Hack Your Heathkit To Trace MOSFET Curves

[TRX Lab] has an old Heathkit model IT-1121 curve tracer, and wants to modify it so he can plot the I-V curves of MOSFETs. For the uninitiated, curve tracers are used to determine the precise characteristics of components by measuring the output for a set of specific inputs – either voltage or current depending on the device you’re testing.

The IT-1121 was introduced in 1973 and supports bipolar and FET transistors of types NPN, PNP, N-channel, and P-channel, along with various other semiconductor devices. But [TRX] wanted to enhance the tester to deal with MOSFETs as well.

The IT-1121 is very flexible, and has selector switches for all the usual polarity and sweep settings — Heathkit also sold a model IT-3121 in later years, but this seems to have been the same basic tester. [TRX] found two shortcomings when plotting the I-V curve of MOSFETs. First, there is no way to apply a Vgs threshold voltage to the curves. Second, when set for FET testing, the polarity of the gate voltage stair step waveform doesn’t match the desired polarity of the drain-source voltage.

In the video below the break, [TRX] first walks us through some of the reasons you’d want a curve tracer in your lab. In the next part of the video, he breadboards up the modification for testing, and finally buttons it up and installs it. Implementing the modification was pretty straightforward. [TRX] designed four op-amp circuit that adds the adjustable offset and a switch to toggle the polarity of the gate voltage waveform. The whole thing fits on a small breadboard inside the case. Two holes are drilled in the panel for the potentiometer and switch.

There’s no GitHub repository for this project, but he presents the full details in the video and says viewers are free to make snapshots of the schematics and layout if they want to build their own.

Modern I-V curve tracers are pretty pricey. Even used, decades-old professional curve tracers are above the budget of most home and small office labs. If you have, or can get one of these at a decent price, this would be a modification well worth considering. You might also consider a home-brew tracer, like this one we covered last year.

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