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Jun 15 2019

Circuit Design Schematic of Adjustable Voltage Regulated Power Supply

Introduction

The adjustable DC regulated power supply adopts the current international advanced high-frequency modulation technology. Its working principle is to widen the voltage and current of the switching power supply, realize the wide range adjustment of voltage and current, and expand the application of the current DC power supply. The control chip of DC regulated power supply adopts the more mature imported components, the power components adopt the newly developed high-power devices in the world, and the adjustable DC regulated power supply design solves the problem of the bulk volume of the traditional DC power supply due to the power frequency transformer. Compared with the traditional power supply, the high-frequency DC power supply has the advantages of small size, light weight, high efficiency, etc., and also creates conditions for the high-power DC power supply to reduce the volume. This power supply is also called a high-frequency adjustable switching power supply. The adjustable DC regulated power supply has complete protection functions. The overvoltage and overcurrent points can be continuously set and can be previewed. The output voltage can be controlled by the touch switch.


Catalog

Article Core

Adjustable Voltage Regulated Power Supply


Ⅰ Adjustable DC Regulated Power Supply

1.1 Working Principle of Adjustable DC Regulated Power Supply

1.2 Circuit Diagram

1.2.1 Circuit Block Diagram

1.2.2 Circuit Schematic


 Hardware circuit design

2.1 Rectifier Circuit


2.2 Filter Circuit


2.3 Voltage Stabilizing Circuit







 LM317

3.1 Summary of LM317


3.2 Characteristics of LM317


3.3 Main Parameters of LM317


3.4 Pin Diagram and Function Description of LM317


3.5 Absolute Maximum Rating of LM317


3.6 Package Form of LM317


3.7 Working Principle of LM317


3.8 Internal Schematic of LM317


3.9 Typical Applications of LM317











 Circuit Design Schematic of Adjustable Voltage Regulated Power Supply

4.1 Circuit Diagram Design of Adjustable Voltage Regulated Power Supply (Ⅰ)

4.1.1 Working Principle

4.1.2 Selection and Production of Components

4.1.3 Production Points

4.2 Circuit Diagram Design of Adjustable Voltage Regulated Power Supply (Ⅱ)

4.2.1 Working Principle

4.2.2 Selection of Components

4.3 Circuit Diagram Design of Adjustable Voltage Regulated Power Supply (Ⅲ)

4.3.1 Working Principle

4.3.2 Selection and Production of Components

4.4 Circuit Diagram Design of Adjustable Voltage Regulated Power Supply (Ⅳ)

4.4.1 Working Principle

4.4.2 Selection of Components

4.5 Circuit Diagram Design of Adjustable Voltage Regulated Power Supply (Ⅴ)


4.6 Circuit Diagram Design of Adjustable Voltage Regulated Power Supply (Ⅵ)


4.7 Circuit Diagram Design of Adjustable Voltage Regulated Power Supply (Ⅶ)



Ⅰ Adjustable DC Regulated Power Supply

1.1 Working Principle of Adjustable DC Regulated Power Supply

When the input voltage is 150V-260V, the output voltage regulator is less than or above the 220V effect, and its efficiency is reduced. The single-chip microcomputer is used for the first step control, so that the input voltage below 310V and above 90V is adjusted and controlled in the range of 190V-250V, and the voltage regulator is used to stabilize the voltage.

The AC voltage input from the municipal electricity is fluctuating greatly. After the overvoltage absorption filter circuit filters out the interference voltage such as the high frequency pulse, it is sent to the DC switching regulated power supply, the AC sampling circuit and the control execution circuit.

The power of the DC switching regulated power supply is small, but the AC voltage of 60-320V can be replaced by the DC voltage of +5V, +12V, -12V. The +5V voltage is supplied to the single-chip microcomputer, and the ±12V voltage is supplied to the high-power switch module of the control circuit.

The single-chip microcomputer takes the input voltage data collected by the sampling circuit, analyzes, judges and sends a control signal to the trigger circuit to control and regulate the output voltage.The control execution circuit consists of an SSR zero-crossing switch high power module and a tapped autotransformer. The SSR uses an RC snubber circuit to absorb overvoltage and overcurrent, so that the SSR will not be damaged during switching. The control execution circuit controls the input voltage of 90-310V to the range of 190V-240V, and then sends it to the parameter regulator for precise regulation.

The parametric regulator consists of an LC oscillator consisting of an inductor and a capacitor with an oscillation frequency of 50 Hz. No matter how the municipal electricity changes, its oscillation frequency does not change, so the output voltage does not change, and the voltage regulation accuracy is high. Even if the input voltage waveform is very distorted, it is a standard sine wave after being oscillated by the parametric regulator, so the regulated power supply has strong anti-interference ability and purification ability.

Protection alarm circuit: When there is a hazard to the safety of the equipment, only sound and light alarms are issued, prompting the operator to take measures to avoid cutting off the output voltage. There will be sound and light alarms when the temperature of the control box is too high, the municipal electricity input is higher than 300V, and the municipal electricity input is lower than 130V in the absence of output voltage. When the input current is too large, the input (output) air automatic switch automatically trips.

 

1.2 Circuit diagram

1.2.1 Circuit block diagram 


Figure 1. Circuit Block Diagram

Figure 1. Circuit Block Diagram


1.2.2 Circuit Schematic 


Figure 2. Circuit Schematic

Figure 2. Circuit Schematic


 Hardware circuit design

2.1 Rectifier Circuit

The function of the bridge rectifier circuit is to rectify the positive and negative alternating sinusoidal alternating voltage into a one-way ripple voltage by using a unidirectionally conductive rectifier element diode. However, this unidirectional voltage often contains a large pulsating component, which is far from the ideal DC voltage.


Figure 3. Bridge Rectifier Circuit

Figure 3. Bridge Rectifier Circuit



13.png

Figure 4. Wave Chart


2.2 Filter Circuit

The filter circuit is composed of energy storage elements such as capacitance, inductance and so on. Its function is to filter out the AC components of the unidirectional pulsating voltage as much as possible, so that the output voltage becomes a relatively smooth DC voltage.


Figure 5. Filter Circuit

Figure 5. Filter Circuit



15.png

Figure 6. Wave Chart


2.3 Voltage Regulated Circuit

The function of the voltage regulated circuit is to take certain measures to keep the output DC voltage stable when the grid voltage or load current changes.

With the development of integration technology, the voltage regulator circuit is also quickly integrated. At present, various types of monolithic integrated voltage regulator circuits have been mass-produced. The integrated regulator has the advantages of small size, high reliability and good temperature characteristics, and is flexible and inexpensive. It is widely used in instruments, meters and other electronic equipment, especially three-terminal integrated voltage regulators.


Figure 7.

Figure 7.


 LM317

3.1 Summary of LM317

The LM317 is one of the most widely used power supply integrated circuits. It not only has the simplest form of a fixed three-terminal regulated circuit, but also has an adjustable output voltage. In addition, it has the advantages of wide voltage regulation range, good voltage regulation performance, low noise and high ripple rejection ratio. The LM317 is an adjustable 3-terminal positive voltage regulator that provides over 1.5 amps of current over an output voltage range of 1.2 volts to 37 volts. This regulator is very easy to use.

 

3.2 Characteristics of LM317

— The adjustable output voltage is as low as 1.2V

— Guarantee 1.5A output current

— Typical linearity adjustment is 0.01%

— Typical load regulation is 0.1%

— 80 dB ripple rejection ratio

— Output short circuit protection

— Overcurrent, overheat protection

— Adjustment tube Safety work area protection

— Standard three-terminal transistor package

— Voltage range of LM117/LM317 from 1.25V to 37V is continuously adjustable


3.3 Main Parameters of LM317

— The output voltage:1.25-37VDC

— The output current:5mA-1.5A

— The chip has an overheat, overcurrent, and short circuit protection circuit inside

— Maximum input-output voltage difference:40VDC

— Minimum input-output voltage difference:3VDC

— Ambient temperature:-10 ~+85 

— Storage ambient temperature:-65 ~+150 


3.4 Pin Diagram and Function Description of LM317

The LM317 has three pins.

The first pin is a voltage regulated pin;

The second pin is a voltage output pin;

The third pin is the voltage input pin.


Figure 8. Pin Diagram of LM317

Figure 8. Pin Diagram of LM317


3.5 Absolute Maximum Rating of LM317

Figure 9. Absolute Maximum Rating of LM317

Figure 9. Absolute Maximum Rating of LM317


3.6 Package Form of LM317


Figure 10. Package Form of LM317

Figure 10. Package Form of LM317


3.7 Working Principle of LM317

The input maximum voltage is more than 30 volts, the output voltage is between 1.5 to 32V and the current is 1.5A. However, you have to pay attention to power consumption and the heat problemwhen using it. The LM317 has three pins. One for input, one for output and one for voltage regulation. The input pin inputs positive voltage, the output pin is connected to the load, the voltage regulated pin has one pin connected resistor (about 200) on the output pin, and the other is connected to the adjustable resistor (several K). The input and output pins are connected to the filter capacitor to ground.


3.8 Internal Schematic of LM317

Figure 11. Internal Schematic of LM317

Figure 11. Internal Schematic of LM317

3.9 Typical Application of LM317

 Application Circuit One 

 

Figure 12. Application Circuit Diagram of Current Regulator

 Figure 12. Application Circuit Diagram of Current Regulator


 Application Circuit Two 


Figure 13. Soft Start Application Circuit Diagram

Figure 13. Soft Start Application Circuit Diagram


 Application Circuit Three 


Figure 14.

Figure 14.


This circuit is the most basic application circuit of LM317. In the process of use, the minimum voltage difference should not be less than 4V and the maximum voltage difference should not be greater than 37V. The circuit below 4V will not work. If it is greater than 37V, the integrated circuit will be damaged.


 Application Circuit Four 


Figure 15. 

Figure 15.


The high-power tube can be used to spread the circuit when a large current is required. This circuit uses the PNP-type high-power transistor to spread the LM317.

 

 Application Circuit Five 


Figure 16.

Figure 16. 


This circuit uses the NPN-type high-power triode for current expansion. The effect is pretty well. I once expanded the current to 5A, and the circuit is still stable.


 Application Circuit Six 


Figure 17.

Figure 17.


The charging circuit with current limiting protection, and adjusting R3 can adjust the charging current.


 Application Circuit Seven 


Figure 18.

Figure 18.


Constant current battery charging circuit. Io=1.25/24=52mA

Changing the value of resistor R1 provides different charging currents.

 

Circuit Design Schematic of Adjustable Voltage Regulated Power Supply

4.1 Circuit Diagram Design of adjustable Voltage Regulated Power supply (Ⅰ)

The simple adjustable voltage regulated power supply adopts three-terminal adjustable voltage regulated integrated circuit LM317, to make the adjustable range of the voltage range from 1.5 to 25V and the maximum load current reach 1.5A. The circuit is shown in the figure 1.

 

Figure 19. 

                 Figure 19.

 

4.1.1 Working Principle 

After 220V AC is reduced by transformer T, 24V AC is obtained, and then the DC voltage of 33V is obtained by full-bridge rectifier and C1 filter composed of VD1~VD4. The voltage is stabilized by the integrated circuit LM317. The output voltage can be adjusted continuously by adjusting the potentiometer RP. In figure 1, C2 is used to eliminate parasitic oscillations, C3 is used to suppress ripples, and C4 is used to improve the transient response of a stabilized power supply. VD5 and VD6 play a protective role in the case of capacitance leakage at the output terminal or short circuit at the adjusting terminal. The LED is the working indicator of the stabilized power supply, and the resistance R1 is the current limiting resistance. The output voltage value can be intuitively indicated by installing a miniature voltmeter PV at the output terminal. 

4.1.2 Selection and Production of Components

There are no special requirements for the components. What you need to do is to select the components according to the figure.

4.1.3 Production Points:

· C2 should be as close to the output terminal of LM317 as possible to avoid self-excitation, resulting in unstable output voltage

· R2 should be close to the output terminal and adjusting terminal of LM317 in order to avoid the change of reference voltage caused by the lead voltage drop between the output terminal and R2 in the state of high current output

· The adjusting terminal of the voltage stabilizer LM317 should not be suspended, especially when connecting to the potentiometer RP in order to avoid suspension of LM317 adjusting terminal caused by poor contact of sliding arm

· The capacity of C4 should not be increased arbitrarily

· The heat sink should be added to the integrated block LM317 to ensure its stable operation for a long time

  

4.2 Circuit Diagram Design of Adjustable Voltage Regulated Power Supply (Ⅱ)

The voltage of high current adjustable voltage regulated power supply can be arbitrarily adjusted between 3.5V~25V, the output current is large, and the adjustable voltage regulated tube circuit is adopted to obtain satisfactory and stable output voltage.

Figure 20. 

               Figure 20.

 

4.2.1 Working Principle

After rectifier filtering, the DC voltage is provided by R1 to the base of the adjusting tube to turn on the adjusting tube. When V1 is turned on, the voltage passes through RP and R2 to turn on V2, and then V3 is also turned on. At this time, the emitter and collector voltage of V1, V2 and V3 no longer change and its function is completely the same as that of the voltage stabilizer. The stable output voltage can be obtained by adjusting RP, and the ratio of R1, RP, R2 and R3 determines the output voltage of the circuit.

4.2.2 Selection of Components: 

Transformer T:  80W to 100W, input AC220V, output double winding AC28V.

FU1: 1A

FU2: 3A~5A

VD1 and VD2:  6A02

RP:  1W ordinary potentiometer with the resistance value ranges from 250K to 330K

C1:  3300 μ F / 35V electrolytic capacitance

C2 and C3:  0.1 μF single stone capacitance

C4:  470μ F / 35V electrolytic capacitance

R1:  180 ~ 220 Ω / 0.1W < 1W

R2, R4, R5:  10K Ω, 1/8W

V1:  2N3055

V2:  3DG180 or 2SC3953

V3:  3CG12 or 3CG80 

 

4.3 Circuit Diagram Design of adjustable Voltage Regulated Power supply (Ⅲ)

This is the circuit of a high stability and high current DC voltage stabilizer with voltage comparator. It is mainly composed of power supply voltage conversion, rectifier filter, reference source circuit, voltage comparison, compound power adjustment, over-current protection circuit and so on. Power supply voltage conversion and rectifier filtering is relatively simple so it will not be much discussed here. ICl (7805) and IC2 (LM317) constitute a precision reference source; IC3 is connected here into an inverse comparator as a voltage comparison circuit. It in-phase terminal is connected to the reference source and sampling voltage is input onto the reverse terminal. After comparing with the in-phase reference in IC3, the result of the output comparison is used to control the conduction degree of the composite adjustment tube in order to adjust the rise and fall of the output voltage. V1 and V2 form a compound power adjustment circuit, which amplifies the control current of the comparator circuit to the load current of several amperes so as to improve the driving ability. V1 does not need to increase the bias resistance between pole c and b like the ordinary "string stable" power supply. V3, R6, R5 constitute a load overcurrent protection circuit. The overcurrent sampling resistance R6 is strung at the negative terminal of the power supply and is not set in the voltage regulated control, so that it has almost no effect on the voltage regulated output. And this is for the circuit in which the sampling resistor R6 is strung at the output terminal of the adjusting tube.

Figure 21.  Circuit of High Current Adjustable Voltage Regulated Power Supply 

Figure 21.  Circuit of High Current Adjustable Voltage Regulated Power Supply

 

4.3.1 Working Principle: 

DC voltage supply voltage regulated circuit is smoothed by rectifier filter after voltage conversion of power supply. One side of it is initially stabilized to 5V by ICl and then provide regulated output for IC2 as the reference voltage of 1.25V, which is directly supplied to the in-phase terminal of the voltage comparator IC3 (LM358), and the other is used as the power supply for IC3. When the power is turned on, the IC3 has no output because V1 and V2 haven't start, and there is no voltage at the inverse terminal (0V). The inverse comparator IC3 will output a high voltage immediately, which makes the V1 and V2 turn on quickly, and the stabilized voltage output starts to rise from 0V. After sampling the partial voltage of R3, RP and R4, the voltage sent to the inverse terminal of IC3 also increases. Compared with the 1.25V reference of the in-phase terminal of IC3, the voltage at the output terminal of IC3 falls down to the set steady voltage value. When the stabilized output voltage has a downward trend due to the connection of the load, the stable process is as follows: stabilized voltage output↓→IC3 reverse terminal voltage↓→IC3 inverse comparison output ↑→V1, V2 are turned on ↑→stable output is normal. The working process of the overcurrent protection tube V3: when the voltage on the overcurrent sampling resistance R6 exceeds 0.7V due to heavy load, V3 will be turned on and the pole b of V1 is earthed to reduce the output voltage to achieve the purpose of overcurrent protection.

The circuit is characterized by high output stability, and the output voltage remains unchanged on the digital table under the condition of rated load current and the normal voltage drop of the adjusting tube V 2 (see attached table for details).

Figure 22. 

Figure 22.

 

4.3.2 Selection and Production of Components: 

First of all, in order to achieve voltage stabilizated output of high current, at least the power of the power transformer should be increased accordingly. In the experiment we choose a 120VA transformer, and this can be selected according to the needs in the practical application.

The rectifier tube:  6A/200V

C1 main filter electrolysis:  ≥ 8200 μ F / 50V

V2:  silicon NPN high power tube whose BVeeo>100V, Icm>10A, PCM ≥ 100W such as C5198, C3263 and so on.

V1, V3:  silicon NPN medium power small volume tube whose BVeeo ≥ 50V, IA, Pcm ≥ 0.6W , β ≥ 180. The recommended model is C8050 and it can be domestic or imported.

ICl:  common three-terminal 7805

IC2:  LM317

IC3:  a single power supply operational amplifier, and the common mode voltage is 0V with small temperature drift.

It is required that the negative terminal of IC3 power supply, C3, R4 sampling, C4, output (circuit board ground wire width is 2em) must be connected together and the cross-line should not be used. Otherwise, the high stable output can not be guaranteed. The attached table is the real reference data measured by disconnecting the R5 terminal of the over-current protection circuit. As long as the welding is correct according to the attached drawings, it can be put into use after simple debugging. If you choose military operational amplifiers and metal resistance, the stability will be even higher.

 

4.4 Circuit Diagram Design of adjustable Voltage Regulated Power supply (Ⅳ)

The power supply circuit is a 0 ~ ±15V linearly adjustable voltage regulated power supply, which can be adjusted from 0V and the maximum output is ±15V. The positive power supply part uses the LM317 three-terminal adjustable integrated voltage stabilizer, and the negative power supply part uses the LM337 three-terminal adjustable integrated voltage stabilizer. The circuit has perfect functions such as current limiting, short circuit protection and thermal protection. Its unique feature is that only one single potentiometer can realize the "synchronous" regulation of positive and negative voltage. It has the characteristics of simple circuit, convenient adjustment, excellent performance, low cost and so on. Therefore, it is very suitable for electronic enthusiasts to DIY.

Figure 23. 

Figure 23.

 

4.4.1 Working Principle

the whole circuit is shown in the figure. The input part of the power supply is the common transformer step-down and bridge rectifier, which increase the capacitance filter, and obtain the up and down symmetrical ±22V DC voltage. The other two groups of ±6.8V auxiliary voltage are also derived, which are respectively connected to the V + and V- terminals of the operational amplifier IC4 and the operational amplifier IC3 to ensure that the working voltage of IC3 and IC4 does not exceed the limited range. The following is a specific description of the voltage stabilization section:

— The positive output circuit consists of the voltage stabilizer IC1 and related components and it is usually connected to the adjusting terminal of the voltage stabilizer IC1 and the other terminals of the potentiometer is grounded. If the value of RP1 resistance is adjusted to 0, then the output voltage Vout is 1.2V, and the 10mA constant current is generated on the resistance R3. As long as the resistance value of the RP1 is changed, the output voltage can be changed. Here, the ground terminal of the RP1 is changed to the output of the operational amplifier IC3, and the output voltage of the IC3 is-1.2V, which is used to offset the reference voltage of the IC1 of + 1.2V, so that the modulation from 0 can be realized. It is also very simple to achieve the above purpose. We just need to connect the operational amplifier IC3 into a differential amplifier and finish subtraction operation. It can be seen from the diagram that the in-phase input voltage is V1, while the inverse input voltage is V2. Because R4=R5=R6=R7, the output voltage of IC3 is VO=R5/R4 × (V1-V2) =-1.2V, and the output voltage of voltage stabilizer IC1 + Vout=5mA × R3+10mA × PR1-1.2V.

— The negative output circuit is composed of a voltage stabilizer IC2 and related components and it omits the potentiometer RP3, which is originally arranged on the IC2 adjusting terminal and now is connected to the output terminal of the operational amplifier IC4, which controls output voltage of the adjusting terminal. It can also achieve the purpose of adjusting the output voltage of the voltage stabilizer. Since the operational amplifier IC4 is connected as an inverse amplifier with a gain of 1, and its inverse input is connected to the output of the voltage stabilizer of the positive output circuit, the negative output voltage stabilizer produces a stabilized voltage with the opposite polarity and equal amplitude. That is, -Vout=-R10/R9 × (+ Vout),). Because R9=R10, -Vout=+Vout. So, the negative output voltage tracks the positive output voltage.

— The diodes D7 and D8 in the circuit are used to prevent the capacitance discharge of the external load from increasing, resulting in damage to the outputs of IC1 and IC3; in addition, diodes D9 and D10 are used to avoid the breakdown of IC1 and IC2 adjusting terminals due to the positive saturation of IC3 output and the negative saturation of IC4 output. And this is because the adjusting terminal of IC1 and IC2 is not allowed to flow into the inverse current.

4.4.2 Selection of Components

There are all general components of this machine and no special specifications. Both IC1 and IC2 are three-terminal adjustable integrated voltage stabilizers, and the positive output model is LM317, and the negative output model is LM337. The packages of them are TO-220 and both of them are all available in the market. Radiators should be installed when they are put into use. IC3 and IC4 are general operational amplifiers, and can also be replaced by OP-07. The resistance is all 1/4W metal film resistance, in which the precision of R4, R5, R6, R7, R9, R10 is 1%. Wire wound potentiometer should be used in RP1 and multi-turn potentiometer will be better. The power transformer T can choose 14-inch black and white TV power transformer. If you want to self-winding, you should choose EI type Gaoxi steel sheet and the range of power can be 35W-45W.

 

4.5 Circuit Diagram Design of adjustable Voltage Regulated Power supply (Ⅴ)

Next, we are going to introduce an adjustable DC regulated power supply circuit based on LM324 operational amplifier. It can automatically change the connection method of secondary winding tap of power transformer during voltage adjustment and select the best input voltage in order to ensure that the input and output voltage difference of the voltage stabilized integrated circuit is kept in a reasonable range. The output voltage of the DC regulated power supply is adjustable in the range of 1.25V to 33V.

The adjustable DC voltage regulated power supply circuit is composed of a main voltage regulated power supply circuit, a secondary voltage regulated power supply circuit and a control circuit, which is shown in figure 6.

The main voltage regulated power supply circuit is composed of power transformer T, rectifier diode VD1~VD4, capacitance C1~C3, three-terminal voltage regulated integrated circuit IC1, potential RP, voltmeter PV and resistance R1. The auxiliary voltage regulated power supply circuit is composed of power transformer T, rectifier diode VD5~VD8, filter capacitance C4, C5 and three-terminal voltage regulated integrated circuit IC2, and the control circuit is composed of operational amplifier IC3 (N1~N4), optical coupler VLC1-VLC5, relay K1~K5 and resistance R2~R13.

After T step down, VD5~VD8 rectification, C4 filtering and IC2 voltage stabilization, the AC 220V voltage provides +12V working voltage for the control circuit. At this time, the reverse input of N1~N4 produces reference voltage of 1V, 3V, 5V and 7V respectively, which is higher than that of each positive input, and the output of N1~N4 is low. The light emitting diode and optically controlled thyristor in VLC1 are turned on and K1 is absorbed. The normally opened contact is turned on, and the 6V AC voltage is rectified by VD1~VD4 and filtered by C1, and then added to the input of IC1.

Adjusting the resistance value of RP can change the output voltage (Uo) after IC1 voltage stabilization. When the output voltage is below 3V, K1 still maintains suction. If it is necessary to increase the output voltage and adjust the RP so that the output voltage is higher than 3V, the N1 output becomes high, so that the light emitting diode and the light controlled thyristor inside the VLC1 are cut off and K1 is released; at the same time, the light emitting diode and the light controlled thyristor inside the VLC2 are turned and K2 is absorbed. The 12V AC voltage is rectified by VD1~VD4 and filtered by C1, and then added to the input of IC1.

Figure 24. Adjustable DC Regulated Power Supply Circuit with LM324 Operational Amplifier 

Figure 24. Adjustable DC Regulated Power Supply Circuit with LM324 Operational Amplifier

 

Similarly, when the output voltage is raised to more than 9V, 15V and 21V, the N2~N4 also outputs high levels one after another, so that K3, K4 and K5 are absorbed in turn, and the 18V, 24V and 36V AC voltages are provided to the bridge rectifier circuit, respectively.

If the RP is adjusted in reverse to reduce the output voltage, the working process is opposite to the above.

 

4.6 Circuit Diagram Design of Adjustable Voltage Regulated Power Supply (Ⅵ)

In this power supply, LM317 is used as the voltage stabilizer, and the adaptive switching circuit is used to automatically switch the input voltage according to the output voltage, so as to reduce the voltage difference between the input voltage and the output voltage and reduce the power consumption of the power supply itself. Among them, VT2, VD5, VW, R5, R6, C10 and relay K form an adaptive switching action circuit. When the output voltage Vo is lower than 14V, VW is cut off because breakdown voltage is insufficient and no current passes through. VT2 is cutoff and K does not act. Its contact K - 1 is normally closed and the secondary 14V AC of the transformer is connected to the voltage regulated circuit. On the contrary, when the output voltage is greater than 14V, VW breakdowns and VT2 conducts. K gets electricity, K-1 actions, and the 28V AC voltage is connected to the voltage regulated circuit. Thus, the input and output voltage difference is not more than 15V. The output voltage of the circuit is 1.25V ~ 30V, which is continuously adjustable, and the maximum output current is 3A. As shown in the figure 7, it is an adaptive adjustable voltage regulated power supply circuit based on LM317:

Figure 25. 

Figure 25.

  

4.7 Circuit Diagram Design of Adjustable Voltage Regulated Power Supply ()

The main device of DC adjustable voltage regulated power supply with overcurrent protection is general voltage regulated integrated block LM723, which contains starting circuit, constant current source, reference voltage regulated source, overcurrent protection and so on. With the high power adjusting tube, it can output continuously adjustable stable voltage in the range of 0 ~ 20. The maximum output current can reach 2A, and has the function of overcurrent protection, which can be used as the maintenance power supply of mobile phone and BP machine, and can also be used for battery charging. The circuit is shown in figure 9. When in normal use, the red and green light emitting diodes shine at the same time. Adjusting potentiometer W can adjust the output voltage in the range of 0 ~ 20. When there is an overcurrent or short circuit at the output terminal, the voltage drop at both terminals of R1 is greater than 0.6V, Q3 and Q4 are conducted, the green light is off, the voltage of LM723 is reduced to nearly 0V, the internal detection circuit is operated, the high voltage of 23V is output and Q1, Q2 are cut off. Therefore, there is no voltage output, which plays a protective role.Output will only be available if the machine is turned off and rebooted. In order to ensure that the adjusting tube Q1 will not be burned out when it outputs the rated current, a heat sink of sufficient size should be installed. The whole power supply can use as the plastic box as a casing, the front plate of it can be equipped with ammeter, voltmeter, switch and potentiomete,the output terminal and the red and green light emitting diode. As long as the components are good, the circuit can work normally without debugging. Among them, it is best to use imported C2819, 2N3395 and other high-power tubes for Q1, LM723, MC1723 and so on can be used as IC.

Circuit Design Schematic 9.png 

Figure 26.

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