Cheap external battery pack for a Sigma EF500 DG Super flash
A month or two back, I built a DIY external flash battery pack for my old workhorse Vivitar 283 flash. The battery pack allows the flash to recycle from a full-power flash burst in about 2 seconds, instead of the 10 seconds that it takes normal AA batteries. Shooting at less-than-full-power yields almost instantaneous recycle times, and near continuous flash use.
The downside of this is that the Vivitar is an old, low-tech, brute-force flash. There are few things you can make it do aside from unceremoniously barfing out a bunch of light when you push the shutter button. In addition to the Vivitar, I also own a Sigma EF500 DG Super flash. The Sigma is a newer, more high-tech piece of equipment, with many more advanced features and capabilities. It is higher power -- it rates a GN (measure of flash power) of 150 vs. the Vivitar's 120, understands Canon's advanced TTL-II metering, has fractional power settings, hi-speed sync mode, and second-curtain sync capabilities. In short, all the bells and whistles of a modern flash.
The main drawback of the Sigma was that it offered no external power port, so you could not (easily) hook up an external battery to the fancy flash. So you had two choices: you could use the fancy Sigma flash, with the slow-recycle AA batteries, or the fast-recycle DIY battery pack with the low-tech Vivitar flash. I wanted to use the fast-recycle DIY battery pack with the high-tech Sigma and have the best of both worlds. Unfortunately, nobody had shown how to do this yet.
Using the instructions (and battery pack) from the DIY page above, then taking a bit of info from the photos of the dissected Sigma flash on this page, then mixing in the male half of a teeny little metal sewing snap from my grandmother's sewing kit (thanks Granny! Dead 7 years and *still* helping me out!) I was able to rig the Sigma to make use of the external battery pack.
The key was to figure out which two (of the four) battery contacts inside the flash transferred power to the flash (and what the polarity of those contacts were). After that, you just had to lay the snap into the bottom of the battery compartment over top of the positive contact, since the contacts themselves are protected by a plastic lip. The nubbin on the sewing snap protrudes below the plastic lip, and touches the metal contact. Finally put the dummy AA batteries into the correct slots in the flash (positive-to-positive, negative-to-negative else bad things will happen), close the cover, hold it in place with something as high-tech as a rubber band, and you're all set.
For anybody wanting to try this themselves, I'll try to explain what goes where:
Tilt the head of your Sigma flash so that the unit lays flat on a desk (if it were attached to your camera, the flash head would be pointing at the ceiling), with the "Sigma" logo right-side-up so you can read it, and the battery compartment on your right-hand side. Open the battery compartment. The two battery chambers you want to use are the ones closest to the desk. The one closest to the flash bulb is the positive contact. The one closest to the flash shoe is the negative contact.