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 original schematic.
This is a high quality power supply with a continuously variable stabilised
output adjustable at any value between 0 and 30VDC. The circuit also
incorporates an electronic output current limiter that effectively controls
the output current from a few milliamperes (2 mA) to the maximum output
of three amperes that the circuit can deliver. This feature makes this
power supply indispensable in the experimenters laboratory as it is
possible to limit the current to the typical maximum that a circuit
under test may require, and power it up then, without any fear that
it may be damaged if something goes wrong. There is also a visual indication
that the current limiter is in operation so that you can see at a glance
that your circuit is exceeding or not its preset limits.
Technical Specifications – Characteristics
Reduced dimensions, easy construction, simple operation.
– Output voltage easily adjustable.
– Output current limiting with visual indication.
– Complete protection of the supplied device against over loads
How it Works
with, there is a step-down mains transformer with a secondary winding
rated at 24 V/3 A, which is connected across the input points of the
circuit at pins 1 & 2. (the quality of the supplies output will be directly
proportional to the quality of the transformer). The AC voltage of the
transformers secondary winding is rectified by the bridge formed by
the four diodes D1-D4. The DC voltage taken across the output of the
bridge is smoothed by the filter formed by the reservoir capacitor C1
and the resistor R1. The circuit incorporates some unique features which
make it quite different from other power supplies of its class. Instead
of using a variable feedback arrangement to control the output voltage,
our circuit uses a constant gain amplifier to provide the reference
voltage necessary for its stable operation. The reference voltage is
generated at the output of U1.
operates as follows: The diode D8 is a 5.6 V zener, which here operates
at its zero temperature coefficient current. The voltage in the output
of U1 gradually increases till the diode D8 is turned on. When this
happens the circuit stabilises and the Zener reference voltage (5.6
V) appears across the resistor R5. The current which flows through the
non inverting input of the op-amp is negligible, therefore the same
current flows through R5 and R6, and as the two resistors have the same
value the voltage across the two of them in series will be exactly twice
the voltage across each one. Thus the voltage present at the output
of the op-amp (pin 6 of U1) is 11.2 V, twice the zeners reference voltage.
The integrated circuit U2 has a constant amplification factor of approximately
3 X, according to the formula A=(R11+R12)/R11, and raises the 11.2 V
reference voltage to approximately 33 V. The trimmer RV1 and the resistor
R10 are used for the adjustment of the output voltages limits so that
it can be reduced to 0 V, despite any value tolerances of the other
components in the circuit.
click for higher resolution
very important feature of the circuit, is the possibility to preset
the maximum output current which can be drawn from the p.s.u., effectively
converting it from a constant voltage source to a constant current one.
To make this possible the circuit detects the voltage drop across a
resistor (R7) which is connected in series with the load. The IC responsible
for this function of the circuit is U3. The inverting input of U3 is
biased at 0 V via R21. At the same time the non inverting input of the
same IC can be adjusted to any voltage by means of P2.
assume that for a given output of several volts, P2 is set so that the
input of the IC is kept at 1 V. If the load is increased the output
voltage will be kept constant by the voltage amplifier section of the
circuit and the presence of R7 in series with the output will have a
negligible effect because of its low value and because of its location
outside the feedback loop of the voltage control circuit. While the
load is kept constant and the output voltage is not changed the circuit
is stable. If the load is increased so that the voltage drop across
R7 is greater than 1 V, IC3 is forced into action and the circuit is
shifted into the constant current mode. The output of U3 is coupled
to the non inverting input of U2 by D9. U2 is responsible for the voltage
control and as U3 is coupled to its input the latter can effectively
override its function. What happens is that the voltage across R7 is
monitored and is not allowed to increase above the preset value (1 V
in our example) by reducing the output voltage of the circuit.
in effect a means of maintaining the output current constant and is
so accurate that it is possible to preset the current limit to as low
as 2 mA. The capacitor C8 is there to increase the stability of the
circuit. Q3 is used to drive the LED whenever the current limiter is
activated in order to provide a visual indication of the limiters operation.
In order to make it possible for U2 to control the output voltage down
to 0 V, it is necessary to provide a negative supply rail and this is
done by means of the circuit around C2 & C3. The same negative supply
is also used for U3. As U1 is working under fixed conditions it can
be run from the unregulated positive supply rail and the earth.
supply rail is produced by a simple voltage pump circuit which is stabilised
by means of R3 and D7. In order to avoid uncontrolled situations at
shut-down there is a protection circuit built around Q1. As soon as
the negative supply rail collapses Q1 removes all drive to the output
stage. This in effect brings the output voltage to zero as soon as the
AC is removed protecting the circuit and the appliances connected to
its output. During normal operation Q1 is kept off by means of R14 but
when the negative supply rail collapses the transistor is turned on
and brings the output of U2 low. The IC has internal protection and
can not be damaged because of this effective short circuiting of its
output. It is a great advantage in experimental work to be able to kill
the output of a power supply without having to wait for the capacitors
to discharge and there is also an added protection because the output
of many stabilised power supplies tends to rise instantaneously at switch
off with disastrous results.
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.
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 also 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
DO NOT file or sandpaper a dirty or worn out tip. If the tip cannot
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 every time.
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 components body and insert
he 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 enlarge 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.
When the solder starts to melt and flow wait till it covers evenly
the area around the hole and the flux boils and gets out from underneath
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 the component.
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.
When you finish your work, cut off the excess of the component leads
and clean the board thoroughly with a suitable solvent to remove all
flux residues that may still remain on it.
PCB – Connections
pcb.gif (60KB) (12,5cm x 8,7cm)
Construction (… continued)
As it is recommended
start working by identifying the components and separating them in groups.
Place first of all the sockets for the ICs and the pins for the external
connections and solder them in their places. Continue with the resistors.
Remember to mound R7 at a certain distance from the printed circuit
board as it tends to become quite hot, especially when the circuit is
supplying heavy currents, and this could possibly damage the board.
It is also advisable to mount R1 at a certain distance from the surface
of the PCB as well. Continue with the capacitors observing the polarity
of the electrolytic and finally solder in place the diodes and the transistors
taking care not to overheat them and being at the same time very careful
to align them correctly.