Homebrew Curve Tracer Competes With The Big Guns

When we first saw the VBA curve tracer, we thought it might have something to do with Visual Basic for Applications. But it turns out it is a mash up of the names of the creators: [Paul Versteeg], [Bud Bennett], and [Mark Allie]. [Paul] designed an original prototype back in 2017. Since then, the project has grown and lessons were learned. The final curve tracer is pretty impressive in more ways than one.

If you’ve never used a curve tracer, they allow you to characterize components using their characteristic curve of voltage versus current. You use an oscilloscope as an output device. This instrument is often used by engineers trying to understand or match devices like diodes, transistors, or — in some cases — even tubes. So if you want to measure the collector-emitter breakdown voltage, for example, or the collector cutoff current, this is your go-to device. You can also match gains in circuits where that matters (for example, a push-pull circuit where two transistors amplify different parts of the same signal).

If you want to understand more about how it works, there are a series of blog posts covering the evolution of the device. You can also find the design files on GitHub. There is also a handy post showing many types of measurements you might want to make.

This is a good-looking project. We’ve seen it done on the cheap, but slowly. Or spend $15 and do better. We doubt any of these have high enough voltages to do most tubes, but they made the same basic instrument for tubes back in the 1950s.

21 thoughts on “Homebrew Curve Tracer Competes With The Big Guns

  1. I have never used Curve Tracer, although a number of place I worked had the big Tektronix one.
    I don’t understand how taking a device beyond its breakdown or avalanche regions doesn’t damage it.

  2. ” You can also match gains in circuits where that matters (for example, a push-pull circuit where two transistors amplify different parts of the same signal).”

    That would help people restoring stereo amps from the 1960s and 1970s, as the original output drivers and transistors are NLA.

  3. In the seventies I remember a few curve tracer projects, real ones not just “the Octopus”. But it was a transition time, “transistor tester” of one form or another were common. I think there was still some residue of early transistors that you might need to push the specs, or buying floor sweepings and needing to critique them.

    The craze soon faded.

    You also needed an oscilloscope, and I’d say they weren’t common among hobbyists back then, at least not DC coupled.

    The concept is certainly simple.

    1. My main use for a transistor tester is to identify the pinout and characteristics (WTF is it) of an unknown 3-pin device on a PCB undergoing reverse engineering. Often you can tell what the pins are from what they are connected to, but it’s harder to determine if it’s a FET, or BJT, or perhaps a diode pair.

  4. “When we first saw the VBA curve tracer, we thought it might have something to do with Visual Basic for Applications.” When I first saw the curve, I thought it might have something to do with a lady that earns money from doing something immoral (another joke understandable by Polush people).

  5. Looking for a modern (like this) vacuum valve tester or better, curve tracer, if found nothing. Only the big, extremely rare (in EU at least) tube tester sell for >1000USD€ seem to exist. I – like many people I guess – sit on a pile of vacuum tubes, some rarities for the 30s and cannot check these. Anybody knows of a project like that ?

  6. It’s a beautiful build! Fairly busy panel but I suppose that’s the physical case limitation. I also appreciate the project page being complete from start to finish rather than broken up into linked short chapters (here’s looking at you, Amazon AWS documentation.. awful).

    I wonder if it could be made more Huntron Tracker-like with the addition of a small screen. But not a CRT or OLED (like so many other projects) but a whimsical miniature laser galvanometer with a rectangular piece of ground glass as the screen. Building on this example:

  7. Hi, how it’s presented the results (the lines) of this curve tracer ? It’s on the computer screen ? If so, it connects directly to the monitor ? … or through usb and uses some kind of software to show the curve ? If not, whitch kind of display is used ? Thanks in advance.

  8. I found and restored an old Heathkit transistor curve tracer a few years back. It would not work with a digital scope, only an analog one due to the persistence of the phosphor.

  9. What you really want is one or more programmable Source Measure Units (SMUs), sometimes called Sourcemeters. Here [1] is a really nice open source clone of the venerable Keithley 236 SMU. Here [2] is a tear-down of a real Keithley 236 SMU. A good working original 236 will set you back roughly $1.5K – $2K, more if it’s calibrated. With a programmable SMU you not only setup the curve tracing sources, you read the results back out. So using something like Gnuplot [3] there is no need for an oscilloscope.

    1. jaromir-sukuba/J-SMU


    2. Keitley 236 Source Meter – Teardown and Review


    3. Gnuplot


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