#245 General Curve Tracer

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.