Introduction: Variable AC Power Supply (Variable Transformer)

I bought an open frame Variable transformer (Variac) from a local flea market for $5. Would need to package it for bench use so decided to use materials at home to create a variable AC supply using a Harbor Freight multimeter (free or less than $4 new), a power switch, IEC socket, fuse holder, a 9V power supply (wall wart) that I had rescued from various scrap electronics. I also made a wooden box from scrap wood (the backpanel of a shelf) as a chassis.

Step 1: Making the Box for the Variable Transformer

  • I laid the parts (the Harbor Freight multimeter), the Variac, the power switch and the AC receptable on a piece of wood to get a sense of the dimensions. Some of the other equipment in my lab is 6 inches tall and 6 inches deep and I thought I would use those two dimensions so the Variable AC supply box would fit in with the other stuff. Six inches was also tall and deep enough to fit the Variac. So finally I decided to use a 12 inch x 6 inch wide front panel. The box would be 6 inches deep.
  • I cut the 1/4th inch hardboard panel to the following dimensions. 2 pieces of 6 inches x 12 inches for the front and back. 2 pieces of 5.5 inches x 12 inches for the top and bottom. 2 pieces of 4.5 inches by 5.5 inches for the two sides.
  • I then drilled the required holes (for ventilation, for the chassis handle (from an ikea drawer pull), for the fuse holder and the power switch - see diagram) and the cutouts for the AC receptable, for the multimeter display, and for the IEC socket on the back panel.
  • I then laid out the parts on the front panel and marked areas where I could place reinforcing pieces of wood. I also marked the location of the holes for the bolts to hold the heavy variable transformer. To the front panel I glued pieces of plywood to reinforce the panel and to serve as an anchor point to screw in the side panels.
  • The box was temporarily assembled with clamps and then holes were drilled for screws to hold the box together. The screws would go through the panel into a glued wooden block on the adjacent panel tying it all together.
  • The wooden panels were painted inside and out with some leftover black enamel paint.

Step 2: Connecting the Parts to the Front Panel and Wiring Components Together

  • The back panel from a Harbor Freight multimeter was removed, holes were drilled and the meter was attached to the front panel by screws. The picture shows a test fit before the painting was done.
  • The back panel was drilled with two holes, one to allow the AC leads to enter the meter from the back and one to allow the 9V supply leads.
  • Wires connecting to the 9V battery clip in the meter were cut and wires from the output of a 9V plug-in power supply were threaded through the hole of the meter backpanel and soldered to the battery leads of the meter. The polarity of the power supply was checked before soldering with a multimeter to make sure + went to red.
  • Connections were covered with heatsink tubing.
  • Two AC wires were threaded through the second hole in the back of the meter and then each wire was pretinned and soldered to the back of the test lead sockets (see diagram).
  • The meter was now attached to the painted front panel with the small screws. The back panel was snapped in.
  • The AC receptable was next attached followed by the power switch.
  • The IEC socket and the fuse holder were attached to the back panel.
  • A wire was soldered from the live terminal on the IEC socket (marked L) on the back panel to the internal (central terminal) of the fuse holder on the back panel. The outer terminal of the fuse holder was connected to the switch with a wire long enough to allow the box to be assembled. A wire from the neutral terminal on the IEC socket was attached to terminal 1 on the transformer (see wiring diagram). I used crimped ring connectors to connect all the wires to the transformer.
  • Please follow the wiring diagram for more details.
  • Wires to the plug in supply were soldered to the metal prongs on the supply. Heatshrink tubing was used wherever possible.
  • Photos show wiring to the transformer, to the power switch, and to the AC receptable.

Step 3: Final Assembly

  • The wiring was checked carefully with a multimeter to make sure that there were no screw ups.
  • The wiper for the variable transformer was then inserted from the back of the transformer (photo) and the black front knob that holds the shaft of the wiper was attached onto the front panel.
  • The panels were now assembled.
  • A 10 A fuse was inserted into the fuse holder in the back panel (photo)

Step 4: Testing

  • The fully assembled variable AC transformer unit was then plugged in with an IEC cord and the voltage was checked at various settings.
  • One mistake I made is that I reversed the live and neutral leads on the transformer, so the voltage increases when I turn the knob to the left rather than decreasing.
  • I most likely will correct this by switching the wires connected to terminal 2 on the transformer with terminal 1.
  • I applied an additional coat of the black paint and let that dry.
  • So for maybe about $10 in parts and not much labor I have now got an incredibly useful device that costs a few hundred dollars on ebay and quite a bit more if I bought new.