Engineering Principles V11

Clive W. Humphris

TRANSFORMERS: Transformer Alternating Voltages and Currents.

Most power supplies are based on a step-down AC voltage derived from a mains-transformer. These, like most components are available to suit every conceivable application. Choosing a suitable supply rail voltage and therefore transformer winding ratio's for your project can save expense on having to reduce surplus overhead voltage elsewhere i.e. through dropping resistors or voltage regulators, which in turn then become hot as energy is wasted, requiring the use of heat sinks.

An ideal transformer as shown here will have an efficiency of 100%. There will be no losses in the flux linking between the primary and secondary windings. In practice this is not so, nevertheless high levels of efficiency can be found.

Any alternating current changes made in the primary winding will be reflected in the secondary. Transformers can be step-up where N2 > N1 or step-down N2 < N1.

As the flux (dotted line) is common to both primary and secondary an induced EMF per turn will be the same for both windings. Once this is found then all other calculations follow. Note the current and voltage relationships between the primary and secondary. Where for a step-up transformer the ratio of primary to secondary current is inversely proportional to the turns ratio.

The power dissipated in the load will be reflected back to the input (primary) circuit, i.e. the power input must at least equal to that consumed by the load.

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