The three-terminal regulator tube is divided into two types: fixed voltage regulator and variable voltage regulator.
First, three-terminal fixed voltage regulator working principle1. The three-terminal fixed voltage regulator IC has two types: positive output (78 series) and negative output (79 series).
2, 78 series schematic:
3. Working principle: The above figure is very similar to the series-regulated regulated power supply composed of general discrete parts. The difference is that the starting circuit, constant current source and protection circuit are added, in order to make the regulator can be in a relatively large voltage variation range. In normal operation, a constant current source circuit is provided in the reference voltage forming and error amplifying portion, and the function of the starting circuit is to establish a working point for the constant current source. Rsc is an overcurrent protection sampling resistor RA, RB constitutes a voltage sampling circuit. The actual circuit is composed of a resistor network. In a voltage regulator with different output voltages, different string and parallel methods are used to form different voltage division ratios. After the error amplification, the operating state of the regulating tube is controlled to form and stabilize a series of predetermined output voltages, so RA is drawn in the form of a variable resistor in the figure.
The 79 series regulator is also a series-regulated regulated power supply, but its regulating tube is in the common-emitter working state. It is a collector output type voltage regulator circuit, and its working principle is similar to that of the 78 series.
Second, three-terminal variable voltage regulator working principle1. As an integrated three-terminal regulator with variable output voltage, there are two types of positive output and negative output.
2. Take the three-terminal regulator LM317 as an example:
3, working principle: LM317 output voltage (that is, the output voltage of the regulated power supply) U0 is the sum of the two voltages, that is, the voltage between the two points A and B and the voltage applied to R2, and IR2 is actually The sum of the two currents, UR1 is a constant voltage of 1.25V, and R1 is a fixed resistor, so IR1 is a constant current. The other way is the current ID flowing out of the LM317 adjustment terminal. When the LM317 is stable, its value is basically constant. Adjusting R2, then UR2=R*IR2 is a function of R2. It can be seen that the output voltage can be adjusted by R2. In addition, the LM317 contains protection functions and is stable and reliable.
The three-terminal adjustable output voltage regulator has a wide variety of positive output series and corresponding negative output series. The working principle is similar.
Typical application of a three-terminal integrated voltage regulator(1) Fixed output connection
In use, you must pay attention to the relationship between (VI) and (Vo). Take W7805 as an example. The fixed output voltage of the three-terminal regulator is 5V, and the input voltage is at least greater than 8V, so there is input/output between 3V differential pressure. Make the adjustment tube work in the amplification area. However, when the voltage difference is large, the power consumption of the integrated block is increased. Therefore, both should be considered, that is, the adjustment tube does not enter saturation at the maximum load current, and the power consumption is not large.
(2) Fixed dual-group output connection
(3) Enlarge the output current connection
Diode D is set with a low T-tube VBE voltage drop. The expanded output current is: The original output current is Io, which can now be approximately β times larger.
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