Let's say one has a typical HP 19.5V Laptop charger, and consider the isolated output DC end. There is an metal outer-pin and inner-pin (which is difficult to touch with fingers). My theoretical understanding is that the outer-pin would have a potential of 0V and inner-pin has 19.5V, relative to ground?

As an aside, would the presence of a grounding-pin affect these?

  • \$\begingroup\$ They are isolated so they are only referenced to themselves... Effectively they will be close to ground since grounded objects are everywhere and it becomes weakly capacitively coupled through the air. \$\endgroup\$ – MadHatter Sep 29 '16 at 2:29
  • \$\begingroup\$ If the output of the charger is isolated from the AC input, and not connected to the AC Earth/Safety Ground, you cannot measure any voltage between either the negative or positive terminal and Ground. \$\endgroup\$ – Peter Bennett Sep 29 '16 at 3:01
  • \$\begingroup\$ For safety reasons, the outputs of laptop chargers aren't left completely floating; they're usually coupled to ground through a capacitor or a direct connection. See this answer. \$\endgroup\$ – Matthew Di Nardo Sep 29 '16 at 3:19
  • \$\begingroup\$ @PeterBennett How would current flow then if that was the case? I thought it'd be like a battery where negative terminal would have ~0.3V and positive ~1.2V, relative to ground \$\endgroup\$ – user40300 Sep 29 '16 at 3:47
  • \$\begingroup\$ A 1.5 volt battery not connected to anything else will have 1.5 volts between its terminals, but you can't measure or specify any voltage between either battery terminal and anything else, because there is no path for current to flow between the battery and anything else, including ground. \$\endgroup\$ – Peter Bennett Sep 29 '16 at 4:16

This depends on how it is designed. I have two different power supplies, one has a mains connector with earth connection, the other one has only live and neutral.

I tried to measure resistance from output side to the input side to see if it was completely isolated (no path for the current) or if they have implemented some sort of reference to anything on the input side.

The one with the ground connector had the outer ring ("0V") connected with a 1 MegOhm resistor to mains earth. So in this case the output was referenced to ground and the output voltage was measurable with respect to ground. (I tried that and I measured the 19VDC from mains earth to output (inner ring))

The other one is different, it had no measurable connection from output to either neutral or live. I tried measuring the voltage with respect to ground, but on every pin (+12V and +5V and 0V in this case) I got the same voltage (314mVDC). This means that there is no reference to ground in any way, otherwise I should have been able to measure the output voltage at least with some offset, but all pins had the same voltage to ground.

My measurement influenced the circuit in the second case (the multimeter created a connection to ground which was not there before).

If there is a grounding pin (in the sense of earth), I would think, that 0V of the output and ground are referenced to each other. If you mean the third pin you see sometimes in a laptop charger, those are often used for communication to identify the charger and their capabilities (you can get 45W, 60W and 90W chargers, so the laptop has to limit the battery charging currents in some cases).

So you see that there are different approaches on how to implement this. I am not a designer of power supplies, so I cannot comment on which way is better, safer or needed by regulations.

  • \$\begingroup\$ Thanks! 1)What do you think would happen if that charger w/ ground pin you tested was plugged into a socket with no ground? (Just a fake third hole) 2) What do you mean by "(+12V and +5V and 0V in this case)"? I thought there were only two pins, and what do those 3 values represent? \$\endgroup\$ – user40300 Sep 29 '16 at 13:02
  • \$\begingroup\$ @user40300 1) nothing, the output would loose its reference to ground, but it would still work correctly. You'd measure something like in the second case. 2) The second power supply generates several voltages (it is not a laptop power supply but for an external hard drive bay for 3.5" HDDs which require 12V and 5V (and 0V to return the current)) \$\endgroup\$ – Arsenal Sep 29 '16 at 13:13
  • \$\begingroup\$ 1)Hmm, so the outer-ring shouldn't be 0V if the ground hole was fake? Would it necessarily be ~300mV then? Any more info about this special case would be greatly appreciated \$\endgroup\$ – user40300 Sep 29 '16 at 13:42
  • \$\begingroup\$ @user40300 I guess you can measure almost anything (might depend on the multimeter). It just doesn't make any sense to try to measure a voltage between isolated circuits and at the time where you measure, you will influence the circuit. I really don't know where you are trying to go, asking all this. If you want to know if your supply is safe, you better start measuring the isolation and not the voltage from primary to secondary side. \$\endgroup\$ – Arsenal Sep 29 '16 at 14:30
  • \$\begingroup\$ Would measuring the potential between the outer-ring and the floor tell me about its safety? \$\endgroup\$ – user40300 Sep 29 '16 at 16:01

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