Curve tracer is an electronic test instrument to analyze the characteristics of transistors and other discrete semiconductors. In this post we construct USB base curve tracer to analyze properties of NPN transistors. This curve tracer is build around Microchip's PIC18F4550 MCU and it use simple Windows based GUI application to plot captured data of a transistor.

In this design PIC18F4550 MCU is used to establish USB connectivity, perform voltage readings and control current/voltage flow into the test subject. To minimize the cost and to make it simple, we use R2R ladder circuit to generate discrete collector-emitter voltage levels for the transistor on test. In each scan session collector-emitter voltage level get increase from 0V to 7.5V in 256 steps. In this design, "tracer" scans the transistor for 7 base current levels which are in between 7µA to 60µA. In viewer application collector-emitter voltage levels are plotted on x-axis and collector current is plotted on y-axis.

In current firmware, PIC18F4550 communicates with the PC over USB-HID class and because of that this may not need any special device drivers on host terminal. In most of Microsoft Windows operating systems the viewer application may be able to communicate with the device directly. Viewer application bundled in this project package are developed using Microsoft .net framework and it can run immediately on any new PC with minimum amount of prerequisites. Based on our experiences the most recommended operating system for this application is either Windows 7 or Windows 10.

To get the perfect results we highly recommended to construct this project on PCB. According to our experiences breadboards and veroboards may generate lot of noises and may finally leads to wrong output(s).