[Amps] Re: Tuned power transformer

[Will Matney]

I thought I'd try to describe in a little more detail about power 
transformer construction. I had received several e-mails asking me about 
this so I hope this will help. Originally I didn't notice the size of 
the capacitor and it is for power correction like was mentioned. Some 
transceivers used RFI filters though on the lines using a couple of 
chokes and some ceramic disc caps going to ground. The transformer core 
size and number of turns are deduced from the frequency it will run at, 
the power output needed, the voltage needed, and the flux density of the 
laminates measured in gauss. For M6 lams made by Temple Steel, The flux 
density runs about 70 gauss if I remember. Mild steel runs about 60 
gauss and is what you use if you have unknown laminates. This would be 
like using an old transformer or rewinding one. The higher the gauss the 
less turns per volt needed and the physical core size drops. The five 
square inches for 1000 watts is from using M6 steel and is about a 
standard, especially in older transformers. Once transformers get over 
1500 watts (1.5 KVa) they generally use Hypersil or other cores. 
Hypersil has a higher flux density resulting in a smaller and lighter 
core. These cores are known as "C" cores. The laminated steel cores 
mostly used are called "E" cores. The reason for this is the shape of 
the core material. The flux densities are obtained from the 
manufacturers spec sheets which you should need in winding new 
transformers. There is an easy formula to calculate the turns per volt 
on any core size but you need to know;

Flux Density (Gauss)
Voltage In
Voltage Out
Frequency in Hertz
Number of Turns (Generally from 1.5 to 5 turns per volt)
Core size in square inches
Duty Cycle

The core sizes to not sized up linearly as the power goes up but form 
something similar to a bell curve. There is a graph in most ARRL 
Handbooks about this I've seen. The best reference material I have 
found, and what I use, is a book published by Howard Sams named 
"Practical Transformer Design". It deals with every aspect of 
transformer, autotransformer, and choke design. Why there is not 
anything published about transformer design is beyond me but this should 
give you a starting point. If anyone else would like, I can send in the 
basic formulas for the calculations. I would check with the local 
library or electronics supply house carrying Sams publications for the 
book. It was published in 1980 but is still obtainable. I hope this 
clears up some things about transformer construction.

Will Matney
US Amp