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From the Q&A

With TJ Byers


Transformer Ratings

Question:

I have a power transformer with the following specifications: 24VCT (12 - 0 - 12) 10A. If the center-tap terminal is not used — and we used it as a 24-volt transformer — then will its rating remain at 10A or will it be half of that value (5A)?

B.Bill


Answer:

Transformers are rated in volt amps (VA). It defines the limit of the magnetizing field inside the transformer. If this limit is exceeded, the transformer goes into saturation and ceases to function properly. This is especially important when the transformer has more than one secondary winding. The resistance of the primary and secondary windings also influences the amount of current a transformer will handle because increased resistance and higher current makes heat — heat that must be dissipated. Your particular transformer has a rating of 240VA which means it will deliver 10 amps at 24 volts.

But I don’t think this is the answer you’re looking for, because I assume you want to turn this AC into DC. The DC current output is determined by the configuration of the rectifier circuit — of which there are three:

  1. Half-wave (single diode)
  2. Full-wave center-tapped (two diodes)
  3. Full-wave bridge (four diodes)

Refer to the diagram for the following discussion. The advantage of half-wave rectification is in its simplicity — one diode and a capacitor. They are generally viable only for power supplies of one-half watt or less, and require more filtering than full-wave rectification.

The full-wave center-tapped rectifier uses only half the transformer secondary at a time — which results in an output voltage that is one-half (0.45, to be exact) the full voltage across the secondary, but takes advantage of the full 10 amps your transformer has to offer. The full-wave bridge rectifier outputs a voltage that is 0.9 percent that of the transformer voltage, but can only use 0.62 percent of the transformer’s current. To achieve the desired DC load current, the transformer current should be 1.6 times higher. For example, to get 10 amps of DC, you need to have 16 amps available to the rectifiers.

Why? Because of the surge current required by the filtering capacitor. Transformers are not ideal and have an internal impedance or “regulation” characteristic. As the load increases, the output voltage decreases. Consequently, the transformer current must be sufficient enough to overcome the extra current imposed by the charging capacitor. A way to eliminate this requirement is to insert a filter choke in series with the line to the filter cap. Doing this will let you use the full potential of the transformer’s current rating — at the cost of an extra inductor and added space.


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