Transformers made for powering large neon signs are inexpensive and very reliable. Most commonly, the secondary is center-tapped, which prevents the use of its full peak-to-peak output in applications where one of the terminals needs to be grounded.
In the power supply described in this post, I took out the high-voltage transformer out of its metallic enclosure to isolate the center tap from ground. This requires very careful application of a thick insulated layer to the transformer. I used a full can of Shellac to patiently coat and re-coat the transformer.
I built a thick Delrin insulating bracket for the modified transformer, and placed it inside a grounded metallic enclosure. I wired it using high-voltage cable that is further insulated by thick silicone tubing. I liberally use high-voltage putty (GC ELECTRONICS # 10-8882) for insulation. I purchased the high-voltage connectors at Surplus Sales of Nebraska.
To obtain a variable AC output, I use a 500 W variac. It must be remembered that neon-tube transformers are commonly designed to first produce a voltage spike to start discharge in the gas, and then enter constant-current mode. As such, the voltage will vary quite a bit depending on the load. A one-to-one voltage relationship between the variac’s output (the input to the neon transformer) and the neon transformer’s output is only possible for very light loads.
Floating DC output is generated by full-wave rectification and filtering of the neon transformer’s output. I used eight NTE542 high voltage diodes (commonly used in small microwave ovens) to build a full-wave-bridge rectifier as shown in the schematic diagram above. Twenty 0.01 uF @ 10 kV capacitors are used to filter the rectified voltage. I used a surplus panel meter (labeled in kV, probably from an X-ray machine) with a string of resistors to measure the output voltage.
I originally built this power supply to run a Chinese DC-powered CO2 laser tube, but it has proven to be a great addition to my collection of high-voltage power supplies for running my plasma generator.
DANGER! Please note that this is a dangerous device! It produces high voltages which can cause very painful or lethal electrical shocks. In addition, spark discharges can be produced which can ignite flammable materials or volatile atmospheres. Remember that the capacitors retain charge long after the power supply is switched off. Thoroughly discharge them before touching the high voltage rails!