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I have tried with many (working) 230V - 9V DC wall adapter power supply (of different brands, some of them coming with an electronic Yamaha device, etc.):

  • they all show a higher voltage when measured in "DC mode" by my multimeter, like 12V - 14V

  • they even show a higher voltage when measuring in "AC mode" of the multimeter: ~ 30V! This doesn't really make sense: the output of the power supply should be DC, so a theoretic AC voltage should be 0V. Ok, I understand the output of such an adapter is never "perfectly" DC, and there might still be a small sinusoidal component... However, why 30V?

What could be the reason of this higher than normal voltage?

Could it be a ripple after the rectifier?

PS: I don't own a oscilloscope to test the output of the wall adapter, but I will try to borrow one!

PS2: if it was only with 1 such adapter, it could just be a faulty one. But now I've tested about 6 different (working) adapters of different brands...

PS3: it's a cheap multimeter (link) but efficient. All other voltages that I measured have always been correct.

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  • \$\begingroup\$ These are probably unregulated supplies. Products designed to use one of these specifically take that into account. For a hobby project consider a switching supply almost implicitly with regulation. \$\endgroup\$ – Chris Stratton Sep 8 at 10:38
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    \$\begingroup\$ Ripple + the fact that without a load their output will be higher (if its just the transformer + diode bridge + capacitor type) \$\endgroup\$ – Wesley Lee Sep 8 at 10:39
  • \$\begingroup\$ A good tutorial here: Hey! Why is my 9V wall-wart outputting 14V?!. \$\endgroup\$ – Basj Sep 9 at 15:04