How Discrete Voltage Regulator with Current Limiter Works

Description: This discrete voltage regulator features capabilities equivalent to modern voltage regulator integrated circuits (ICs). While constructing a discrete version may not be cost-effective, studying the schematic diagram provides valuable insights into its operation. The circuit includes a resistor of 6.8k ohms, which is connected at the junction between transistors. This resistor supplies the base current to transistor T2, and the voltage at this base will follow the output voltage. The base voltage is influenced by two other transistors: T4, located on the right side, and T3, positioned below T2. Initially, when the circuit is powered, both T4 and T3 are off, causing the base voltage to rise. This increasing voltage will be reflected in the output. At a certain threshold, determined by a 5k potentiometer and a 5.1V zener diode, the output voltage reaches the activation point of T4, which then turns on and shorts the base voltage of T2, preventing further increases and potentially decreasing it if it becomes too high. This mechanism forms the basis of the regulation process.

Regarding T3, it remains inactive under normal conditions. It can only be activated when a voltage builds up across Rx and reaches a specific level (forward bias voltage, Vbe). Rx is typically a low-value resistor that determines the maximum current permitted by the regulator. Ignoring the current from the 470-ohm resistor and the 5.1V zener diode, nearly all current through Rx originates from the power supply load. For instance, if T3 has a Vbe of 0.6V, a 0.6-ohm resistor for Rx will develop a voltage of 0.6V when 1A of current flows from the load, activating T3 to short the base voltage of T2 and thereby reducing the output voltage to prevent further increases in load current. This configuration achieves a current limiting function of 1A if a transistor T3 with a Vbe of 0.6V and a 0.6-ohm resistor for Rx is selected.

The circuit can be illustrated as follows:

1. **Transistors**: The core components include T2, T3, and T4, which are configured to form a feedback mechanism for voltage regulation. T2 acts as the primary pass transistor, while T3 serves as the current limiter and T4 as the voltage regulator.

2. **Resistors**: The 6.8k ohm resistor is crucial for supplying the base current to T2, while Rx is a low-value resistor that determines the maximum current threshold for the load. The 470-ohm resistor and 5.1V zener diode help stabilize the circuit and provide the necessary reference voltage.

3. **Potentiometer**: The 5k potentiometer allows for fine-tuning of the output voltage, enabling adjustments based on specific application requirements.

4. **Feedback Mechanism**: The interaction between T2, T3, and T4 creates a feedback loop that ensures the output voltage remains stable under varying load conditions. When the output voltage exceeds the set point, T4 activates, reducing the base voltage of T2 and thus controlling the output.

5. **Current Limiting Feature**: The arrangement of T3 and Rx provides a current limiting feature, preventing excessive current draw from the power supply, which could potentially damage the circuit.

This discrete voltage regulator design illustrates the principles of voltage regulation and current limiting, showcasing how individual components work together to maintain stable output under varying conditions.This discrete voltage regulator has complete features that is equivalent with modern voltage regulator integrated circuit (IC). It`s not cheaper if you build discrete one like this, but it`s always interesting to study the schematic diagram to learn how it works.

Here is the schematic diagram of the circuit. Look at the 6, 8k resistor, it should be dotted at the cross section between transistors. This resistor supply the base current of T2. The voltage at this base will be followed by the output voltage. This base voltage will be affected by two other transistor, T4 on the right side and T3 below T2. At the first time the circuit is powered, T4 and T3 is off, and this base voltage increase. This increasing voltage will appear on the output. At certain point (set by 5K pot and 5. 1 zener diode), the voltage output voltage reach the level of T4 activation point, where T4 becomes active to short this T2 base voltage, preventing further increase, and even decrease it if too high. This mechanism is the basic of the regulation. What about T3 In normal condition, T3 is always inactive. The only way to get activated is when a voltage is build up across Rx and reach a certain level (Vbe forward bias).

Rx is normally a resistor with small value, depends on how much maximum current is allowed for the regulator. Neglecting the current from resistor 470R and 5. 1v Zener diode, almost all current at Rx is coming from the load of this powers supply. Let say we have T3 with 0, 6V Vbe (forward base-emitter bias voltage), a 0. 6 © Rx resistor will build 0, 6 Volts voltage if 1A current is returned from the load, activating T3 to short T2 base voltage, reducing the output voltage to prevent further current increase at the load.

This way a 1A current limiter is accomplished if we chose a transistor T3 with 0. 6V Vbe and 0. 6 © resistor for Rx. We aim to transmit more information by carrying articles. Please send us an E-mail to wanghuali@hqew. net within 15 days if we are involved in the problems of article content, copyright or other problems. We will delete it soon.