NOTE: This question concerns working with live high voltage transformers. Do not undertake this lightly, or in an altered state (drunk, high, etc.)

My original coil (6" diameter) has been powered by one 12kv Neon Sign Xfmr at 30mA. I have recently obtained five more NST's to hook into the coil. I am writing to ask how you determine the polarity of the transformer so you can hook up all of the -ves and +ves together?

Answer:

The SAFEST method to get two neon sign transformers properly phased for parallel operation is to connect their cases together and then to a good AC ground. Then wire the primaries in parallel and attach a wire with adequate HV insulation to one the first transformer's HV terminals. Use caution for this next part, as there is the possibility of having a 12KV potential difference across the gap that the wire will be forming. I recommend not directly holding the wire, but rather using an insulated rod to manipulate the hot wire.

When you plug the 120VAC in you can then touch the free end of the wire to the HV terminals on the second transformer. You will get a 12KV arc to the one that is OUT of phase, and hardly any arc at all to the HV terminal that is IN phase. If the phasing is the opposite of that desired, then simply rewire the parallel connection wires that join the first transformer to the second. In other words, leave the first transformer alone and just exchange the two primary wires on the second transformer ONLY.

I know that some people test their transformers by first connecting one of the HV terminals of the first transformer to one of the HV terminals of the second transformer. THIS IS VERY DANGEROUS!! If the HV terminals are wired so that they are AIDING, then the cases of the two transformers will float to a 12KV difference, and the other two HV electrodes will rise to a 24 KV difference. This in itself can lead to transformer destruction, not to mention case to case arcing and possible case to primary arcing.

Since neon transformers are center tapped to the case, CONNECT the cases, and ground the cases to a good AC ground. By the way, the same goes for when you are actually using the paralleled transformers. Always connect the cases and ground them to a good AC ground.

Coiling has enough dangers as it is. Let's not take any unnecessary chances!

By the way, if you feel uncomfortable working with the HV output of a neon transformer, then power the primaries of the transformers with the output of a 6 or 12 volt transformer. With a 12 volt transformer the voltage is reduced from 12Kv to 1,200 volts.