Electronics > Beginners
Choosing the right transformer for a power supply
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Ian.M:
The average DC current you can draw without exceeding the transformer's RMS secondary current rating depends on the rectifier topology, and whether or not it directly feeds a reservoir capacitor.  See Hammond  Manufacturing Ltd (transformer division)Design Guide for Rectifier Use for derating factors.   The derating factors can be confirmed by SPICE simulation if you prefer a more analytic approach.

When you intend to run a regulator circuit close to its limits, another important consideration is the voltage range of the unregulated DC bus.   Typically, its necessary to allow for a mains supply variation of +/-10% from its nominal voltage (though some regions have asymmetric tolerances specified in their standards for mains supply), which will result in an approximately +/-10% variation in RMS secondary voltage and thus DC bus voltage.  Transformer regulation is another issue - a transformer specified for 15% regulation, will have an output voltage 15% higher unloaded than at full load, so its easily possible for the unloaded voltage with 10% high mains supply to be 40% more than the fully loaded voltage with 10% low mains supply that one has to design to if you wish to avoid regulator dropout.  This makes it extremely difficult to design a lab grade linear regulator circuit for an output voltage more than 50% of its max rated input voltage, unless you add a pre-regulator.   

A Zener clamped capacitance multiplier may be worth considering - it will follow the troughs of the ripple when the output is significantly loaded, which will vastly reduce the influence of the ripple on the main regulator, and the Zener clamping the base of the capacitance multiplier's pass transistor can be selected to limit the maximum unloaded voltage to keep it within the main regulator's input voltage rating.
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