DIY Variable AC Power Supply

For some reason, I decided my bench needed a variable AC power supply.

First I started by trying to reproduce the project at http://www.eleccircuit.com/ac-variable-power-supply-0-30v-3a/, but failed miserably. Either I did something wrong, or it just doesn’t work the way I thought it would. I got horribly chopped waves, and chops on alternating cycles of the wave. When asking around, people told me ‘well, duh! why didn’t you just use a variac’ ?

This got me started on building my own supply, which turned out to be pretty simple:

• Variac, part number 149324, from Jameco electronics
• 28 VAC transformer, part number 149797, from Jameco electronics
• a relay and socket, courtesy of my junk box
• 12 VAC transformer, part number 273-1365A, from radio shack
• AC panel meter from random China eBay seller
• Metal instrument case, 209358, from Jameco electronics
• Double binding post, 125197, from Jameco electronics

Here’s a picture of the completed power supply:

AC power supply, inside view

Here’s the front view:

Variable AC power supply, front view

My whiteboard from the video, describing the design:

variable AC power supply, white board schematic

The power supply provides approximately 0-30 VAC at the front panel binding posts. This is fully isolated due to the 28 VAC transformer. At the back panel, there’s a standard AC receptacle where 0-130 VAC is available (non-isolated). The toggle switch on the front of the supply selects which of the two outputs is enabled (0-30 VAC on the front, or 0-130 VAC on the back)

Not shown in the diagram is the fuse, which is actually pretty darn important if you want to protect your variac. You want to fuse both the primary and the secondary of the variac.

The small radio shack transformer is used to supply the ~ 6 VAC that’s needed for the digital panel meter. The relay is used to switch the DPM between the front panel binding posts and the back panel receptacle.

Scott