Construction Help - Wire Preparation

CONSTRUCTION HELP

Wire Preparation

The first step is to determine the length of the wire needed in coiling it about the core and them making the proper connections. Experience has shown that 4 to 5 extra inches are needed beyond the coiled winding. About a third should be at the beginning of the winding and the other two-thirds at the end. A simple technique for determining the length is to take a piece of string, wind it about the core with the proper number of turns and then add 4 to 5 inches to that length. Another technique is to calculate the length-per-turn, multiply it by the required number of turns and then add 4 to 5 inches. Generally, the kits are supplied with enough wire, insulation and other components (except cores) to allow for two attempts in constructing the transformers. One will find that the wire cannot be reused because of work-hardening.

The second step is to straighten out the wire as much as possible. This can be accomplished by holding the wire at one end with a pair of pliers and then pulling the wire between the clenched thumb and forefinger several times. Some work-hardening will take place, but it will not prove to be a problem. Each wire is then skinned-back (insulation removed) about 3/4ths of an inch at one end. This is usually the beginning end of the winding. One will find that scraping off the insulation (with a jackknife, usually) is probably one of the more difficult (and boring) tasks in constructing these transformers.

Three of the more popular wires (known as magnet wires) used in these transformers are Formvar, Thermaleze and Imideze. Formvar is generally available with a single coating of insulation (0.08) mils thick) and has the designation SF. Thermaleze and Imideze usually have a thick coating (1.6 mils) and have the designation H. All three have the same dielectric-breakdown-per-unit-thickness (2000 volts/mil). They only differ in their thermal capabilities which are important in motors, generators and 60 hertz power transformers and not transmission line transformers. Incidentally, Imideze has the greatest thermal tolerance (220 degrees centigrade) and Formvar the least (105 degrees centigrade).

Extra insulation on the wire, like Scotch #92 or Teflon tubing is necessary in many cases in order to achieve the proper spacing between the wires and hence obtain the desired optimum characteristic impedances. When striving for characteristics impedances of 50 ohms, sometimes one layer of Scotch #92 tape (a thickness of 2.8 mils) is required. In other cases, two layers are needed. The author has found that by laying the Scotch #92 tape (which is 1/2 inch wide) edgewise on the wire and then rolling it around like a rug or window shade, two layers are practically realized on #12, #14 and #16 wire. A little less will be on #12 wire and a little more on #16 wire. Electrically, the objectives will be met. A u-frame will be found helpful in holding the wire. The two-layer thicknesses can also be accomplished by controlled-spiral winding.

The goal to achieve a single layer thickness of Scotch #92 (2.8 mils) is a little more difficult. One has to slit the 1/2-inch-wide tape so it just covers the circumference of the wire. The width can be calculated by multiplying the diameter by 3.14 (p). For #14 wire, this amounts to about 4/10ths the width of the 1/2 inch tape. Since generally two pieces now become available from the 1/2 inch wide tape, the starting length is one-half of the final length needed. The slitting process involves laying the Scotch #92 tape on a smooth surface (like glass or metal) and using a safety razor blade and ruler with a metal edge. Incidentally, the thickness of Scotch #92 tape is made up of 1 mil of polyimide and 1.8 mils of adhesive. This tape has excellent electrical and thermal properties and is highly recommended.

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