Problem with grounding??

[bernardgreen]

Am I missing something?

Yes there is stray capacity between components connceted to the mains and components connected to the ELV output. The largest stray capacity is between the windings on the output transformer.

Some PSUs have a very high value resistor between the output and the mains ground. This is intended to allow static electrical charge to be disapated to ground without damaging the equiment the PSU is powering.

 

[JohnW2]

Am I missing something?

Yes there is stray capacity between components connceted to the mains and components connected to the ELV output. The largest stray capacity is between the windings on the output transformer.

Errr? I'm most certainly not missing that - since it was my very point! If you quoted me more fully, you'll see that, in response to Simon's suggestion that the explanation was something other than stray capacitance, I wrote:

...but, in the absence of any capacative (or other) coupling between input and output of PSU, I don't really see why the presence of the filter capacitors should influence what voltage the equipment will be floating at. Am I missing something?

Some PSUs have a very high value resistor between the output and the mains ground. This is intended to allow static electrical charge to be disapated to ground without damaging the equiment the PSU is powering.

Hmmm. If/when such a resistor exists, it will surely obviously move the goalposts - but would it not then tend to bring that output potential close to the supply ground - rather than, as suggested, ~115V above it?

Kind Regards, John

 

[bernardgreen]

but would it not then tend to bring that output potential close to the supply ground - rather than, as suggested, ~115V above it?

If the ground rail of the PSU's mains side ( of the output isolation ) is grounded by an earth wire to the earth at the socket then yes this resistor will be able to do it assigned task. But when the PSU has only Live and Neutral connections then the filter capacitors will act as a voltage divider and bring the ground rail up to mid point of the mains supply.

 

[JohnW2]

but would it not then tend to bring that output potential close to the supply ground - rather than, as suggested, ~115V above it?

If the ground rail of the PSU's mains side ( of the output isolation ) is grounded by an earth wire to the earth at the socket then yes this resistor will be able to do it assigned task. But when the PSU has only Live and Neutral connections then the filter capacitors will act as a voltage divider and bring the ground rail up to mid point of the mains supply.

I'm lost!! I thought we were talking about the situation in which there were filter capacitors between supply L & N and the supply earth - which obviously assumes that there is an 'earth' (CPC) connection to the PSU. If not, then exactly what would these 'filter capacitors' be connected to? - connecting them to any part of the PSU output would seemingly make no sense (quite the converse, I would have thought!).

Kind Regards, John

 

[SimonH2]

If the ground rail of the PSU's mains side ( of the output isolation ) is grounded by an earth wire to the earth at the socket then yes this resistor will be able to do it assigned task. But when the PSU has only Live and Neutral connections then the filter capacitors will act as a voltage divider and bring the ground rail up to mid point of the mains supply.

Exactly, and I've just done some measurements on the 85W PSU for my MBP.
There does appear to be a 1k resistor between output and the earth terminal. There appears to be about 36M between one mains pin and the earth terminal, but open circuit between the other mains pin and earth. Obviously these are DC measurements, I don't have a capacitance meter handy.

I think the 36M is high enough to discount, but the 1k does mean that whatever is powered by the supply will be connected loosely to the earth terminal. If we assume a "strong" potential divider between L&E with the earth pin at the middle, then we have around 120V through 1k applied to the laptop casing.
As it is, it must be a fairly weal divider. Using my "not very high impedance" multimeter, I find 90V AC on the laptop case when using the unearthed adapter, and can draw 35microA to earth. Somewhat crude measurements, but assuming 120V driving 0.35mA, that's an impedance in excess of 3M.

Interestingly, when using the earthed adapter, I can still pull 25microA, but there's no significant O/C voltage.

And I've had a good look with a strong magnifier - there is definitely no double insulated symbol on the PSU.

 

[SimonH2]

I'm lost!! I thought we were talking about the situation in which there were filter capacitors between supply L & N and the supply earth - which obviously assumes that there is an 'earth' (CPC) connection to the PSU. If not, then exactly what would these 'filter capacitors' be connected to? - connecting them to any part of the PSU output would seemingly make no sense (quite the converse, I would have thought!).

Exactly - and no, I'm none the wiser either !

Apart from the "electric fizziness" I've felt from my laptop, anecdotally I've heard of people observing sparks when connecting/disconnecting TVs and stuff. Ie, the TV has one of these floating grounds, then you earth it through one of it's connecting leads.

 

[SimonH2]

Mind you, it does bring me back to something I experienced many years ago on a farm. We had a hot water heater (like a giant metal bucket on a stand, with a lid on top and a tap on the side). The farmer commented that he was getting a bit of a tingling from it - and when I looked, the contacts on teh earth in the old 15A socket were splayed out. I squeezed them back in, plugged in and switched on.
BANG as it blew the fuse, and a hole in the heating element that had come loose and moved - I think they were bare elements clamped between the bottom of the "bucket" and a metal plate, with mica sheets for insulation.

Lucky we didn't touch anything else that was properly earthed and bypass our insulating rubber wellies

 

[JohnW2]

There does appear to be a 1k resistor between output and the earth terminal. There appears to be about 36M between one mains pin and the earth terminal, but open circuit between the other mains pin and earth. Obviously these are DC measurements, I don't have a capacitance meter handy. ... I think the 36M is high enough to discount, but the 1k does mean that whatever is powered by the supply will be connected loosely to the earth terminal. If we assume a "strong" potential divider between L&E with the earth pin at the middle, then we have around 120V through 1k applied to the laptop casing.

I'm getting increasingly confused. What 'earth terminal' are you talking about?

As it is, it must be a fairly weal divider. Using my "not very high impedance" multimeter, I find 90V AC on the laptop case when using the unearthed adapter, and can draw 35microA to earth. Somewhat crude measurements, but assuming 120V driving 0.35mA, that's an impedance in excess of 3M.

[I take it that you mean 0.035mA]... which, if such were what it was due to, would correspond to a capacitance of ~1000 pF - which I assume is way out of the ballpark of any 'stray capacitance'.

As a matter of interest, what multimeter have you got that can measure AC microamps?

Kind Regards, John

 

[JohnW2]

... anecdotally I've heard of people observing sparks when connecting/disconnecting TVs and stuff. Ie, the TV has one of these floating grounds, then you earth it through one of it's connecting leads.

Indeed, I've personally experienced that on many an occasion (usually when connecting/disconnecting the arial plug).

Kind Regards, John

 

[bernardgreen]

Where the switched mode PSU is properly designed to work without an earth connection the power is transfered to the output via a high frequency transformer with the minimum possible capacitive coupling between primary and secondary windings. In these the filter capacitors are between the Live and Neutral and their main function is to reduce high frequency go OUT from the PSU and into the mains wiring of the building.

Some of the "lower quality" PSUs use a common design of PCB that is designed for use with an earth but often reaches the market with a lead that does not have an earth but with the Live to earth and Neutral to earth capacitors fitted. With the exception of the tingle effect these are for the most part safe to use as the isolation between mains and ELV output is adequate to limit the amount of energy that can cross the isolation to an amount that is non lethal.

Some of the " cheap and nasty " PSUs and many of those from dubious suppliers do not have adequate isolation and in use the isolation may break down and raise the ELV to mains potential. The effective output voltage may remain the correct DC voltage so the equipment supplied continues to operate correctly. But the supply to the equipment is 230 volts above ground without any current limiting other than the input fuse so a severe, maybe fatal, electrical shock is possible when the isolation in the PSU breaks down.

 

[bernardgreen]

(usually when connecting/disconnecting the arial plug).

I found it on a TV aerial whose mounting bracket was earthed. The lead and the dipole ( the actual aerial connected to the TV ) were a nice tangy voltage above the rest of the aerial. Not nice to find that when balancing on a ladder,

 

[JohnW2]

Where the switched mode PSU is properly designed to work without an earth connection ... the filter capacitors are between the Live and Neutral and their main function is to reduce high frequency go OUT from the PSU and into the mains wiring of the building. ... Some of the "lower quality" PSUs use a common design of PCB that is designed for use with an earth but often reaches the market with a lead that does not have an earth but with the Live to earth and Neutral to earth capacitors fitted.

I don't see the difference, at least qualitatively. In the latter case, two capacitaors in series between L&N with their midpoint connected to an unused 'earth terminal' seems no different from a single capacitor (of half the capacitance) just been L&N (i.e. the former case) - unless you are saying that, in the latter case, the 'unused earth terminal' is connected to something else internally.

Some of the " cheap and nasty " PSUs and many of those from dubious suppliers do not have adequate isolation and in use the isolation may break down and raise the ELV to mains potential. ...

I'm sure that's true, but there we are talking about badly designed, or faulty, eqipment - which is a rather different matter.

Kind Regards, John

 

[JohnW2]

(usually when connecting/disconnecting the arial plug).

I found it on a TV aerial whose mounting bracket was earthed. The lead and the dipole ( the actual aerial connected to the TV ) were a nice tangy voltage above the rest of the aerial. Not nice to find that when balancing on a ladder,

Indeed ... I was rather alarmed when I first experienced it! In my case, there are any number of amplifiers/splitters (as well as the actual aerial mounting) in the melting pot, so there's no real telling where the connection to earth was arising.

Kind Regards, John

 

[bernardgreen]

unless you are saying that, in the latter case, the 'unused earth terminal' is connected to something else internally.

the "ground" is also the 0 volt of the switching system which is connected to the primary winding of the output isolating transformer.

Badly, even dangerously badly, designed equipment is produced and sold and customers looking for cheap solutions buy it from dubious sellers.

Criminal manufacturers falsify testing certificates and reputable companies are deceived into buying second rate ( and some times dangerous ) items based on these false certificates. The reputable companies react and stop dealing with these manufacturers. Other dubious sellers do not react and continue selling the items.

 

[SimonH2]

I'm getting increasingly confused. What 'earth terminal' are you talking about?

The earth "pin" on the PSU - which has an oddball crackpot connector dreamt up by Apple - because clearly, for Apple, there aren't enough standards they don't already own.

Clearest image I could find quickly is on this page - though I don't recommend the alternative use !

It varies - some PSUs have a plastic stud, some have a metal one. There are two types of adapter to actually connect these to the mains : Some are what are often called "duckhead" (because with the US version fitted to certain PSUs, it can look a bit like a ducks head) - and at least for the UK version doesn't have an earth contact. They also do cables which for the UK and European versions do have an earth contact.

[I take it that you mean 0.035mA]... which, if such were what it was due to, would correspond to a capacitance of ~1000 pF - which I assume is way out of the ballpark of any 'stray capacitance'.

As a matter of interest, what multimeter have you got that can measure AC microamps?

It's an older version of this YF-3503 which I was surprised to find as what looks like a current product. The version I have is third down here.

As you can guess, finest Taiwanese engineering - but it's done me good service for many years, I think I must have had it for at least 25 years ! My biggest gripe is that the sockets don't take fully shrouded plugs. They have a recess, but it's not the full depth. And IIRC, the tracks on the PCB for the 20A connection were "a bit on the light side" but I was able to fit the smoke back in and fix it.

And as I looked up those images, I had a bit of an idea. While the meter has capacitance measurement, I've never actually used it. I twigged that the fine test prods I have would fit the socket, and I have test clips to fi the other end. So a quick measurement later, and I can tell you that in my supply, there is something in the order of 250pF between each mains pin of the supply and the earth stud.