srturner:
I have puzzled a lot over this. I have several power supplies that have 3-prong plugs despite being enclosed within plastic enclosures and having 2-wire outputs.
Which country are you in?
In the UK the earth pin is needed as it also mechanically unlocks the shutters in the socket that stop you putting your fingers in the holes.
PerryBebbington:
Which country are you in?In the UK the earth pin is needed as it also mechanically unlocks the shutters in the socket that stop you putting your fingers in the holes.
Sorry, should have specified. I'm in the U.S.
S.
"In the UK the earth pin is needed as it also mechanically unlocks the shutters in the socket that stop you putting your fingers in the holes"
We don't need shutters in the USA. (we know better)
raschemmel:
"In the UK the earth pin is needed as it also mechanically unlocks the shutters in the socket that stop you putting your fingers in the holes"We don't need shutters in the USA. (we know better)
In the USA you have that wimpy, low voltage nonsense, in the UK we have proper mains voltage.
"In the USA you have that wimpy, low voltage nonsense, in the UK we have proper mains voltage."
You blokes go 'all in' eh ?
We don't need shutters in the USA. (we know better)
Given your last president that comes as a bit of a surprise.
srturner:
...And what would be the purpose of connecting a laptop chassis to earth ground?
Perhaps for RS-485 or other hardware communications that need an Earth/Ground reference??
I agree that all exposed metal should be grounded but internal DC electronics need not be (it's usually design specific).
Transformerless power supplies are more common than you may think ... I use them daily with most of my power tools and appliances and have never got a shock (even in wet/damp situations) due to double insulation. These tools appliances are 2-prong connected (no ground).
There's lots of non-insulated PMIC parts available here.
Connecting the protective ground to the negative of the laptop (or in my case, USFF machines) power supply presumably relaxes the insulation requirements of the switchmode transformer as it need not actually be double-insulated. 
dlloyd:
Transformerless power supplies are more common than you may think ... I use them daily with most of my power tools and appliances and have never got a shock (even in wet/damp situations) due to double insulation. These tools appliances are 2-prong connected (no ground).
Sorry, but to repeat #13, that is simply misleading nonsense!
These power supplies all contain a transformer. It just happens to be a very small one operating at a high frequency. Perhaps you may require spectacles to locate it. 
Adding to #8, I was fascinated to perform a postmortem of the LED bulb in our en-suite this week after it flickered and finally went out.
It had eleven LEDs for a nominal 9.5 W luminaire, one had a large black burn and a second had a small black burnt spot. This is the (exact) style:
As it turns out, the opaque lower section covers a solid aluminium heatsink of the same shape on which an aluminium plate sits and on which the aluminium "PCB" LED board is mounted (with screws). Clearly a nominal 9.5 W LED system requires a quite substantial heatsink and why this part occupies a large proportion of the "bulb".
Further down in the base, a PCB assembly includes a tiny rectifier bridge, a even more tiny 8 pin (four are common) ASIC, three electrolytic capacitors, a possibly half watt resistor and an inductor. Clearly a complete constant-current "buck" converter and not the simply capacitive dropper which would be the basis of lesser lamps. The size of the chips is fascinating considering our 240 V (nominal) supply voltage.
These power supplies all contain a transformer. It just happens to be a very small one operating at a high frequency. Perhaps you may require spectacles to locate it.
Well, a transformer is a device made of two or more inductors, so one high frequency inductor doesn't count. If you have a link to a micro-sized CSA approved isolation transformer, I'm very interested!
Check the link provided from Digi-Key, there are over 200 components (however, skip the ones not recommended for new designs). Check "Typical Application" for schematics.
For example, in this non-isolated circuit, the GND and Neutral are separate:
Another example would be a Dremel Tool, something many hobbyists would use. The schematic for this is proprietary so cannot be found on the web. However, I assure you, its a non-isolated PCB design. Here's a universal PCB replacement. Note the AC Neutral is the ground reference of the circuit.
dlloyd
That circuit would go bang!
Neutral and earth are connected, by connecting a bridge rectifier like that you are shorting the diode between N and GND.
The output is not isolated, not withstanding what I just said half the time it is connected to L and the other half to N.
It is ONLY any use where the load is completely and safely enclosed from external contact.
That circuit would go bang!
It doesn't because that circuit would be all on one PCB which is double insulated from earth grounding ... the GND symbols used in these documents only pertain to the PCB's schematic. It's too bad they don't have a unique international symbol for the power supply's negative for mains based schematics.
Anyways, that's from one of the over 200 components sold (and in stock) from a reputable supplier (MP100LGN-Z) ... just click on the datasheet and check the typical application.
Here's another (MP171AGS-P) Typical Application:
For an example of a 5V, 50mA non-isolated supply, see Figure 10 on page 18.
The list goes on and on (so does the list of products that mains based electronics is used).
It is ONLY any use where the load is completely and safely enclosed from external contact.
I 100% agree.
It doesn't because that circuit would be all on one PCB which is double insulated from earth grounding ... the GND symbols used in these documents only pertain to the PCB's schematic. It's too bad they don't have a unique international symbol for the power supply's negative for mains based schematics.
When I wrote the common ground tutorial I agonised over this and similar points and whether they should be explained. Earth, ground and 0V are used interchangeably as if they mean the same thing, probably because they are often tied together in many circuits. I also suspect that they mean slightly different things in different parts of the world, making explaining them even more difficult in an international forum. As you will know from the tutorial I decided not to discuss them.
dlloyd:
Transformerless power supplies are more common than you may think ... I use them daily with most of my power tools and appliances and have never got a shock (even in wet/damp situations) due to double insulation. These tools appliances are 2-prong connected (no ground).
OK, so we apparently have been suffering a semantic problem here - not surprising I suppose.
My interpretation of a "power supply" is a module which you connect to the power mains and which has an output termination which allows you to connect it to a separate device. Examples would be the power "brick" for a laptop, a phone charger or the adapter for most appliances which have a plug and socket between such as outdoor Christmas lights.
You were referring to an integral part of a double-insulated appliance and of course you can use a "buck" regulator within such an appliance. Efficient switchmode regulators necessarily use a series inductor such as you illustrate in #32.
That MP100L is a rather cunning device - but not particularly efficient which is why it require a proper heatsink. It is a hybrid of a phase dimmer turned down "low" and a linear regulator. 
This topic was automatically closed 120 days after the last reply. New replies are no longer allowed.
