Removing neon transformer center tap?

Question

I have recently acquired an old neon transformer (rated 12kV) for conducting some low temperature plasma experiments. It is the mid-point grounded style with a center tap on the secondary coil. I had overlooked these details and I really need to have a single lead at full potential, so I'd like to remove the center tap ground and ground one side of the secondary. I'm under the impression that this will allow the opposite lead to supply a full 12 kV potential (open circuit, that is).

Will removing the center tap and grounding one end pose any threat to the transformer (such as arcing through the potting material) and how should I ground the opposite end/is it necessary to install a separate ground rod or can I use mains?

PS the NST (neon sign transformer) is an Allanson 12kV 30mA (no-GFI)

Thanks in advance, Mitchell

Answer 1

Removing the center tap, even if it was something you could just do, would be a very bad idea. The beauty of having a center-tapped transformer is that each side of the tap only has to be rated for half the total voltage, whereas if you have the full voltage on one end you risk destroying the insulation, and thus the transformer.

Your best bet would probably be to find another transformer, one that is end-point grounded. Whether it be a neon transformer, an oil burner ignition coil, or (NOT RECOMMENDED), a microwave oven transformer, getting a transformer that is designed to have the full potential on one lead would be ideal.

So my recommendation would be to not remove the center tap, as it could cause all sorts of problems, even ones that you would never expect.

Answer 2

The transformers are not usually connected internally to ground or the casing if they offer a center tap, however, the transformer manufacturer could also build them and internally bond the casing of the transformer to one side of the coil, or, to the centertap, possibly tying the feed's neutral from the input to the same thing so the inductive coupling works across the transformer works.

When the coils are resonantly coupled, as in a neon sign transformer, this may not be the case because without labeling of it's input, it could throw the phasing off if the outer terminals were not connected correctly. An out of phase transformer will give no output, so, it is important to use a megohmeter and test to see if any of the taps are connected to the case before beginning.. This willl eliminate case electrocution if using an older transformer without gfci or short circuit protection and potentially save one's life. The danger of the center tap is that the amperage is double of the higher voltage and potentially lethal on the high side of the transformer. The transformers are basically rated at 30 ma-60 ma max. If we touch a 15kV transformer at it's maximum load before cutout, it could easily kill us. Do the math to see the potential wattage at 15kV x 60ma.. it equals 900 watts. That will definitely leave a mark!

The high voltage systems I work with for a living are much higher amperages, but, overall, not all that much higher.. Take a look at the wattage on these systems.. We work with 13,800 volts a lot. and, the line to ground voltage is 7970 volts. 7970 x 10 amps = 79,700 watts. This will blow a limb off and instantly kill somebody. 5kV is even more dangerous because the system feeds more amperage to equal the same wattage in it's output, so, with the 5kV input, which is either 4800v or 4160, to equal 80kW, we need 19.2 amps instead of 10 amps. That's double the damage because the amperage in the system is what decimated the layers of skin , other tissues and nerves, all basically doing sdamage the same way, however, the 5kV may not allow one to walk away from the electrocution, the 15kV may.. Don't go thinking this is possible because high voltage systems are all very dangerous... there isn't one labeled to be safer than the other.

Marc