FAQ Variac - how to connect to a neon transformer

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Richard Hull
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FAQ Variac - how to connect to a neon transformer

Post by Richard Hull » Fri Aug 02, 2013 5:58 pm

The "variac" or "powerstat", (manufacturers brand names), is an "autoformer". The autoformer and is a single winding type of transformer that allows the voltage of the line input to be smoothly varied at its output from 0 to over 110% of the input line voltage. For U.S. 120 wall outlets, that would be from 0 volts to about 140 volts. They can be wired for 0-120 volts variable as well, but most people like the ability to boost the voltage a little bit over line voltage.

There are 240 volt variacs as well that are similar and can vary their output smoothly from 0 to about 260 volts.

Your variac should be able to handle about a 20% greater volt-ampere (VA) rating than expected in your load. A good neon transfomrer should have its VA rating on the nameplate. In general, a 10 amp variac will handle any neon transformer made with capability to spare. However, if you plan to move past the demo phase and do real fusion, it is highly recommended to obtain a 20 amp variac. Many folks use an x-ray transformer for a real fusor and most of these are 220-240 volt transformers. This would mean you might obtain a 20 amp 240 volt variac if you are serious about fusion and only want to make one purchase of a variac. Note* a 240 volt variac can be used on 120 volts just fine with no re-wiring needed when going from 120 to 240, but a 120 volt variac will not accept 240 volts.

Most full featured variacs have 5 screw terminals on the connector panel in back. Some rare few have 3 terminals, some have 4 and also rare are 7 terminal variacs. These 7 terminal units are mostly found on rare 240 volt units.

I attach a single diagram that shows how to wire a normal 5 terminal, 120 volt variac, but would also be true for a 5 terminal 240 volt unit was well.

The terminal group is rarely numbered as in my diagram, but most variacs use the same convention and some include a schematic of the coil with lines connecting the coil to the respective terminal, (shown in my diagram). Most demo fusor transformers, especially neon sign transformers can easily be pushed to an over voltage of 140 volts yeilding a bit more high voltage. However, if you desire to just have a variable voltage out of 0-120 volt line voltage move the hot line input wire from 4 to 5.

Why the unused terminals? The variac usually sits on its base with the knob on top as show in my diagram. The variac's knob follows the convention of fully counter clockwise is 0 voltage and a clockwise turn raises the voltage.

There are usually three or four heavy tabs along the base to allow for securing the variac with screws. If the variac is to be mounted behind a panel where only a knob is to protrude, the shaft set screws are loosened, the knob removed and the shaft is then pushed through the bottm of the variac and the set screws retightened. A panel is then drilled for the mounting holes for the tabs and a hole fo the shaft. Now, the rotation is reversed all the way ccw is on full blast and turning it all the way cw is 0 volts. Not good! Now to return the order to be correct rotation and to correspond to what we expect, you need to take the neutral off of #1 and connect it to #5, the output hot remains on #3 but the input hot must be removed from #4 and go to #2. Pretty slick.

Richard Hull
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