Simple voltage regulator circuit

Alternating current can be converted into direct current after rectification, but its voltage is unstable: changes in the supply voltage or current consumption can cause fluctuations in the power supply voltage. To obtain a stable and unchanging direct current power supply, a voltage stabilization circuit must be added .

To understand the working of the voltage stabilizing circuit, we must start with the voltage stabilizing tube.

1. Diode regulator tube with "special functions"

Generally, triodes are forward-conducting and reverse-cutoff; if the reverse voltage applied to a diode exceeds the diode's tolerance, the diode will break down and be damaged. However, there is a type of diode whose forward characteristics are the same as those of an ordinary diode, but whose reverse characteristics are rather special: when the reverse voltage is applied to a certain degree, although the tube is in a breakdown state and passes a large current, it is not damaged, and this phenomenon has good repeatability; conversely, as long as the tube is in a breakdown state, although the current flowing through the tube changes greatly, the voltage at both ends of the tube changes very little, playing a voltage stabilizing role. This special diode is called a voltage regulator.

The models of Zener diodes include 2CW, 2DW and other series, and its circuit symbol is shown in Figure 5-17.

The voltage-stabilizing characteristics of the Zener diode can be clearly expressed by the volt-ampere characteristic curve shown in Figure 5-18.

The voltage regulator works by using the voltage regulation characteristics of the reverse breakdown region. Therefore, the voltage regulator should be connected in reverse in the circuit. The reverse breakdown voltage of the voltage regulator is called the stable voltage. The stable voltages of different types of voltage regulators are also different. The voltage regulation value of a certain type of voltage regulator is fixed in a certain range. For example: the voltage regulation value of 2CW11 is 3.2 volts to 4.5 volts. The voltage regulation value of one tube may be 3.5 volts, and the voltage regulation value of another tube may be 4.2 volts.

In practical applications, if you cannot choose a voltage regulator with a voltage stabilization value that meets your needs, you can choose a voltage regulator with a lower voltage stabilization value, and then connect one or more silicon diodes in series as "pillows" to increase the stabilization voltage to the required value. This is to use the forward voltage drop of the silicon diode to stabilize the voltage at 0.6 to 0.7 volts. Therefore, the diode must be connected forward in the circuit, which is different from the voltage regulator.

The voltage regulation performance of the Zener diode can be expressed by its dynamic resistance r:

Obviously, for the same current change ΔI, the smaller the voltage change ΔU across the Zener diode, the smaller the dynamic resistance, and the better the performance of the Zener diode.

The dynamic resistance of the voltage regulator changes with the working current. The larger the working current, the smaller the dynamic resistance. Therefore, in order to achieve a good voltage stabilization effect, the working current should be selected appropriately. A larger working current can reduce the dynamic resistance, but it cannot exceed the maximum allowable current (or maximum dissipated power) of the tube. The working current and maximum allowable current of various types of tubes can be found in the manual.

The stability of the voltage regulator is affected by temperature. When the temperature changes, its stable voltage will also change. The temperature coefficient of the stable voltage is often used to express this performance. For example, the stable voltage Uw of the 2CW19 voltage regulator is 12 volts, and the temperature coefficient is 0.095%℃, which means that for every 1℃ increase in temperature, its stable voltage increases by 11.4 millivolts. In order to improve the stability of the circuit, appropriate temperature compensation measures are often used. When the stability performance requirements are very high, a voltage regulator with temperature compensation is required, such as 2DW7A, 2DW7W, 2DW7C, etc.

2. Silicon voltage regulator circuit

A simple voltage regulator circuit composed of a silicon voltage regulator is shown in Figure 5-19 (a). The silicon voltage regulator DW is connected in parallel with the load Rfz, and R1 is a current limiting resistor.

How does this circuit stabilize voltage? If the grid voltage increases, the output voltage Usr of the rectifier circuit will also increase, causing the load voltage Usc to increase. Since the voltage regulator DW is connected in parallel with the load Rfz, as long as Usc increases a little, the current flowing through the voltage regulator will increase sharply, causing I1 to increase, and the voltage drop on the current limiting resistor R1 to increase, thereby offsetting the increase in Usr and keeping the load voltage Usc basically unchanged. Conversely, if the grid voltage decreases, causing Usr to drop, causing Usc to also drop, the current in the voltage regulator will decrease sharply, causing I1 to decrease, and the voltage drop on R1 will also decrease, thereby offsetting the decrease in Usr and keeping the load voltage Usc basically unchanged.

If Usr remains unchanged and the load current increases, the voltage drop on R1 increases, causing the load voltage Usc to drop. As long as Usc drops a little, the current in the voltage regulator tube will decrease rapidly, causing the voltage drop on R1 to decrease again, thereby keeping the voltage drop on R1 basically unchanged and stabilizing the load voltage Usc.

In summary, it can be seen that the voltage regulator plays the role of automatic current regulation, while the current limiting resistor plays the role of voltage regulation. The smaller the dynamic resistance of the voltage regulator, the larger the current limiting resistor, and the better the stability of the output voltage.

So how to choose the voltage regulator and current limiting resistor?

1. Because the voltage regulator is connected in parallel with the load, the stable voltage of the voltage regulator should be equal to the DC voltage of the load, that is, Uw=Usc. The selection of the maximum stable current of the voltage regulator should take into account the maximum current passing through the voltage regulator under special circumstances: one case is that when the load current Ifz = 0, the entire maximum load current Ifzmax passes through the voltage regulator; another case is that the input voltage Usr increases, which will also cause the current passing through the voltage regulator to increase. Generally, the voltage regulator with small dynamic resistance and small voltage temperature coefficient is selected according to the maximum current of the voltage regulator, which is conducive to improving the stability of the voltage.

2. The current limiting resistor R1 can be calculated from the formula: Because Usr, and Ifz are both variable, in order to ensure that Iw does not exceed the maximum stable current of the Zener tube when Ifz=0, R1 must be large enough. In order to ensure the stability, it must be ensured that when Usr is the minimum, Iw is greater than the minimum stable current of the Zener tube. Combining the above two considerations, the selection range of the current limiting resistor R1 is:

The circuit shown in Figure 5-l9 (A) is simple and reliable, but the stable voltage cannot be adjusted and the load current is too small. It is generally used as a voltage regulator for the front stage of the circuit and as a reference voltage for other power supplies.

The two-stage silicon voltage regulator circuit can output two stable voltages U1 and Usc, and further improve the voltage regulation effect. The circuit is shown in Figure 5-19 (b)

3. Series voltage regulator circuit

The series voltage regulator circuit is a commonly used circuit. The circuit is shown in Figure 5-20(a).

The transistor BG is an adjustment element in the circuit. It has the ability to "act according to circumstances". Whenever the output voltage of the circuit fluctuates due to changes in power supply or power consumption, it can be adjusted in time to keep the output voltage basically stable. Therefore, it is called an adjustment tube. Because the transistor as an adjustment element in the circuit is connected in series with the load, this circuit is called a series voltage regulator circuit. The voltage regulator DW provides a reference voltage for the adjustment tube to keep the base potential of the adjustment tube unchanged. R1 is the protection resistor of DW, which limits the current passing through DW and protects the voltage regulator. Rfz is the load resistor and is the DC path of BG.

BG and DW work in harmony to ensure that the circuit outputs a stable voltage. The circuit voltage stabilization process is as follows: If the input voltage Usr increases, the output voltage Usc increases. Since Ub=Uw is fixed, the base-emitter voltage Ube of the adjustment tube =Ub-Usc will decrease, the base current Ib will decrease accordingly, and the tube voltage drop Uce will increase accordingly, thus offsetting the increase in Usc and making Usc basically stable. If the load current Isc increases, the output voltage Usc decreases. Since Ub is fixed, Ube will increase, making Ub increase, and Uce decrease, which also makes Usc basically stable.

From the above analysis, we can see that the adjustment tube is like an automatic variable resistor: when the output voltage increases, its "resistance" increases, sharing the increased voltage; when the output voltage decreases, its "resistance" decreases, making up for the decreased voltage. In either case, the circuit maintains a stable output voltage. What "commands" the adjustment tube to change is the change in the output voltage ?Usc; it is ΔUsc that controls the base current Ib of the adjustment tube, which makes the adjustment tube change with ΔUsc. In other words, it is the unstable output voltage that drives the adjustment tube to stabilize the output voltage. If the form of the voltage regulator circuit shown in Figure 5-20 (a) is slightly changed and drawn as Figure 5-20 (b), it is not difficult to see that the original series voltage regulator circuit is an emitter follower. R1 is the upper bias resistor, the voltage regulator DW is the lower bias resistor, and the output voltage is taken from the emitter resistor Rfz.