Transformers are essential components in many radio projects, often accounting for a significant portion of the overall cost. However, since they can usually be reused in various ways, salvaging power transformers can be a cost-effective approach.
Assessing Power Handling Capability
The weight of a power transformer provides a clue to its power handling capability. For unpotted transformers, those weighing 15 pounds (6,8 Kg) or more generally fall within the kilowatt (KW) range. For example, an 800-volt, 20-pound transformer in a voltage tripling circuit should be able to intermittently power 1,000 watts of RF.
Testing Salvaged Transformers
When testing salvaged transformers, always use a variac or place a light bulb in series with the primary. Wiring color codes can be found in the appendix of most electrical and electronic handbooks, including Orr’s great reference.
Considerations for Rewinding Transformers
Power transformers can be used directly or rewound to power tube filaments or provide various specific voltages. When deciding whether to rewind a transformer, consider the following factors:
- Cost of purchasing a replacement: If the required secondary is readily available and inexpensive, it may not be worth the effort to rewind the transformer.
- Complexity of the secondary: With experience, practically any secondary combination can be rewound. However, the time required for a casual builder to reach that level of experience may not be practical. It is encouraged to limit ambitions to single secondary windings.
Methods for Rewinding Transformers
There are two methods for rewinding a transformer, both based on the same principle:
- Complete disassembly method: This involves completely disassembling the transformer, replacing the secondary winding, and then reassembling the laminated core around the coil form. This method produces tighter windings and more predictable results.
- Partial disassembly method: This requires replacing the secondary winding without disassembling the laminated core. It is limited to situations where sufficient space exists between the windings and the metal core, and when only a limited number of secondary turns makes this method practical.
Step-by-Step Guide for Complete Disassembly Method
- Remove the mounting bolts and carefully pry the laminates apart using a knife or narrow-bladed screwdriver.
- Keep track of the manner in which the laminates were assembled to facilitate reassembly.
- Remove the coil, wound upon a phenolic or treated cardboard form, without disturbing its size or shape.
- Carefully remove the secondary winding one turn at a time, keeping a written record of the total turns.
- Calculate the turns per volt ratio by dividing the total turns by the secondary voltage.
- Consult a wire table to ensure the selected wire size can handle the required current.
- Wrap the new secondary turns in the same direction as those removed, using craft or wax paper between layers.
- Reassemble the laminates in the same order as when disassembled, using C-clamps or a vise to aid the process.
- Insert the mounting bolts, apply a coat of varnish, and compress with 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.
By following these steps and utilizing the wealth of information available in handbooks related to wire capacity and projected performance, you can successfully rewind transformers for your radio projects. Happy winding!