Linear voltage regulators

In the last lesson, we examined a parametric voltage stabilization circuit using a bipolar transistor and a Zener diode (zener diode), but the industry does not stand still, and engineers have repeatedly improved this circuit, now it has stable operation with a good temperature spread, short-circuit protection and much more Total. All this was crammed into an LDO chip LDO (low-dropout linear regulator) linear voltage stabilizer with low voltage drop. Those. a microcircuit that receives input voltage and converts it into output, while stabilizing the output, which can be, for example, 8 volts or 15 volts, and at the output we get a clear 5 volts. Linear voltage stabilizers come with a fixed output voltage, for example L7805 (5 volt output) or with an adjustable voltage divider on two resistors (LM317), depending on the circuit, we will get an output from 1.25 to 35 volts, provided that the input voltage allows this do, i.e. You cannot use a voltage regulator to increase the voltage, for example 12 volts came in, but at the output we got 30 volts, NO, in order to get 30 volts at the output, more should come to the input, for example 35 volts.
Схема подключения L7805.jpg
Схема подключения L7805.jpg

L7805 fixed output voltage

Among linear voltage regulators, L78 series microcircuits occupy one of the first places in popularity and demand. This microcircuit allows you to obtain a stable output voltage of 5 volts from a high input voltage of 7..36 volts. For example, if the device is powered by a “KRONA” battery, then as the battery discharges, the voltage will drop, at the same time, for the operation of the microcontroller or logic chips, a clear 5 volts are needed, independent of the input voltage, which the L7805 successfully copes with
In the marking of microcircuits, the last two digits mean the output voltage, which can be 5; 6; 8; 8.5; 9; 12; 15; 18; 24 V
Polarity - Positive
Output type - fixed
Number of outputs - 1
Output voltage 5; 6; 8; 8.5; 9; 12; 15; 18; 24 V
Maximum load current 1.5 A
Voltage drop at Iout, 2(1) V (A)
Maximum input voltage 35 V
Operating temperature -40…+125°C
Housing to-220sg
L7805 на макетной платы для питания светодиода.jpg

LM317 adjustable output voltage

In addition to standard voltages and their widespread use, very often a non-standard voltage is required, for example in laboratory power supplies, where various experiments require, for example, smoothly changing the voltage from 3 to 5 volts (to check the operation of the circuit, for example). For such tasks, the industry produces linear regulators with the ability to set the desired output voltage.

The output voltage cannot be lower than 1.25 volts and not higher than 35 volts, then it depends on the ratio of the resistors according to the following formula.

V=1.25 (1+R1/R2) + (Current Adj * R1)

If R1 is small (less than 1000 Ohms), then the second part can be neglected, since it will be a very small number.
Микросхемы стабилизации и их применение.jpg

Current regulator on LDO

If you connect the ADJUST and OUTPUT pins of the linear regulator LM317 through a small resistor, for example 24 ohms, then the maximum current that can pass through it will be limited by the formula

I (max) = 1.2/R1 , where R1 is the resistor between the ADJUST connection to OUTPUT and the OUTPUT

So with a 24 ohm resistor, we get a current of 0.05 Ampere, and the generally accepted rule states that the battery charging current should not exceed 10% of the battery capacity, while a small current is not as harmful as a large one. Therefore, using LM317, in addition to regulating the voltage, you can also regulate the current, for example, to charge the battery
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