**Introduction to Three-Terminal Regulator**
A three-terminal Zener diode is a semiconductor device that exhibits high resistance until it reaches a specific reverse breakdown voltage. Once this critical voltage is reached, the device enters a reverse breakdown state, where the voltage across its terminals remains nearly constant within a certain current range or power dissipation range. This stable voltage characteristic makes it ideal for use in regulated power supplies and limiting circuits.
**Classification of Three-Terminal Regulator Tubes**
There are two main types of three-terminal voltage regulator tubes. The first type has a fixed output voltage and is known as a fixed-output three-terminal regulator. The second type allows for adjustable output voltage and is referred to as an adjustable-output three-terminal regulator.
**Working Principle of Three-Terminal Regulator**
The fixed three-terminal regulator operates based on a series regulator circuit, which functions similarly to a basic series voltage regulator.
The circuit includes a voltage divider made up of resistors R1, Rp, and R2, which serves as a sampling network. A portion of the output voltage, UB2, is taken from the output terminal and applied to the base of transistor T2. Meanwhile, the Zener diode Dz provides a stable reference voltage, Uz, at the emitter of T2. Resistor R3 limits the current through the Zener diode.
Transistor T2 acts as a comparator amplifier, comparing the sampled voltage (UB2) with the reference voltage (Uz). It then adjusts the base voltage of transistor T1 accordingly. The input voltage, Ui, is supplied to a circuit where T1 is connected in series with the load RL. By varying the collector-emitter voltage (UCE1) of T1, the output voltage (Uo) across RL can be controlled. Thus, T1 is called the "adjusting tube" because it regulates the output voltage. Since T1 is connected in series with the load, this configuration is known as a series regulator.
Resistors R4 and T1 also serve as the collector load for T2. When the input voltage drops or the load resistance decreases, causing a drop in output voltage, the sampled voltage (UB2) decreases. This leads to a reduction in the base potential of T2, while the emitter voltage (Uz) remains constant. As a result, the collector current of T2 decreases, increasing the collector voltage (Uc2), which in turn raises the base voltage of T1. This causes an increase in the collector current of T1, reducing the voltage drop (UCE1), and stabilizing the output voltage.
Similarly, if the output voltage rises due to changes in the input or load, the circuit compensates by increasing the voltage drop across T1, thus keeping the output voltage steady.
Adjusting the potentiometer (Rp) allows for fine-tuning of the output voltage. The adjusting tube (T1) and the load resistor (RL) form an emitter follower configuration, which ensures a stable output voltage.
During operation, the adjusting tube is always in an active (amplified) state. It must handle the full load current, so a high-power transistor is typically used, along with a heat sink for thermal management. To protect against short circuits or overloads, most regulators include protection circuits such as current limiting or thermal shutdown.
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