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REFERENCE MATERIAL
Transformers
Salvage and Winding

Transformers are components used in a large number of radio projects, and represent a significant percentage of the cost of building. Since they can usually be reused in one form or another, I salvage most of the power transformers I get my hands on.

The weight of a power transformer holds a clue to its power handling capability. For unpotted transformers, 15 pounds on up will generally place it well within the KW range. This is to say that an 800 volt, 20 pounder, in a voltage tripling circuit, should be able to intermitently power 1,000 watts of RF.

Always use a variac, or place a light bulb in series with the primary when testing salvaged transformers. Wiring color codes are available in the appendix of most electrical and electronic handbooks, including Orr's great reference.

In general power transformers may be used directly or rewound to power tube filaments, or provide a variety of specific voltages. When deciding whether to rewind a transformer, however, several factors should be considered. First, can the transformer be purchased inexpensively - rewinding a transformer involves an amount of work that in some instances may not be justified. For example, if the required secondary is 12 V at 5A, it may be more cost efficient to simply slip down to the local Rat Shack and purchase one. On the other hand, if the secondary needs to be, say, 5 V, CT, at 30 amps, winding it may be worth the effort. Consideration should also be given the complexity of the secondary. With experience, practically any secondary combination can be rewound. However, the time required of the casual builder to meet that level of experience may not be practical. Consequently, the casual builder is encouraged to limit ambitions to single secondary windings.

There are two methods that may be used to rewind a transformer. However, both are based upon the same principal. The first involves completely disassembling the transformer, replacing the secondary winding, and then reassembling the laminated core around the coil form. The second method requires replacing the secondary winding, without disassembling the laminated core, and is limited to situations in which sufficient space exists between the windings and the metal core, and when only a limited number of secondary turns makes this method practical. As a general rule, however, the first method produces tighter windings and consequently, more predictable results.

The first step in disassembling a transformer for rewinding, is removal of the mounting bolts. Once they are removed, a knife or narrow bladed screwdriver may be used to carefully pry the laminates apart. Each laminate is made of steel or iron, shaped like a capitol letter "E". In addition, spacing strips the same thickness are employed to fill the gap that remains when the laminates are assembled by alternating their direction, one on top of the other. Most often, aside from the mounting bolts, the laminates are held together with a thin coating of shellac applied between the laminates. The idea here is to keep track of the manner in which the laminates were assembled in order to better facilitate reassembly.

Once the laminates are removed, there will remain the coil, wound upon a phenolic or treated cardboard form. Do not disturb the size or shape of the form, since it's shape is crucial to reassembly of the transformer. The primary should be the inner most winding, and the secondary(s), the outermost. At this point, the secondary winding may be carefully removed.

Remove the secondary winding one turn at a time, counting each as you go. It may be wise to maintain a written running total as the turns are removed. Should you lose track, attempting to rewind and recount may not be feasible.

Once all the secondary turns are removed, tally the number of turns and divide by the secondary voltage. The result, is the turns per volt ratio that may be used to calculate the number of turns to rewind. Prior to rewinding, however, a wire table (located in the appendix of most handbooks) should be consulted to ensure that the wire size selected is capable of handling the current that may be demanded of it. Formavar, or other quality enameled wire should be used.

It is important when you begin winding the secondary, that you wrap the turns in the same direction as those removed. Wrap them one next to the other. Use craft or wax paper between the layers. If a tap is required, pull up about 6 inches, twisting it together and carry it out to one side. Be sure to place a piece of craft or wax paper over the turns it crosses as it's taken out to the side. Continue winding until the total number of turns have been applied. Leave a 6 inch soldering length and wrap the windings with a single layer of silk tape, which may be purchased in the drug department of most retail stores.

The next step is to reassemble the laminates. An effort should be made to follow the same order as when they were disassembled. The use of C-clamps or a vise will greatly aid the process. However, if, near the end, a laminate or two do not seem to fit, it is better to leave them out than to try to force them in. Once in place, insert the mounting bolts and apply a coat of varnish. By moving the unit from side to side, you should be able to "wick" the varnish down between the laminates. After applying the varnish, compress with a couple C-clamps until dry. If the core seems loose, shim it with small wood wedges or cardboard and apply a coat of varnish to hold them in place. The transformer should now be ready to use in the circuit for which it was intended.

A second method involves simply removing the secondary windings and wrapping the new ones with the core and laminates in place. One of the amplifiers I recently built called for 5 volts, CT, at 30 Amps. I started with was a 12 volt, 8 amp transformer, with the secondary wound on top of the primary. In this instance, there was enough room between the laminates and the secondary windings to simply unwrap the primary windings with the laminate core intact. As I unwrapped them, I kept count - there were 24 turns. That meant that there were two turns per volt, and my new secondary, of a gauge capable of handling the increased current, required 10 turns, center-tapped at 5. The completed transformer worked great in the project I wound it for.

While little to no information on the practical details of rewinding transformers is available, most handbooks have a wealth of information related to wire capacity and projected performance. Consequently, before attempting to rewind your own, make sure to avail yourself of that information. Good Luck and Happy Winding!


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