Curve Tracer

[martin manning]

I found some time to assemble this thing for fun this weekend. Often called an “octopus,” it’s a diagnostic tool used with a ‘scope to display current vs. voltage in X-Y mode. These are commonly powered by a filament transformer, but using a function generator lets you see frequency effects, like the two diode traces below.

The first one is at 100Hz, and it shows about 0.7V forward voltage drop at around 3mA. The second one is at 10kHz, and it shows the on-to-off transient conducting in the reverse bias direction for a bit. This is a used 1N4007 that was lying about on the bench. See? You don't really need fast diodes for a 60Hz supply! :^)

Characteristic patterns are produced when different things are connected to the test terminals. An open circuit makes a horizontal line (all voltage, no current), and a short makes a vertical line (all current, no voltage). Resistors show a diagonal line indicating a linear current-voltage relationship. Reactive components like caps and inductors make ellipses, showing the phase lead/lag between current and voltage. Transistor junctions and Zener diodes (if you can reach the reverse breakdown voltage) produce a dog leg.

I’ve hooked these things up with clip leads before, but a Hammond enclosure and a handful of other parts makes a quicker, neater set-up (the tangle of the free-form version is the source of the name, I guess). In my version (as seen from the outside), the signal is fed into the left pair of binding posts, and the device under test is connected to the right pair. The left switch selects a current sensing resistor, 10k or 1k for 0.1 or 1.0 mA/V, and the right switch is a x1/x10 attenuator for the output. The left BNC is the X, and the right is the Y.

[katopan]

Looks great Martin! I've temporarily hooked up this sort of thing and captured a heap of diode curves, but mounted up in a box like that is very nice.

[martin manning]

Thanks, katopan; yes just plug up a few cables and it's ready to go. I thought since there are frequently posts here by people wanting to learn some basic electronics this might be useful.