Hardware

The hardware used is based on a simple microcontroller (good old Atmel ATMega8) which generates the varying voltages with two PWM-DAC based outputs. The measurements are done with the internal ADC. For switching between NPN and PNP transistors (or N-mosfets and P-mosfets) one of the digital outputs is used. The USB connection is based on the V-USB package of OBDev V-USB. The power section for the DUT contains an opamp for buffering the DAC output, so it can sink and source currents. The resistor measures the current and creates at the same time a current limiting function. The second opamp is arranged as a current to voltage convertor with some gain so that about 10 mA gives a full scale for the ADC. Note that the ground reference for the floating current convertor is switched to accommodate for positive and negative currents. Also be aware the opamp’s inputs and outputs must reach rail to rail voltages. The control connection for the DUT (the base, or the gate) has fairly simple set-up from the second PWM-DAC with one RC-filter and a high value resistor; FETs get a voltage, bipolar transistors get a small current.

Firmware

To be able to connect to the device without needing a software driver, the system will work as a HID device. Communication will be on the features report channel. The device and the used protocol has the capacity for 16 different modes with each 8 functions. At the moment only the mode for measuring a trace and resetting (for firmware updates) are implemented. The functions are initializing, reading measurements, and reading and setting calibration data. More modes and funtions can be created in the future; there is still enough room on an ATMega8. One can think of an automated hFE measurement mode and likewise modes for FETs. Creating one trace is done by the initializing command containing the info for the used bias voltage (and this determines directly the bias current in cases of bipolar transistors), the number of measurement points, and the polarity to be used. After the initialization the requested number of measurements can be retrieved. There are also commands for setting and reading calibration values, which are needed for correction of the current to voltage conversion. The calibration is used at PC software level, and stored on the device because it is 'attached' to the components of that device.

The firmware contains a hid-based usb bootloader. This helps in a simple system where updates of the firmware can be transported conveniently to the device. The software supports this mechanism (it was used heavily during development).

Software

The controlling GUI is built using a Python script. The graphics are made using the PyQT and pyqtgraph packages. The connection with the device is through USB using the pywinusb package. Starting the GUI through main.py gives the following window: 

It defaults to top-right quadrant for 5 traces, good for a normal npn transistor.

Within the application it is possible to update the firmware, so new features can be introduced. There is also a bootload.py script which gives the possibility to update the firmware or give it the initial firmware using the console.

Results and calibration

Connecting the hardware to the computer, and starting the software conveniently through the CurveTracer.bat file gives the opportunity to calibrate it. This calibration is necessary only once. Put a 1kOhm resistor (1%) in the hardware between the collector and emitter pins, and do a 'single shot' measurement. You should see a straight line through points 0mA/0V, 1mA/1V, 2mA/2V, and 3mA/3V. If the line isn't correct, you can adjust it by changing the 'Current correction voltage', 'Set' it, and do a new measurement. 

A view of some other measurements are in the next pictures. A LED, a pnp transistor (BC213) and a npn transistor (2SC945). The last one in full graph mode: you get a single trace in the opposite quadrant, without...