1998 Qex Build A High Voltage Transformer For A Low Cost

Build a High- Voltage

Power Supply at Low Cost

Looking for HV supply parts? A few dead microwave ovens may supply all the parts you need.

By Randy Henderson, WI5W

Hot from the kitchen, here's a full-course supply for power-hungry vacuum-tube amplifiers. Have you had thoughts of building a high-power amplifier or resurrecting a used "bargain" amp? I've thought about such a project for a long time. Initially, it seemed like a good idea when compared to the price of a new amplifier. After researching the cost of power-supply parts, however, I began to have second thoughts.

I needed the components and materials to make a unit capable of supplying 2700 V at 500 mA or more. I also needed a well regulated 500 V output for the screen grid. I have met several hams who have built such supplies with sur-

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plus parts and achieved good results.

When searching surplus and used parts, I never seemed to find just what I wanted. I was also discouraged by the thought of finding replacements for expensive, scarce parts—such as transformers—should future repairs become necessary.

Microwave Ovens

As a source of high-power, highvoltage parts, large (defunct) vacuum-tube color televisions and militarysurplus gear are no longer as plentiful as in the past. The ubiquitous microwave oven is in some instances, a good replacement source. The new ones have become so inexpensive that repair is often impractical, so nonwork-ing units can often be obtained at little or no cost.

To better exploit the innards of these appliances, it helps to have some knowledge of how they work, or at least how they are wired.

A single electron tube (a magnetron) generates microwave energy that travels to the cooking cavity by way of a short wave guide. The magnetron uses strong permanent magnets to make electrons swirl past resonant cavities inside the tube. The interaction of electric and magnetic fields sustains oscillation in the cavities.

The circuit powering the magnetron in almost all microwave ovens is similar to Fig 1.' There are slight circuit differences between brands and models. Some may not have the bleeder resistor across the capacitor. A few

1 Notes appear on page 51.

models use separate transformers for the filament windings.

Here's a surprising fact: The various models of microwave ovens have so many similarities that you can mix and match their components. You can build a very reliable power supply by using these components correctly.

The transformer high-voltage winding typically puts out 2100 V RMS under a light load. The capacitor and rectifier are electrically similar in most units. The greatest variation in the components of Fig 1 will be mechanical details such as shape and mounting configurations.

The circuit in Fig 1 is a voltage doubler. The magnetron serves double duty as rectifier and load. There is no second capacitor to smooth the current pulses flowing through the magnetron. Apparently, this modulates the frequency of the magnetron output to help prevent standing-wave patterns that might cause uneven heating in the cooking cavity.

The capacitors in these circuits are usually oil filled, 1 |iF or smaller and rated at 2200 V (ac). Generally, you can use them as filter capacitors for dc voltages up to 3200 V.

The rectifiers come in a variety of shapes. They are tough, reliable and much more convenient than strings of smaller diodes.

There are other useful parts, too. Look for fuses, fuse holders, relays, switches, low voltage electrolytic capacitors, resistors, connectors, highvoltage wire, hardware and other goodies for the power supply. I even used part of the cabinet for this project.

While on the subject of salvage parts, I recently saw an interesting source of parts that could be used in the screen supply.2 Some disposable flash cameras have electrolytic capacitors with ratings of 160 pF at 330 V (dc). Perhaps you can ask a local film processor for some of the used camera bodies destined for the trash bin or recycling center.

Souped-Up Transformers

The high-voltage transformers in typical microwave ovens have features that must be considered when using them in Amateur Radio power supplies. One obvious characteristic is that they have no case; the windings are exposed. Be careful not to nick or damage the windings as you work with these heavy, awkward components. This is especially important with the secondary (high-voltage) winding.

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