Ripple-free, Short-circuit protected variable output voltage and current power supply.

Almost all electronics hobbyists have their own power supply unit. Here is a circuit of variable voltage and current power supply (0V – 25V and 60mA – 1500mA) having unique features like short-circuit protection, ripple-free etc. Ripple-free, short-circuit protected variable output voltage and current power supply in this website is unique and verified in the series of power supply.

Circuit Description Ripple-free, short-circuit protected variable output voltage and current power supply

The logic of this power supply circuit is very easy and even a common hobbyist can easily understood it, series pass transistor with zener diode is used for regulation.

Here diodes D1 through D4 act as a rectifier bridge where capacitor C1 and C4 is used for smoothing. Finally, zener diode ZD1 regulates the output voltage.

Glowing RED LED indicates the supply has tripped where GREEN LED indicate normal operating. Parallel connected resistor R3 through R5 conduct transistor T3 which further turns on the transistor T4. Voltage drop across resistor R9 due to collector current of transistor T4 enables transistor T5 through T7 turns-on.

The output voltage from transistor T1 becomes zero when collector voltage of transistor T5 is fed to its base. Transistor T6 active protection circuit because it holds the base current from transistor T4. In trip condition GREEN LED remains off with the help of transistor T7.

Switch SW1 is used to reset, for again use. Capacitor C5 is used to prevent T3 from turning on at surge current drawn by load. Preset VR1 and VR2 is used as voltage and current controller respectively, where VR3 adjusting for maximum current.

Advanced power supply

Capacitor C3 zener diode prevent ripple from going into the base of transistor T1. Base capacitor at transistor T5 is used to prevent the false triggering of T5.


Resistors (all ¼-watt, ± 5% Carbon)

R1 = 390 Ω/1W

R2 = 22 Ω

R3 – R5 = 1.2 Ω/1W (Wired Wound)

R6 = 100 Ω

R7 = 30 KΩ

R8 = 2.7 KΩ

R9 = 330 Ω

R10, R11 = 10 KΩ

R12 = 39 KΩ

R13 = 27 KΩ

R14­ = 3.3 KΩ

VR1 = 10 KΩ/1W LIN

VR2 = 10 Ω/1W Wire Wound

VR3 = 1 KΩ


C1 = 0.1 µF/100V

C2 = 2200 µF/50V

C3 = 100 µF/35V

C4 = 0.1 µF/50V

C5 = 4.7 µF/35V

C6 = 1 µF/10V

C7 = 0.01 µF

C8 = 0.22 µF/100V


T1 = SL1L100

T2 = 2N3055

T3, T5, T6, T7 = BC147B

T4 = BC157B

D1 – D5 = 1N4003

ZD1 = 27V/1W


SW1 = ON/OFF switch

SW2 = PUSH–TO–ON switch

F1 = 200mA fuse

X1 = 230V 50Hz or 110V 60Hz primary 0–25V, 1.5A secondary.



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