A Variac or autotransformer is something I've always wanted. They're an item I just don't see every day though in my travels. One day I did happen to come across one while I was shopping at a flea market. The fellow selling it wanted $5 for it which I thought was fair, so I bought it. Only thing was all I got for $5 was the bare Variac and a panel plate for it too.

In this project I had a real well I never moment that I hope I can share with everyone that reads this. Enjoy!

Step 1: Well Now I Have a Variac

Sure I'd heard about Variacs, or autotransformers as they are generically called I even had a fair idea what one did, but not exactly how it does it. Just what makes this gadget tick, and more importantly how do I hook it up?

It can't be all that hard can it? After all a Variac is just an adjustable transformer and I've hooked up plenty of transformers in my time. All any transformer is is just a coil of wire right? The five studs that stick out of the thing are obviously the connection lugs for it. Why I'll just get out my trusty multimeter and ring the thing out.

Well those were my thoughts at the outset of this project but once I began things quickly descended into what can only be described as chaos. those 5 studs yield 20 possible combinations of pairs of connections and I rung some sort of a reading out of every one of them!

The graphics on this page are digital reproductions of the tables I made on a piece of paper while I was in the initial discovery phase of my Variac. I made two of these, because of course there is a control on a Variac and these readings change as the control is manipulated. The numbers in the tables are Ohm readings I recorded measuring pairs of terminals to each other.

Measurements that are supposed to match and don't is because my trusty multimeter isn't the best multimeter I guess.

Step 2: Variac Schematic

After much puzzling over my tables I'd made this schematic. It is my best guess as to how the terminals on my Variac are connected.

Step 3: Bench Test

It still took me quite some time to find the correct connections for how I wanted my Variac to operate. Depending on connection combinations I could get 132 VAC out of it, or have the voltage rise if I spun the control shaft clockwise, or counter clockwise. Turns out I only wanted a top voltage of input voltage with the voltage rising if I spun the control clockwise so this is how I had to hook my Variac up.

Any other connection than this got me something else I did not want. What surprised me was that switching the HOT for the neutral on leads 1 and 4 made the Variac dial reverse its operation from clockwise raises the output voltage, to counter clockwise raises the output voltage. And here I always thought AC was AC. Not with a Variac!

Step 4: Making My Case

Now that I am seeing things work the way I want them to it is time to box this deal up. I don't want my case to be any bigger or bulkier than it needs to be so I made a custom enclosure. A simple U shape was bent out of sheet metal with tabs. A bottom was cut to fit out of a scrap piece of wood. Holes were drilled into the top of the U chassis to mount the Variac. Holes were added on a side for a fuse holder and HEYCO cord strain relief. A hole was drilled into the top for a toggle switch.

Two more sides needed to be made for the remaining sides. I chose a scrap piece of chassis stock to cut the back out of with some vent slits in it, and used some phenolic sheet stock to make a front panel. The entire case was assembled with self drilling sheet metal screws.

The sheet metal U was painted with spray paint prior to assembly. In the image the U can be seen after it was bent and the Variac mounting holes were drilled. Also shown is the panel plate and a homemade knob I made out of a plastic gear and plastic screw top hot glued together.

Step 5: Adding a Voltage Meter

The more dials and switches and meters I can throw at a project the cooler it is I think so in that vein I put a voltage meter onto my Variac. The dial is not completely linear throughout its range so the meter does actually serve somewhat of a useful function on this project. Although with it's hand written scale I wouldn't consider it the last word in accuracy.

Step 6: Meter Schematic

I drew out the schematic for my meter. Values used will depend on what parts are available so will have to be adjusted accordingly. As an example I will list the parts I used

Shunt Resistor 680
Trim Resistor 50K
Step Down Transformer 9V Secondary
100 ma Meter Movement

Adjust trim resistor for full deflection at full output voltage.

Step 7: What Good Is It?

So far about all I've done with my Variac is dim light bulbs and test unknown transformers. In this image I am testing an unknown transformer and using a light bulb as a current limiter. I am monitoring the voltage with a multimeter, and the current with an amprobe. The current draw was so low in order to get the meter to deflect I had to wrap the wire through the clamp several times.

Variacs are not isolated so they are super dangerous to play around with. Be careful!