How to design simplest series regulator circuit

Voltage regulators are an essential part of any electronic circuit, ensuring a stable output voltage despite changes in input voltage or load conditions. One of the simplest and most effective designs is the series voltage regulator using a Zener diode and a transistor. In this blog post, we will walk through the design of a basic series regulator, its calculations, and its advantages over other types of regulators.

Circuit Overview

The following shows the circuit diagram of the simplest series regulator circuit.

The simplest series regulator circuit consists of:

Transistor Q1: The pass element (e.g., a BJT like 2N2222 or TIP31C).
Zener diode DZ: Provides a reference voltage.
Resistor Rs: Connected between the collector and base to set the bias current.
Load resistor RL: Represents the load.

This circuit operates by maintaining the output voltage close to the Zener voltage minus the base-emitter drop of the transistor ( $V_{BE}$ ).

Component Values and Calculations

Given circuit parameters:

Input voltage ( $V_i$ ) = 20V
Zener diode voltage ( $V_Z$ ) = 12V
Series resistor ( $R_s$ ) = 1kΩ
Load resistor ( $R_L$ ) = 220Ω and 1kΩ

1. Output Voltage Calculation

The output voltage is given by:

V_o = V_Z - V_{BE

Substituting values:

V_o = 12V - 0.7V = 11.3V

2. Voltage Drop Across the Transistor

The voltage across the pass transistor is:

V_{CE} = V_i - V_o

V_{CE} = 20V - 11.3V = 8.7V

3. Current Through the Resistor ( $I_R$ )

For $R_s = 1kΩ$ :

I_R = \frac{V_i - V_Z}{R_s} = \frac{20V - 12V}{1kΩ} = 8mA

4. Load Current ( $I_L$ )

When $R_L = 220Ω$ :

I_L = \frac{V_o}{R_L} = \frac{11.3V}{220Ω} = 51.4mA

If $R_L = 1kΩ$ :

I_L = \frac{V_o}{R_L} = \frac{11.3V}{1kΩ} = 11.3mA

5. Zener Current ( $I_Z$ )

The Zener current is:

I_Z = I_R - I_B

Base current ( $I_B$ ) is:

I_B = \frac{I_L}{h_{FE}}

Assuming $h_{FE} = 100$ :
For $R_L = 220Ω$ , $I_{B} = \frac{51.4 m A}{100} = 0.514 m A$ .

I_Z = 8mA - 0.514mA \approx 7.5mA

Advantages of Series Regulators

Better Efficiency

Series regulators are more efficient than shunt regulators. They replace the series resistor in shunt regulators with a pass transistor, which minimizes power loss.

Quiet Operation

Unlike switching regulators, series regulators do not produce high-frequency noise, making them ideal for low-noise applications.

Simplicity

While not as efficient as switching regulators, series regulators are simpler to design and build. They provide efficiencies between 50% and 70%, which is sufficient for most low-power applications.

Load Current Adaptability

In series regulators, the input current changes with the load current, making them better suited for variable load conditions.

Video Demonstration

The video demonstrates how the simplest series voltage regulator circuit operates. Using a transistor (Q1) as the series control element, and a Zener diode (DZ) to provide a stable reference voltage, we show how the output voltage is regulated. The circuit also includes a base resistor (Rs) to control current flow and a load resistor (RL) to simulate real-world load conditions. Watch as we explain how the Zener diode maintains a constant voltage, and the transistor adjusts to varying load currents, ensuring a stable output voltage for your electronics.

Comparison with Other Regulators

Shunt Regulators: Simple but inefficient due to constant power dissipation in the series resistor. Best suited for small loads where efficiency is not critical.
Switching Regulators: Extremely efficient ( $75\%-95\%$ ) but complex and prone to radio-frequency interference (RFI). Suitable for high-power applications.

To explore how shunt and switching regulators compare to series regulators, check out this detailed guide: LM7805 vs. LM317: Which Voltage Regulator to Choose?.

Applications of Series Regulators

Series regulators are widely used in circuits where low noise and moderate efficiency are required. For small electronic applications like powering microcontrollers, designing a simple transformer-based series regulator circuit can be efficient and reliable. You can also use tools like this Transformer Calculator Online to optimize transformer parameters for your design.

Key Takeaway

The series voltage regulator is an excellent choice for low-noise, moderate-efficiency power regulation. It balances simplicity with performance, making it ideal for many low-power applications.

If you're looking to design a transformer for a series regulator or want to explore other regulator types, check out this article on DIY Transformer Design for Small Applications and use this handy LM317 Voltage Regulator Online Calculator.