In the previous Part 4 of this series Build Your Own Curve Tracer, Part 4: TPA202 DVM and Supplies, the TPA202 circuit description was completed, describing what it is. In this Part 5, we look at how to use it and for what purposes.
The TPA202 is line-powered and has a rear power switch. Commercial power supplies can be substituted for the Power Supply of the circuit diagram. (For line-powered circuits, dangerous voltages are present and the usual safety precautions apply.)
The front-panel has the following controls:
Input Port Voltage (IVDR) - 10-turn pot that adjusts the INP voltage source. The INP range switch reduces by about ×10 the output voltage for more control resolution at low voltages (< 1 V).
Output Port Voltage (OVDR) - 10-turn pot that adjusts the OUT voltage source. The OUT polarity switch selects output voltage and current polarity.
DVM INP or OUT switch - selects either input or output port for DVM display.
DVM V or I switch - selects whether port voltage or current is displayed.
Input Port Current Sense Range - IISN ranges are selected. The range settings also select port source resistances.
Output Port Current Sense Range - OISN ranges are selected. The range settings also select port source resistances.
Input Port (INP) Output - BNC with COM ground outer conductor.
Output Port (OUT) Output - BNC with COM ground outer conductor.
LED analog output-port voltage indicator.
The front-panel outputs are the 3.3 decade LED display with sign (polarity), the three current unit LEDs of mA,
A, and nA, and the analog output voltage LED indicator.
A simple transistor test fixture is constructed by connecting INP, OUT, and their COM terminals to the three transistor terminals of a transistor socket or 4-pin female header with connections ordered as EBCE. (For completeness, a 5-pin header with EBCEB is recommended.) Transistors with center base terminals are plugged into the left three pins as EBC; transistors with center collector terminals (such as some JFETs or MOSFETs) plug into the right three pins as BCE.
The OUT output of the TPA202 connects to the collector or drain (c) and the INP output connects to the base or gate (b). The outer-conductor grounds both connect to the emitter or source (e) terminal which is also the common (COM) terminal. These various sockets can be mounted on a small box as shown below. (This particular TPA202 prototype is in a box instead of on rails and was found difficult to access in development - hence the change in packaging to a rail-mount assembly.)
To operate, power on, and before inserting the transistor or other DUT into the test fixture, select INP and OUT current ranges for the measurement. Insert DUT. Then select input port to set input conditions by setting the DVM INP to be displayed. Select current or voltage source. To set current or voltage, select the DVM input-port V or I display and adjust the front-panel input-port knob to the desired value.
Set the DVM display to output port voltage. Adjust the front-panel output voltage until the DVM output-port voltage display shows the desired voltage.