This example introduces an adjustable DC regulated power supply circuit with a current indication made of LEDs. The output power is 50W and the output voltage range is 1.25-28V.
Circuit Operation Principle The adjustable DC stabilized power supply circuit consists of a power input circuit, a regulated output circuit, and a current indicating circuit, as shown in the figure. 
The power input circuit is composed of a power switch fuse fuse FU1, FU2, a power transformer T, rectifier diodes VD1, VD2, and a filter capacitor Cl.
The regulated output circuit is composed of a three-terminal adjustable voltage regulator integrated circuit ICl, a capacitor C2, a C4, a diode VD3, a VD4, a resistor Rl and a potentiometer RPl.
The current indicating circuit is composed of a resistor R2. R4, Zener diode VS, LED driver integrated circuit IC2 and LED VLl, VLlO.
After the power switch S is turned on, the AC 220V voltage is divided into two by T step-down, VDl and VD2 rectification and Cl filter: one channel is regulated by ICl and RPl, and the other is VS regulated to lOV. IC2's working power supply.
R2 is a current sampling resistor. When the output current flows through R2, a voltage drop is generated on R2. This voltage signal is input from the 5 pin of IC2. After buffer amplification and comparison with the reference voltage dividing circuit, the LED VLl-VLlO is driven to emit light, indicating the current value.
Adjustable DC stabilized power supply component selection
Rl and R4 select 1/4W metal film resistor for use; R2 selects 2W wire wound resistor for use; R3 selects lW metal film resistor for use.
Both RPl and Rå¨ use linear potentiometers.
Cl-C3 selects aluminum electrolytic capacitors with a withstand voltage of 5OV; it is called monolithic capacitor.
VDl and VD2 select lN5401 or lN5404 silicon rectifier diode; VD3 and VD4 select 1N4007 silicon rectifier diode.
VS selects silicon steady voltage diode of lW, lOV.
VLl-VLlO selects φ3mm LEDs.
lCl selects LM317T type three-terminal voltage regulator integrated circuit; IC2 selects LM3914 type LED display driver integrated circuit.
T selects 5OW, the secondary voltage is a dual 24V power transformer.
After the adjustable DC stabilized power supply circuit debugging circuit is installed, power calibration is performed.
Calibration voltage: Use a multimeter to measure the output voltage, adjust RPl, and mark the corresponding voltage value on the corresponding scale of RPl.
Calibration current: Connect a 200Ω, lOW cement resistor to the output of the regulated power supply, then adjust RPl to make the output voltage lOV. Then adjust Rå¨ so that VLlO just lights up, and the corresponding scale is 5OOmA. The entire adjustable DC regulated power supply current scale can be divided or doubled by this point.
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