DC stabilized power supply circuit

　　Stabilized power supply generally consists of three parts: transformer, rectifier and voltage regulator, as shown in Figure 5-21. The transformer converts the mains AC voltage into the required low-voltage AC power. A rectifier converts alternating current into direct current. After filtering, the voltage regulator converts the unstable DC voltage into a stable DC voltage output.

1. Technical indicators of regulated power supply and requirements for regulated power supply

　　The technical indicators of a regulated power supply can be divided into two categories: one is characteristic indicators, such as output voltage, output electric filter and voltage adjustment range; the other is quality indicators, which reflect the advantages and disadvantages of a regulated power supply, including stability. , equivalent internal resistance (output resistance), ripple voltage and temperature coefficient, etc. There are mainly four requirements for the performance of a regulated power supply:

1 . good stability

　　When the input voltage Usr (rectified and filtered output voltage) changes within the specified range, the change in the output voltage Usc should be very small. General requirements.

　　The degree of change in the output voltage caused by changes in the input voltage is called the stability index, which is commonly expressed by the voltage stabilization coefficient S : The size of S reflects the ability of a regulated power supply to overcome changes in the input voltage. Under the same input voltage change conditions, the smaller S is, the smaller the change in output voltage is, and the higher the stability of the power supply is. Usually S is approx .

2. Small output resistance

　　When the load changes (from no load to full load), the output voltage Usc should remain basically unchanged. The performance of this aspect of the regulated power supply can be characterized by the output resistor.

　　The output resistance (also called equivalent internal resistance) is represented by rn , which is equal to the ratio of the change in output voltage to the change in load current.

　　rn reflects the ability to maintain a constant output voltage when the load changes. The smaller rn is, the smaller the change in the output voltage will be when Ifz  changes. For a regulated power supply with excellent performance, the output resistance can be as small as 1 ohm or even 0.01 ohm.

3. Small voltage temperature coefficient

　　When the ambient temperature changes, it will cause the output voltage to drift. A good regulated power supply should effectively suppress the drift of the output voltage and keep the output voltage stable when the ambient temperature changes. The drift of the output voltage is represented by the temperature coefficient KT.

4. Output voltage ripple is small

　　The so-called ripple voltage refers to the 50 Hz or 100 Hz AC component of the output voltage, which is usually expressed as the effective value or peak value. After voltage stabilization, the ripple voltage after rectification and filtering can be greatly reduced, and the multiple of reduction is inversely proportional to the voltage stabilization coefficient S.

　　The series voltage stabilizing circuit introduced in the previous section is used as a simple voltage stabilizing power supply, which can meet the needs of ordinary radio enthusiasts. However, this kind of power supply still has many "innate" flaws. To improve the performance requirements, some improvements must be made. By improving its performance from the following four aspects, you can make a regulated power supply with practical value. This is: adding amplification links to improve stability and making the output voltage adjustable; using composite tubes as adjustment tubes to increase the output current; adding protection circuits to make the power supply safe and reliable.

2. Stabilized power supply with amplification link

　　The change amount of the output voltage △ Usc is very weak, and its control effect on the adjustment tube is also very weak, so the voltage stabilization effect is not good enough. A regulated power supply with an amplification link is to add a DC amplifier to the circuit to convert the weak The output voltage change is first amplified, and then the adjustment tube is controlled, thereby improving the control effect on the adjustment tube and improving the stability of the regulated power supply. Figure 5-22 is a regulated power supply circuit with an amplification link.

　　In the figure, BG1 is the adjustment tube and BG2 is the comparison amplifier tube. A part of the output electric field change △ Usc is compared with the reference voltage Uw, and is amplified by BG2 and then enters the base of BG1 . Rc is the collector resistance of BG2 and the upper bias resistance of BG1 . R1 and R2 are the upper and lower bias resistors of BG2 , forming a voltage dividing circuit. A part of Δ Usc is used as a sample of the output voltage and sent to the base of BG2 , so it is also called the voltage Ub2 on the sampling circuit R2: it is called the sampling voltage. . DW and R3 form a voltage stabilizing circuit to provide a reference voltage .

　　It can be seen from the circuit that when the output voltage Usc drops, the base potential Ub2 of BG2 also drops through the action of the voltage divider circuit composed of R1 and R2 . Since the reference voltage UW keeps the emitter potential of BG2 unchanged, Ubc2:=Ub2, UW decreases accordingly. Therefore, the collector current Ic of BG2 decreases and Uc2 increases, that is, the base potential Ub1 of BG1 increases, causing Icl to increase and the tube voltage drop Uce1 to decrease, resulting in the output voltage Usc remaining basically stable. The greater the amplification of BG2 , the stronger the adjustment effect and the more stable the output voltage.

　　If the output voltage Usc increases, in the same way, Usc will decrease through feedback , keeping the output voltage basically unchanged.

　　Let’s talk about the selection principles of each component. As mentioned before, Rc is the load resistance of the amplifier stage, which is also equivalent to the bias resistance of the adjustment tube. A large Rc has a large amplification factor, which is beneficial to improving the voltage regulator index. However, an excessively large Rc will make the current of BG2 and the adjustment tube too small, limiting the load current and adjustment range. Usually Rc is selected according to the following formula:

　　Usr min is the minimum voltage of the rectified output. Ic2 can take 1~3 mA. The stable voltage Uw of the voltage regulator tube DW has a wide selection range, as long as BG2 is not saturated (that is, Uw is less than 2 volts lower than Usc ). If Uw is large, the sampling voltage can be larger, which is beneficial to improving the voltage stabilization performance. The current I3 passing through the current limiting resistor R3 should be equal to the stable current of DW, which should satisfy the following relationship:

　　The input voltage Usr should be 3 to 8 volts greater than the output voltage Usc . If Usr is too small, the adjustment tube will be easily saturated and cannot adjust. If Usr is too large, the loss of the tube will be increased and power will be wasted. If the rectifier ripple is small, Usr should be lower; if the ripple is large, Usr should be higher. The β value of the adjustment tube BG1 should be as large as possible. For this purpose, a compounding tube can be used. The power consumption of the regulating tube must also be large enough and should meet the requirements of the following formula: Usr max is the rectified output voltage when the grid voltage is the highest.

　　The amplifier tube BG2 should also use a tube with a large β value to enhance the control effect on the adjustment tube and make the output more stable. In the voltage stabilizing circuit with a larger Usc , you should also pay attention to the reverse voltage that BG2 can withstand and the transistor that should be selected.

　　The voltage dividing resistor ( R1 + R2 ) should be appropriately smaller to improve circuit performance. Usually the current flowing through the voltage dividing resistor is 5-10 times greater than the base current of the amplifier tube. The voltage dividing ratio is determined by the output voltage Usc and the reference voltage Uw, and is determined by the following formula: Generally, R1 or R2 can be selected first, and then the other resistor can be adjusted through calculation. The voltage dividing ratio should be selected larger, generally 0. 5～0.8.

3. Stabilized power supply with adjustable output voltage

　　As can be seen from the circuit above, the relationship between the output voltage and the reference voltage is "adjusted" by the voltage divider circuit. When the reference voltage is constant, changing the voltage dividing ratio can change the output voltage within a certain range. By adding a potentiometer W between R1 and R2 (see Figure 5-23), the output voltage can be continuously adjusted within a certain range.

4. Use composite tubes as regulated power supplies for regulating tubes

　　In a regulated power supply, the load current Ifz must flow through the regulating tube. A power supply that outputs a large current must use a high-power regulating tube. This requires a large enough current to be supplied to the base of the regulating tube, and the comparison amplifier circuit cannot supply the required amount. The high current required, on the other hand, the adjustment tube needs a higher current amplification factor to effectively improve the voltage stabilization performance, but the current amplification factor of high-power tubes is generally not high. The way to solve these contradictions is to add one or several "assistants" to the original adjustment tube to form a composite tube. The voltage-stabilized power supply circuit using composite tubes as adjustment tubes is shown in Figure 5-24.

　　When a composite tube is used as a regulating tube, the reverse current Iceo2 of BG2 will be amplified. Especially when a high-power germanium tube is used, the reverse cut-off current Icbo is relatively large and increases exponentially as the temperature increases, which can easily cause high-temperature no-load conditions. The loss of control of the regulated power supply causes the output voltage Usc to increase. The error signal ΔUsc is amplified and added to the stage base of BG2 to reduce Ic, possibly forcing BG2 to cut off. In order to make the adjustment tube work in the amplification area at different temperatures, a resistor (R7) is often added to the base of BG1 and connected to the positive pole (as shown in Figure 5-24) or the negative pole of the power supply. When there is little change in temperature or load or when all silicon tubes are used, this resistor does not need to be added.

　　The value of R7 can be approximately determined by the following formula:

5. Stabilized power supply with protection circuit

　　In the voltage stabilizing circuit, short-circuit protection measures must be taken to ensure safe and reliable operation. Ordinary fuses melt slowly, and the protection effect cannot be achieved by adding a fuse, and a protection circuit must be installed.

　　The function of the protection circuit is to protect the diaphragm tube from being burned when the circuit is short-circuited and the current increases. The basic method is that when the output current exceeds a certain consistent value, the adjustment tube is placed in a reverse biased state and thus cut off, automatically cutting off the circuit current.

　　There are many forms of protection circuits. Figure 5-25 is a diode protection circuit, which is composed of diode D and detection resistor R0. During normal operation, although the voltage across the diode decreases, the diode is still in the reverse blocking state. When the load current increases to a certain value, the depression ROIe on the resistor RO increases, causing the diode to conduct. Since UD=Ube1+RO Ie, and the conduction voltage UD of the diode is certain, Ube1 is forced to decrease, thereby limiting Ie to a certain value to achieve the purpose of protecting the adjustment tube. When using, the diode should be selected with a large UD value.

Figure 5-26 is a triode protection circuit. It consists of triode BG2 and voltage dividing resistors R4 and R5. When the circuit is working normally, the base potential of BG2 is higher than the emitter potential through the pressure effect of R4 and R5, and the emitter junction withstands the reverse voltage. So BG2 is in a cut-off state (equivalent to an open circuit) and has no impact on the voltage stabilizing circuit. When the circuit is short-circuited, the output voltage is zero, and the emitter of BG2 is equivalent to grounding, then BG2 is in a saturated conduction state (equivalent to a short circuit), so that the base and emitter of the adjustment tube BG1 are close to a short circuit and are in a cut-off state. Cut off the circuit current to achieve protection purposes.