Power001

                                          

0-30 VDC Stabilized Power Supply With Current Control 0.002-3 A
~ Power Supply categories   ~
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.
General Description :
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. 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. 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.
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. 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. 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.
Part Components :
Capacitors	Resistors
C1      = 3300 μF/50V Electrolytic C2,C3 =    47 μF/50V Electrolytic C4      =  100 nF Polyester C5      =  200 nF Polyester C6      =  100 pF Ceramic C7      =    10 μF/50V Electrolytic C8      =  330 pF Ceramic C9      =  100 pF Ceramic Semiconductors U1,2,3 = TL081 Q1      =  BC548 or BC547 Q2      =  2N2219 Q3      =  BC557 or BC327 Q4      =  2N3055 Diodes D1,2,3,4   = 1N5402/..3/..4 (2A Diode) D5,6,9,10 = 1N4148 D7,8        = 5.6 V Zener D11         = 1N4001 (1A Diode) D12         = LED	R1                =   2.2 KΩ 1W R2                =      82 Ω ¼W R3                =    220 Ω ¼W R4                =   4.7 KΩ ¼W R5,6,13,20,21=   10 KΩ ¼W R7                =   0.47 Ω 5W R8,11           =    27 KΩ ¼W R9,19           =    2.2 KΩ ¼W R10              =   270 KΩ ¼W R12,18         =     56 KΩ ¼W R14             =     1.5 KΩ ¼W R15,16         =       1 KΩ ¼W R17              =     33 KΩ ¼W R22              =     3.9 KΩ ¼W RV1             =     100 KΩ Trimmer P1,P2          =       10 KΩ Lin.Potentiometer Others T1 Mains transformer : 220V Prim., 24V/3A Sec.  Heatsink
Technical Data
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 % (max)
CAUTION !
While using electrical parts, handle power supply and equipment with great care, following safety standards as described by international specs and regulations. 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.

Source : Electronics Lab