I have the following power sources:

  • AC/DC adapter (5V-36V)
  • 5V USB

I connected the two power inputs properly (diode) but I am not sure about the reverse polarity protection. I haven't found any sources on the internet which mentions if there is any problem with ground short circuit. My problem is if neither the adapter nor the usb is isolated then reverse polarity connection could cause short circuit through the zero line in the "wall".


simulate this circuit – Schematic created using CircuitLab

So what is your opinion about this? Are the adapters isolated? How can I protect the circuit if the power sources are not isolated?


If the DC power supply or charger was not isolated, you could be electrocuted by just touching the output connector. Hence, all are isolated. Anyway you can't simply check If it is isolated by measuring the AC voltage between the output of the AC-DC supply and between the AC line. You may (and you probably will) get some reading because of two reasons: 1. small parasitic impedance found between the output and input of the isolating transformer. 2. capacitance between any object and earth, which is the neutral of the AC line voltage.

check this model to understand the first: http://www.cliftonlaboratories.com/audio_transformer_data_and_modeling.htm you can see a capacitor between first winding and second.but don't worry since the impedance is large enough to limit any current flowing through it and harming you. but this impedance form a path of the AC voltage to the voltmeter.

the second is the reason why you can get readings on an oscilloscope if the measuring probe was left floating. also when connecting a voltmeter probe to the mains socket and leaving the other probe floating...

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