Simple Series Voltage Regulator Design

In this article, we present a series voltage regulator design featuring two transistors (2N2222A) and a Zener diode. The series voltage regulator circuit diagram is explained first, followed by the selection of component values, the Zener diode, and detailed series voltage regulator calculations. This type of design is especially relevant in projects where stable voltage regulation is crucial, such as building a DIY 5V regulated power supply for microcontroller circuits.

Circuit Diagram and Working Principle

An improved series voltage regulator circuit diagram is shown below.

The circuit diagram shows both the unregulated power supply and the voltage regulator. For learning about the rest of the power supply circuit, see how DC power supply works, DIY transformer design, capacitor input filter works and output DC voltage and Frequency.

The regulated power supply is encircled with blue color. The simplest power supply were covered in the previous tutorials Zener Diode Voltage Regulator and the simplest voltage regulator which uses one diode and one transistor. The voltage regulator covered here uses two transistors and one diode.

In this configuration, resistors $R_{1}$ and $R_2$ form a sampling network, while the Zener diode $D_{Z}$ provides a stable reference voltage. The regulating transistor $Q_2$ dynamically adjusts the base current of $Q_1$ , which stabilizes the current flow and output voltage. Such designs are fundamental in battery-powered applications, including automatic cutoff battery chargers for lithium-ion cells.

When the output voltage increases, the sampled voltage $V_2$ rises. Since the Zener voltage $V_Z$ remains constant, the base-emitter voltage $V_{BE2}$ of transistor $Q_2$ also increases, causing $Q_2$ to conduct more current. This reduces the base drive to $Q_{1}$ , stabilizing the output voltage. This feedback mechanism is key in systems like those described in LM317-based battery chargers, ensuring the load voltage remains consistent despite variations.

Zener Diode Selection and Voltage Divider

The Zener diode voltage is determined based on the desired output voltage of the regulator. For instance, when designing a reverse polarity protection circuit, selecting the right Zener voltage ensures the circuit handles input variations effectively.

Component Selection

1. Zener Diode Voltage Selection

The Zener diode voltage depends on the desired output voltage. For example, in a DIY 5V regulated power supply, the Zener diode voltage would typically be slightly lower than 5V, ensuring the regulating transistor operates correctly. For a 12V output, as used in some battery charger circuits, the Zener diode voltage might be around 5.1V, with a feedback network scaling the output voltage.

2. Feedback Resistors

The resistor values are selected to set the feedback voltage $V_{2}$ , which matches the sum of the Zener voltage $V_Z$ and the base-emitter voltage $V_{B E 2}$ . For instance:

V_2 = V_{BE2} + V_Z

The ratio of $R_1$ and $R_2$ determines the output voltage.

For a 12V output:

\frac{R_2}{R_1 + R_2} = \frac{V_2}{V_o} = \frac{6}{12} = 0.5

Choosing $R_1 = R_2 = 10k\Omega$ results in a feedback current of 0.6mA, ensuring sufficient current for the Zener diode and feedback circuit.

3. Series Transistor

The series transistor adjusts the current dynamically to maintain output stability. This technique is commonly used in projects like low-cost automatic cutoff battery chargers, where stable regulation is critical for safety.

Design Example: 12V Series Voltage Regulator

Key Components

Zener diode: $V_Z = 5.1V$ (standard)
Feedback resistors: $R_1 = R_2 = 10k\Omega$
Output voltage: $V_o = 12V$

Calculation

Using the feedback equation:

V_Z = V_2 - V_{BE2} = 6V - 0.7V = 5.3V

A standard 5.1V Zener diode is selected, balancing precision and availability.

For example, if the desired output voltage is 12V, a Zener diode with a breakdown voltage of approximately 5.1V or 5.6V can be paired with a suitable voltage divider network. Calculations involve ensuring the feedback network matches the reference voltage, similar to the setup used when calculating lithium-ion battery life.

The following shows that the output voltage is12V as expected. Also shown are the unregulated power supply voltage.

The following shows graph of signals at the output of the secondary transformer, the unregulated power supply voltage and the regulated power supply output voltage.

The following video demonstrates the operation of the series voltage regulator.

Applications of Series Voltage Regulators

The series voltage regulator is versatile and finds applications in:

Battery Chargers: Ensuring safe charging of lithium-ion batteries in setups like automatic cutoff chargers.
Power Supply Units: Achieving stable output voltage in designs such as the DIY 5V regulated power supply.
Voltage Protection Circuits: Protecting sensitive electronics with systems like reverse polarity protection.

Conclusion

The series voltage regulator is a fundamental circuit in electronics, offering stability and precision in various applications. Whether used in power supplies, battery chargers, or protection circuits, its robust feedback mechanism ensures reliable performance. For more insights into related designs, check out guides on lithium polymer battery builds or calculating battery life.