Many times the hobbyist wants to have a simple, dual power supply for a
project. Existing powersupplies may be too big either in power output or
physical size. Just a simple Dual Power Supply is required.For most
non-critical applications the best and simplest choice for a voltage
regulator is the 3-terminal type.The 3 terminals are input, ground and
output.
The 78xx & 79xx series can provide up to 1A load current and it have
onchip circuitry to prevent damage in the event of over heating or
excessive current. That is, the chip simply shuts down rather than
blowing out. These regulators are inexpensive, easy to use, and they
make it practical to design a system with many PCBs in which an
unregulated supply is brought in and regulation is done locally on each
circuit board.
Circuit diagram:
Dual Power Supply Schematic Circuit diagram
This Dual Power Supply project provides a dual power supply. With the
appropriate choice of transformer and 3-terminal voltageregulator pairs
you can easily build a small power supply delivering up to one amp at
+/- 5V, +/- 9V, +/- 12V, +/-15V or +/-18V. You have to provide the
centre tapped transformer and the 3-terminal pair of regulators you
want:7805 & 7905, 7809 & 7909, 7812 & 7912, 7815 &
7915or 7818 & 7918.
Note that the + and - regulators do not have to be matched: you can for
example, use a +5v and -9V pair. However,the positive regulator must be a
78xx regulator, and the negative a 79xx one. We have built in plenty of
safety into this project so it should give many years of continuous
service. The user must choose the pair he needs for his particular
application.
Parts :
Transformer
This Dual Power Supply design uses a full wave bridge rectifier coupled
with a centre-tapped transformer. A transformer with a power output
rated at at least 7VA should be used. The 7VA rating means that the
maximum current which can be delivered without overheating will be
around 390mA for the 9V+9V tap; 290mA for the 12V+12V and 230mA for the
15V+15V. If the transformer is rated by output RMS-current then the
value should be divided by 1.2 to get the current which can be supplied.
For example, in this case a 1A RMS can deliver 1/(1.2) or 830mA.
Rectifier
We use an epoxy-packaged 4 amp bridge rectifier with at least a peak
reverse voltage of 200V. (Note the part numbers of bridge rectifiers are
not standardised so the number are different from different
manufacturers.) For safety the diode voltage rating should be at least
three to four times that of the transformers secondary voltage. The
current rating of the diodes should be twice the maximum load current
that will be drawn.
Filter Capacitor
The purpose of the filter capacitor is to smooth out the ripple in the
rectified AC voltage. Theresidual amount of ripple is determined by the
value of the filer capacitor: the larger the value the smaller the
ripple.The 2,200uF is a suitable value for all the voltages generated
using this project. The other consideration inchoosing the correct
capacitor is its voltage rating. The working voltage of the capacitor
has to be greater than thepeak output voltage of the rectifier. For an
18V supply the peak output voltage is 1.4 x 18V, or 25V. So we
havechosen a 35V rated capacitor.
Regulators
The unregulated input voltage must always be higher than the regulators
output voltage by at least 3V inorder for it to work. If the
input/output voltage difference is greater than 3V then the excess
potential must bedissipated as heat. Without a heatsink 3 terminal
regulators candissipate about 2 watts. A simple calculation of the
voltage differential times the current drawn will give the watts tobe
dissipated. Over 2 watts a heatsink must be provided. If not then the
regulator will automatically turn off if theinternal temperature reaches
150oC. For safety it is always best to use a small heatsink even if you
do not think youwill need one.
Stability
C4 & C5 improve the regulators ability to react to sudden changes in load current and to preventuncontrolled oscillations.
Decoupling
The monoblok capacitor C2 & C6 across the output provides high
frequency decoupling which keepsthe impedence low at high frequencies.
LED
Two LED's are provided to show when the output regulated power is
on-line. You do not have to use theLED's if you do not want to. However,
the LED on the negative side of the circuit does provide a minimum load
tothe 79xx regulator which we found necessary during testing. The
negative 3-pin regulators did not like a zeroloadsituation. We have provided a 470R/0.5W resistors as the current limiting resistors for the LED's.
Diode Protection
These protect mainly against any back emf which may come back into the
power supply when itsupplies power to inductive loads. They also provide
additional short circuit protection in the case that thepositive output
is connected by accident to the negative output. If this happened the
usual current limiting shutdownin each regulator may not work as
intended. The diodes will short circuit in this case and protect the 2
regulators.
