Refurbishing a Variac
Variacs are useful tools but are highly dangerous when not handled properly. The output is not isolated and they also produce large voltages (+140AC!). This is more than enough to kill you should you come in contact with its output or anything connected to its output!
I purchased a variac from an institution that had been used for demos/labs ect. It arrived as described, dirty and used but fully functional and apparently with a new AC plug. Variacs are tremendously useful to the hobbyist who wishes to run their circuits at AC voltages and/or above the standard DC power supply outputs (5V, 12V, 30V). The transformer is an autotransformer (primary connected to secondary) with a center tap that adjusts across all the windings. Because it is an autotransformer, the secondary is NOT isolated from the primary and therefore any circuit you connect to the Variac will NOT be isolated from the mains! Many circuits need to be isolated from the mains with a transformer not configured as an autotransformer. I decided to clean it up and check if anything needed attention before I used it for my own projects.
My process for refurbishing was to take the variac apart and clean/check everything. Since there are moving parts inside (slide with brush), you can expect there to be mechanical wear.
The teardown began with removing the front cover which was two flathead screws. Notice there are multiple connections with only some being used. This is because many variacs can be wired for either 120 or 240VAC. I used masking tape and a pen to mark where all the wires had to be reconnected. It is critical these wires go back in the same spot.
With the front cover off, we can clearly see the connections available. In this unit, terminals 1 and 3 are used for input from the mains. Terminal 3 is the output connected to the center tap and the bottom left screw is used to ground the case and provide a ground option to the output on the front cover.
I removed the dial knob and then the main cover. The dial knob is held in with two set screws and the main cover with three flat heads. The disk with the brush and the center shaft are connected. To remove the whole thing I had to remove the snap ring from the bottom.
As you can see, it is very dusty and dirty from years of use.
A better shot to show you why it needed some cleaning, Notice the damaged wires on the upper right hand side. This happens when too much current is drawn and the wires melt down and destruct. You can see some of the wires were cut around the area indicating that someone tried to keep the unit operational by removing the affected area.
I cleaned everything with some Windex, scotch pad, and penetrating oil. Just a little oil and some muscle is all you need to clean the contacting portion of all the wires. I decided to leave the blown out section as it was. The transformer was still functional throughout its entire range just had a little less adjustability around 119VAC
The slide brush looked to be in pretty decent shape which is fortunate as I did not feel like tracking down a new one.
After putting the whole unit back together and cleaning the case, I now have a slightly better performing variac.