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I am trying to establish a suitable power supply for a device that I have that runs of 24VDC.

In the operating manual that I have for the device it says to ensure that a "grounded power supply" is used to prevent damage to both the device and the user. Also included in the manual was a schematic something similar to the following:

schematic

simulate this circuit – Schematic created using CircuitLab

The manufacturer of the device also sells a suitable but somewhat pricey power supply for the the device that looks like a normal power supply to me with a live, neutral and earth on the input.

All of this basically got me wondering about the internal anatomy of a bog standard power supply...

Does the power supply that the manufacturer sells for this device do something like this:

schematic

simulate this circuit

Or is this how all power supplies with L,N & E pins work?

As you may be able to tell, I am a little confused but hopefully you get the general jist of my problem.

If someone could shed some light, it would be very much appreciated. Thanks.

To clarify, this is the sort of thing that I am expecting to have to buy: http://uk.rs-online.com/web/p/desktop-power-supply/0188781/

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No, it is not the way all supplies work. In some cases, just the power supply case is grounded and the output may be floating or have an optional tie to ground (this is the way most bench supplies are wired, excluding some of the big SMPS types).

However, in this case we can see that the case is made of plastic and there is nothing to ground, so the output is most likely connected to the earth pin.

There is a third possibility- that earth pin of the supply is only connected to the output through a Y capacitor (for noise reduction purposes), but that is probably less likely.

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The for electrical safety reasons the ground connection must be connected to the housing, frame, etc. of the power supply. This makes that you need a three prong power plug. The gnd connection on the secundary of the transformer is then again connected to the same ground connection. This makes that both the power supply and the device are grounded.

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It depends. You can have an unearthed mains supply in some situations (for instance, emergency battery sets) even, so your earthing/grounding depends on application. Many devices have no referencing to mains potential at all (which is what earthing something basically does, references it to the nominal 0V on the mains supply system. By the way, personally I always think of "grounding" as serving an active purpose in the circuit whereas "earthing" is done for safety purposes, though obviously an arrangement can do both at the same time.

So where you want an earthed point on your secondary side is when you've got exposed conductive parts connected to the circuit. An obvious example is audio, in which you've got all those connectors with their metal bodies and pins. So that if an exposed conductive part gets a dangerous potential on it, the circuit breaker pops rather than the person touching it getting a shock.

Hence why the old hack of lifting earth wires to solve ground loops is considered unsafe.

I took a laotop PSU apart a while ago and noted that the earth connection which grounds the 0V side of the 20V output was routed through a small hole in a live heatsink. It was flimsy single insulated wire. Hmmm.

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