Neon Transformer Dismantling \ Repairing.

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Disclaimer: The information presented on this page is intended for people of 21 years of age or older who have a good working knowledge of electrical and electronic safety. The author is not responsible for any loss of any kind brought about by using the information given.
  • Background Information.
  • Transformer Sources.
  • Common Faults.
  • Dismantling.
  • Modifications.
  • Reassembly.
  • Neon Transformers in Tesla Coil Use.
  • Neon Transformer Manufacturers.

    • Background Information

     The transformers that are used to convert mains voltages to those suitable for powering neon signs are commonly used for the power supplies of tesla coils. Typical Ratings for a neon transformer are:

    Transformer Sources

    Common Faults

     Some of the transformers that are obtained in this way are fine, but have been dismantled from faulty signs. Those units that are faulty typically have one or both high-voltage secondaries burnt out, or the insulation on one or both secondaries carbonises and shorts. Another common problem is that the secondary ground wire becomes detached from the core.

    Dismantling

    Commercial neon transformers are placed in a container which has appropriate insulated lead-out wires, and the container is then filled with an insulating material (or potting compound) that completely engulfs the transformer. The reason for encasing the transformer in this manner is not only for the purposes of high-voltage insulation, but also to protect the delicate secondary windings, and to prevent the ingress of water when in service. Neon transformers can be potted in asphalt, though some newer units use other types of potting compounds (such as epoxy resins) which may not be removable. The diagram below shows a typical neon transformer that has been removed from its case and potting compound:
    Typical Neon Xfmr
    The primary and secondary windings are usually prewound - that is they are wound before being slotted onto the core. The grounded iron core is made from stamped iron laminations that have a thin enamel coating (to act as an insulator from adjacent laminations). This laminated core is made to come apart into two "U" or "L" shaped sections which is how the pre-wound primary and secondary windings are fitted on to it. The base wires of the two secondary windings are electrically grounded to the core laminate.

    Preparation for Unpotting.

    Remove all hardware, and insulators if possible. Take a hammer and a chisel and remove the cases by splitting them down the corners. Often a small steel strap which electrically connects the core to the case (at the same location where the grounding lug is screwed in) must be removed. Break off any stubborn insulators, but try to preserve the lead wire. You are left with a block of asphalt.

    Methods of Unpotting.

    One of the following methods for unpotting may be used.

    Seperating the Core

    If you look carefully on a neon transformer core, there will usually be fasteners that are accesible even with the shunt plates and windings installed. The fasteners are likely to be in the corners, or between the HV windings and the primary. The fasteners can be removed to dismantle the core. An example of a core with two mating sections, much like a tongue and groove construction used in furniture making, is shown below:
    Neon Core 1
    Once the core is apart, you can see how the windings are removed/installed. The windings simply slide right off of the ends of the "bars" of the disassembled core:
    Neon Core 2
    Another type of core construction (used on Jefferson Electric transformers) is shown below:
    Neon Core 3
    The core of this transformer is driven out with a punch at the x's.

    Modifications

    While the core is apart, the current capacbility of the transformer secondary may be increased by removing a few of the shunt laminations. Never take out more than 2 or 3 of these laminations per side, as the additional power output will burn out the secondaries. 70-75 ma can be achieved from a 60 ma unit using this method.

    Reassembly

     Wood, bakelite, and plastic wedges are often used commercially to wedge the secondary windings against the core to prevent vibration. When reassembling windings onto a core, 30 mil polyethylene can be used as a wedge material. The polyethylene sheet is softened in boiling water, and the transformer core is warmed in an oven. The heated soft polyethylene sheet is dried and wrapped around the warm transformer core. The secondary windings can then be gently forced onto the core. Once cooled, the windings are insulated from the core, and they will not vibrate. The polythene is a better insulator at the RF frequencies encountered in tesla coil use than asphalt. Soft rope caulking is another good insulator for Neon transformer secondaries. It is cheap, and can be reused. Anti-Corona dopes do not give as good a performance. It will often be necessary to provide new connections to the secondary from short lengths of the enalled copper wire of the winding. The wire used does not need to be of the same standard as that used in neon sign use, as it is anticipated that for tesal coil use it will not be exposed to the elements or used for long lengths of time. In addition, thicker gauges of connection wire will tend to promote breakage of the secondary wire in use. Some sources for HV wire include:

    Neon Transformers in Tesla Coil Use

     Neon sign transformers that have been rebuilt may be fired dry. The asphalt used to pot the cores in commercial neon transformers does not really insulate well against the RF and kickback from the Tesla Tank. The units last longer when they are freed of the asphalt potting. Rebuilt units may alternatively be immersed in mineral or transformer oil.

    Neon Transformer Manufacturers

    The following companies make neon transformers:
    ©C J Hill 1998. Last modified on 15th March 1998.
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