On an ATX PSU, 3.3V output was dead short with ground (checked with DMM),once i powered ON the PSU 3.3V output was providing the rated output (3.3V) how this is possible?
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asked Mar 13, 2016 at 7:34
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1 Answer
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Nobody knows how your particular PSU works internally, and the schematic is usually not public available.
But typically, an ATX PS delivers about 20-25A on the 3.3V rail. If it were a simple transformer, the wire on the 3.3V side would be quite short and thick. Now, a power supply is more complex. There is a rectifier, typically made of schottky diodes. They already have low forward voltages of about 0.4V, but if your DMM draws a really small test current in the order of 1mA through a diode which is made for >20A, the voltage drop can be negligible, and your DMM won't notice the Diode.
Last, there is a quite large capacitor (several mF) right before the output, and if you attach your DMM, the cap will look like a short circuit for a several seconds.
Finally, your question is a bit like I have a 3000W motor which works fine. How is it possible that I can easily turn the shaft by hand when power is off?
