Power Box

Introduction

The completed power box. I built a pedestal from some construction pine, which I placed at the corner of my workbench.
My power box combines the functions of an isolation transformer and a variac, with an additional full-wave rectifier and a set of built-in instrumentation. It can be used as an isolation transformer, a variable AC power supply with or without isolation, or a full-wave rectified variable AC power supply, also with or without isolation. It also has a built-in circuit breaker for fault protection.

Each of those functions is useful for power supply testing. The rectified AC mode can be injected directly into the input capacitor of a power supply under test. The isolation modes can be used to isolate instrumentation, or to float the AC supply and allow for arbitrary grounding. The variable voltage modes can be useful for slowly powering up suspect line-powered electronics, reducing the chance of a catastrophic failure. And by adding an external capacitor, I can even use this as a variable voltage DC supply, although due to the exceptionally bad power factor that would result, it's not always a good idea.

The resulting box is almost 100lbs. Overall, the design is somewhat archaic but extremely functional. I like the 1950s retro military look.

The Story

A few years back, I picked up a General Radio brand 10A variac from eBay. Unlike standard bench variacs, this one did not come with a case. It also seems a lot larger than a similarly rated bench variac, which leads me to believe that the rating is quite conservative.

When I bought it, my intention was to build myself a simple AC power supply with isolation. I also won an auction for Simpson analog panel meters, also with the intention of using them on the power box.

Unfortunately, I was unable to find a suitable isolation transformer or enclosure. Years passed and the project went nowhere.

Finally, I found the last two major components. The isolation transformer is a Hammond 1200VA 1:1 toroid, which I got for a song at a swap meet. The chassis started life as a Honeywell safe that I found at a discount store. It was quite a lot of 1/8" plate steel for the price, and it seemed to be the perfect size.

Construction

A test fitting of my power box. The variac is mounted, but the isolation transformer is sitting on the floor to the left.
My first task was stripping the safe down. It came with a digital keypad on the front and locking mechanisms behind. Most of it came off with a screwdriver, and I used a Dremel to hack off the steel bits from the locking mechanism. There were a few small holes left behind, but with some careful layout, the components on the front panel could overlap those.

I captured CAD models of all the major components in QCad, and printed copies of the layouts. My printer is reasonably accurate, so I simply glued the paper layouts to the front panel with some rubber cement to locate all the drill centers and cut lines.

Cutting the front panel was very challenging to do but not very interesting to write about. Much use was made of a hand drill, a drill index, a step drill bit, a Dremel and a right-angle attachment for the Dremel. I burned through a couple dozen Dremel cut-off wheels in the process, but it worked out, a little at a time. Suffice it to say that 1/8" steel is challenging to work with.

I mounted the isolation transformer to the rear wall with heavy duty cable ties, and bolted the variac to the front panel. I installed all the remaining components, and wired it all up. There was a lot of wiring and heat-shrinking involved, but it was uneventful.

The small labels were made on a Brother labelmaker, while the big variac scale was made using my own meter scale software, as featured in Circuit Corner 5. I modified the software slightly to support colored areas, rotated labels, and a few other minor details.

During initial power-up testing, disaster struck. The DC volt meter had been modified, removing the series resistor, and it blew up as soon as the power was switched on. Luckily, I managed to win an auction for some 1mA DC current meters in the same chassis, and with a little careful surgery, I repaired the meter.