Diagram Guide

The Adjustable Switching Regulator Circuit with LM2576 are monolithic integrated circuits that provide all the active functions for a step-down (buck) switching regulator, capable of driving 3A load with excellent line and load regulation. These devices are available in fixed output voltages of 3.3V, 5V, 12V, 15V, and an adjustable output version.

Requiring a minimum number of external components, these regulators are simple to use and include internal frequency compensation and a fixed-frequency oscillator. The Adjustable Switching Regulator Circuit with LM2576 offers a high-efficiency replacement for popular three-terminal linear regulators. It substantially reduces the size of the heat sink, and in some cases no heat sink is required.

A standard Adjustable Switching Regulator Circuit with LM2576 of inductors optimized for use with the LM2576 are available from several different manufacturers. This feature greatly simplifies the design of switch-mode power supplies.

Other features include a guaranteed ±4% tolerance on output voltage within specified input voltages and output load conditions, and ±10% on the oscillator frequency. External shutdown is included, featuring 50 μA (typical) standby current. The output switch includes cycle-by-cycle current limiting, as well as thermal shutdown for full protection under fault conditions.

It is a simple and less efficient method to control DC voltage is using a voltage divider and transistor emitter follower configuration. The figure below is illustrated use a 1K potentiometer to set the base voltage of a medium power NPN transistor. The collector of the NPN feeds the base of a larger PNP power transistor which supplies most of the current to the load. The output voltage will be about 0.7 volts below the voltage of the wiper of the 1K pot so the output can be adjusted from 0 to the full supply voltage minus 0.7 volts. Using two transistors provides a current gain of around 1000 or more so that only a couple milliamps of current is drawn from the voltage divider to supply a couple amps of current at the output. The figure is;


Note that this circuit is much less efficient than the 555 timer dimmer circuit using a variable duty cycle switching approach. In the figure below, the 25 watt/ 12 volt lamp draws about 2 amps at 12 volts and 1 amp at 3 volts so that the power lost when the lamp is dim is around (12-3 volts * 1 amp) = 9 watts. A fairly large heat sink is required to prevent the PNP power transistor from overheating. The power consumed by the lamp will be only (3 volts * 1 amp) = 3 watts which gives us an efficiency factor of only 25% when the lamp is dimmed. The advantage of the circuit is simplicity, and also that it doesnt generate any RF interference as a switching regulator does. The circuit can be used as a voltage regulator if the input voltage remains constant, but it will not compensate for changes at the input as the LM317 does.

This be simple Adjustable voltage regulator power supply circuit, at use integrated number circuit TL431. By from the circuit can fine volt 3V – 30V, depend on feed volt supply input and change the value R2 , R1. It follow a formula calculates Vout = (1+R1/R2) , Vref = 3V-30V , but this circuit gives current get not tall 100mA only. It just if want to enhance current , must use the transistor helps to enlarge current, such as 2N3055, TIP41 numbers or the other.

source:http://www.eleccircuit.com/adjustable-regulator-by-tl431/