Summer 2003 Issue

If the transformers in your junkbox never seem to be exactly what you need for a project, VE3ERP has a solution–wind your own–and a computer program to do all the math.

By George Murphy,* VE3ERP

Did you ever have the urge to get the exact power transformer needed for a project by rewinding an old transformer from your junk box? There are several methods of selecting a transformer to rewind for a particular application. Most of them, at least in my experience, result in my trying them all and finally using the one that blows the fewest fuses.

Recently, I had occasion to design a multi-voltage DC output power supply to replace the multitude of wall-wart DC “power supplies” cluttering up every wall outlet and power bar in a friend’s entertainment and computer center. The design was no problem; it was just an enlarged version of the SUPER ACADAPT1 I have in my hamshack, but I needed a larger transformer to run it. Rather than use my usual pragmatic FUD (Fumble Until Done) technique, I decided to do it right.

What it boils down to is this: The best way to transform a transformer is to design what you want from scratch the way engineers do, look for a transformer in your junk box with a core size close to what is needed, and transform it!

A Few Definitions

I will try to spare you one of my personal pet peeves. Often when delving into an intriguing article, I am frustrated by the author using esoteric terminology I do not understand, on the assumption I am already an expert on the subject. Thus, if you are unfamiliar with “transformerspeak,” here are a couple of terms you should be aware of:
Current Density (measured in circular mils [CM] per ampere): This defines the current carrying capacity of a conductor. The higher the current density, the more current can be carried. In a power transformer this means that for a given current, high-current-density wiring will run cooler but takes up more space and requires a larger and heavier core than low-current-density wiring. It is good practice to design for the lowest current density that will do the job. Some typical densities commonly used for small power transformers are:

500 CM/amp—intermittent light-duty service (e.g., small appliances)
700 CM/amp—continuous-duty commercial service (e.g., communications equipment, computers)
1000 CM/amp—continuous heavy-duty service (e.g., indutrial generators, military equipment)
Core Flux Density (measured in gauss): The number of magnetic force lines per unit area. The flux density employed depends on the application, the power rating, the core material, and the frequency. Designing for a flux density higher than actually required results in larger cores and heavier wires than necessary. For small transformers (up to about 50 volt-amperes, or VA) flux densities of about 14,000 gauss are commonly used.

Selecting inappropriate values of current density and core flux density may result in excessive size, inefficiency, and/or possible overheating of the transformer.

The Anatomy of a Power Transformer

The most common core form for small power transformers is the EI configuration, so named because the shapes of the segments resemble the letters E and I. Figure 1 shows how these segments are stamped from rectangles of sheet-iron alloy with no waste whatsoever. The two shapes are interleaved through a coil as shown in figures 2 and 3 to form the transformer core.

It is interesting to note how dimensions C and D are related to dimension A (C = A/2; D = 1.5A). When assembled, this forms two rectangular C wide paths for the magnetic lines of force, joined along their long sides where they become a single tongue A (2 ¥ C) wide. Thus, it is only necessary to establish dimensions A and B to design the entire core. Once you have these dimensions, you can rummage in your junk box for a transformer with a similar-size core and carry on with the design process to determine the specifications for the new windings.
 

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