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Project Notes

#245 GeneralCurveTracer

Test a DC-powered general curve tracer circuit.

GeneralCurveTracer_protoboard_build

Notes

I’ve borrowed this general curve tracing circuit from stoneslice’s prototype and video. Rather than rely on an AC power supply, this uses a simple DC-powered oscillator to drive a test signal across the device under test. It features a DPDT switch to toggle and compare two devices under test.

X-Y signals are plotted on an oscilloscope to visualise the characteristic curve for the component.

  • X is the ground-referenced voltage at the anode of the DUT
  • Y is the voltage across the resistor at the cathode of the DUT, which is proportional to the current flowing through the DUT.

NB: see w2aew’s #49: Simple Component Tester using Oscilloscope - Octopus Curve Tracer for an example of a classic AC-powered curve tracer.

100nF Capacitor

The charge-discharge cycles result in an oval pattern:

curve_capacitor

10kΩ Resistor

Response is predictably linear:

curve_resistor

1N4730 3.9V Zener Diode

The curve clearly illustrates characteristic Zener diode behaviour:

  • Forward-biased, it behaves like a normal diode rectifier
  • Relatively low reverse breakdown (Zener) voltage

curve_1n4730

Testing Rectifier Diodes

If a single rectifier diode is placed under test, this circuit generally fails to produce the characteristic forward voltage drop curve. I believe this is because at low voltages (when powered by 9V battery), it never hits reverse breakdown and the 1µF C4 never gets to cycle. The result is that the test signal ends up getting pushed just below the forward voltage threshold.

My fix for this is to test rectifier diode in a back-to-back pair. This allows C4 to cycle and a nice forward-voltage curve to be generated. the downside is that the reverse voltage curve is simply the inverse mirror of the forward-biased curve.

1N5819 Schottky Barrier Diode

Schottky Barrier Diodes exhibit extremely low forward voltage drop. NB: this curve is of two 1N5819’s back-to-back - see note above.

curve_1n5819

1N4148 High-speed Diode

A common rectifier diode. NB: this curve is of two 1N4148’s back-to-back - see note above.

curve_1n4148

Construction

Breadboard

The Schematic

GeneralCurveTracer_protoboard_build

GeneralCurveTracer_protoboard_layout

Credits and References

About LEAP#245 BJTOscillators
Project Source on GitHub Return to the LEAP Catalog

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is my personal collection of electronics projects, usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting (IMHO!).

The projects are usually inspired by things found wild on the net, or ideas from the sources such as:

Feel free to borrow liberally, and if you spot any issues do let me know. See the individual projects for credits where due. There are even now a few projects contributed by others - send your own over in a pull request if you would also like to add to this collection.