1.2 to 33 VDC digital voltage supply design

　　illustrate

　　This is a simple to build microcontroller controlled power supply that can switch between pre-set voltages (or 32 or more) between 1.2 and 33 volts DC and up to 3 amps. This guide will go through every aspect of the building process, however, some basic familiarity with electronics and microcontrollers will be required to program the microcontroller. Specifications

　　Input voltage: 33 V DC maximum

　　Input current: 3 amps maximum

　　Output voltage: 5 to preset voltage 1.2 to 33**V DC

　　Output current: 3 amps max *There is no bridge rectifier, so input voltage must be DC **Output voltage

　　illustrate

　　The hart of the circuit is an LM350 adjustable positive voltage regulator (T2). The regulator is capable of providing over 3 amps over an output voltage range of 1.2VDC to 33Vdc. It's ease of use, thermal overload protection, large voltage range, current limiting, and high ripple rejection make it a great choice for a variable power supply. The voltage is controlled by the current driving the ADJ pin through a resistor to ground (Vout pin). Therefore, by changing the resistor the output voltage will change. Changing the resistance is controlled by an ATTINY2313 Atmel microcontroller (U1). The microcontroller has 2 main functions, collecting user input, and changing the output. Collecting user input is easy, there are two buttons (S1 - S2), one to go to the next voltage, and the other to the previous voltage. The buttons are connected to pins PD2 and PD3 of the microcontroller. When a button is pressed, the microcontroller sees a signal (+5 volts) on the corresponding pin. At rest, when a button is pressed the microcontroller sees a low signal (0 volts) on the corresponding pin because of the pull-down resistor (R2 - R3) connecting the pin to ground.

　　When the microcontroller sees a change from low to high on an input pin, it sends a high signal (+5 volts) to the output pin. There are 5 output pins PB0 PB1 PB2 PB3 PB4, each with a small current limiting resistor (R4 - R8) going through an LED (D2 - D6) so you can see what the currently selected voltage is, and then a small 2N2222 transistor (Q1 - Q5) at the base pin.

　　Each transistor has a resistor connected to its collector pin and its emitter pin connected to ground. When the transistor receives voltage on its pin, power will flow from the collector to the emitter. This basically turns a resistor change on the ADJ pin of the LM350 (T2) turning the current on or off. The LM7805 (T1) is just a basic fixed 5 volt DC regulator to provide power to the microcontroller

　　The diode D1 is connected from a positive voltage to ground to protect the circuit

　　Capacitors C1 C2 C3 C4 and C5 are used to maintain a stable power supply and to separate the LEDs from the parts of the circuit that need to be mounted to the PCB. They can be mounted on the panel to easily display the selected voltage, or completely excluded and replaced with jumpers. Currently, they are set to the following values:

　　D2 -> D3 - 3.3V, 5V, D4 -> 7V, D5 -> 9V, D6 -> 12V

　　Changing the values ​​of R9 - R15 will change the preset voltage to whatever voltage you want.

　　Where Ra is component R9 and Rb is the parallel component of R10 - R14 & R15. Remember that R10, R14 & R15 and their own values ​​need to be calculated in parallel like this.

　　Schematic diagram

　　Diagramms

　　Parts List

MCU Programming

　　C# code file for the microcontroller: MCU_Power_Supply.c

　　It can be easily modified with this same circuit to control 36 different voltages. Set SUT_CKSEL fuses:? Int. RC Oscillator. 4 MHz; Startup Time:? 14 CK + 65 ms Make sure the CKDIV8 fuse is not set The PCB also has connections for RX TX and PD6 so that an LCD screen, computer control, and additional inputs and outputs can easily be added.

　　A printed circuit board