QEX

The primary and secondary windings are normally wound separately instead of one over the other (see Fig 2). The filament winding, however, is usually wound over the high-voltage (secondary) winding. One side of the high-voltage winding is often grounded to the frame. This is the "cold" or low voltage end of the winding. Even transformers with ungrounded leads do not have sufficient insulation to safely withstand the full secondary voltage at the end of the winding that is normally grounded. For this reason, the transformers must be used in pairs in a full-wave supply.

One feature of the transformer must be modified: A small block of laminations is wedged into the core. These magnetic shunts allow some of the magnetic field to bypass the secondary winding. The effect is similar to put ting an inductance in series with the primary. In a microwave oven, this practice helps protect the magnetron from overloads. In a power supply, it will cause poor voltage regulation.

Fortunately, the shunts are pressed in place and can be driven out with a hammer and punch. When doing this, support the transformer core so that the windings are safe from force exerted on the shunt and core. One method is to place wooden 2x4s under each side of the core so that the windings are clear of the work surface. Another method is to bolt the core securely to a frame, such as the one used to construct the power supply.

Some of the shunts are pressed in very tightly, and they may be held together with a rivet. For best results, use a two-pound hammer. The punch

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