I have the following diagram for a transformerless power supply. I tried for real and worked, however I'm not sure what would be its max output amperage. I have reviewed a few sites about this but all of their schematics seems to be < 1A. In some of the sites it states this kind of artifact can't reach say 10A...but no reason is given. I understand all the risks this kind of power supply implies.

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  • 2
    \$\begingroup\$ Have you tried to do any analytical circuit solving techniques to understand how this circuit behaves under high current load? A look at R3 and Ohm's Law might be a good place to start - at some point its voltage drop will become too high and the output voltage will no longer reach the rated value. \$\endgroup\$
    – nanofarad
    Commented Mar 26, 2023 at 3:12
  • \$\begingroup\$ Note: 100 mA through 220 ohms is over 2 W, so I wouldn't advise using this at 100 mA unless you beef up R3 considerably. \$\endgroup\$
    – The Photon
    Commented Mar 26, 2023 at 5:26
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    \$\begingroup\$ Time to fire up your favorite simulator, play around with all values and build your own understanding. \$\endgroup\$
    – winny
    Commented Mar 26, 2023 at 7:46

1 Answer 1


The capacitor C1 is the limiting factor. 230VAC is 325V peak, so the capacitor stores at most 2.2uF * (325V)^2 = 0.232J at each peak of the input. For 50Hz that implies that the power the diode bridge can pull out of the capacitor is less than 0.232J * 100Hz = 23.2W, or an RMS current of 0.1A.

This is neglecting the diode drops and the losses in R2 and R3 (which will of course make things lower), and ignoring R1 (which is just a bleeder, it doesn't pass significant current).

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    \$\begingroup\$ Although it is understood, you should say "The capacitor C1 is the limiting factor.", just for added clarity. \$\endgroup\$ Commented Mar 26, 2023 at 9:11

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