Tech note: making your own V-I plots at home

When working on my latest book, The Secret Life of Circuits, I wanted to keep the artwork real. My beef with the diagrams in popular electronics textbooks and online tutorials is that most of them are fake. At best, they’re retraced from ancient texts; at worst, they’re sketched from memory and can be charitably described as “inspired by true events”:

An assortment of V-I plots for diodes, collected on the internet.

The Secret Life of Circuits contains about 290 original illustrations and does its best to avoid such shenanigans. I painstakingly gathered real data for everything from quartz crystal frequency response, to battery discharge curves, to signal reflections in a 100 ft run of coax cable strewn around the workshop, to the behavior of vacuum tubes.

A snippet from the sample chapter, available here.

Most of it was straightforward to capture, but I can’t say the same about the parametric plots that show the relation between voltage and current in semiconductor devices. In some portions of the curve, the currents are too miniscule to record with the most common graphing instrument, the oscilloscope. In other portions, the current suddenly skyrockets — and before you know it, the device lets out the magic smoke. Even in the in-between region, there’s no respite: the characteristics of semiconductor junctions change with temperature, and that includes self-heating due to currents as low as 1 mA. Do nothing and watch the point on the oscilloscope screen drift away.

To tackle this problem, I eventually ditched the oscilloscope in favor of a benchtop multimeter (DMM) and pulsed power from a lab supply. The perk of the multimeter is that it can easily measure down to microamps and microvolts; the perk of pulsed power is that heating-indued drift can be kept in check.

Oh — I would also submerge the device under test in non-conductive liquid for cooling purposes. Mineral oil is a sensible choice, but many other options should do:

A close-up of the final setup.…