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# Mini variable Power supply from 0 to 30V, 1A

This mini variable power supply is totally transistorized, it can vary its output in a wide range of voltages and can be used for projects where the current consumption is maximum 1A.

The circuit uses only three transistors and, due to its simplicity, is a special project for beginners who want to have a small power supply with great features.

## Mini Variable Power Supply Operation

The power supply uses a step-down transformer to obtain a voltage of 24 V in alternating current. This voltage is rectified in full wave by a group of four rectifying diodes (D1, D2, D3, D4) and then smoothed with capacitor C1, which acts as a filter. You can visit the PS block diagram for more information about a Power supply.

The voltage is then applied to the collectors of transistors Q1 and Q2. The voltage at the base of transistor Q1 is governed by the collector of transistor Q3 which depends on potentiometer R5.

The potentiometer modifies the bias of transistor Q3 and therefore its collector-emitter voltage. The collector voltage of transistor Q3 in turn modifies the voltage at the base of transistor Q2 via transistor Q1.

Transistor Q3 will conduct according to the voltage at its base and in turn will modify the voltage at the base of transistor Q2, which is the transistor that delivers the voltage and current to the load. Transistors Q1 and Q2 are connected in such a way that they behave like a Darlington transistor.

Varying the position of the potentiometer arrow varies the voltage of transistor Q3, which in turn controls the voltages and currents of transistors Q1 and Q2.

The set of resistors R4, R6 and potentiometer R5 form a variable voltage divider that allows the output voltage to be modified.

Electrolytic capacitor C2 helps stabilize the voltage at the base of transistor Q1 and electrolytic capacitor C3 helps stabilize the output voltage of the source.

## Circuit Component List

• 1 120/240 to 24 VAC, 1A transformer (T)
• 2 BC337 NPN 50 volts, 800 mA transistors (Q1, Q3)
• 1 NPN TIP41 50 volts, 2A transistor (Q2)
• 5 1N4002, 100V, 1A diodes (D1, D2, D3, D4, D5)
• 1 10K potentiometer (R5)
• 2 10K resistors (R1, R2)
• 1 1K resistor (R3)
• 2 100 ohms resistors (R4, R6)
• 1 1000 uF, 35 volts electrolytic capacitor (C1)
• 2 100 uF, 35 volts electrolytic capacitors (C2, C3)
• 1 heat sink for the NPN TIP41 (Q2) transistor
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