Since Tesla coiling can involve fair amounts of power and voltage, we really like the idea of having some type of control over the situation. Variacs allow us this opportunity to control the voltage going to the transformer which supply the voltage to our coils. Smaller coil supplies like NST's dont really need a variac, but it is sure nice to have one when tuning a coil in for the first time and when experimentation is being performed while changing major parameters of the coil. Larger coils such as piggie powered coils definately benefit from the use of a variac. There are variacs out there that can handle the power requirements of small pigs, but when we start needing 25+ amps of power to fuel the piggie, variacs need to be paralleled. Paralleling variacs allow one to "equalize" the current flow between multiple variacs, thus increasing the amount of current available to be used.
The item needed to parallel a set of variacs is called a paralleling choke. It consists of a toroidal ferrite core of a largeish size with 2 sets of windings on it. Most variac companies offer such an animal, but the cost of these chokes are somewhat on the high side ( 150$+ each). What can we do to avoid spending this much on such a simple item?? We make our own. Find yourself a small variac ( 120 V 2-7 amp range check out Ebay for some) and remove the existing windings from it. Next step is to get yourself some # 8 solid Cu. wire with the THHN type of insulation. ( you can use # 10 THHN solid Cu. but # 8 is better) , wind 8 - 10 turns of wire thru the core hole on one side of the toroid form, and then again wind a second winding ( in the same direction ) on the other side of the core with the other wire. ( 2 lengths of wire will be needed).
Wiring this choke to the varaics consists of taking wire #1 and attaching it to the output of variac #1 and the wire #2 attaches to the second varaics output. The wire that exits the choke ( the other ends of #1 and #2) are placed together as a combined output from both variacs.
compared to the picture... wire #1 is H1 and H2 wire #2 is H4 and H3 hook H1 to variac #1, H4 to variac #2, H2 and H3 connect to each other for the total out.
here are some pictures to give you an idea of what is going on...
