With the 78/79 series of three-terminal regulator IC to form a regulated power supply, few external components are required. The circuit also has overcurrent, overheating, and protective tube protection circuits that are reliable, convenient, and inexpensive. The numbers following the 78 or 79 in this series of integrated voltage regulator IC models represent the output voltage of the three-terminal integrated voltage regulator circuit. For example, 7806 indicates that the output voltage is positive 6V, and 7909 indicates 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 sufficiently large heat sink should be installed on the three-terminal integrated voltage stabilization circuit (not used under low power conditions, of course). When the temperature of the regulator tube is too high, the voltage regulation performance will be deteriorated or even damaged.
When the production requires a regulated power supply that can output more than 1.5A current, usually a few three-terminal voltage regulator circuits are connected in parallel to make the maximum output current of N 1.5A, but the application should pay attention: the integrated use of parallel connection Pressure circuit should use the same manufacturer, the same batch of products, in order to ensure the same parameters. In addition, there is a certain margin on the output current to avoid chain burn-down of other circuits when individual integrated voltage regulator circuits fail.
The TO-220 and TO-202 packages are most commonly used in the 78 ** and 79 ** series of three-terminal regulators. The two package patterns and pin numbers and pin functions are shown in the figure.
From the front, the 123 pins are marked in order from left to right. When the circuit is connected, the voltage of pin 1 is higher than that of pin 2, and pin 3 is the output bit. For 78** positive pressure series, 1 foot high potential, 2 feet grounded, 3 feet as output bit; and for 79 ** negative pressure series, 1 foot grounded, 2 foot negative voltage, 3 feet as output bit. As shown in the figure.
In addition, it should be noted that the heat sink is always connected to the second pin of the lowest potential. In the 78** series, the heat sink is connected to pin 2 (ground). In the 79** series, the heat sink is connected to pin 2 (input).
The internal circuit of the 7805 three-terminal regulator IC has overvoltage protection, overcurrent protection, and overtemperature protection, which makes its performance very stable. Can achieve more than 1A output current. The device has a good temperature coefficient, so the product has a wide range of applications. Local adjustments can be used to eliminate noise effects and solve the dispersion problems associated with single-point regulation. The accuracy of the output voltage error is divided into ±3% and ±5%.
7805 typical application circuit diagram: (Figure error pin 2 pin ground 3 pin output) 78XX series integrated voltage regulator typical application circuit as shown below, this is a positive output 5V DC voltage regulator power supply circuit. The IC uses an integrated voltage regulator 7805. C1 and C2 are the input and output filter capacitors respectively, and RL is the load resistor. When the output current is large, the 7805 should be equipped with a heat sink.
Transfer the multimeter to the DC voltage test at 20 volts and connect it to the output of the power supply. Then prepare a small light bulb with an operating current of 0.23 A and a voltage of 12 volts. Turn on the regulator and turn on and off the lamp. The voltage is stable and it is a good drop.
Because the regulator is a component. It consists of many transistors and peripheral circuit elements (diodes, regulators, constant current sources, resistors, etc.). The multimeter does not realistically simulate its working environment, so even if the measured positive and negative resistance of the pin does not differ much, it cannot be determined to be good. On the contrary, if there is a significant difference in the measured resistance values, it is basically certain that it is bad.
Of course, for some extreme situations, such as: the resistance between the input and output poles should be large. However, it tends to zero. It also shows that it is bad. Or the short circuit between the output terminal and the ground terminal can be judged to be bad.
In addition, in the road electricity test (can be disconnected from the circuit), such as welding down the test: right there is a literal, about the black table pen to measure the middle leg, when the red table to measure the two feet when the resistance is small, about 3K or so (different The measured value of the table will not be the same), while the red table pen measures the middle leg and the black table pen has a resistance of 20-50K when measuring the other two legs, which can be considered as good approximation.
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