Curve Tracer Adaptor

This unit employs a dual trace oscilloscope with X-Y function as a display to test and demonstrate the action of circuits and components such as transistors, diodes, zener diodes, and terminated and unterminated transformers. A low frequency sinewave (ie 10Hz - 1kHz) is applied to op amp IC2a via potentiometer VR1 to set the "X" and "Y" levels for the X-Y display on the scope. The output of IC2a is applied to the X input via R4 and IC2b and also to Probe 1 via the contacts of relay 1. IC2b provides a low impedance drive for the X input and also isolates the X input cable capacitance from probe 1. The current flowing into the probes develops a voltage across R4 which is processed by IC2d and applied to the CRO Y input to represent current.

Circuit diagram:
Curve Tracer Adaptor circuit schematic
Curve Tracer Adaptor Circuit Diagram

The scope display thus represents an X-Y graph where voltage across a circuit under test is displayed on the X axis (horizontal) and the current though it displayed on the Y axis (vertical). With a calibrated scope this equates to 1mA/V. IC1 and a relay are included to enable two probes to be used and comparisons made between a known good device and a faulty one. The relay should be a low capacitance reed type. By using the scope’s X and Y gain controls, the sinewave applied to the device under test should be adjustable from a few millivolts up to 24V peak-peak to get a very useable display.

Thus, the unit can be used on voltage sensitive devices and at the other end of the scale apply enough voltage to check the operation of, say, a 10V zener diode. Note that all devices should be tested in the unpowered condition. If used for in-circuit tests, the effects of circuit components will need to be taken into account. Shielded coax leads should be used for the X and Y inputs and the probe leads should have zero resistance. Normal scope probes should not be used as these usually have significant built-in resistance which will interfere with measurements.
Author: R. Rayner
Copyright: Silicon Chip Electronics