What is the role of the three-terminal regulator? Three-terminal regulator 7805 application details

It is generally used in the protection circuit of DC circuit to reduce the voltage and stabilize the voltage. +The commonly used 78 series and 79 series, 78XX are all positive voltage output, 79XX are all negative voltage output, the input voltage is generally not too large, less than 36V. L or H or a blank often appear after +78 and 79 to represent the rated current. For example, 78L12 represents the output voltage of %2B12V+0.5A. + Wiring: When the character faces itself, the leftmost pin is 1 pin, the middle is 2 pin, and the far right is 3 pin. When wiring, pin 1 is connected to voltage input, pin 2 is connected to terminal c, and pin 3 is connected to output terminal.

The 7805 three-terminal voltage stabilized integrated circuit, among electronic products, the common three-terminal voltage stabilized integrated circuit includes the 78 series with positive voltage output and the 79 series with negative voltage output. As the name implies, the three-terminal IC refers to this kind of integrated circuit for voltage stabilization. There are only three pins for output, namely the input terminal, the ground terminal and the output terminal. It looks like an ordinary triode, the standard package of TO-220, and the TO-92 package of 9013.

Composition structure

Using 78/79 series three-terminal voltage regulator IC to form a regulated power supply requires very few external components,

There are protection circuits for over-current, over-heating and adjustment tubes inside the circuit, which are reliable, convenient and inexpensive to use. The number after 78 or 79 in this series of integrated voltage regulator IC models represents the output voltage of the three-terminal integrated voltage regulator circuit. For example, 7806 means that the output voltage is positive 6V, and 7909 means that the output voltage is negative 9V.

Because the three-terminal fixed integrated voltage regulator circuit is easy to use, it is often used in electronic production.

In practical applications, a large enough heat sink should be installed on the three-terminal integrated voltage regulator circuit (of course, it is not used under low power conditions). When the temperature of the voltage regulator tube is too high, the voltage regulator performance will become worse or even damaged.

When a stabilized power supply capable of outputting more than 1.5A is required in the production, several three-terminal stabilized circuits are usually connected in parallel to make the maximum output current of N 1.5A, but the application needs to pay attention to: the integrated stabilizer used in parallel The voltage circuit should use products of the same manufacturer and batch number to ensure the consistency of the parameters. In addition, a certain margin is left on the output current to avoid chain burnout of other circuits when individual integrated voltage regulator circuits fail.

In the 78 ** and 79 ** series of three-terminal regulators, the most commonly used packages are TO-220 and TO-202. The graphics, pin numbers and pin functions of these two packages are shown in the attached drawings.

From the front, ①②③pins are marked in order from left to right. When connecting to the circuit, the voltage of ①pin is higher than that of ②pin, and ③pin is the output bit. For example, for 78** positive pressure series, ①pin is high potential, ②pin is grounded, and ③pin is the output position; for 79** negative pressure series, ①pin is grounded, ②pin is connected to negative voltage, and ③pin is the output position. As shown in the attached picture.

In addition, it should be noted that the heat sink is always connected to the lowest potential pin ②. So in the 78** series, the heat sink is connected to the ② pin (ground), while in the 79** series, the heat sink is connected to the ② pin (input terminal).

Internal circuit

The internal circuit of the 7805 three-terminal regulator IC has over-voltage protection, over-current protection, and over-heat protection functions, which makes its performance very stable. Able to achieve output current above 1A. The device has a good temperature coefficient, so the product has a wide range of applications. The local adjustment can be used to eliminate the influence of noise and solve the dispersion problem related to single-point adjustment. The output voltage error accuracy is divided into ±3% and ±5%.

7805 three-terminal voltage regulator IC should pay attention to the following items in circuit application:

The input and output pressure difference should not be too large. If it is too large, the conversion efficiency will decrease rapidly, and it will be easily broken down. The maximum input voltage cannot exceed 35 volts; the output current cannot be too large, 1.5A is its limit value. For high current output, the size of the heat sink should be large enough, otherwise it will cause high temperature protection or thermal breakdown; the input and output pressure difference should not be too small, and the voltage regulation efficiency will drop sharply below 2 volts.

Application circuit

7805 typical application circuit diagram: (Identified error in the figure, 2 feet grounded, 3 feet output) The typical application circuit of 78XX series integrated voltage stabilizer is shown in the figure below. This is a regulated power supply circuit that outputs a positive 5V DC voltage. IC adopts integrated voltage regulator 7805, C1 and C2 are input and output filter capacitors respectively, and RL is load resistance. When the output current is large, 7805 should be equipped with a heat sink.

The voltage stabilizer diode VD1 is connected in series between the 78XX voltage stabilizer pin 2 and the ground, which can increase the output voltage Uo to a certain extent. The output voltage Uo is the sum of the 78XX voltage stabilizer output voltage and the voltage stabilized value of the voltage stabilizer VC1. VD2 is the output protection diode. Once the output voltage is lower than the regulated value of VD1, VD2 turns on, bypassing the output current to protect the output stage of the 7800 regulator from damage.

Due to the role of R1 and RP resistor network, the output voltage is increased, and the increase depends on the ratio of RP to R1. Adjust the potentiometer RP to adjust the output voltage within a certain range. When RP=0, the output voltage Uo is equal to the 78XX regulator output voltage; when RP gradually increases, Uo also gradually increases.

VT2 is an external expansion tube, VT1 is a push tube, and the two are Darlington connections. R1 is the bias resistor. The maximum output current of this circuit depends on the parameter of ov and VT2.

Input voltage

The limit input voltage of 78** series stabilized integrated block is 36V, and the minimum input voltage is 3-4V higher than the output voltage. Also consider the power loss caused by the pressure difference between output and input, so the general input is between 9-15V.

If the voltage of 7V wants to output 5V, you need to use a low-dropout voltage regulator integrated block, as shown in the figure.

You can also use 3 ordinary rectifier diodes to step down, and you can also get a relatively stable voltage of 5V, and the allowable current of the diode is greater than the current you need.