How to Modify a Transformer

Commonly, the perfect transformer might not be readily accessible for purchase, or if it is available, it can be costly. Nonetheless, there is a possibility of modifying the transformer's secondary winding, especially when producing small voltage and current outputs. This approach can provide a highly cost-effective solution for creating a customized power supply.

Transformer Power Rating and Wire Thickness

Before delving into the modification process, several essential guidelines need consideration. The power rating of a transformer is determined by its mass, i.e., the number of laminations, and cannot be altered.

Consequently, modifications to the secondary winding with the intention of increasing the load should be avoided.

However, if a higher voltage output is desired, one can achieve this by increasing the number of turns on the secondary winding, which results in a smaller current.

Conversely, completely rewinding the secondary with thicker wire will reduce the number of turns and lead to a decreased output voltage. But this will yield proportionately more current.

It is crucial to exercise moderation when increasing the secondary voltage to prevent insulation issues.

Additionally, any changes made should solely focus on the secondary winding, leaving the primary winding untouched.

Winding Turn Ratio Formula

An essential rule to follow when modifying or winding transformers is expressed by the equation:

Vsec / Vprim = Tsec / Tprim.

In this equation, Vsec represents the secondary voltage, Vprim denotes the primary voltage, Tsec is the number of secondary turns, and Tprim represents the number of primary turns.

Keeping the primary voltage fixed, the secondary voltage is directly proportional to the number of turns on the secondary winding.

For this calculation, the secondary voltage is considered to be the voltage when the transformer is operating without a load.

How to Remove the Laminations and the Bobbin

Practical aspects of the operation involve the challenging task of removing the bobbin that holds both primary and secondary windings from the transformer's core.

The core comprises iron laminates, typically arranged in a figure-of-eight pattern, sometimes shaped as Es and Is or Us and Ts.

To safely remove the laminates, the transformer should be held in a vice while using a flat-bladed screwdriver, a hammer, and fine-nosed pliers to carefully prize them off one by one.

The goal is to extract the laminations without causing damage. While the first couple of laminates may get bent, it is acceptable as getting every single one back in place is often impractical.

Removing the Lacquered Tapes

Once the bobbin is free, the layer of lacquered paper or tape covering the secondary windings should be removed to access them. The method of attaching the leads to the windings should be carefully noted for future reference when creating new leads.

The next step involves unwinding the secondary while maintaining a neat coil and counting the turns.

After completing this task, one can calculate the number of turns needed for the new windings and determine the positions of any taps.

Rewinding Process

The rewinding process should be carried out in reverse order.

To safeguard the windings, it is essential to apply a few layers of insulating tape and a generous coat of lacquer or varnish.

Finally, the laminations need to be reassembled. Although this can be a challenging task, persevering and restoring as many laminations as possible is crucial.

Missing out on a few laminations might not significantly impact the power rating and regulation but could result in an audible 50Hz buzz from the finished transformer.

As a precautionary measure, it is advisable to liberally cover the laminates with varnish and ensure they are thoroughly dried.

Summarizing the Steps

Step 1: Consider the Need for Modification

• Assess the availability and cost of an ideal transformer off the shelf.
• Recognize that modifying the transformer's secondary winding might be a cost-effective solution, especially for one-off power supply needs or small-scale production.

Step 2: Understand Transformer Power Rating and Limitations

• Acknowledge that the transformer's power rating depends on its mass, determined by the number of laminations, which cannot be changed.
• Avoid modifying the secondary winding with the aim of loading it more heavily, as it can adversely affect the transformer's performance.
• Realize that increasing the number of turns in the secondary winding will raise the voltage output but result in a smaller current, while rewinding with thicker wire will reduce the number of turns and lower the output voltage.
• Exercise moderation when increasing secondary voltage to prevent insulation problems.
• Only make changes to the secondary winding and leave the primary winding untouched.

Step 3: Apply the Rule for Transformer Modification

• Utilize the formula Vsec / Vprim = Tsec / Tprim, where Vsec represents the secondary voltage, Vprim is the primary voltage, Tsec is the number of secondary turns, and Tprim is the number of primary turns.
• Understand that, with a fixed primary voltage, the secondary voltage is proportional to the number of turns in the secondary winding.

Step 4: Prepare for Modification

• Secure the transformer's core in a vice, being careful not to clamp it too firmly.
• Use a flat-bladed screwdriver, a hammer, and fine-nosed pliers to gradually prize off the laminates from the core.
• Start from one side and alternate sides to remove the laminates, working towards the middle, and ensure to remove them undamaged.

Step 5: Access the Secondary Windings

• Once the bobbin holding the windings is free, remove the layer of lacquered paper or tape to expose the secondary windings.
• Observe the method of attaching the leads to the windings for future reference.

Step 6: Unwind and Count the Secondary Windings

• Carefully unwind the secondary winding while maintaining a neat coil.
• Count the number of turns in the secondary winding.

Step 7: Determine New Winding Specifications

• Calculate the number of turns required for the new secondary winding to achieve the desired voltage output.
• Identify the positions of any taps needed for specific voltage requirements.

Step 8: Rewind the Secondary Winding

• Rewind the secondary winding with the new specifications, following the calculated number of turns and tap positions.

Step 9: Insulate and Protect the Windings

• Apply a few layers of insulating tape to protect the windings.
• Coat the windings with lacquer or varnish to ensure proper insulation.

Step 10: Reassemble the Transformer

• Carefully put back the laminations, aiming to restore as many as possible.
• Cover the laminations with varnish and ensure they are thoroughly dried to prevent issues.

By following these step-by-step instructions, you can successfully modify a transformer's secondary winding to suit your specific power supply needs.