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A Beginner’s Variable
Dual-Rail Power Supply
If you’re just beginning in electronics, then
you’ll probably baulk at building a mainsoperated power supply. This project uses a
plugpack which means that you can make
your own variable dual-rail power supply
without worrying about mains wiring.
By DARREN YATES
When it comes to experimenting in
electronics, power supplies are a bit
of a “chicken and egg” situation. To
experiment with circuits, you need a
power supply but unless you have the
necessary knowledge already, building
a mains-powered supply is beyond
most beginners.
The alternative is to run all of your
circuits from batteries or buy a readymade supply. Either option is expensive. So in the interests of making it
easier to start experimenting, we’ve
26 Silicon Chip
come up with this dual-rail power
supply which runs from a 16V AC
plugpack. It’s capable of providing output voltages ranging from ±1.25V DC
to ±15V DC at currents up to 500mA
(see Fig.1).
The beauty of this design is that it
doesn’t require any external mains
wiring! All the mains wiring is contained inside the plugpack, leaving
you with just the low-voltage AC
output which connects straight into
the project.
In order to keep costs down, the
output voltage is varied in 11 switched
steps. This eliminates the need for
an output voltage meter since the
precise value can be directly read off
the switch position. The 11 switched
voltage ranges are: 1.25V, 1.5V, 3V,
4.5V, 5V, 6V, 7.5V, 9V, 12V, 13.5V &
15V. Both supply rails are protected
against short circuits and voltages
generated by external loads, while
a LED indicator lights if the supply
stops regulating.
Another worthwhile feature is the
provision of a “load” switch. This
allows the power to the load to be
switched on and off while keeping the
supply switched on.
The output current capabilities of
the supply are relatively modest but
should be more than adequate for
most projects. Fig.1 plots the maximum current that can be delivered
at various output voltages. As can be
seen, the supply is capable of deliv
ering 250mA or more for voltages
Fig.1: this graph plots the
maximum output current
from the supply for
voltage settings between
1.5V & 15V (16VAC 1A
plugpack). The supply
is capable of delivering
250mA or more over most
of the range.
from 1.5V up to about 14V, with a
maximum of 500mA at 7.5V. Note
that these figures assume a 16VAC
1A plugpack supply.
By now, some readers will be asking
“what is a dual-rail power supply?”
It’s quite straightforward really – a dual-rail power supply has both positive
and negative output voltage rails, as
well as the ground (or zero volt) rail.
Most projects and circuits you build
will only require the positive output
and the ground rail. This is basically
the same as if you connected a battery
of the same voltage to the circuit you’re
building.
However, you’ll also come up
against circuits which use operational
amplifiers (op amps) and these require
both posi
tive and negative supply
rails. That’s where the dual-rail power
supply comes in. It can power op amp
circuits with ease and so is just that
much more versatile than a standard
single rail supply.
An important feature of this design
is that the negative supply rail automatically tracks the positive supply
rail. This means that the two rails
always have the same absolute value.
Thus, if you set the positive output to
+12V, the negative rail will be at -12V.
And here we should clear up a
common misconception regard
ing
dual rail supplies. Despite what many
people think, it’s quite possible to
use the positive and negative rails to
obtain a much higher output voltage
than is possible by simply connecting
between one of these rails and the
0V rail.
For example, if you want a 30V single-rail supply, simply set the supply
to give ±15V and connect the circuit
across these outputs. Another way of
looking at this is simply to consider
that there is 30V between the two outputs. So a dual-rail ±1.25-15V variable
power supply can also function as a
2.5-30V single rail supply.
How it works
The circuit for the Beginner’s Dual
Rail Power Supply uses only standard
components which you can find in any
virtually electronics store. If you’ve got
a parts bin handy, you’ll probably have
a few parts that are suitable already.
Let’s take a look at the circuit – see
Fig.2. The plug pack takes care of all
of the mains wiring and steps the
240VAC mains voltage down to a
suitable 16VAC for our circuit. This
is fed via power switch S1 to rectifier
diodes D1 & D2 to produce unregulated
plus and minus DC rails of about 20V.
These DC rails are filtered by two
470µF electrolytic ca
pacitors and
fed to LM317 and LM337 3-terminal
regulators. These provide the adjustable plus and minus supply outputs
respectively.
In the case of the positive rail, the
LM317 (REG1) does most of the work.
Its output voltage is set by the 120Ω
and 2.7kΩ resistors on its ADJ terminal and by the resistive divider string
associated with switch S3. These components form the feedback network
around the regulator IC.
Basically, switch S3 sets the output
voltage from REG1 by setting the resistance between the ADJ terminal and
the 0V rail. When the ADJ terminal is
connected to 0V, the output voltage
is +1.25V. This voltage can then by
PARTS LIST
1 plastic case, 198 x 113 x
62mm
1 PC board, code 04110941,
102 x 57mm
1 front panel label
1 red 4mm binding post
1 black 4mm binding post
1 blue 4mm binding post
1 SPDT toggle switch (S1)
1 DPDT toggle switch (S2)
1 12-position 1-pole rotary
switch (S3)
1 knob to suit S3
2 LED bezels
1 16VAC 1A plugpack
1 3.5mm power socket
2 mini U heatsinks
4 rubber feet
Semiconductors
1 LM358 dual op amp (IC1)
1 LM317 3-terminal regulator
(REG1)
1 LM337 3-terminal regulator
(REG2)
6 1N4004 rectifier diodes
(D1-D6)
6 1N914 diodes (D7-D12)
2 15V 1W zener diodes
(ZD1,ZD2)
2 5mm red LEDs (LED1,LED2)
Capacitors
2 470µF 25VW electrolytics
2 100µF 25VW electrolytics
4 1µF 63VW electrolytics
1 0.1µF 63VW MKT polyester
Resistors (0.25W, 1%)
1 4.7MΩ
2 330Ω
2 47kΩ
1 270Ω
1 22kΩ
1 220Ω
2 3.3kΩ
1 180Ω
1 2.7kΩ
2 150Ω
3 1kΩ
2 120Ω
1 680Ω
1 56Ω
1 560Ω
1 27Ω
1 470Ω
Miscellaneous
Machine screws & nuts,
washers, hook-up wire.
stepped up to a maximum of +15V by
using S3 to progressively switch in
additional resistors in the string.
The 1µF capacitor between the ADJ
pin and ground ensures that any residual noise from the mains is kept to a
minimum. Finally, the output voltage
October 1994 27
28 Silicon Chip
POWER
LED1
1k
470
25VW
470
25VW
D1
1N4004
330
ZD2
15V
ZD1
15V
330
-15V
47k
+15V
47k
22k
1
8
+15V
-15V
IC1a
2 LM358
4
1
1
1
OUT
2.7k
LM317
REG1
ADJ
3
IN
BEGINNER'S POWER SUPPLY
D2
1N4004
FROM
16VAC
PLUG-PACK
POWERT
S1
D3
1N4004
1
120
100
25VW
15V
13.5V
12V
9V
7.5V
6V
5V
4.5V
3V
1.5V
1.25V
S3
D4
1N4004
REG2
ADJ
IN LM337 OUT
1k
560
470
680
270
220
150
56
180
150
27
120
D5
1N4004
3.3k
3.3k
0.1
D7
AO I
LM317
D8
2x1N914
100
25VW
6
5
4.7M
IC1b
D6
1N4004
A IO
LM337
7
1k
A
K
4x1N914
D9-D12
LED2
DROPOUT
M1
R2
R1
S2b
LOAD
S2a
0V
V
V
This is the view inside the prototype. Note the two small heatsinks fitted to the
two 3-terminal regulators. Take care to ensure that the regulators are correctly
oriented – each device is installed with its metal tab towards the centre of the
PC board.
from REG1 is filtered by a 100µF electrolytic capacitor and fed to the load
via switch S2a.
Negative regulation
The negative regulator (REG2) works
in a similar manner to REG1. It’s made
to track the positive rail by using IC1a
to provide a mirror of the voltage on
the ADJ terminal of REG1. For example, if the ADJ voltage of REG1 is at
10.75V (to produce a 12V output),
then IC1a will act to produce -10.75V
on the ADJ terminal of REG2.
This is achieved by connecting IC1a
as a unity gain invert
ing amplifier.
Its inverting input (pin 2) is fed from
the ADJ terminal of REG1 via a 47kΩ
Fig.2 (left): the circuit uses two
adjustable 3-terminal regulators
(REG1 & REG2) to provide the positive
& negative supply rails. IC1a inverts
the control voltage applied to the
ADJ terminal of REG1 to drive REG2,
while IC1b drives D9-D12 & LED 2 to
provide dropout indication.
resistor, while the associated 47kΩ
feedback resistor sets the gain to -1.
The non-inverting input is biased to 0V
via a 22kΩ resistor to ensure minimum
output offset.
The output of IC1a drives the ADJ
terminal of REG2 via a 1kΩ resistor.
This 1kΩ resistor is inside the feedback loop and is there so IC1a can
actually drive the ADJ terminal to the
maximum required value of -13.75V
(when the output voltage is set to
±15V). This is outside the operating
range of the LM358 because its supply
rails are ±15V. The result of all this
is that the negative output voltage
of REG2 tracks the positive output
voltage of REG1.
The ±15V supply rails for IC1 are
produced by zener diodes ZD1 and
ZD2, while LED1 provides power
indication. Diodes D3, D4, D5 and D6
protect the regulators from any reverse
voltages which may be generated by
capacitive or inductive loads con
nected across the outputs.
Dropout detection
When the regulators are working as
intended, the ripple voltage superimposed on the DC rails will be very low.
However, if the current drain is higher
than the regulators can supply while
still maintaining about 2V between
their IN and OUT terminals, the ripple
voltage will suddenly become quite
high. At this point, the output voltage
will fall quite rapidly if even more
current is called for and the ripple will
go even higher.
What this means of course is that
the power supply is unable to provide
sufficient current to the load and is
dropping out of regulation. This undesirable condition is indicated by
the dropout indicator circuit and this
is based on IC1b and diodes D9-D12.
IC1b is connected as an inverting
amplifier with a high gain, as determined by the ratio of the 4.7MΩ feedback resistor to the impedance of the
0.1µF input capacitor and the 3.3kΩ
resistors which monitor the positive
and negative supply rails. The two
back-to-back diodes, D7 & D8, limit
the maximum input signal to ±0.7V.
When ever either regulator drops
out of regulation (eg, if an output is
shorted to ground), the ripple output
increases greatly. Because it operates
with such high gain, IC1b squares up
this signal to produce a square-wave
October 1994 29
LED1
K
S1
V+
180
150
27
A
0V
V-
150
56
1
S3
1
11
56
0
0W
22
27
0
680
47
LED2
K
S2
0
2
3
4
A
D3
ZD1 1
1k
1uF
REG2
1k
PLUGPACK
SOCKET
output at pin 7. This output drives a
bridge rectifier consisting of D9-D12
via a 1kΩ current limiting resistor.
The bridge rectifier in turn drives
LED 2 and this begins to glow when the
ripple at one of the regulator outputs
exceeds about 4mV peak-to-peak. By
the time the ripple reaches 19mV p-p,
the LED is fully alight.
An optional metering circuit is also
shown on Fig.2, although we haven’t
included it in the prototype (the appropriate connection points are on
the PC board). All you have to do is
calculate what resistance should be
added in series with the meter to give
a full-scale reading at 30V.
For example, if you have a 0-1mA
meter movement, then by Ohm’s Law
R = V/I = 30/.001 = 30kΩ. Making R1
30 Silicon Chip
4 3
IC1
LM358
470uF
330
2 1
120
3.3k
0.1
D10
D7
D6
D12
100uF
D11
1uF
100uF
R1
3.3k
D2
470uF
D9
47k
330
1uF
ZD1
D8
22k
D1
1k
4.7M
2.7k
1uF
120
47k
REG1
D5
R2
METER
D4
Fig.3: use medium-duty (24 x 0.2mm) hookup wire for all wiring connections
& take care to ensure that switch S3 is wired exactly as shown. Resistors R1 &
R2 can be left out of circuit if you don't intend installing an output meter.
= 27kΩ and R2 = 2.7kΩ will be near
enough, especially when the internal
impedance of the meter is taken into
consideration.
Construction
All of the components for the Beginner’s Power Supply are installed
on PC board coded 04110941 and
measuring 102 x 57mm. Before commencing construction, check the
board carefully against Fig.4 for any
shorts or breaks in the tracks. If you
find any, use a dash of solder or a
small artwork knife where appropriate to fix the problem.
Fig.3 shows the parts layout on the
PC board. Start by installing PC stakes
at the external wiring points, followed
by the wire links, resistors, diodes,
capacitors and ICs. Make sure that all
polarised parts are correctly oriented
and check the resistor values on your
multimeter before mounting them on
the board. Table 1 shows the resistor
colour codes.
Note that diodes D1-D6 are all
1N4004 types, while the remaining
diodes are the smaller 1N914 types.
Pin 1 of the IC is adjacent to a small
notch or dot in one end of the plastic
body.
The metal tabs of the two 3-terminal
regulators must be oriented exactly as
shown on Fig.3; ie, the metal tab of
each device goes towards the centre
of the board. Do not confuse these
two regulators – REG1 is an LM317
type while REG2 is an LM337. Once
mounted, they can be fitted with small
finned heatsinks to aid cooling.
After the board assembly has been
completed, you can install the resistors
around switch S3. As supplied, this
switch will be a 12-position type. It
is easily converted to an 11-position
type by lifting the locking ring at the
front of the switch bush and rotating
it to position 11. This done, solder
the resistors to the switch terminals
exactly as shown on Fig.3, starting
at terminal 1 and continuing in an
anticlockwise direction to termi
nal
11 (note: in most cases, the terminal
numbers are marked on the back of
the switch).
If you have a switch that doesn’t
have the terminals marked, here’s
an easy way to find terminal 1. All
you have to do is rotate the switch
fully anticlockwise, then use your
multi
meter to find which terminal
is now connected to the wiper. This
will be terminal 1 and you can begin
by soldering the 27Ω resis
tor to it.
The remaining resistors can then be
installed exactly as shown.
Check the resistor values carefully
as they are mounted. If you make a mistake, then one or more of the voltage
ranges will be wrong. It’s also a good
idea to trim the resistor leads back as
you go so that you don’t end up with
a tangled mess. Don’t forget the wire
link between the switch wiper (near
Fig.4: this is the full-size etching pattern for the PC board
the centre) and terminal 11.
The Beginner’s Power Supply is
designed to fit into a plastic zippy
case measuring 198 x 113 x 62mm.
The front panel is actually one of
the long sides of the case, while the
PC board is mounted on the bottom
of the case. The whole unit is then
turned upside down so that the lid
becomes the base.
The first step is to attach the front
panel label (bottom nearest the lid),
then use this as a drilling template for
the front panel items. The PC board
can also be used as a template to mark
out its four mounting holes, while an
additional hole will be required in the
rear panel to accept a 3.5mm power
socket.
Note that it’s best to initially drill
all holes to 3mm. These can then be
enlarged where necessary using a
tapered reamer.
Final assembly
Once the holes have been completed, mount the various items in place.
Fig.3 shows where each component
should be placed. Note that the range
switch (S3) must be oriented so that
RESISTOR COLOUR CODES
❏
No.
❏ 1
❏ 2
❏ 1
❏ 2
❏ 1
❏ 3
❏ 1
❏ 1
❏ 1
❏ 2
❏ 1
❏ 1
❏ 1
❏ 2
❏ 2
❏ 1
❏ 1
Value
4.7MΩ
47kΩ
22kΩ
3.3kΩ
2.7kΩ
1kΩ
680Ω
560Ω
470Ω
330Ω
270Ω
220Ω
180Ω
150Ω
120Ω
56Ω
27Ω
4-Band Code (1%)
yellow violet green brown
yellow violet orange brown
red red orange brown
orange orange red brown
red violet red brown
brown black red brown
blue grey brown brown
green blue brown brown
yellow violet brown brown
orange orange brown brown
red violet brown brown
red red brown brown
brown grey brown brown
brown green brown brown
brown red brown brown
green blue black brown
red violet black brown
5-Band Code (1%)
yellow violet black yellow brown
yellow violet black red brown
red red black red brown
orange orange black brown brown
red violet black brown brown
brown black black brown brown
blue grey black black brown
green blue black black brown
yellow violet black black brown
orange orange black black brown
red violet black black brown
red red black black brown
brown grey black black brown
brown green black black brown
brown red black black brown
green blue black gold brown
red violet black gold brown
October 1994 31
For a free catalogue, fill in & mail
or fax this coupon.
✍
Please send me a free catalog
on your satellite systems.
Name:____________________________
Street:____________________________
Suburb:_________________________
P/code________Phone_____________
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33-35 Wickham Rd, Moorabin 3189
Ph (03) 553 1763; Fax (03) 532 2957
32 Silicon Chip
Additional heatsinking
As the unit stands, the output current capability is limited by the modest
amount of heatsinking. That’s because
the two 3-terminal regulators have
inbuilt thermal overload protection
which means that they automatically
throttle back when they start to get
too hot.
As an option, you can slightly increase the output current capability
by increasing the heatsinking. This
-
DROPOUT
0V
13.5
1.5
+
15
1.25
POWER
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Now for the smoke test. Connect a
16VAC 1A plugpack supply, switch
on and use your multimeter to check
the voltage between the “+” and “0V”
terminals for each switch posi
tion.
In each case, the measured voltage
should correspond to the switch position. The negative rail can then be
checked in similar fashion; ie, by connecting the multimeter between the
“-” and “0V” terminals.
If everything checks out, the power
supply is ready for use. If you strike
problems, check the supply rails
to the 3-terminal regulators and to
IC1. You should find +20V on the
IN terminal of REG1, -20V on the IN
terminal of REG2, +15V on pin 8 of
IC1, and -15V on pin 4 of IC1. If any
of these voltages are incorrect, switch
off and check D1, D2, ZD1 and ZD2
as appropriate.
If the measured output voltages
don’t correspond to the switch settings, check the resistor string around
S3. You may have some of the resistors
in the wrong positions.
12
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the pointer on the knob aligns with
the 1.25V marking on the front panel
when the switch is rotated fully anticlockwise.
Binding posts are used for the three
output terminals. We suggest that you
use red for positive, black for 0V and
blue for the negative. The PC board
is secured in the case using machine
screws and nuts, with additional nuts
under each corner of the board acting
as spacers.
The wiring can now be completed
as shown in Fig.3. It’s a good idea to
use different coloured wire for each
section, as this will make it easier to
check your wiring later on. Take care
with the orientation of the LEDs – the
anode lead is always the longer of the
two and the cathode will be adjacent
to the flat edge on the LED bevel.
Fig.5: this full-size artwork can be
used as a drilling template for the
front panel.
additional heat
sinking can be obtained by substituting an aluminium
lid for the plastic lid of the case. The
two regulators are then bolted to the
lid using TO-220 isolating kits (ie, a
mica washer and insulating bush) to
provide electrical isolation and their
leads connected to the PC board via
flying leads.
SC
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