Three-terminal voltage regulator is a semiconductor device with high resistance up to the critical reverse breakdown voltage. In the reverse breakdown of the voltage regulator tube, the terminal voltage is almost constant within a certain current range (or within a certain power loss range), showing the characteristics of voltage regulator, so it is widely used in the regulated power supply and limiting circuit.
classification
There are mainly two kinds of three-terminal regulator tube, one of which has a fixed output voltage and is called fixed-output three-terminal regulator tube, the other has an adjustable output voltage and is called adjustable output three-terminal regulator tube.
Three-terminal regulator tube
The principle of
Since the fixed three-terminal voltage regulator belongs to series voltage regulator circuit, its principle is equivalent to series voltage regulator circuit.
Wherein, the voltage dividers composed of R1, Rp and R2 are sampling circuits, and part of the voltage UB2 is taken out from the output end as the sampling voltage and added to the base electrode of the triode T2. The stabilizer tube, Dz, USES its stable voltage Uz as the reference voltage and is applied to the emitter of T2. R3 is the current limiting resistance of the regulator tube. The triode T2 consists of a comparative amplifying circuit, which compares and amplifies the sampling voltage UB2 and the reference voltage Uz, and then controls the base potential of the triode T1. The input voltage Ui is applied on the circuit in series between the transistor T1 and the load RL. Therefore, UCE1 can adjust the voltage Uo at both ends of RL by changing the voltage drop between T1 collectors. In other words, the output voltage of the regulator circuit can be adjusted by the Uo transistor, so T1 is called the regulator tube. As the adjusting element is a transistor tube and in series with the load in the circuit, it is called a transistor series voltage regulator circuit. The base offset resistance of resistors R4 and T1 is also the collector load resistance of T2.
When the output voltage decreases due to the decrease of the grid voltage or the load resistance, the sampling voltage UB2 decreases correspondingly and the base potential of T2 decreases. However, since the T2 emitter potential, which is the stable Uz of the stabilizer tube, remains unchanged, the emitter voltage UBE2 decreases, leading to the T2 collector current decrease and collector potential Uc2 increase. As the collector of the amplifier tube T2 is connected to the base of the regulator tube T1, the base potential of T1 increases, resulting in the collector current increasing and the tube pressure drop UCE1 decreasing. Since T1 is in series with RL, the output voltage Uo is essentially unchanged.
Similarly, when the power grid voltage or load changes and causes the output voltage Uo to increase, through sampling, comparison, amplification, adjustment and other processes, the tube pressure drop UCE1 of the regulating tube will be increased, resulting in the suppression of the increase of the output voltage, the output voltage remains basically unchanged.
Adjust the potentiometer Rp to fine - tune the output voltage. The adjusting tube T1 and the load resistance RL constitute the emitter output circuit, so it has the characteristics of stable output voltage.
In the working process of the series stabilized voltage supply circuit, the adjustment tube is always in the amplified state. The current passing through the adjusting tube is equal to the load current, so appropriate high-power tubes must be selected for the adjusting tube, and heat sinks must be installed as specified. In order to prevent short circuit or long-term overload burn out the adjustment tube, in the DC voltage regulator is generally equipped with short circuit protection and overload protection circuit.
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