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  • Contents

I Description

Do you know the DC regulated power supply?

DC regulated power supply is widely used in industrial production and daily life, and its design occupies a very important position in power supply technology.

Therefore, based on the analysis of the problems of the traditional DC stabilized power supply, the method and calculation of each part of the 5 V, 1 A DC stabilized power supply based on L7805CT are described. Besides, it can also provide a 5 V DC power supply for counters, decoders, and digital tubes to realize addition and subtraction counting.

The system circuit based on L7805CT introduced in this blog is simple, stable, easy to control, cost-effective, and is of high use-value.

l7805

Catalog

I Description

II Introduction

III Design requirements for DC stabilized power supply

IV Hardware unit circuit design

4.1 Overall structure of the power supply

4.2 Component selection

V Application

VI Conclusion

FAQ

Ordering & Quantity

II Introduction

With the continuous development of electronic technology, the design requirements of electronic equipment for power supply are getting higher and higher, from traditional requirements to high-quality, high-efficiency, and high-stability to meet the needs of objects.

The normal operation of electronic devices requires DC power. DC power sources include solar batteries, accumulators, and dry batteries. But do you know which one is the most cost-effective? Of course, the most cost-effective method is to convert the AC power provided by the grid into the required DC power. Most electronic equipment uses this mode, and the three-terminal regulator is the most widely used.

Although people use many types of DC stabilized power supplies with different functions, the principles are similar. The voltage regulator chip used in the system introduced in this blog is L7805CT. L7805CT has the advantages of small size, simple external wiring, stable operation and strong applicability. It can meet people's requirements in life, study, and work.

III Design requirements for DC stabilized power supply

The design requirements of the DC regulated single-channel power supply based on L7805CT are as follows:

  • Input AC voltage 220 V;
  • Output DC voltage +5 V;
  • Output DC current 1 A;
  • The output ripple of the circuit is less than 50 mV;
  • The equivalent internal resistance is less than 0.15 Ω;
  • Ripple coefficient is less than 0.002%;
  • Voltage adjustment rate ≤0.001%;
  • Voltage stabilization coefficient ≤0.005%.

IV Hardware unit circuit design

4.1 Overall structure of the power supply

This design focuses on the knowledge and skills involved in the main links of the DC power supply, and completes the circuit design and production of 220 V AC input and 5 V, 1 A DC output.

The DC power supply is usually composed of four links:

  • Power Transformers;
  • Rectifier circuit;
  • Filter circuit;
  • Regulator circuit .

As shown in Figure 1.

Block diagram of DC power supply

Figure 1. Block diagram of DC power supply

4.1.1 Power transformer

The power transformer converts the high-voltage AC mains power into the appropriate voltage AC and sends it to the single-phase bridge rectifier circuit.

When choosing a power transformer model, the parameters we need to consider are capacity and output voltage.

4.1.2 Rectifier circuit

The rectifier circuit uses the unidirectional conductivity of the diode to complete the rectification. Commonly used rectifier circuits are as follows: single-phase half-wave rectifier circuit, single-phase full-wave rectifier circuit and single-phase bridge rectifier circuit.

Through comparative research, in order to overcome the shortcomings of half-wave rectification, the L7805CT-based system uses a single-phase bridge rectifier circuit. It consists of 4 diodes VD1~VD4 connected in the form of a bridge. Whether it is a positive half-cycle or a negative half-cycle, the direction of the current flowing to the load RL is the same. Both transform the AC voltage output by the secondary winding of the power transformer into a pulsating DC voltage. The reverse voltage that each device bears is the peak value of the power supply voltage, and symmetrical current flows through the positive and negative half cycles of the transformer, the utilization rate is higher than that of the full-wave rectifier circuit, and the current ripple is reduced. Therefore, there is no unidirectional magnetization.

4.1.3 Filter circuit

The rectified direct current contains a relatively large alternating current component, and the pulsation coefficient is relatively large, so it cannot be directly used as the power supply for electronic circuits. Filter circuits are commonly used to remove or reduce the AC component in the output voltage, so that pure AC power is used on the device.

This design uses the energy storage characteristics of capacitors and inductors to set appropriate parameters.

  • When the circuit voltage rises, the energy storage element stores energy.
  • When the voltage drops, the energy storage element releases energy, so as to achieve the purpose of reducing pulsation.

The capacity of the capacitor is related to the discharge time. To make the filtering effect better, a capacitor with a larger capacity can be used as the filter capacitor. The filter capacitor is generally selected as RLC≥(3~5)T/2.

4.1.4 Voltage stabilizing circuit

The DC voltage obtained after the AC power is rectified and filtered often fluctuates with the fluctuation of the grid voltage, temperature changes, and load resistance changes. Then, the quality of power supply will be reduced, affecting the operation of the equipment. Therefore, a voltage stabilizing circuit needs to be added between the filter circuit and the load link to achieve the purpose of stabilizing power supply.

The chip L7805CT has the characteristics of good voltage regulation performance, high reliability, easy installation and low cost. Therefore, the linear stabilized power supply replaces the discrete stabilized circuit and is widely used. In order to reduce the interference and make the voltage stabilizing circuit work normally, the input voltage should be at least 2.5~3V higher than the output voltage.

4.2 Component selection

4.2.1 Selection of power transformer

Considering the diode forward voltage drop, wire resistance, and power grid fluctuations, the output voltage UI of the three-terminal integrated regulator should meet:

 

Where:

  • Uomax is the maximum output of regulated power supply;
  • (UI-UO) min is the minimum voltage difference between the input and output of the integrated regulator;
  • ΔUI is the change of the input voltage caused by the fluctuation of the power grid (generally taken as 10% of the sum of UO, (UI-UO) min, and UIP).

For the integrated three-terminal regulator, when (UI-UO) min=2~10V, it has better voltage regulation characteristics.


When U1 = 10 V


When U2 = 9 V

In a single-phase bridge rectifier circuit, the relationship between the transformer secondary winding current I2 and the capacitor filter output current II is:

In summary, choose a transformer with a capacity of 15VA and 9V.

4.2.2 Selection of rectifier diode

Because, the current flowing through each rectifier diode in the bridge circuit is:

 

The maximum reverse peak voltage of each rectifier diode is:

 

In summary, select the transistor IN4001, its parameters are: ID=1A, URM=50V.

4.2.3 Selection of filter capacitor

Although the current passing through the diode is a pulsating current, because the capacitor C has an energy storage and discharge function, this reduces the degree of voltage pulsation across the load RL and increases the average value.

The average value and smoothness of the DC voltage on the load are related to the discharge time constant τ=RLC. The larger the value of C or RL, the slower the discharge of C. The larger the output DC voltage value, the better the filtering effect; otherwise, the worse. Generally take:

The AC power supply in some Asian countries is a 50Hz sine wave, with 50 identical waveforms per second. After full-wave rectification, both the positive and negative half waves become pulsating direct current in the same direction. At this time, there are 100 identical waveforms per second, that is, f=100Hz.

So the filter capacitor capacity is:

among them,

The output DC voltage of the capacitor filter circuit is estimated as:


In summary, two electrolytic capacitors of 2200μF and 50V can be used, and they are connected in parallel. In order to filter out the interference of high-frequency signals and improve the dynamic characteristics of the power supply, a 105pF, 50V high-frequency ceramic capacitor is connected in parallel at both ends of the filter capacitor.

4.2.4 How to choose a three-terminal integrated voltage regulator

The condition that the power consumption of the three-terminal integrated voltage regulator must meet is greater than 5W and the output voltage is 5V. This design chooses L7805CT, the output voltage is 4.8~5.2V, the quiescent current is 4.2~8mA, and the maximum output current can be 1.5A, which meets the design requirements. Considering the large output current, a heat sink needs to be added.

V Application

Adopt the design based on L7805CT to provide 5V DC power supply for counter, decoder and digital tube.

Use the button to generate a single clock, realize the count of modulo 10 through the standard counter circuit, and generate the corresponding code. The value is output to the integrated decoding circuit through the decoder to display the numbers 0-9. Use the "DP" section of the integrated decoding circuit as a power-on indication. The counter has a reset button, and the integrated decoding circuit adopts a common cathode structure to realize addition and subtraction counting. as shown in figure 2.

l7805 circuit

Figure 2. L7805 Circuit

VI Conclusion

The 5V, 1A DC power supply design based on the L7805CT described in this blog can provide 5V DC power supply for counters, decoders, and digital tubes to achieve addition and subtraction counting.

The design circuit of the L7805CT is simple, with few components, low power consumption and stable. Therefore, it can be extended to other object applications as a DC stabilized power supply to provide electrical energy.


FAQ

  • What is l7805?

A voltage regulator IC maintains the output voltage at a constant value. 7805 IC, a member of 78xx series of fixed linear voltage regulators used to maintain such fluctuations, is a popular voltage regulator integrated circuit (IC). ... 7805 IC provides +5 volts regulated power supply with provisions to add a heat sink.

  • What is the working principle of IC 7805?

A voltage regulator IC maintains the output voltage at a constant value. 7805 IC, a member of 78xx series of fixed linear voltage regulators used to maintain such fluctuations, is a popular voltage regulator integrated circuit (IC). The xx in 78xx indicates the output voltage it provides.

  • How do I test a 7805 with a multimeter?

Turn on the DC power supply and adjust the output voltage of about 8V or slightly larger. Or alternatively you can use a battery 9V-12V as voltage source. Look at the voltmeter panel when you set the voltage. Prepare a DC voltmeter readings on voltage range 50V to measure the output voltage of the IC 7805.

  • How does 7805 regulate voltage?

For 7805 IC, it is +5V DC regulated power supply. This regulator IC also adds a provision for a heat sink. The input voltage to this voltage regulator can be up to 35V, and this IC can give a constant 5V for any value of input less than or equal to 35V which is the threshold limit.

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