Curve Tracer Adapter


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.

Curve Tracer Adapter

Curve Tracer Adapter

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


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