This is the resistive ballast I built for my 5KVA pole transformer. It consists of three, 240V/3000 Watt hot water heater elements immersed in a tank of water. The heater elements are mounted vertically on the sheet metal lid of the water tank and hang down into the water. I bored the holes for the heater elements using a hole saw and a hand drill. Also on top is a 2-pole, 277V/30A power switch housed in a galvanized junction box. Logically the heater elements should be located near the bottom of the water tank so that cooling can be assisted by convection. However, I built it the way I did for a good reason. The water tank is one of several standard US military .50 caliber ammo cans in my junk collection. The lid isn't hinged--it is held on only by a latch on each end, so it can be removed in a jiffy. If the tank of water gets too hot, I can simply pull the lid off and move it to another ammo can with cold water in it and be back in business in seconds. I think I'll stick a meat thermometer through the lid so I can monitor water temperature.

Using short jumper wires, I can select various combinations from very low to fairly high AC input power. Possible combinations include:

3 elements in series for 1000 Watts

2 elements in series for 1500 Watts

1 element in series with 2 parallel elements for 2000 Watts

1 element for 3000 Watts

2 elements in parallel for 6000 Watts

3 elements in parallel for 9000 Watts

Resistive ballasts do have certain shortcomings. One drawback is inefficiency. A large fraction of the total AC input power is used to heat water, leaving the remainder to power the Tesla coil. Another drawback is voltage loss. The average AC voltage output of the pole transformer is reduced, especially in the early part of the tank capacitor charging cycle, because much of the 240VAC is dropped across the heater elements. However, as the tank capacitor approaches peak charge, the voltage drop across the heater elements becomes less significant and the pole transformer output approaches its rated peak potential.

The inefficiency problem can't be eliminated, but it can be mitigated by operator technique. I can simply run more power. If, for example, I wish to stuff 3000 Watts into my Tesla coil, then I'll set up the ballast to draw 6KW. What do I really lose by this? Electricity is cheap.

On the plus side, the resistive ballast is simple, predictable, and cheap. Anyone with the ability to apply Ohm's Law can design one in a few minutes. Also, I have roughly zero chance of unwanted power surges or kickbacks in my mains. Besides limiting current, the high resistance adds "smoothness" to the power supply circuit. I didn't even bother to test my ballast as a lashup job. I just built it in finished form with full confidence that it would work fine on the first try, and it did. The ammo can and power cord were already in my junk collection. The heater elements were $3 each from a surplus electronics dealer. The only things I paid retail price for are the power switch, the 240V plug, and the galvanized junction box.