OK, the title doesn’t sound too inspiring, but it does capture a question that always comes up when selecting a transformer to use in a full wave rectifier system.

The basic capacitor input schematic is shown above.

From a first guess, we might imagine that the DC current flowing to the load will be equally divided by the rectifiers. So when the top rectifier is conducting, half the current flows, and when the bottom rectifier is conducting another half of the current flows. Thus we might come to the conclusion that the RMS current through each half of the secondary is half the DC current. So if 100mA DC is needed, we would say that we need a secondary current rating of 50mA. This is WRONG!!

If we did go down this route you might “get away with it” for a while, even a long while, but by selecting such a small current for the secondary, means we are selecting a thinner wire to use for the secondary turns. Thinner wire has higher resistance and so will warm up more than a thicker wire. All the wire in the secondary will warm up uniformly and, in the extreme, either fuse open or catch fire. Even if this sad state of affairs did not happen, chances are that over a prolonged period of use, any faults in the wire will probably show themselves, and result in an open circuit secondary.