On a recent thrift store expedition (you never know what you’re going to find), I came across an interesting-looking box. It was apparently designed to be wall-mounted, and had three bolts for electrical contacts and a large heatsink.
It was apparently an electrical device of some sort — if the tie-point bolts didn’t give that away, the large heatsink would. The heatsink also implied that it was expected to dissipate a fair amount of power.
The three leads on the top were marked 1, 2, and A (A being the center bolt). My initial guess was that it was a rectifier of some sort — perhaps a battery isolator for marine or automotive applications. This was confirmed by an ohmmeter test: connecting the negative lead to A showed open circuits to pins 1 and 2, but reversing the leads showed 33 to 35 kilohms resistance.
Diodes, being nonlinear devices, can produce strange resistance readings, so I did a quick I-V curve test using a lab supply. Current was near zero up to 0.6V, then rose very sharply to over an amp near 0.7V. Silicon diodes, then, from A to both 1 and 2. It’s a rectifier.
Now that I had the puzzle basically solved, I felt it was no longer cheating to do a Google search. A few links later, I found something very similar: a two-battery, 70-amp battery isolator. Mystery solved!