# 0-30V Lab Variable Power Supply

Posted on Jun 12, 2017

At the left side (input AC) goes the secondary of a 220 volt (primary) 18 Volt (secondary) / 3 Ampere mains transformer. The alternating voltage of 18 volts from its secondary transformer is rectified by the diodes D1, D2, D3 and D4, which are connected to a bridge arrangement and make a double rectifier. The pulse from the bridge exit is applied to the ends of the R1-C1 net and filtered. The circuit around the integrated U1 produces the necessary reference voltage for the stable operation of the circuit. The Zener D8 diode operates at the minimum current for zero thermal coefficient to ensure stable operation of the power supply. The resistors R5 and R6 regulate the output of the reference generator.

## The circuit works as follows:

The voltage at the output of U1 increases until the D8 starts running, the circuit stabilizes and the reference voltage of the Zener (5.6 Volt) is shown at the ends of R5. The current flowing through the non-inverting input of the effector amplifier is negligible, so all current flowing through R5 also passes through R6 and so the output voltage of U1 is twice the diode voltage.

U2, by means of the resistors R11 and R12, increases the voltage according to the formula A = R11 + R12 / R11, thus increasing the voltage to approximately 20 volts. The RV1 trimmer and resistor R10 are used to set the output voltage limits to go down to zero.

When the power supply is in operation and a load is connected to its output, then all output current passes through resistor R7. At this point, the reversible input (-) of U3 has a potential of 0 and is polarized by the resistor R21 while the non-inverting (+) input of U3 is polarized by a small voltage ranging from 0 - 2 volts depending on the position of the rotor Potentiometer P2. Suppose the voltage at this point of the circuit is 1 volt and the output voltage is set to a few volts. If the load increases then the voltage at the ends of R7 will increase, so U3 will be activated, and it will lead U2 through the D9 passage, and consequently the U2 is controlled by a voltage value and above.

From this moment, the output current limiting circuit acts, since U3 controls the operation of U2 so that the voltage drop across the edges of R7 is constant. C8 capacitor provides the U3 reconnection loop required stability.

As long as the power supply works and provides constant voltage, LED D12 is off. While the current exceeds the preset by P2 value, the LED lights up and the power supply from a constant voltage source is a constant current source and at the same time the output voltage is changed so as to keep the current constant for that load. R19, ​​R20 defines the necessary operating conditions of Q3 that detects when the current limiting current circuit starts and operates at that moment and LED D12 lights up via the R22 that defines the operating current of the LEDs. However, the current limitation is made by the R7 that serves as a transducer, and the current value of the limit circuit is set by the potentiometer P2. Thus, depending on the position of the potentiometer P2, a different voltage occurs at each end of the R7, and is divided by the internal reference voltage of the integrated circuit U1.

From the junction of D2, D4, through R2 and the D5, D6, C2, C3, R3, produces a negative voltage necessary for the operation of U2, U3 so that they can go down to 0 volts. D7 diode stabilizes The negative voltage caused to terminals 4 of U2 and U3.

Resistor R4 ensures polarity at U1 and capacitors C4 and C5 release the circuit from high frequencies that could cause instability in the integrated circuits.

Transistor Q1 protects the power supply from the negative voltage drop caused when the power supply ON-OFF switch is closed, keeping the output of U2 low.

Resistor R14 cuts off the transistor when the power supply is working normally, and in this simple way the power supply is very useful for experimentation because it drastically cuts the output voltage without having to wait until the capacitor unloads, thus losing plenty of time.

Finally, via R15 the voltage is fed to the transistors Q2 and Q4 which provide the required power of 0.5 A. The resistor R16 finally polarizes the transistors Q2 and Q4, which are in DARLINGTON. ## Components List

 Resistors Capacitors Transistors R1 = 2.2 KΩ 1W R7 = 0.47Ω 5W Brick R2 = 82Ω 1/4W R3 = 220Ω 1/4W R4 = 4.7KΩ 1/4W R5,R6,R13,R20,R21 = 10KΩ 1/4W R8,R11 = 27KΩ 1/4W R9,R19 = 2.2KΩ 1/4W R10 = 270KΩ 1/4W R12,R18 = 56KΩ 1/4W R14 = 1.5KΩ 1/4W R15, R16 = 1KΩ 1/4W R17 = 33Ω 1/4W R22 = 3.9KΩ 1/4W RV1 = 1MΩ Trimmer P1,P2 = 10KΩ Linear Potentiometer C1 = 3300μF/40V Electrolytic C2,C3 = 47μF/63V Electrolytic C4 = 100nF Polyester C5 = 220nF Polyester C6 = 100pF Ceramic C7 = 10μF/50V Electrolytic C8 = 330pF Ceramic Q1 = BC547 NPN Transistor O2 = 2N2219 NPN Transistor Q3 = BC327 PNP Transistor Q4 = BD249C NPN Power transistor Diodes Operational Amplifier ICs D1,D2,D3,D4 = 1N5402 D5,D6 = 1N4148 D7,D8 = 5.6V Zener D9,D10 = 1N4148 D11 = 1N4001 D12 = LED U1,U2,U3 = TL081 Transformer T1 = 220V - 18V/3A

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