Overview
This is a 5V switching regulator implemented with transistors. It reduces voltage, functioning as a buck converter. The circuit is a switching converter regulator that produces an output voltage lower than the applied input voltage.
The input voltage range is 6V to 15V, and the circuit provides a 5V output at about 100mA because it operates in switching mode.
How it works
The basic operating principle is shown below.
When the power supply is applied, current flows through Q1, Q2, R1, R2, R3, R4, C1 and C2, which are connected as an astable multivibrator. This generates a pulse-frequency signal at the collector of Q2.
The pulse signal is applied to the base of Q3. Q3 acts as a switch between its collector and emitter. When Q3 conducts, its collector-emitter path behaves like a closed switch. That allows base current to flow from ground through Q3 CE and R6 into the base of Q4, activating the PNP transistor Q4.
High current from VCC flows through Q4 CE and charges C3 via L1. The output voltage then rises.
When the output voltage exceeds about 5V, part of the current flows through R7 and the zener diode ZD1 into the base of Q5.
Q5 then operates like a switch between its collector and emitter to stop the astable multivibrator from oscillating. The oscillator stops producing frequency when Q5 is driven, which cuts off base current through R4 into the base of Q3 as if Q3 were off.
With Q3 and Q4 off, Q4 stops conducting and the output voltage falls below 5V. Q5 then loses base bias and the astable multivibrator resumes oscillation. This cycle repeats at high speed, regulating the output around 5V.
Construction
The circuit can be assembled on a breadboard or the components can be soldered directly according to the layout below.
The output voltage is set by the zener diode. You can try other zener voltages for different outputs, for example 3V or 6V. D1 may be substituted with a 1N4007 in some tests, which can produce similar behavior in this configuration.
If the load is increased beyond the current the circuit can supply, the output voltage drops to about 4.68V. During testing, the unloaded output is 5V; under heavier load beyond the circuit’s capability, the voltage falls to approximately 4.68V, which is expected for this simple design.
Component List
Resistors: 0.25W, 5%
- R1, R4, R7: 4.7 kΩ
- R2, R3: 47 kΩ
- R5: 1 kΩ
- R6: 100 Ω
Capacitors
- C1: 0.0015 μF, 50V polyester
- C2: 0.01 μF, 50V polyester
- C3: 470 μF, 16V electrolytic
Semiconductors
- Q1, Q2, Q3, Q5: BC548 or BC549, NPN, 45V, 100mA
- Q4: BC327, PNP, 45V, 800mA
- D1: 1N5819, 40V, 1A, Schottky diode
- ZD1: 5.1V, 500mW (1N5231) zener diode
Other components
- L1: Toroidal core, diameter 2.5–3.0 cm
- Winding wire: 0.4 mm copper
Conclusion
The circuit is simple and can hold the output around a constant voltage, but it is not very stable. Compared with a ready-made switching regulator IC, a dedicated regulator IC offers better performance and greater convenience. Nevertheless, this circuit is useful for learning. In practical designs, transistors often complement ICs well, so combining older discrete approaches with modern ICs can be beneficial.
