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Part
2
High Power
Linear Bench Supply
Last month, we introduced our new Linear Bench Supply, capable of delivering
8A at 45V or 2A at 50V. It’s based around a 500VA toroidal transformer, a PCB
control module fitted to a finned heatsink and two thermally controlled fans to
keep it cool. These all mount in a metal instrument case. This month we cover
the assembly and testing details of the PCB module.
T
here are quite a few steps involved in building this Supply,
but none are terribly complicated. So if you follow our instructions,
you shouldn’t have any trouble getting
it to work and ensuring that it’s safe.
You’ll need most or all of the parts
in the list at the end of this article, so
the first job is to gather those.
There’s a bit of screwing, drilling,
tapping and cutting needed to complete the hardware side of this project.
Ideally, you should have a drill press,
although you can get away with a decent hand drill.
You’ll also need assorted drill bits,
an M3 tap set, files and a hacksaw on
hand.
Around half the assembly time is
in building the control module, with
the other half preparing the case and
putting it all together. We’ll have the
case assembly and wiring details next
68
Silicon Chip
month. This month’s article concentrates on building that control module.
We’ve made it as easy as possible
by using almost entirely through-hole
parts and mounting them all on a single PCB. So let’s get started building it.
Construction
Before mounting any parts on the
control board, use the blank PCB and
some of the other parts to mark out
where holes will need to be drilled
on the heatsink. The hole locations
are shown in Fig.5, but it’s better to
use the actual PCB and devices to determine where to drill.
Start by fitting the PCB with the
9mm tapped spacers at each corner.
Then temporarily place transistors Q3,
by Tim Blythman
Australia’s electronics magazine
Q4, Q5, Q6, Q7 and REG3 into their
respective mounting holes, but don’t
solder them yet.
Place the acrylic spacer under the
heatsink to lift it up by 3mm, then centre the PCB on the face of the heatsink.
Making sure that each component
is sitting up straight and at the same
height, mark where the centre of each
mounting hole sits on the heatsink (eg,
using a felt tip pen).
Hold the bridge rectifier in place
above the main devices, centred on
the heatsink (see photos) and mark its
mounting hole too.
While you’re at it, use the acrylic
insulating plate to mark out the positions of the four mounting holes on
the underside of the heatsink, two on
each side.
Now take the heatsink away and
carefully drill all the marked holes
with a 2.5mm bit to a depth of at least
siliconchip.com.au
6mm (or deeper if you don’t have an
M3 finishing/bottoming tap), making
sure they are drilled perpendicular to
the face of the heatsink. Use kerosene
or light machine oil to lubricate the
drill bit and regularly clean out swarf.
Once all the holes have been drilled,
tap them for an M3 thread to a depth
of at least 6mm, again using plenty of
lubricant and regularly clearing swarf
from the tap.
Be careful not to use too much force
to turn the tap, or you could break it,
ruining both it and the heatsink.
As long as you regularly remove
the swarf and re-lubricate the tap and
hole, a consistently moderate amount
of torque should be required.
If you do encounter increased resistance, unwind the tap a little bit and
then try winding it clockwise again. If
the resistance is still there, take it out
and clean and re-lubricate the hole,
then try again.
You can use a finishing tap to get the
tapped holes to the required depth, or
drill them a bit deeper and use the intermediate tap to cut threads at least
6mm into each hole. When finished,
deburr all the holes and clean out all
the swarf.
You may like to wash the heatsink
with soapy water and let it dry off to
get rid of some of the lubricating oil
and the remaining swarf.
Before proceeding, it’s also a good
idea to use the bare PCB to mark out
where its mounting holes will go in
the bottom of the case.
Use the heatsink acrylic spacer to do
the same for the four heatsink mounting holes, and position the mains
transformer as shown in the photos,
to mark out its central mounting hole.
Make sure you leave enough space
behind the heatsink fins for the fans.
The fins should be around 45mm from
the inside rear of the case.
It’s a good idea to use an unassembled PCB and the acrylic heatsink spacer as a
template to mark the mounting hole positions inside the case bottom. It’s easier
to do this now, rather than later!
resistor and shunt monitor IC4, in an
8-pin SOIC package, which is mounted
near the shunt.
Start with IC4. Apply flux paste to
its pads, then locate IC4 over them.
Make sure that its pin 1 is orientated
so that it’s closest to the shunt pads.
Pin 1 is typically marked with a dot
or divot on top of the IC package and
a bevelled edge on that side.
Once it is in the correct location,
solder one of its pins. Check that all
of its pins are lined up with their
pads. If not, re-heat the solder joint
and gently nudge the part into place
with tweezers.
Once you are happy that the part is
aligned and flat against the PCB, solder
the remaining pins by applying some
solder to the iron tip and carefully
touching each pin in turn. The solder
should flow from the iron to the pin.
Once the other pins are soldered, go
back and re-touch the first pin.
If you are having trouble, apply
some more flux. Excess solder can be
removed with solder wick and a bit
of extra flux paste. If a bridge occurs,
don’t remove it right away, but solder
any unsoldered pins first. Then use the
wick on one side at a time to remove
any bridges.
The shunt is the next part to be fitted. It is relatively easy to solder but
is connected to a wide power trace,
so it may need a bit more heat. It is
not polarised.
Apply solder to one pad, then rest
the part on top and apply heat again
to allow the part to sink into the solder and down onto the pad (pressing
down on the part with tweezers helps
with this process).
When the first solder joint is good,
solder the other side, then go back and
re-touch the first joint.
With these two parts in place, it’s a
good idea to clean up any excess flux
on the PCB using isopropyl alcohol or
a specialised flux remover.
Through-hole parts
You can now fit all the smaller axial parts, ie, resistors under 1W, zener
diode ZD1 and small signal diodes
D1-D4. Make sure that the diodes are
orientated as shown in the overlay
diagram.
PCB assembly
With that out of the way, we can now
proceed to assemble the PCB using
the overlay diagram, Fig.6, as a guide.
The Bench Supply is built on a
double-sided PCB coded 18111181,
measuring 150 x 120mm. The following description assumes the PCB
is orientated as shown in Fig.6, with
the heatsink mounted devices at the
bottom edge.
There are two surface-mounted
parts on this PCB, which should be
fitted first. These are the 15mΩ shunt
siliconchip.com.au
CL
Fig.5: a half-size
drilling template for the
heatsink. All holes are
drilled and tapped for
an M3 thread, to a depth
of at least 6mm. While
this should give you an
idea of what to expect,
as mentioned in the text,
it’s better to temporarily
insert the actual devices
and mark where their
mounting holes sit if
possible.
(SCALE 50%)
22
A
15
A
A
60
A
2
30
60
30
A
A
A
75
1
A
30
6.5
5.5
150
75
HOLES A: DRILL 2.5mm DIAMETER, TAP FOR M3 SCREW AND DEBURR.
Australia’s electronics magazine
November 2019 69
R EG 3
33W 5W
IC4
INA282
3W
15mW
10mF
may need to bend their leads out with
small pliers to fit the PCB pad patterns.
Next, mount DIL pin header CON6,
followed by the trimpots. Orientate
them so that the adjustment screws
are positioned as shown in the overlay
diagram. They are all the same value.
Follow with the two 5W resistors,
which can be installed slightly above
the PCB surface to improve convective
cooling, although this is not critical.
Note that, as explained last month,
you may need to change the value of
the 33Ω 5W resistor if you’re using
different fans from the ones specified
(which we don’t recommend!).
Now fit the terminal block (CON1),
with its wire entry holes facing the
edge of the board, and polarised headers CON2-CON5, CON7 and CON8.
The polarised headers should be
mounted with the orientations shown
in Fig.6.
Onboard regulators
REG1 (7824) and REG4 (7812) both
need flag heatsinks as REG1 drops
around 20V and REG2 drops 8V. Both
are mounted identically but rotated
180° relative to each other.
Start by lining up the component
Australia’s electronics magazine
FJA4313
D5 D6
0.1W
22W
Q5
5404
4700mF
0.1W
LM317HV Q3 BD140
CON1
DC OUT
100nF
1kW
FJA4313
Thermistor
CON7
6.8V
ZD1
22W
Q4
18
111181
18111181
2019
0.1W
FANS
100nF
10kW
SB380
IC3
555
4700mF
BRIDGE+
C
22 W
Q6
10kW
BRIDGE–
68W
18111181
100mF
35V
+
FJA4313
1nF
+
Q7
CON5
1mF CON4
+
+
0.1W
2.2kW
+VE GND
7812
+
4700mF
22W
78L05
100nF
100mF
63V
4700mF
10kW
10kW
D3
4148
220W 5W
1kW
R EG 5
100nF
REG4
IC2
LM358
1kW
10kW
+
Q1 0
1MW
5V A1 A2 A3 A4 A5
VR8
10kW
100mF
35V
68W
7824
10kW
100nF
D1
100mF
35V
4148
R EG 2
7905
D2
1MW
GND
CON6
100W
100nF
100nF
10kW
10kW
10kW
R EG 1
100nF
CON2
D4
4148
100mF
35V
9.1kW
IC6
LM358
+
IRF540
10kW
1kW
100nF
100nF x2
VR6
10kW
4148
BC546
100mF
35V
100kW
10kW
10k W
100m F
35V
1kW
100kW
BC546
CON3
IC1
LM358
1M W
10kW
100nF
10kW
10kW
R ev G
BC546
+
Silicon Chip
22kW
100nF
100kW
Q9
1
VR7
10kW
– +
A
Q1
BC546
IC5
LM358
+
1m F
Q8
10kW
+
Q1 1
Q1 3
CON8
100nF
BC546
BC546
100nF
+
50V Linear Bench PSU
Q2
VR5
10kW
VMAX IMAX BC546
– +
Q1 2
100nF
T P 5 TP 6
VR1
10kW
VR2
10kW
100nF
100nF
27kW
K
IACT
1M W
GND VSET VACT ISET
While the resistors have colour-coded bands, these can be hard to distinguish, so it’s best to check each with
a multimeter set to measure ohms before soldering them in place.
Next, fit the six 1W resistors and the
two larger diodes (D5 & D6), again ensuring their cathode stripes are facing
in the directions shown in Fig.6. Watch
out as they are orientated differently.
The next job is to fit DIP ICs IC1IC3, IC5 and IC6. These are all LM358
op amps except for IC3, which is a
555 timer.
You don’t need to use sockets; in
fact, it’s better to solder these all directly to the PCB. But make sure that in
each case, the pin 1 dot/notch is facing
as shown in the overlay diagram and
the IC is pushed down fully onto the
board before soldering all of its pins.
The next components to mount are
the MKT and ceramic capacitors. The
MKT capacitors are mostly 100nF in
value, although one is 1nF so don’t get
them mixed up. The location for each
capacitor is shown in Fig.6.
You can now solder the seven BC546
transistors in place, along with REG5.
The transistors and regulator look similar so don’t get them mixed up. You
70
VOLTAGE
TP0 TP1 TP2 TP3 TP4 CURRENT
+
Fig.6: most of the
Bench Supply
components mount
on this control
board. Ensure
that the diodes,
transistors, ICs
and electrolytic
capacitors are fitted
with the correct
orientations as
shown. It’s also
a good idea to
check carefully
that the different
value resistors and
capacitors go in the
right places.
Note that one of
the 100µF electros
is rated at 63V
(below and to the
right of the 220Ω
Ω
5W resistor) where
all others are 35V.
Fit the four 4700µF
capacitors last, after
the power devices
(that mount on the
heatsink along with
the bridge rectifier)
have been soldered
in place.
FJA4313
with its footprint to determine where
the leads need to be bent down by 90°.
Having bent the leads, check that the
tab mounting hole lines up with them
inserted. If not, adjust the bend.
When you are happy with this,
smear a small amount of thermal
compound on the back of the regulator and mount it by sandwiching the
flag heatsink between the regulator
and the PCB.
Fasten with a 6mm machine screw
from the bottom and a nut on the top
of the tab. Ensure the nut is tight but
be careful not to twist the regulator
and its leads.
Ensure the regulator and heatsink are
square within their footprints and not
touching any other components before
soldering and trimming their leads.
You can fit most of the electrolytic
capacitors next; all but the four large
4700µF units. They are polarised; in
each case, the longer (positive) lead
must be soldered to the pad marked
with a “+” on the PCB. The cans have
stripes on the opposite (negative) side.
Follow with the two remaining onboard TO-220 components, REG2 and
Q10. These do not need heatsinks as
their dissipation is quite low. They can
siliconchip.com.au
Compare the PCB layout opposite with this shot of the completed board, albeit
with its transistors (and bridge) already fixed to the heatsink
be fitted vertically, but make sure that
their tabs are facing as shown in Fig.6.
Connecting the off-board
components
Presuming that you are using the
Five-way Panel Meter module for display, you will need to build that separately (see the article starting on page
90). If you’re using individual panel
meters, we’ll leave that part of the construction up to you. Most of the work
is in cutting holes for them in the front
panel and wiring them up.
Voltage and current adjustment potentiometers VR3 and VR4 mount on
the front panel and connect to the PCB
using flying leads and polarised plugs.
This prevents them from being accidentally connected backwards if the
unit is later disassembled.
Separate a 150mm length of 10-way
ribbon cable into two three-way pieces and three two-way pieces. Trim the
siliconchip.com.au
two three-way pieces to around 10cm
each, separate the wires at each end,
strip them and solder one end of each
to the leads of VR3 and VR4. You may
wish to protect the solder joins with
short pieces of small diameter heatshrink tubing.
Now crimp the polarised plug pins
onto the other ends of the wire. If you
don’t have the correct tool, it may be
easier to solder the wires, although
the tabs of the pins will still need to
be bent over to fit into the housing.
You can crimp them using small pliers in a pinch (no pun intended), but
it’s a bit tricky. These will plug into
CON2 and CON3.
The square pads of CON2 and CON3
are connected to ground, so should go
to the ends of the potentiometer tracks
which have a low resistance to the wipers with the pots fully anti-clockwise.
The middle connections of CON2 and
CON3 go to the wipers, and the third
Australia’s electronics magazine
pin goes to the other end of the tracks.
You can check this by verifying that,
with the pot cables plugged into the
board, the middle pins have a low resistance to ground (TP0) when the relevant knob is wound fully anti-clockwise. If this is not the case, you may
have the outside leads reversed.
LED1 is also attached using flying
leads and mounted off the PCB, via
CON8. Solder a length of the two-way
ribbon cable to the pins for a matching
polarised plug, then solder the other
ends of the wire to the LED. The longer lead of the LED must be soldered to
the wire that goes to the pad on CON8
marked with a plus sign.
If using a pre-wired panel mount
LED, simply crimp or solder the wires
to the plug pins and push them into
the housing. If you have a bare LED,
you should heatshrink the wires to
insulate and protect them, and use a
bezel for mounting.
If your fans are not already terminated with 2.54mm-pitch headers, attach
a keyed plug as for the LED. Note that
the positive lead for both fans (ordinarily red) goes to the pin closest to
output connector CON1.
A similar header is used to connect
the NTC thermistor for monitoring the
heatsink temperature. It is not polarised like the other components, but
you can still fit the same style plug to
connect to the locking header on the
PCB, so do that now.
The bridge rectifier (BR1) is mounted on the heatsink and connected
to the transformer and PCB via four
stout (10A-rated) wires. Cut two wires
around 7cm long and crimp or solder
spade terminals to one end of each.
Protect the outside of the spade using
heatshrink tubing insulation.
Solder the other end of the wires to
the PCB. The red wire should go to the
terminal marked BRIDGE+ (and the
bridge rectifier terminal with a plus)
and the black wire to the terminal
marked BRIDGE- (and the diagonally opposite bridge rectifier terminal).
Initial testing
Now detach all the external components except for the two potentiometers, VR3 and VR4, and the NTC
thermistor. This will allow you to do
some basic checks.
Before powering the board up,
double-check the construction so
far, making sure that all the onboard
components have been fitted, with
November 2019 71
We’ve “opened out” this otherwise completed Supply to give you a better idea of what goes where and with what. Note the
Presspahn insulation (fawn colour) which isolates the bitey bits from the rest of the circutiry – just in case,.
the correct polarity. Check also that
the solder joints all have good fillets,
do not look dry and that there are no
shorts between solder joints on the
underside of the board.
The initial tests are only made at
low power, but there is still enough
energy present to damage components
if something has been installed incorrectly. There is the possibility of components becoming very hot if a fault
occurs, hence the initial low-power
tests which should hopefully find any
problems before delivering enough energy to do any damage.
Note that there can be 70V differential voltage between various parts
of the circuit when it is powered on.
This is enough to give a shock. Make
sure the PCB is mounted on insulated
tapped spacers and there is nothing
underneath the board which might
cause a short circuit (eg, do not place
it on a metal surface!).
Before powering up the unit, wind
all the trimpots and variable resistors
to their minimum positions. This includes the six trimpots on the PCB
72
Silicon Chip
and the two externally mounted adjustment potentiometers.
The best way to do the initial tests
is with a variable DC supply fed into
the BRIDGE+ and BRIDGE- leads with
the appropriate polarity. You will need
around 40V to ensure that REG1 is delivering the full 24V at its output.
If you don’t have a 40V DC supply,
you can feed 27-39V DC directly into
REG1’s input (with the positive lead
clipped to the right-hand lead of the
220 5W resistor). Or you can feed
24V into REG1’s output, via the lefthand lead of the 68 1W resistor. But
in the latter case, any faults in REG1
itself may not show up.
It would be ideal if you can monitor the current drawn by the circuit;
if your supply lacks an ammeter, you
can monitor the voltage across the
220 5W resistor, assuming that you
are not bypassing this due to a lower
test supply voltage.
Power up the circuit and check the
current draw. It should be around
60mA, which corresponds to 13.2V
across the 220Ω resistor. If there is a
Australia’s electronics magazine
severe fault, then you will see a much
higher voltage across this resistor and
it could get very hot. In that case, shut
off power as soon as possible and
check for faults. Any more than 20V
across this resistor means that something is wrong.
Assuming the current draw is OK,
you can now check the various voltage rails for correctness. Connect the
negative multimeter probe to ground
via TP0 and check the voltages with
the positive probe. The 24V rail can
be measured at the left end of the 68Ω
resistor (assuming you aren’t feeding
power in there, as there would be little point in checking it then).
You should get a reading close to
24V, although it may be lower if your
test supply does not have a high enough
output. As long as it is above 18V, the
remaining voltage rails should still be
correct.
But you will not be able to complete
the calibration until 24V is available
from REG1, nor can you accurately
calibrate the device if feeding power
into the 24V rail.
siliconchip.com.au
PARTS LIST – LINEAR 45V 8A BENCH POWER SUPPLY
{
1 double-sided PCB coded 18111181, 150 x 120mm
1 vented metal instrument case [Jaycar HB5556]
1 Five-way Panel Meter module (see article starting on page 90)
WITH 1 acrylic bezel [SILICON CHIP ONLINE SHOP Cat SC5167]
OR 1 set of separate 5V panel meters and suitable mounting
hardware
1 acrylic spacer for heatsink
[SILICON CHIP ONLINE SHOP Cat SC5168]
1 40V 500VA toroidal transformer [element14 2817710]
1 35A 400V bridge rectifier (BR1)
[Jaycar ZR1324, Altronics Z0091]
1 IEC mains input socket with fuse and switch
[Jaycar PP4003, Altronics P8340A]
1 150 x 75 x 46mm diecast finned heatsink [Jaycar HH8555]
2 24V DC 80mm high-flow fans [Digi-key P122256]
2 80mm fan filter/guard [Jaycar YX2552]
2 TO-220 flag heatsinks, 6073B type (for REG1 & REG4)
[Jaycar HH8502, Altronics H0630]
1 16V DC/230V AC 16A SPST or DPDT panel-mount toggle
switch [Jaycar ST0581/ST0585]
1 208 x 225mm sheet of Presspahn or Elephantide [Jaycar
HG9985]
2 TO-220 insulated mounting kits (for Q3 & REG3)
[Jaycar HP1176]
1 2-way terminal block, 5mm pitch (CON1)
[Jaycar HM3172, Altronics P2032B]
2 3-way polarised headers (CON2,CON3)
[Jaycar HM3413, Altronics P5493]
2 3-way polarised plugs (for VR3 & VR4)
[Jaycar HM3403, Altronics P5473 + P5470A)
4 2-way polarised headers (CON4,CON5,CON7,CON8)
[Jaycar HM3412, Altronics P5492]
4 2-way polarised plugs (for LED1, thermistor & fans)
[Jaycar HM3402, Altronics P5472 + P5470A]
1 6x2-pin header (CON6) [Jaycar HM3250, Altronics P5410]
2 12-pin IDC headers (to connect CON6 to Panel Meter)
[Digi-Key 2057-FCS-12-SG-ND]
1 10kW stud-mount or lug-mount NTC thermistor
[Digi-key 495-2138, Altronics R4112]
11 6.3mm spade crimp connectors (for BR1 and mains socket)
1 red chassis-mount banana socket/binding post
1 black chassis-mount banana socket/binding post
1 green chassis-mount banana socket/binding post
1 6A fast-blow M205 fuse (F1)
2 knobs (to suit VR3 and VR4)
4 instrument case feet and associated mounting hardware
Wire, cable etc
1 1m length of 3-core 10A mains flex
1 1m length of 12-way ribbon cable (to connect CON6 to the
Panel Meter module and to connect VR2, VR3, LED1 and
the thermistor)
1 1m length of 10A-rated red wire (for BR1 and output
terminals)
1 1m length of 10A-rated black wire (for BR1 and output
terminals)
1 small tube of thermal paste
various lengths of 3mm and 6mm diameter heatshrink tubing
pack of small (2mm) cable ties
pack of self-adhesive wire clips
74
Silicon Chip
Fasteners
8 M3 x 32mm machine screws (for mounting fans)
1 M3 x 15-16mm machine screw and flat washer (for
mounting BR1)
5 M3 x 12mm machine screws (for rear panel Earth and
mounting Panel Meter)
13 M3 x 9-10mm machine screws (for mounting fans and Q3-Q7)
18 M3 x 6mm machine screws (for panel Earths, PCB
mounting, REG1, REG3 & REG4)
4 M3 x 10mm Nylon machine screws (for mounting heatsink)
8 M3 x 15mm tapped Nylon spacers (for mounting fans)
4 M3 x 9mm tapped Nylon spacers (for mounting PCB)
13 6.3mm spade crimp connectors (for BR1, the mains socket
and output switch)
6 M3 crinkle or star washers (for panel Earths)
16 M3 hex nuts (for panel Earths, REG3, REG4 and mounting
Panel Meter)
12 crimp eyelet lugs, 3mm inner diameter (for panel and
output Earths)
Semiconductors
4 LM358 op amp ICs, DIP-8 (IC1, IC2, IC5, IC6)
1 555 timer IC, DIP-8 (IC3)
1 INA282 shunt monitor IC, SOIC-8 (IC4) [Digikey 296-27820-1]
1 7824 24V linear regulator, TO-220 (REG1)
1 7905 5V linear regulator, TO-220 (REG2)
1 LM317HV high-voltage adjustable regulator, TO-220 (REG3)
[Digikey LM317HVT/NOPB]
1 7812 12V linear regulator, TO-220 (REG4)
1 78L05 5V linear regulator, TO-92 (REG5)
7 BC546 NPN transistors, TO-92 (Q1,Q2,Q8,Q9,Q11-Q13)
1 BD140 PNP transistor, TO-126 (Q3)
4 FJA4313 NPN power transistors, TO-3P (Q4-Q7) [SILICON CHIP
ONLINE SHOP Cat SC4096]
1 IRF540N N-channel Mosfet, TO-220 (Q10)
1 5mm red LED with bezel (LED1)
[Jaycar SL2610, Altronics Z0220]
1 6.8V 1W zener diode (1N4736 or equivalent; ZD1)
4 1N4148 signal diodes (D1-D4)
1 1N5404 400V 3A diode (D5)
1 SB380 80V 3A schottky diode (D6)
Capacitors
4 4700µF 63V electrolytic [Altronics R5228]
1 100µF 63V electrolytic
6 100µF 35V electrolytic
1 10µF 63V electrolytic
2 1µF 50V multi-layer ceramic
18 100nF MKT
1 1nF MKT
Resistors (all 1/2W 1% metal film unless otherwise stated)
4 1MW
3 100kW
1 27kW
1 22kW
16 10kW
1 9.1kW
1 2.2kW
5 1kW
1 220W<at> 1 100W
2 68W#
1 33W<at>
4 22W
4 0.1W# [Digi-Key 0.1GCCT-ND, Mouser 603-KNP1WSJR-52-0R1]
1 0.015W 2W or 3W, SMD 6432/2512 size
[Digikey YAG2165CT, Mouser 603-PE252FKE7W0R015L]
6 10kW vertical multi-turn trimpots (VR1,VR2,VR5-VR8)
2 10kW linear 24mm potentiometers (VR3,VR4)
# 1W 5% <at> 5W 10%
Australia’s electronics magazine
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have a ‘scope. With the thermistor near
25°C, the fan PWM output at pin 7 of
IC2 should be off, so a voltmeter will
read 0V.
If the thermistor is warmed up (such
as by being held in a warm hand), the
average voltage at pin 7 should rise to at
least 3V, representing a 12V PWM signal with a duty cycle of around 25%.
This indicates that the thermistor circuit is working as expected.
Fig.7: this shows how to make the ribbon cable
which connects the Five-way Panel Meter to the Bench Supply main PCB.
Whether your cable looks like the pictures inside the upper or lower circles
depends on the style of IDC connector that you are using.
The 12V rail can be measured at pin
4 or 8 of IC3. If the 12V rail is correct,
then the negative rail generator should
be working, and the tab of REG2 should
have around -9V on it. The output of
REG2 is connected to pin 4 on IC1,
IC5 and IC6 and these should all be
close to -5V.
Finally, the output of the +5V rail
can be found at pin 1 of CON6 (marked
“5V”). The outputs on CON6 marked
A1-A4 correspond to the signals for the
external panel meters. They should all
read 0V if trimpots VR3 & VR4 are fully clockwise.
Pin A5 on CON6 should read around
3-4V if the thermistor is working correctly, but it may be a bit lower at high
ambient temperatures.
If this is correct and you have built
the Five-way Panel Meter, it can now
be connected to CON6 to allow it to be
calibrated (see the section on making
the ribbon cable below, if you haven’t
already done so).
All the readings, apart from the temperature, will be incorrect until calibration is complete.
If you are using individual panel meters, they can be connected now. Due
to the limited current available from
REG5, separate digital panel meters
may need a separate 5V supply.
Initial calibration
Now check the voltages TP5 and
TP6. TP5 should be at around 12V if
VR1 has been wound to its minimum.
Once you’ve verified that, adjust VR1
until TP5 measures 15.6V.
This sets up VR3 to provide 50V at
the output when fully clockwise. This
depends a little on the exact properties of trimpot VR3 itself, but this setting can be fine-tuned when construction is complete and you can measure
the actual output voltage to full scale.
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Similarly, adjust VR2 to get 6V at
TP6, corresponding to approximately
8A at the output. This too can be finetuned later. If you wish to set a more
conservative maximum current limit,
you can adjust VR2 for a lower voltage at TP6.
At this stage, TP1 and TP3 should all
be showing very close to 0V. If not, adjust VR3 and VR4 respectively so that
this is the case. This ensures a minimum output voltage when the unit is
fully powered up later.
TP2 and TP4 should also be near (or
even below) 0V. This shows that the
output voltage and current are both
zero. You should not proceed unless
this is the case, as there should be no
output with REG3 absent. If you get
positive readings here, check around
IC1 and IC4 for circuit problems before
proceeding with any high-power tests.
We will need to adjust VR4-VR7 later; this is not possible until the Supply
is fully assembled.
Other checks
If you have a frequency meter or oscilloscope, you can check the two oscillators. Their exact frequency is not
critical, but significant variations can
indicate other problems.
The oscillator for the negative rail
generator is at pin 3 of IC3 and should
measure around 60kHz. You should
also check the duty cycle if possible; it
should be close to 50% for maximum
efficiency. If the duty cycle is wrong,
and the negative rail is not reaching -5V,
the values of the components around
IC3 may be incorrect.
The frequency of the fan PWM circuit can be measured at pin 1 of IC2.
This should be around 280Hz, with a
50% duty cycle. Pin 1 delivers a square
wave while pin 2 can be probed to
check the ‘triangular’ waveform if you
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Mounting the power devices
Once you are happy with the results
of the tests outlined above, the power
components can be added to the board.
Disconnect the power and allow the capacitors to discharge, which may take
a minute or so.
The components in this area connect
via thick tracks and may need more heat
than the earlier components to solder.
Re-check now that the heatsink is
free of swarf and metal dust, as these
can puncture the transistor insulating
pads and cause a short circuit. The face
of the heatsink should be smooth. A
light sanding with fine sandpaper will
help to flatten any raised areas.
First, mount transistors Q3-Q7 and
REG3 loosely to the heatsink. Use a
6mm M3 machine screw, insulating
bush and insulating washer for REG3.
The mounting for Q3 is the same as
REG3 except that you’ll need a longer,
10mm screw. Mount the four large transistors using 10mm-long M3 machine
screws, with a thin smear of thermal
paste over the side of the devices which
touch the heatsink.
While Q3 is in a TO-126 package, a
TO-220 insulating mounting kit will
work fine with some careful trimming.
Note that Q3 has its plastic face mounted against the heatsink, so the washer
is more to ensure good contact than it
is for insulation.
Check for continuity between the
heatsink and leads of Q3 and REG3;
there should be no continuity on any
of the leads. You will need to probe the
non-anodised face of the heatsink. If
there is, remove that part, check the insulation and reattach. You must do this
before soldering or fitting the PCB, as
Q3’s emitter is effectively connected to
the heatsink via the collectors of Q4-Q7.
Now position the 3mm acrylic spacer
next to the PCB, with the latter sitting
on its 9mm tapped spacers. Line up
the power device leads with the PCB
pads and drop them into place, with the
heatsink resting on the acrylic spacer.
November 2019 75
Check the device mounting heights
and adjust if necessary. Then solder
one lead at each end of each device.
You can then carefully flip the whole
assembly over and solder all the pins
thoroughly, with the PCB resting on
something to prevent it sagging under
its own weight. When finished, trim
the leads short.
Tighten up all the screws holding
the devices to the heatsink and check
that they are firmly attached, as once
the large electrolytic capacitors are fitted, access will be limited. You might
also like to re-check that REG3 and Q3
are still insulated from the heatsink.
Next, smear the face of BR1 with thermal paste and attach it to the heatsink
using a 16mm-long M3 machine screw
and flat washer. Install it with the positive terminal at the bottom. This means
that the wires do not need to cross over
to reach the PCB terminals. The bridge
has a bevel to identify the positive terminal, and will typically also be printed with a “+” symbol on the side.
Connect the BRIDGE+ and BRIDGEterminals to the bridge rectifier by
pushing the spade connectors onto
its tabs.
The final components to fit are the
four 4700µF 63V capacitors mounted
directly in front of the output transistors. Their negative stripes must face
towards the front edge of the PCB. Solder them in place and trim the leads
to complete the component assembly.
Now is a good time to attach the thermistor to the heatsink. If using the studmount type, thread it into its hole on the
heatsink. If using the lug type, attach it
with a machine screw and shakeproof
washer. Mount it on the flat side of the
heatsink so that it is not directly cooled
by airflow from the fans.
Check the thermistor leads for continuity against the heatsink; there should
be none. If there is, check the mounting
and re-insulate as necessary.
IDC ribbon cable assembly
Now is a good time to make up the
IDC cable that will connect the Fiveway Panel Meter to the control board
(assuming you’re using that meter and
not some other arrangement). Cut a
175mm length of 12-way ribbon cable
and attach the IDC sockets at each end
with the same orientation. So with the
cable stretched out flat, the two polarising tabs on the IDC connectors should
face the same way.
If you can’t get 12-way ribbon ca76
Silicon Chip
ble, take some wider ribbon cable, cut
between the 12th and 13th wires and
then gently pull the two sections apart.
They should separate cleanly.
See Fig.7 for details on how to make
this cable. Usually, IDC connectors are
supplied as three pieces: the main part
of the connector, with holes to mate
with the pin header on the bottom and
blades to slice through the cable insulation on the top; a plastic clamp which
is pressed down on the top of the cable
to force it into the blades, and a locking bar which provides strain relief and
holds it all together.
The way the cable is fed through
these three-piece IDC connectors is
shown at the top of Fig.7. But the 12way IDC sockets we purchased only
consisted of two pieces, with the clamp
and locking bar integrated and no provision for cable strain relief. This arrangement is shown in the lower two
circles. Make your cables to match one
or the other, depending on the style of
IDC sockets that you have.
I t ’s e s s e n t i a l
to use sufficient
clamping force
to ensure that the
blades properly
pierce the cable insulation and make
contact with the copper strands within,
without pressing so
hard that you break the
plastic.
You can do this in a
vice; however, a proper
IDC crimping tool generally makes the job easier
(eg, Altronics Cat T1540).
of BR1. You can use 24-40V AC or 3058V DC.
If you can limit the current to a few
hundred milliamps, that’s a good idea,
but note that this will mean that it takes
some time for the main capacitor bank
to charge, and it will draw the maximum current as it does so.
Once the Supply is powered up,
check that the Panel Meter powers
up too. You may need to tweak the
brightness and contrast if these have
not been set.
The voltages and currents should all
read zero as VR5, VR6, VR7 and VR8
should have all been set to their minimum and have not been calibrated. The
temperature shown on the Panel Meter
should be around ambient if the thermistor is wired up correctly.
Assuming that it checks out OK,
power it off; it’s time to start preparing the case.
We’ll have the full details on the
final assembly and testing in part 3,
next month.
SC
More testing
Now that you’ve finished assembling the
control board, assuming you have a suitably safe source of AC
or DC power, you can
do some more testing.
Plug in the Fiveway Panel Meter,
VR3, VR4, thermistor and LED and
then apply power
to the two unconnected terminals
We’ll cover the final assembly of the supply in the third
and final part of this project next month.
Australia’s electronics magazine
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