USB Variable Power Supply

Occasionally we needed 2.8, 3.3 or 3.8 volts to power a little project while debugging firmware or hardware. Fortunately, lugging around a 15 pound BK precision DC regulated power supply forced us into a creative mood so we designed a variable mini power supply. It can supply several hundred milliamps via a USB cable from your laptop. If that isn't enough, just plug it into a USB wall adapter for more power.

Our projects never use 5V anymore. These days, we usually require 2.8V, 3.3V or LiIon simulation from 3.6V to 4.2V. This little variable power supply covers them all.

A PIC16F677 is used to monitor the voltage with 100mV accuracy. Press the monitor button to monitor current with 10mA accuracy.

The power supply is continuously adjustable from 1.7 to 4.6 volts using a simple pot. The low and high voltages are shown on the display below along with some of our most used voltages:

Documents:

• Schematic page: USB_Power_schematic.pdf
• Source code: USB_Power_src.zip
• Source schematic: USB_Power_eagle.zip*
• Microchip PIC16F677 datasheet

*The Source schematic file includes:

• USB_Power_02.sch: Eagle schematics
• MPI-PCA02-01 rev 01.doc: Manufacturing Process Instructions
• PCA-PW002-01-BOM_02.xls: PCA parts list
• Pacificware.lbr: Eagle component library (schematic and layout symbols) of newly created components specific to this project