This is only a preview of the April 1994 issue of Silicon Chip. You can view 28 of the 96 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Articles in this series:
Items relevant to "Remote Control Extender For VCRs":
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Items relevant to "Discrete Dual Supply Voltage Regulator":
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Need a dual
supply regulator in
a hurry but don’t
have any LM317 or
7805
3-terminal
regulators handy?
This simple circuit
can provide
regulated supply
rails from ±5V to
±12VDC at up to
800mA.
I
F YOU’RE NOT in the component
buying business, then you’ll probably be unaware that there has
been a severe world-wide shortage of
parts during the last 12 months – particularly 3-terminal regulators. Now
since these devices get a guernsey in
just about every project designed, we
recently decided to see if we could
come up with some sort of replacement based on readily available components.
Since doing this work, the supply
situation has vastly improved but
we still feel that the design may be
suitable for many applications. The
fact that it uses only junkbox parts is
a major plus.
All the parts for the regulator are
built onto a small PC board. This contains everything necessary to convert
the AC voltage from a centre-tapped
mains transformer to regulated plus
and minus DC supply rails, including
a bridge rectifier and filter capacitors.
It will provide an output voltage of
between ±5V and ±12V DC at currents
up to 800mA.
Circuit diagram
Fig.1 shows the circuit diagram for
the Dual Regulated Power Supply. It
uses four power diodes, an LM358
dual op amp, a zener diode, a couple
of transistors and sundry resistors and
capacitors.
Power is derived from a 12-24V
centre-tapped mains transformer. Its
output is fed to a bridge rectifier consisting of diodes D1-D4 to produce
positive and negative rails which are
then filtered using two 470µF electrolytic capacitors. These rails are then
fed to the collectors of transistors Q1
and Q2 respectively and are also used
to power the dual op amp (IC1).
Discrete dual supply
voltage regulator
By DARREN YATES
The assembled PC board can form the basis
of a simple variable power supply or can be
used to provide fixed regulated supply rails
from ±5V to ±12V DC.
April 1994 29
R2
10k
6
4x1N4004
A
240VAC
N
D4
R1
10k
D1
5
F1
2A
6-12V
Q1
BD139
8
7 100
IC1a
LM358
B
E
D4
.047
1N4148
1k
C
+VOUT
0V
6-12V
D3
470
25VW
D2
10
16VW
ZD1
4.7V
400mW
470
25VW
Fig.1: the regulated
positive supply rail is
derived by using ZD1 to
set a reference voltage
on pin 5 of inverting
amplifier stage IC1a.
This in turn drives
current amplifier stage
Q1. Inverting amplifier
stage IC1b & current
amplifier Q2 are used to
derive the negative rail.
GND
PLASTIC
SIDE
100k
470
25VW
E
C
470
25VW
100k
-VOUT
.047
2
B
1
IC1b
3
100
4
F2
2A
B
Q2
BD140
E
C
DUAL REGULATED POWER SUPPLY
IC1a and its associated zener diode
(ZD1) form the heart of the regulation
circuit. This op amp is connected as
a bootstrapped-diode reference source
and drives current amplifier stage Q1.
IC1b, on the other hand, simply functions as a unity gain inverter stage; it
drives current amplifier Q2
Zener diode ZD1 functions as the
reference element and is part of a positive feedback path around IC1a. This
feedback path may not be all that clear
at first glance – it starts at the output
of IC1a (pin 7) and goes via the 100Ω
resistor, the base-emitter junction of
Q1, diode D4 and the 1kΩ resistor,
before ending at the non-inverting
input (pin 5). This loop ensures that
the output voltage remains constant.
The 10µF capacitor across ZD1 filters
out any noise on the line and improves
the regulation.
Note that it is necessary to include
the transistor (Q1) in the feedback loop
so that the op amp can compensate for
the voltage drop across the base-emitter junction to give the required output
voltage.
Setting the output voltage for the
positive rail is now just a case of selecting the negative feedback network
to set the gain of IC1a. This feedback
network consists of two resistors (R1
and R2) connected in the usual way;
ie, one from the output to the inverting
input (pin 6) and the other from the
inverting input to ground.
30 Silicon Chip
The formula for the output voltage
is: Vout = 5.3V x (R2 + R1)/R1
where the 5.3V reference is equal to
the voltage across ZD1 plus the voltage
across D4 (ie, 4.7 + 0.6 = 5.3V). With
the current values, IC1a’s gain is set
to two and so the output voltage is set
to 10.6V. However, this can be easily
PARTS LIST
1 PC board, 04103941, 107 x
53mm
6 PC stakes
2 M205 (2AG) fuse clips
2 M205 2A fuses
2 Micro-U heatsinks
1 centre-tapped mains
transformer to suit (see Table 1)
Semiconductors
1 LM358N dual op amp IC
1 BD139 NPN transistor
1 BD140 PNP transistor
1 4.7V 400mW zener diode (ZD1)
4 1N4004 rectifier diodes
1 1N914 signal diode
Capacitors
4 470µF 25VW electrolytic
1 10µF 16VW electrolytic
2 0.047µF 63VW MKT polyester
Resistors (0.25W, 1%)
2 100kΩ
1 1kΩ
2 10kΩ
2 100Ω
altered by changing the value of R1,
R2 or ZD1.
The negative rail is much simpler
to produce because all we need do is
invert the output of the positive rail.
This is done by feeding the voltage
on the emitter of Q1 to the inverting
input (pin 2) of IC1b via a 100kΩ
resistor.
As previously mentioned, IC1b
functions as a unity gain inverting amplifier. Its output at pin 1 drives PNP
power transistor Q2 via a 100Ω current
limiting resistor. As before, the output
transistor is included in the feedback
loop to ensure that its base-emitter
voltage is compensated for. In this way,
the negative rail mirrors the voltage on
the positive rail.
The two .047µF capacitors connected across the base-emitter junctions of
Q1 and Q2 reduce the sensitivity of the
circuit to noise or glitches and improve
the regulation. The final outputs are
taken from the emitters of Q1 and Q2
and filtered by two 470µF capacitors.
A maximum of 800mA can be supplied
by both sections.
Construction
All of the components for the Discrete Power Supply, including the two
2A fuses, are installed on a PC board
coded 04103941 and measuring 107
x 53mm.
Before you begin construction, it’s
a good idea to check the PC board
TABLE 1
1k
R2 10k
F1
D1-D2
470uF
0V
IC1
LM358
D3-D4
470uF
AC2
D5
470uF
ZD1
R1 10k
AC1
.047
100W
Q1
10uF
100k
+VOUT
0V
100k
470uF
-VOUT
Q2
Fig.2: install the parts on the PC board as shown here & note that small finned
heatsinks should be fitted to Q1 & Q2. Resistors R1 & R2 are selected to set the
required output voltage – see text.
Fig.3: this is the full-size etching pattern for the PC board.
for any shorts or breaks in the tracks.
You can do this by carefully checking
your etched board against the full-size
pattern. Generally, there won’t be any
problems here but it’s always a good
idea to make sure.
Begin the board assembly by installing the two wire links, followed by
the resistors, diodes and capacitors.
Be sure not to confuse the zener diode
with the rectifier and signal diodes.
After that, install the IC and power
transistors. Be particularly careful
with these components – check the
orientation of the IC carefully and note
that the transistors are installed with
their plastic faces towards the adjacent
.047µF capacitors.
Transformer
5V
12V CT
6V
15V CT
8V
18V CT
12V
24V CT
.047
100
F2
DC Output (V)
Note also that Q1 is an NPN transistor while Q2 is a PNP type, so be
sure to use the correct transistor at
each location.
Finally, solder in six PC stakes at
the external wiring points, install the
fuse clips and bolt two small finned
heatsinks to the power transistors.
There’s no need to isolate the transistors from the heatsinks but don’t let
them short against any of the other
parts on the board.
To test the circuit, you need a
centre-tapped mains transformer (or
you can use an AC plugpack supply
with a centre tap). Table 1 shows the
transformer input voltage you need
for a given DC output voltage. Wire up
the secondary windings of the transformer to the PC board as shown on
the overlay diagram and the primary
to a mains terminal block.
Warning: use extreme caution when
installing the mains wiring – 240VAC
can kill! The transformer and the PC
board should be mounted inside a
metal case and this must be securely
earthed. Cover all mains connections
with heatshrink tubing to avoid the
possibility of electric shock.
Before applying power, check your
wiring carefully for any wrong connections. Once you’re sure that everything
is OK, switch on and check the output voltage with your multimeter. If
you have used the values shown on
the circuit, you should get a reading
of about 10.6V on both rails with
respect to ground (this will depend
on the exact voltage across the zener
diode). If need be, you can substitute
a trimpot for resistor R2 and trim the
output voltage until it is exactly what
you require.
A variable supply
By replacing R2 with a 20kΩ linear
potentiometer, you can make a simple
dual-tracking variable power supply
capable of covering the range from ±5V
to about ±12VDC. The circuit could
thus form the basis of a very useful
benchtop power supply for powering
experimental lash-ups.
For lower output voltages, you could
replace ZD1 with a number of signal
diodes. If only one diode is used, the
output voltage will be about ±2.4V.
Remember, the formula for the output
voltage is: Vout = (VZD1 + VD4) x (R2
SC
+ R1)/R1.
RESISTOR COLOUR CODES
❏
❏
❏
❏
❏
No.
2
2
1
2
Value
100kΩ
10kΩ
1kΩ
100Ω
4-Band Code (1%)
brown black yellow brown
brown black orange brown
brown black red brown
brown black brown brown
5-Band Code (1%)
brown black black orange brown
brown black black red brown
brown black black brown brown
brown black black black brown
April 1994 31
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