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I saw this project of a linear power supply, and I couldn't understand how the output can deliver 4 amps assuming maximum current of the transformer of 4 amps before the voltage starts to drop.

I understand that 4 amps(RMS) can deliver 5.5 amps peak, but still, from my experience when the filtered output is close to the AC peak voltage (which is here), we assume that the transformer's current is twice the load current or more, because of the short duration in which the transformer's voltage is larger, right?

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Just a few points for clarity. The transformer's voltage will drop if any load at all is added. The rated current is the current at which the transformer will give it's rated voltage output. For instance, I recently bought a +/-12v center tap transformer to build a benchtop power supply and it actually has a no-load voltage of 36V.

Second, be aware that RMS voltage and current do not represent the same energy delivery as an equal voltage DC. An equal RMS voltage means that the circuit will produce equivalent ohmic heating(power loss) to that voltage in DC.

From that fellow's project, it doesn't seem to be clear whether he has tested his PSU at 30V and 4A continuous. There is a chance that running it continuously would burn out the transformer. That being said, a transformer's durability ratings relate to not going over the insulation thresholds and not running at a current that will cause it to overheat. It is quite possible that the PSU exceeds the rated load of the transformer, but not the safety margins, whatever they are, built into the transformer, and as a result, runs at greater than it's rated load. He is also using a fan in his case, which may be keeping the transformer cool enough not to thermally cascade, the same as you can run greater than AWG rated current on just a copper wire so long as you cool it well enough, and have to derate it (consider it to be able to carry less current) when it will be exposed to higher ambient temperatures.

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  • \$\begingroup\$ Well, the story behind the question is: I wanted to build a power supply, so I got a 24V-3A transformer, given that I'll need 23V at the regulator + 2V across the diodes, that leaves me with 7V (ripple and voltage drop under loading), right ?. \$\endgroup\$ – M RK Jul 27 '18 at 14:55
  • \$\begingroup\$ The supply should deliver 1.5A, but after testing it with a resistive load, the voltage after the caps dropped to 19V at only 1A, apparently the current spikes reached 3A+ and loaded the transformer enough to drop the voltage to 20V ac, so, this is what I couldn't understand, especially that I see circuits that uses 3A transformer to give 3A out, wouldn't they at least need a 6A trans. ? \$\endgroup\$ – M RK Jul 27 '18 at 15:05
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Those assumptions are incorrect.

VA output of linear transformer must be derated at least 20% into bridge cap linear supply because charge current is much greater than load current at low duty cycle and losses are quadratic.

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