Copyright of this circuit belongs to smart kit electronics.
In this page we will use this circuit to discuss for
improvements and we will introduce some changes based on
This is a very useful project for anyone working in electronics.
It is a versatile power supply that will solve most of the supply
problems arising in the everyday work of any electronics work
shop. It covers a wide range of voltages being continuously
variable from 30 V down to 3 V. The output current is 2.5 A
maximum, more than enough for most applications. The circuit is
completely stabilised even at the extremes of its output range and
is fully protected against short-circuits and overloading.
Technical Specifications - Characteristics
Input voltage: 24V DC
Output current: 2.5 A
Output volytage: 3-30V DC
How it Works
The power supply is using a well known and quite popular VOLTAGE
STABILIZER IC the LM 723. The IC can be adjusted for out put
voltages that vary continuously between 2 and 37 VDC and has a
current rating of 150 mA which is of course too low for any
serious use. In order to increase the current handling capacity of
the circuit the output of the IC is used to drive a darlington
pair formed by two power transistors the BD 135 and the 2N 3055.
The use of the transistors to increase the maximum current output
limits the range of output voltages somewhat and this is why the
circuit has been designed to operate from 3 to 30 VDC. The
resistor R5 that you see connected in series with the output of
the supply is used for the protection of the circuit from
overloading. If an excessively large current flows through R5, the
voltage across it increases and any voltage greater than 0.3 V
across it has as a result to cut the supply off, thus effectively
protecting it from overloads. This protection feature is built in
the LM 723 and the voltage drop across R5 is sensed by the IC
itself between pins 2 and 3. At the same time the IC is
continuously comparing the output voltage to its internal
reference and if the difference exceeds the designer’s standards
it corrects it automatically. This ensures great stability under
different loads. The potentiometer P1 is used to adjust the out
put voltage at the desired level. If the full range from 3 to 30 V
is desired then you should use a mains transformer with a
secondary winding having a rating of at least 24 V/3 A. If the
maxi mum voltage output is not desired you can of course use a
transformer with a lower secondary voltage output. (However, once
rectified the voltage across the capacitor C2 should exceed by 4-5
volts the maximum output expected from the circuit.
First of all let us consider a few basics in building electronic
circuits on a printed circuit board. The board is made of a thin
insulating material clad with a thin layer of conductive copper
that is shaped in such a way as to form the necessary conductors
between the various components of the circuit. The use of a
properly designed printed circuit board is very desirable as it
speeds construction up considerably and reduces the possibility of
making errors. Smart Kit boards also come pre-drilled and with the
outline of the components and their identification printed on the
component side to make construction easier. To protect the board
during storage from oxidation and assure it gets to you in perfect
condition the copper is tinned during manufacturing and covered
with a special varnish that protects it from getting oxidised and
makes soldering easier. Soldering the components to the board is
the only way to build your circuit and from the way you do it
depends greatly your success or failure. This work is not very
difficult and if you stick to a few rules you should have no
problems. The soldering iron that you use must be light and its
power should not exceed the 25 Watts. The tip should be fine and
must be kept clean at all times. For this purpose come very handy
specially made sponges that are kept wet and from time to time you
can wipe the hot tip on them to remove all the residues that tend
to accumulate on it.
DO NOT file or sandpaper a dirty or worn out tip. If the tip can
not be cleaned, replace it. There are many different types of
solder in the market and you should choose a good quality one that
contains the necessary flux in its core, to assure a perfect joint
DO NOT use soldering flux apart from that which is already
included in your solder. Too much flux can cause many problems
and is one of the main causes of circuit malfunction. If
nevertheless you have to use extra flux, as it is the case when
you have to tin copper wires, clean it very thoroughly after you
finish your work. In order to solder a component correctly you
should do the following:
Clean the component leads with a small piece of emery paper.
Bend them at the correct distance from the component body and
insert the component in its place on the board.
You may find sometimes a component with heavier gauge leads than
usual, that are too thick to enter in the holes of the p.c.
board. In this case use a mini drill to increase the diameter of
the holes slightly. Do not make the holes too large as this is
going to make soldering difficult afterwards.
Take the hot iron and place its tip on the component lead while
holding the end of the solder wire at the point where the lead
emerges from the board. The iron tip must touch the lead slightly
above the p.c. board.
When the solder starts to melt and flow wait till it covers evenly
he area around the hole and the flux boils and gets out from
underneath the solder. The whole operation should not take more
than 5 seconds. Remove the iron and allow the solder to cool
naturally without blowing on it or moving the component. If
everything was done properly the surface of the joint must have a
bright metallic finish and its edges should be smoothly ended on
the component lead and the board track. If the solder looks dull,
cracked, or has the shape of a blob then you have made a dry joint
and you should remove the solder (with a pump, or a solder wick)
and redo it.
Take care not to overheat the tracks as it is very easy to lift
them from the board and break them.
When you are soldering a sensitive component it is good practice
to hold the lead from the component side of the board with a pair
of long-nose pliers to divert any heat that could possibly damage
Make sure that you do not use more solder than it is necessary as
you are running the risk of short-circuiting adjacent tracks on
the board, especially if they are very close together.
After you have finished your work cut off the excess of the
component leads and clean the board thoroughly with a suit able
solvent to remove all the flux residues that may still remain on
Start building the circuit by placing the pins on the board and
soldering them. You must be very careful when soldering the
components that are going to carry heavy currents as your joints
must be capable of withstanding the maximum current without
getting hot. Solder the IC socket in its place taking care not to
insert it the wrong way round and then put the resistors in their
places on the board. Resistor R5 should be soldered in such a way
as to keep its body slightly separated from the p.c. board to let
the air circulate around the component and cool it. Continue your
work with the capacitors. Be careful not to insert the
electrolytic the wrong way round. The polarity is marked on the
capacitors and the p.c. board is also marked accordingly. Insert
the rectifier bridge in its place. The bridge is a heavy duty type
and has leads made of heavier gauge wire than usual. If you have
any difficulty inserting them in the p.c. board you can enlarge
the holes with a mini drill. (Automatic production of p.c. boards
requires all the holes on the board to be of the same diameter.)
Do not however make the holes too wide as you are going to find
soldering the leads much more difficult afterwards. Solder TR1 in
its place and mount TR2 on the heatsink following the diagram and
making sure there is no electrical connection between the heat
sink and the transistor. Don’t forget the insulators, and use heat
transfer compound between the transistor body and the heat sink.
Using heavy gauge wires connect TR2 to the board and finally using
a flat ribbon cable connect the potentiometer with the rest of the
circuit. Insert the VOLTAGE REGULATOR in its socket and your power
supply is ready. Now make a final inspection of your work to
ensure that there are no mistakes that could cause a lot of
trouble later. If everything looks OK you can connect the input of
the circuit (it is marked «24 VAC» on the board) to the secondary
winding of the transformer. Connect a voltmeter to the pins marked
«OUT 3-30 V» and using a mains lead connect the primary of the
transformer to a convenient power out let. If everything was done
properly the voltmeter should give a reading and turning the
potentiometer should make it change.
Slight variations from the minimum and maximum voltages specified
are normal, are caused from component tolerances and should not
worry you. Although the circuit works with low voltages and is
quite safe to touch any part while it is in operation it needs a
mains transformer to supply this low voltage and the primary of
the transformer is connected to the mains which makes it very
dangerous. The best idea is to use a case for everything in order
to make a complete stand alone power supply for your experiments.
Smart Kit also makes a suitable case for this supply with a
printed front panel, ready drilled for the output connectors,
switches, fuse holder and panel instruments.
|R1 = 560R 1/4W
||C1 = 100nF
|R2 = 1,2 K 1/4W
||C2 = 2200uF 35-40V
|R3 = 3,9 K 1/4W
||C3 = 100 pF
|R4 = 15K 1/4W
||C4 = 100uF/ 35V
|R5 = 0,15R 5W
|D = B40
C3300/2200, 3A rectifier bridge
|P1 = 10K potesiometer
||TR1 = BD 135
|IC = LM723
||TR2 = 2N3055
This circuit works from the mains and there are 220 VAC pre sent
in some of its parts. Voltages above 50 V are DANGEROUS and could
even be LETHAL. In order to avoid accidents that could be fatal to
you or members of your family please observe the following
DO NOT work if you are tired or in a hurry, double check every
thing before connecting your circuit to the mains and be ready to
disconnect it if something looks wrong.
DO NOT touch any part of the circuit when it is under power.
DO NOT leave mains leads exposed. All mains leads should be well
insulated. -DO NOT change the fuses with others of higher rating
or replace them with wire or aluminium foil.
DO NOT work with wet hands. -If you are wearing a chain, necklace
or anything that may be hanging and touch an exposed part of the
circuit BE CAREFUL. USE ALWAYS a correct mains lead with the correct plug
and earth your circuit correctly. If the case of your project is
made of metal make sure it is properly earthen. If it is possible
use a mains transformer with a 1:1 ratio to isolate your circuit
from the mains.
When testing a circuit that works off the mains wear shoes with
rubber soles, stand on dry non conductive floor and keep one hand
in your pocket or behind your back.
If you take all the above precautions you are reducing the risks
you are taking to a minimum and this way you are protecting your
self and those around you. A carefully built and well insulated
device does not constitute any danger for its user. BEWARE: ELECTRICITY CAN KILL IF YOU ARE NOT CAREFUL.
Here are some photos from
this power supply finished and installed in a box.