5.1.3 NST - Neon sign transformers

1. Neon sign transformers:

The most common power supply for powering a Tesla coil is the neon sign transformer. These step up the 120 VAC from the wall socket to the several thousands of volts necessary for spark gap operation.

The most common neon transformer voltage ratings are 6000, 7500, 9000, 12000, and 15000 volts with current ratings of 30 or 60 ma. Occasionally you will find 120ma neon transformers but these are not very common.

These transformers are ideal for Tesla coils as they are internally current limited with magnetic shunts. This means that neons can take direct short circuits with no ill effect.

Generally speaking the higher the transformer voltage and current rating the more output you can expect from your Tesla coil. The most useful neons for Tesla coils are the 9000 and 12000 volt models. The 15000 volt ones are quite hard on capacitors. For a first coil project 9000 volt neons are recommended.


2. Where to obtain these transformers:

First thing to do is to get out the yellow pages. Look under signs and take note of all the sign companies in your area. Make special notice of any that advertise sales and service of neon signs.

Now take your list and go to these sign companies and ask them for used neon sign transformers. Tell them what you want them for. This will often generate some interest.

If you strike out at the first place go to the next one.

Expect to pay about $20 for a used 30ma neon and $35 for a 60ma neon. If they want considerably more than this move on to the next place. Sometimes you can get lucky and find a place with a pile of transformers in unknown condition. Negotiate a price and take your chances or see if you can do some testing on the spot. You can check the secondary windings for continuity with an ohmeter. The resistance will vary considerably from transformer to transformer. Just check for opens or short circuits.

Or take along an AC line cord and plug the units in. Attempt to draw an arc from each high voltage terminal to the case. Each side should show an equal arc. Careful you dont get shocked. The smaller neons can deliver painful shocks and the larger ones can kill.


3. Unpotting and repair of bad transformers:

How about getting a Tesla power supply for free? Check out those same sign companies in your area and ask for dead neon transformers. Any place the services signs will have bad transformers. Some places pile them up while others pitch them into the the trash dumpster. Check these places out and ask them to save their bad units for you.

What good will dead transformers do? You can unpot these dead units and disassemble the transformers. Often just unpotting them will revive them. Neons transformers are potted in tar. Tar is a cheap and not very effective insulator. Over time the tar dries out. High voltage arcs within the tar block slowly carbonizes some of this tar. Once carbonized it becomes more conductive inviting more arcing and more carbonization. Eventually high voltage secondary winding shorts out. Because of the current limiting in neons the shorted secondary is normally not damaged.

So removing the carbonized potting compound completely restores the neons to operation. Other problems like broken wires and connections can also be repaired once unpotted. New insulation like hot glue or silicone RTV is used to insulate the secondary windings from the core. Or the transformer can be put into a container of mineral or transformer oil.