for a 24V output, I have the option to purchase a 0-24V 5A transformer, or a 24-0-24V 5A transformer. Can I draw 24V 7A from the 24-0-24V transformer (full wave rectification with 2 diodes)? Assuming I^2*R heating loss, since each winding is being used half the time, I should be able to increase the current sqrt(2) times, right? Any other possible problems (core saturation, core heating etc) that I should be worried about? Will the 24-0-24 transformer provide a bigger safety margin if I'm only drawing 5A continuously?
edit: sorry for the stupid question. Neil_UK is right. To answer my own question, I may be able to draw 7A, but the voltage will drop, as it is defined by the secondary winding resistance (and the primary side voltage and turns ratio). 48V 5A is not the same as two 24V 5A transformers. I'm probably better off getting a single 24V 7A (or 8 or 10A) transformer. Less headache, more stable voltage and more safety margin at not too much higher cost (considering transformers live for decades).
PS: also, this is for a bench power supply, so SMPS will not do (without DC-DC tracking pre-regulators and stuff). In hindsight, I think I will need 12V and maybe 6V taps.
Best Answer
You can safely draw about 5A from a 48V CT 5A(RMS) transformer (rated at 240VA) with a full-wave (2-diode) rectifier and filter capacitor.
The voltage will be about 33V peak, so about 170W total (including rectifier losses) with a large capacitor.
With a linear regulator you will get 5A out of it, plus about 50W of waste heat. With a buck switching regulator you could get perhaps 6A depending on the efficiency of the regulator.
All in all, though you'd be better off buying a decent brand of commercial off-line switcher that can supply 24VDC regulated at 7A or 10A or 15A for most, not all, applications. A 14.6A (350W) supply is less than $50 US, almost 90% efficient, and weighs relatively little.