I'm messing with a little microprocessor board I've been putting together, and tried powering it up for the first time today. It promptly blew the 0.5 A on-board fuse.

Can't find any obvious shorts. Tried "forcing" it with a croc lead bridging across the blown fuse. Fortunately I'm using a bench power supply possessing both an Ammeter AND current limiting, which, very usefully, works down to tens of milliamps, or less. (!

)
The chip's booted up so must be working, ditto the 78L05 regulator etc. Correct Voltages. Neither are getting hot. There's a ULN2003 driver for output, not hot either. Yet it's pulling nearly 1 Amp. I decided to let it have more current and see if anything starts smoking.
Yep! An electrolytic, on the 12V supply input. Wrong polarity!
