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I'm just wondering what's the difference between a simple power supply with a ground prong and one without. I'm talking about those little DC power supplies with barrel plugs you can easy find on any small appliances. As far as I know a barrel plug has two contacts: one positive and one negative. What's the role of the third ground prong on the AC side for those have it? I hope I'm not mistaken but a small DC power supply like that use floating voltage right? With all plastic construction I don't think it can do much in terms of safety.

Actually I took two PS and toyed with it. One is relatively higher quality with a 3 pronged IEC C13/C14 plug. The other one has a cheapo 2 pronged design. I measured the DC voltage (with a multimeter) between the "wall-socket ground" and the sleeve of the barrel plug. The 3 pronged one resulted a 0 V, while the 2 pronged one showed a fluctuating ~300 mV potential difference. So does that mean the 3 pronged power supply has the negative terminal actually grounded? If it is what's the pros and cons of doing that?

I'll greatly appreciate any input! Thank you!

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    \$\begingroup\$ Redo your measurements: (1) Unplug the power supplies. (2) Switch your meter to resistance. (3) Measure from earth pin to barrel jack. \$\endgroup\$ – Transistor Nov 4 '17 at 0:49
  • \$\begingroup\$ @Transistor The resistance is <1 Ohm. And the meter showed positive in continuity test. So can I deduce the sleeve is grounded? \$\endgroup\$ – Den Nov 4 '17 at 0:58
  • \$\begingroup\$ Do they both have a "square within square" symbol on them? \$\endgroup\$ – ThreePhaseEel Nov 4 '17 at 3:59
  • \$\begingroup\$ fyi, an example of a small appliance is a toaster or a blender. i think that you mean a small electronic device \$\endgroup\$ – jsotola Nov 4 '17 at 4:42
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    \$\begingroup\$ @ThreePhaseEel Only the 2 pronged one has that symbol. That's a double insulation symbol right? \$\endgroup\$ – Den Nov 4 '17 at 5:51
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Grounding the output is a cheap and effective way to reduce electromagnetic interferance. but your equipment is then succeptable to ground loops.

Y capacitors are another way but they produce barely preceptable electric shocks. (rub the plug lightly on your external ear and you can hear the electric current)

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