This is only a preview of the August 1988 issue of Silicon Chip. You can view 40 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 "High Performance AC Millivoltmeter":
Items relevant to "Universal Power Supply Board":
Items relevant to "Building the Discolight, Pt.2":
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Low-cost circuit gives +15V, -15V or -+15V
Universal power supply
There is more than one way to skin a cat they
·reckon, and the same applies to designing a power
supply. This small board enables you to obtain
+ 15V, - 15V or ± 15V DC from a number of
different transformer and rectifier combinations.
By LEO SIMPSON
It's a problem that has confronted us on a number of occasions
over the years; many circuits require ± 15V DC rails or one or the
other and, in each case, a suitable
printed circuit board has to be
designed. So we decided to· solve
this problem for a number of different transformer combinations.
One common situation is when
you are powering a circuit from a
12VAC plugpack transformer but
you want ± 15V rails, using
3-terminal regulators. Sounds difficult? Nope, piece of cake. Just use
two half wave rectifiers to obtain
the positive and negative rails and
then follow with the regulators.
Or maybe you have a more conventional situation with a 12VAC
transformer such as the Ferguson
PF2851 (or equivalent). To obtain
± 15V DC the circuit is the same.
But if you have a 30VAC centretapped transformer such as the one
from Altronics (Cat. No. M-2855),
you then use a bridge rectifier,
followed by the filter capacitors
and 3-terminal regulators.
Anyway, you get the general
idea. We are presenting one PCB
pattern and showing how to use it
in four different ways, depending
on what your requirements are and
This version of the universal power supply board uses half-wave rectifiers
and two 3-terminal regulators to give ± 15 rails (see Fig.1). Note that the LED
indicator circuit was added after this photo was taken.
38
SILICON CHIP
what transformer you are using.
Actually, there are other options
and we'll mention those later.
The board measures 71 x 52mm
(code 04106881) and was used for
the first time in the Studio 200
Stereo Control Unit, part two of
which was featured in the July
issue. We are using it again in the
AC Millivoltmeter described
elsewhere in this issue.
The circuit variations
Fig.1 shows the first circuit situation presented above and could be
used with 1 a 12VAC plugpack or
with any chassis mounting transformer with an output voltage or 12
to 15 volts AC. You can regard the
circuit in two ways. First, as two
half-wave rectifiers, Dl and D2, .
producing filtered but unregulated
DC supply rails of ± 18-22V, depending on the transformer secondary voltage.
The other way of regarding the
circuit of Fig.1 is as a conventional
half-wave voltage doubler circuit
which has been "centre-tapped" at
the junction of the two 1000µF
capacitors. Either way, the result is
the same.
Because Dl and D2 function as
half-wave rectifiers, the ripple
voltage superimposed on the DC
supplies will be 50Hz. This may or
may not be a problem but, for a
given current drain from the supplies, the ripple voltage will be
slightly more than twice the 100Hz
ripple obtained if the bridge rectifier circuit of Fig.2 is used.
Fig.2 may be used with the 30V
centre-tap M-2855 transformer
supplied by Altronics or the 24V
centre-tap model from Tandy (Cat.
No 273-7013). Following the bridge
rectifier diodes, the unregulated DC
voltage will be again be ± 18-22V,
depending on the transformer.
The 3-terminal regulators to be
used will normally be 7815 or
LM340T-15 (or other variants) for
supply rails
board for op amp circuits
POSmVE
REGULATOR
01
A
OUT
PARTS LIST
+V
+
02
100
0.1
LED
OV
N
0.1
-V
OUT
NEGATIVE
REGULATOR
SINGLE WINDING HALF-WAVE RECTIAER DUAL POLARITY
Fig.1: this dual polarity version uses a 12-15V transformer to drive
half-wave rectifiers (D1 and D2) and two 3-terminal regulators.
A
± 15V Supply (See Fig.1)
1 PCB, code 04106881, 71 x
52mm
1 2851 power transformer with
12.6V secondary
2 1 N4002 silicon diodes
1 7815 3-terminal regulator
1 7915 3-terminal regulator
2 1 000µF 25VW PC-mount
electrolytic capacitors
2 1 00µF 25VW PC-moun_t
electrolytic capacitors
2 0.1 µF metallised polyester
capacitors (greencaps)
1 red LED
1 1.5k0 0.25W resistor
+v
100
N
+
0.1
LED
ov
.,.
0.1
1000
-V
OUT
NEGATIVE
REGULATOR
CENTRE TAPPED, FULL WAVE DUAL POLARITY
Fig.2: in this circuit, a centre tapped transformer and bridge
rectifier are used to drive the 3-terminal regulators.
01
:J
IN
POSmVE
REGULATOR
OUT
+v
GND
+
+
1000
100
Alternative ± 15V Supply
(See Fig.2)
1 PCB, code 04 106881 , 71 x
52mm
1 M-2855 power transformer
(Altronics)
4 1N4002 silicon diodes
1 7815 3-terminal regulator
1 7915 3-terminal regulator
2 1 000µF 25VW PC-mount
electrolytic capacitors
2 100µF 25VW PC-mount
electrolytic capacitors
2 0. 1µF metallised polyester
capacitors (greencaps)
1 red LED
1 1.5k0 0.25W resistor
0.1
LED
.
ov
POSmVE
REGULATOR
CENTRE TAPPED, FULL WAVE
Fig.3: this single supply circuit uses D1 and D2 to provide full wave
rectification from a centre-tapped transformer.
the + 15V supply. For the negative
rail, the equivalent types are 7915
or LM320T-15 (or other variants).
The output side of the regulators
have lOOµF and 0. lµF capacitors
connected to ensure good transient
response and stability. We also
made provision for a LED (light
emitting diode) power indicator fed
via a resistor from the positive
supply.
"~oo,
GND
NEGATIVE
REGULATOR
~~M
IN
Fig.5: here are the pinouts for the
3-terminal regulators and the LED.
Single supply versions
Fig.3 is a single supply version of
the board using a 30V centre-tap
(CT) or 24V CT transformer. Here,
Ill and D2 provide full wave rectification and the resulting ripple is
100Hz. The unregulated DC voltage
will be between + 18-22V.
Fig.4 is a single supply circuit using a 12 or 15V transformer feeding
a bridge rectifier. Again, the
AUGUST 1988
39
can be 1.5k0 for + 12 or + 15V outputs, reduced to 6800 for 8V outputs and to 4700 for 5V outputs.
The LED can be omitted, of course,
in which case the limiting resistor
can be left out too.
A
+
100
N
0.1
LED
Other variants
.,.
SINGLE WINDING BRIDGE RECTIFIER
Fig.4: single supply version using a 12-15V transformer and a
bridge rectifier. The unregulated DC voltage will be between
18-22V.
unregulated DC voltage will be between + 18-22V with ripple at
instead of the 1000µF units shown
on the circuits.
100Hz.
Less than 15V supplies
All the circuits presented here
are good for several hundred
milliamps but those which use full
wave rectification will naturally be
able to extract more current from
the given transformer.
If you want more current, a bigger transformer will be required
and the regulator(s) will have to be
fitted with heatsink(s). It is also a
good idea to go for bigger filter
capacitors; ie, 2200µF or 2500µF ,
Want less than 15V? No problem,
you can install 12V regulators instead although for a given current
drain their dissipation will be
slightly higher.
You can also go for 5V or 8V
regulators if you wish although
then it would be wise to reduce the
transformer secondary voltage
from 30V CT to 15V CT for Fig.2
and to around 6-7V for Fig.1.
The LED current limiting resistor
NEGATIVE
REGULATOR
--
TRANSFORMER
SECONDARY
•••
i ·i!:5
.......
~
02
C)~--V
I
=~ijr::
100uF
I
TRANSFORMER
SECONDARY
You can also produce a single
(positive) supply version of Fig.1 if
you wish, by leaving out D2 , the
negative 3-terminal regulator and
the three associated capacitors. Or,.
if you want a negative supply version, leave out Dl, the positive
3-terminal regulator and its three
associated capacitors.
Wiring them up
We have shown four wirmg
diagrams for the circuits depicted
in Figs.1 to 4. There are only two
particular wrinkles to watch out for
when wiring up these boards. First,
watch out for the polarity of diodes
and capacitors. Second, note that
the pinouts for the positive and
negative regulators are not the
same.
~
......
.......
-__L .....
.......
I
01-04
er-
J-rCD
•••
~ ~-+v
o~
lE°lli 5 POSITIVE
•••REGULATOR
iEllia POSITIVE
•••REGULATOR
I
··~~,-·
-'ov
NEGATIVe
REGULATOR
I
LEO
Fig.6: parts layout diagram for the circuit shown in
Fig, 1. The value of R depends on the supply rail
(see text).
-+ ...........
TRANSFORMER SECONDARY
I
CT--
01
1o~F
=~
-
lov
:®--+v
iEllia POSITIVE
•• •REGULATOR
Fig.7: this parts layout diagram corresponds to the
circuit shown in Fig.2. Take care with component
polarity.
......
lI .......
.......
- -g
TRANSFORMER
SECONDARY
01 -04
-~Ilov
C)
Cl-
~
~-+v
iEllia POSITIVE
• • • REGULATOR
1
Fig.8: parts layout the single supply version shown in
Fig.3. A centre-tapped transformer must be used.
40
SILICON CHIP
Fig.9: parts layout for the circuit shown in Fig.4.
Don't forget to install the wire link.
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