Power Supply 2.5 – 12 V

Power Supply is probably the most common electronics project you can build.This is a basic design for digital power supply. It was built around 3 main components. Digital potentiometer – MCP4251-502E/P, Low  Dropout Voltage Regulator – MIC29152 and PIC16F877A.The idea behind it is simple. Take a standard LDO design – 2 resistors, first resistor between ADJ pin and Out, second resistor between ADJ and ground and modify it to include digitally controlled resistor in the desired range. Now PIC can control the resistor value by sending SPI commands/data thus changing the output voltage. For this project MCP4251-502E/P was used. It’s a dual 5kOhm Potentiometer with SPI interface. Special attention should be put on verifying that digital potentiometer is operating within its Max operational rating. For MCP4251 the current through resistor network should not exceed 2.5mA and voltage on resistor terminals at 5v Max. Full datasheet can be downloaded here.Few words about LDO.MIC29152 is a Low – Dropout Voltage Regulator with overcurrent and over temperature features built in and its 5 pin version has Enable input that can be controlled by PIC microcontroller. Enable pin was used to shut down the regulator if Max allowed current is reached. Although MIC29152 can handle 1.5A the Max current in this version  was limited to 400mA.For higher currents a bigger Heat Sink should be used and current sensing function will require some changes. Due to greater heat dissipation higher power rating resistors should also be used. This design was built and tested only up to 400mA.For proper operation 2 independent power supplies are needed. One for PIC power, second for 14-16 V Input to LDO. If PIC is supplied from the same power source as LDO for example by 7805 or LM317 current limiting function will not work correctly and LDO will not be shut down at desired current.   Instead in case of a short at the LDO output PIC will stop operating leading to MIC29152 overheating till its over current/over temperature protection will shut it down.

Briefly about power supply operation: Input voltage 14-16V can be controlled by SW2 from 2.5V to 12V in 1 V steps. Push/release SW2 to cycle through different voltage values. Default voltage  is 2.5V.Currently the voltage values are predefined in the software. If a custom voltage is required just change the resistors values in R_Value0, 1.For more details see attached assembly code at the bottom of the page. Output voltage and current are shown on a standard  LCD Display.Voltage divided  by 10 from LDO output is measured by Analog Channel 1   and current sensing  is performed by measuring voltage that is present on 1 Ohm resistor (10 Ohm x10  in parallel for heat dissipation) by AN2 .See design schematic below.

If 400mA is reached in any voltage range the PIC over current function will shut down the LDO resulting in zero voltage and current and LED will be ON until SW1 is pushed to release it.Future possible improvements. Presently only basic over current capabilities are implemented. The design can be upgraded to include Constant Current\Voltage feature, also instead of cycling through predefined voltage values a keypad can be used to directly enter the voltage value. Digital Power Supply software – Assembly/Hex Ver 1.0.0