# center tap transformer full wave rectifier current rating

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.

• A 24v 5A transformer will not power a 24V 7A load, period. That leaves you with only one option! Mar 5, 2018 at 15:59
• The RMS current drawn from the secondary will be approx 2 times the DC load current. This applies to bridge rectifiers. And CT full-wave rectifiers have the worst utilization factor (You probably know why), so the RMS current drawn from the secondary winding could be higher than x2. Mar 5, 2018 at 16:04
• By a 24-0-24 transformer, do you mean a 48V center tapped transformer? Mar 5, 2018 at 16:06
• @Rohat Thanks, so RMS AC current will not provide average DC current? i.e. I need e.g. 2A RMS AC for 1A average DC? So you're saying that since I can pull 2x RMS (10A AC) from the 24-0-24 transformer, I will end up with 5A average DC? So the 0-24 transformer would actually give only around 2.5A DC, even though it is rated 5A AC? But VA in = VA out, right? So what am I losing? Mar 5, 2018 at 16:56
• @crj yes I think it is called a 48V CT. Two 24V windings connected in the middle. 24-CT-24. Mar 5, 2018 at 17:56

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.

• Why the downvote? This is the correct answer. Mar 6, 2018 at 15:12
• Thanks for the reply. This is for a bench power supply (my first proper one), so a SMPS will not do. I do not want to go into DC-DC tracking pre-regulators right now. I think at 33V peak the current will drop to 24*5/33=3.6A or less. Which probably answers my own question. Mar 6, 2018 at 16:05

simulate this circuit – Schematic created using CircuitLab

But you are better off defining your DC load resistance and get a suitable SMPS.