Microwave Transformer Repurposing for High Current PSU

Can you use a transformer in a microwave oven? Yes, and here's how!

The quest for a reliable and affordable high-current power supply unit (PSU) often leads DIY enthusiasts down a path of exploration and innovation. When a friend requested a 13.8V, 50A PSU, the readily available options presented a dilemma: they were either prohibitively expensive or of questionable quality, often originating from far-east manufacturers. This is where the ingenuity of a seasoned electronics hobbyist shines. Faced with this challenge, the decision was made to build one from scratch, a task that, while demanding, promises a rewarding outcome. The most significant hurdle in constructing such a PSU is the transformer itself. High-current applications necessitate a transformer with substantial power output, often upwards of 800 watts. Fortunately, a readily accessible and virtually free source for these powerful components exists within the heart of discarded microwave ovens. These ovens, designed to generate the high voltages required for their magnetrons, house transformers that, with a bit of modification, can be transformed into the linchpin of a new, high-performance PSU.

The Microwave Transformer: A Hidden Gem

Microwave ovens typically contain transformers with a primary winding designed for mains voltage (230V in Europe) and a secondary winding that outputs a very high voltage, often around 2000V AC. This high voltage is crucial for the magnetron's operation. However, for our purpose of creating a low-voltage, high-current PSU, only the primary winding is initially of interest. The process of salvaging this component involves carefully removing the high-voltage secondary winding. This is typically achieved using basic tools like a hammer and an old wood chisel. Once the thin, high-voltage wires are hammered out, the transformer core remains, now ready to be rewound with a new secondary winding suited for our low-voltage, high-current requirements.

Winding the New Secondary: The Art of Voltage and Current

With the transformer core prepared, the next crucial step is to wind a new secondary coil. The goal is to achieve approximately 13.8V, with a buffer for voltage drops across rectifying components, so aiming for 16-18V AC is a sensible approach. The number of turns required per volt is determined by a test winding. In this instance, winding five turns of insulated wire and measuring the output voltage revealed that the transformer yielded approximately 0.9V per turn. This empirical data allows for precise calculation: 17 to 18 turns would be sufficient to achieve the desired voltage.

The choice of wire for the secondary winding is paramount. It must be thick enough to handle the substantial current without overheating. Consulting AWG (American Wire Gauge) tables is essential. For a 50A requirement, a wire capable of handling at least this current is needed. A 2 AWG wire, for example, can typically handle around 66A. However, the physical dimensions of the salvaged transformer's core can limit the number of turns possible. A thick wire, including its insulation, might be too large to fit the necessary 17-18 turns. In this particular project, a 2 AWG wire, with its insulation, had a diameter too great to allow for the required number of turns, leaving space for only five, which would yield just 4.5V.

Innovative Solution: Copper Strips for High Current

When conventional wire proves too bulky, creative solutions are required. In this project, a stockpile of inherited copper sheets, measuring 200 cm x 60 cm and 0.6 mm thick, provided the answer. By calculating the feasibility of using strips cut from these sheets, it was determined that this approach would be highly effective. A strip of 0.6 mm copper, with a width calculated to fit the available space and provide sufficient cross-sectional area, could comfortably handle around 100 Amps. The cross-sectional area of a 26.5 mm wide strip of 0.6 mm thick copper is approximately 15.9 mm², which is superior to a single core 6 AWG wire in terms of current-carrying capacity. This innovative use of readily available materials overcomes the limitations of standard wire gauges.

Insulation: The Key to Safety and Longevity

Effective insulation between the windings is critical for preventing short circuits and ensuring the longevity of the PSU. The search for suitable insulating material led to an unexpected but ingenious source: the kitchen. A roll of "silicon impregnated non-stick oven paper" proved to be an ideal candidate. This material can withstand temperatures up to 220°C and, despite its thinness (0.04 mm), offers excellent strength. To ensure maximum safety and reliability, four layers of this material were used between each turn of the copper strip.

Precision Cutting and Winding

The process of preparing the copper strips involved meticulous measurement and cutting. A mini-lathe, equipped with a steel core and a razor blade, was employed to cut the copper sheets into strips of 27 mm width. This provided the precision needed for the winding process. For cutting the longer lengths of copper foil, an old drill-powered sheet metal cutter was utilized. While not the most refined tool for the job, it efficiently cut the two 2-meter long strips required for the secondary winding, with minor filing needed in a few areas.

The winding itself requires careful attention to detail. The copper strips, after being cut and marked for connection points, were carefully folded around the transformer core, with the four layers of insulation placed between each turn. Wooden wedges were used to press the sides of the windings, ensuring they remained secure and did not unfold. This meticulous process, taking approximately an hour for each 2-meter strip, resulted in a neatly wound secondary coil. Once the winding was complete, the transformer underwent final checks for continuity and short circuits. A "cool cure" varnish, designed for coil winding, was then poured over the windings to provide additional protection and insulation.

Testing and Results: A Powerful Outcome

The moment of truth arrived with the testing of the newly wound transformer. After allowing the varnish to cure, the transformer was connected to the mains. The output voltage measured a healthy 15.63V AC, perfectly within the desired range. While the exact current output could not be measured due to the absence of a 100A ammeter, the results were highly encouraging. A 40A circuit breaker, connected to the secondary, tripped instantly when the low-voltage circuit was shorted through it, indicating a substantial current capacity, estimated to be around 50 Amps. This suggests the transformer is more than capable of meeting the 50A requirement for the PSU.

Future Steps and Considerations

With the transformer successfully modified and tested, the next steps involve assembling the rest of the PSU components, which are currently on order from China. This project highlights the immense potential for repurposing seemingly obsolete electronics. Microwave oven transformers, often discarded, are a valuable resource for DIY electronics projects requiring high current. However, it is crucial to remember the inherent dangers of working with mains voltage and high currents. Safety precautions must always be the top priority. Proper insulation, secure connections, and the use of appropriate safety equipment are non-negotiable. Furthermore, understanding the principles of transformer design, including voltage per turn and current-carrying capacity of conductors, is essential for success.

Frequently Asked Questions (FAQ)

Q1: Is it safe to dismantle a microwave oven?
Microwave ovens contain capacitors that can store a dangerous electrical charge even after the oven has been unplugged. It is vital to discharge these capacitors safely before attempting to dismantle the oven. If you are unsure, seek professional guidance.

Q2: Can I use the high-voltage secondary winding from the microwave transformer?
No, the high-voltage secondary winding is extremely dangerous and is not suitable for low-voltage, high-current applications. It must be removed and replaced with a new winding.

Q3: What kind of wire should I use for the new secondary winding?
Use thick, insulated copper wire or, as demonstrated in this project, copper strips. The gauge or thickness of the wire must be sufficient to handle the required current without overheating. Consult AWG tables for guidance.

Q4: How do I determine the number of turns for the secondary winding?
Wind a few turns of insulated wire and measure the AC voltage output. Divide the measured voltage by the number of turns to determine the volts per turn. Then, multiply this value by your desired output voltage (plus a buffer for voltage drops) to calculate the total number of turns needed.

Q5: What are the risks involved in this project?
The primary risks include electric shock from mains voltage and potential overheating or fire hazards if the windings are not properly insulated or if the current capacity is exceeded. Always prioritize safety.

Comparative Table: Wire Gauges for High Current

Note: Current ratings are approximate and can vary based on insulation, ambient temperature, and installation method. The copper strip used in the project (15.9 mm² cross-section) is roughly equivalent to or better than 2 AWG in terms of heat dissipation and current handling for this specific application.

Conclusion

Repurposing a microwave oven transformer for a high-current PSU is a challenging yet achievable project that can yield significant cost savings and a highly customized power supply. The ingenuity demonstrated in using copper strips and oven paper highlights the resourcefulness often found in the DIY electronics community. With careful planning, precise execution, and an unwavering commitment to safety, such projects can be both educational and incredibly rewarding, providing reliable power for a multitude of applications.

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