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0-30 VDC Stabilized
Power Supply with Current Control 0.002-3 A
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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 |
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Technical
Specifications
Input Voltage:
................ 24 VAC
Input Current: ................ 3 A (max)
Output Voltage: ............. 0-30 V adjustable
Output Current:
............. 2 mA-3 A adjustable
Output Voltage Ripple: .... 0.01 % maximum
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Features
-
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 and
malfunction. |
To
start 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.
The
circuit 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.
Schematic
diagramm
![](PSU_files/schem_th.gif)
click
for higher resolution
Another
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.
Let
us 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.
This
is 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.
The
negative 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 on it.
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.
board.
-
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
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 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.
Construction (...
continued) |
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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.
Mount
the power transistor on the heatsink. To do this follow the diagram
and remember to use the mica insulator between the transistor body
and the heatsink and the special fibber washers to insulate the
screws from the heatsink. Remember to place the soldering tag on one
of the screws from the side of the transistor body, this is going to
be used as the collector lead of the transistor. Use a little amount
of Heat Transfer Compound between the transistor and the heatsink to
ensure the maximum transfer of heat between them, and tighten the
screws as far as they will go.
Attach
a piece of insulated wire to each lead taking care to make very good
joints as the current that flows in this part of the circuit is quite
heavy, especially between the emitter and the collector of the
transistor. It is convenient to know where you are going to place
every thing inside the case that is going to accommodate your power
supply, in order to calculate the length of the wires to use between
the PCB and the potentiometers, the power transistor and for the
input and output connections to the circuit. (It does not really
matter if the wires are longer but it makes a much neater project if
the wires are trimmed at exactly the length necessary). Connect
the potentiometers, the LED and the power transistor and attach two
pairs of leads for the input and output connections. Make sure that
you follow the circuit diagram very care fully for these connections
as there are 15 external connections to the circuit in total and if
you make a mistake it may be very difficult to find it afterwards. It
is a good idea to use cables of different colours in order to make
trouble shooting easier.
The
external connections are: - 1 & 2 AC input, the secondary of
the transformer. - 3 (+) & 4 (-) DC output. - 5, 10 & 12
to P1. - 6, 11 & 13 to P2. - 7 (E), 8 (B), 9 (E) to the power
transistor Q4. - The LED should also be placed on the front panel of
the case where it is always visible but the pins where it is
connected at are not numbered.
When
all the external connections have been finished make a very careful
inspection of the board and clean it to remove soldering flux
residues. Make sure that there are no bridges that may short circuit
adjacent tracks and if everything seems to be all right connect the
input of the circuit with the secondary of a suitable mains
transformer. Connect a voltmeter across the output of the circuit and
the primary of the transformer to the mains. DO NOT TOUCH ANY PART
OF THE CIRCUIT WHILE IT IS UNDER POWER.
The
voltmeter should measure a voltage between 0 and 30 VDC depending on
the setting of P1, and should follow any changes of this setting to
indicate that the variable voltage control is working properly. Turning
P2 counter-clockwise should turn the LED on, indicating that the
current limiter is in operation.
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If you want
the output of your supply to be adjustable between 0 and 30 V you
should adjust RV1 to make sure that when P1 is at its minimum setting
the output of the supply is exactly 0 V. As it is not possible to
measure very small values with a conventional panel meter it is
better to use a digital meter for this adjustment, and to set it at a
very low scale to increase its sensitivity.
While using
electrical parts, handle power supply and equipment with great care,
following safety standards as described by international specs and
regulations.
CAUTION
This circuit works off the mains and there are 220 VAC present 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 rules:
- 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. - ALWAYS use a
proper mains lead with the correct plug and earth your circuit
properly. - If the case of your project is made of metal make sure
that 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 you are 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 -
yourself 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.
Check your
work for possible dry joints, bridges across adjacent tracks or
soldering flux residues that usually cause problems. Check
again all the external connections to and from the circuit to see if
there is a mistake there. - See that there are no components
missing or inserted in the wrong places. - Make sure that all
the polarised components have been soldered the right way round. -
Make sure the supply has the correct voltage and is connected the
right way round to your circuit. - Check your project for faulty
or damaged components.
R1 =
2,2 KOhm 1W |
R2 = 82
Ohm 1/4W |
R3 =
220 Ohm 1/4W |
R4 =
4,7 KOhm 1/4W |
R5, R6,
R13, R20, R21 = 10 KOhm 1/4W |
R7 =
0,47 Ohm 5W |
R8, R11
= 27 KOhm 1/4W |
R9, R19
= 2,2 KOhm 1/4W |
R10 =
270 KOhm 1/4W |
R12,
R18 = 56KOhm 1/4W |
R14 =
1,5 KOhm 1/4W |
R15,
R16 = 1 KOhm 1/4W |
R17 =
33 Ohm 1/4W |
R22 =
3,9 KOhm 1/4W |
RV1 =
100K trimmer |
P1, P2
= 10KOhm linear pontesiometer |
C1 =
3300 uF/50V electrolytic |
C2, C3
= 47uF/50V electrolytic |
C4 =
100nF polyester |
C5 =
200nF polyester |
C6 =
100pF ceramic |
C7 =
10uF/50V electrolytic |
C8 =
330pF ceramic |
C9 =
100pF ceramic |
D1, D2,
D3, D4 = 1N5402,3,4 diode 2A - RAX GI837U |
D5, D6
= 1N4148 |
D7, D8
= 5,6V Zener |
D9, D10
= 1N4148 |
D11 =
1N4001 diode 1A |
Q1 =
BC548, NPN transistor or BC547 |
Q2 =
2N2219 NPN transistor |
Q3 =
BC557, PNP transistor or BC327 |
Q4 =
2N3055 NPN power transistor |
U1, U2,
U3 = TL081, operational amplifier |
D12 =
LED diode |
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