s
just a matter of a few finishing touches.
Secure any loose wiring with the cable ties. The wires on our transformer
were not too long, so they did not need
to be fixed to anything.
If yours are significantly longer, you
can use self-adhesive plastic cable
clamps to tidy them up.
The fan and thermistor wires can
be bundled together and fixed against
the right-hand side panel with adhesive wire clips. Similarly, the output
wires to the binding posts should be
attached to the base of the case with
adhesive wire clips.
The other wires to the front panel
can be bundled together with cable ties.
Since they do not travel far, they should
not need to be secured to anything else.
The earth wires should be clipped in
place if there are any that might move
around excessively. Take care with the
lead for the top of the case if it has a lot
of slack. You could fit a cable clip to the
inside of the top of the case to secure it.
Variations
While we aimed for 50V output voltage in our design, necessitating the 57V
rail, you can use a lower-voltage transformer too. As long as the 24V regulator
can still deliver 24V, the Bench Supply
will still work.
To use a lower-voltage transformer,
you may need to reduce the value of
the 220Ω 5W resistor, to ensure the input of REG1 always stays above 26V.
You can also adjust the upper outputvoltage limit downwards using VR1.
VR1 may even need to be increased in
value (eg, to 20kΩ or 50kΩ) if a very
low output voltage is desired.
The current capacity of the output
transistors is much higher than the
2A each we have chosen, but thermal
considerations limit their operation.
You could tweak the PSU to provide a
higher output current if the input voltage (and thus total dissipation at zero
output voltage and maximum current)
is reduced.
The PCB tracks, CON6 and the wiring can handle up to 10A, so this is
about the practical limit without making major changes. Note that you may
need to reduce the value of the 27kΩ
resistor in series with trimpot VR2 to
set the current limit to 10A.
Fan considerations
We chose a particularly high-powered
pair of fans to ensure that the output
transistors will be cooled as much as
possible. The 33Ω series resistor is suitable for these fans, but may not drop
enough volts if different fans are used,
particularly those with a lower current draw.
Its value should be chosen to provide
a 9V drop (from 57V to 48V) at the typical current draw of the chosen fans. A
5W resistor should be suitable for up to
around 500mA under these conditions.
Reproduced by arrangement with
SILICON CHIP magazine 2020.
www.siliconchip.com.au
Fig.9: this frontpanel artwork
is shown here
at 50% life size.
The full-size
version can be
downloaded from
the December
2020 page of the
PE website.
40
Practical Electronics | December | 2020
