Photo 2 (left): Oscilloscope pattern showing output noise (top trace) and the current through the plate winding, using the vertical EI-core transformer. If an AC magnetic field that leaks from the power transformer intersects a wire (or PCB trace), it will induce a small current into that wire, just as if it were a secondary winding on the transformer. The resultant noise voltage produced is generally very small, but in audio equipment, even a few millivolts of noise in a sensitive circuit can be audible. This field, called “stray flux,” is a primary concern in the selection of a transformer for audio equipment. Of course, nothing is perfect, and there is always at least some part of the magnetic field that escapes from the transformer. In a perfect transformer, the entire magnetic field generated by the primary winding would be contained entirely within the transformer and pass through the secondary winding(s). Sometimes this problem is simply the result of not enough filtering or regulation of the DC power supply, or perhaps a ground loop in an input circuit.īut often, magnetic coupling from the AC power transformer is the culprit. If you’ve built much audio equipment, chances are that at one time or another you’ve run into a problem with line-frequency noise getting into the audio signal signal path.
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Magnetically Induced Noise And Stray Flux I’ll describe a number of the more common types in detail (Photo 1) a little later. There are many different ways to build the core using different materials, processes, and shapes. This is done because the permeability, or magnetic conductance, of iron is much higher than that of air, which allows a transformer to work much more efficiently. All the windings are placed on a “core” made of an iron alloy. The voltages required for the rest of the system are generated in “secondary” windings. In a power transformer, the “primary” winding is driven with AC line voltage - the power that comes out of the wall.
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Conversely, the field passing through another winding induces a current in it, which is used to drive the load. Coiling wire into a “winding” and passing current through it produces the magnetic field. Grossly oversimplified, a transformer works by converting an AC current to a varying magnetic field, and then back into an AC current. Clockwise from top: EI-core plate/filament transformer, toroidal transformer, PC-mount UI-core transformer, and C-core transformer. Photo 1: An assortment of power transformers.