Electrical tingling question

[ban-all-sheds]

I used to have a German desk lamp which used a 6V automotive bulb (I believe that in its day 6V Beetles were still common). It had a transformer in the base, a telescopic arm, and a 2-pin plug.

Depending on which way I plugged it into an adapter (before I put a 1363 on it) I could get that not-quite-a-tingle-but-a-slightly-"sticky/rough" feeling when dragging a fingertip lightly along the telescopic arm. (Body & shade were plastic).

 

[aptsys]

Can you expand on that ?, it implies that an SMPS can never provide a fully isolated ELV supply, something that a magnetic transformer can do.

It's highly unlikely that complete galvanic isolation is possible. You would at least see a Y2 capacitor between the primary and secondary which by it's nature will pass some current from the mains through it. Without this, everything attached to the secondary side just becomes one big antenna for all of the switching noise.

Except for cheap knockoffs, there are no mains powered SMPSUs that have a truly floating secondary.

 

[bernardgreen]

Without this, everything attached to the secondary side just becomes one big antenna for all of the switching noise.

Careful design of the drive to the isolation transformer primary and proper filtering of the output would remove the need for the Y2 capacitor. It might be that it is "cheap" design that creates the need for a Y2 capacitor to be fitted.

 

[aptsys]

Careful design of the drive to the isolation transformer primary and proper filtering of the output would remove the need for the Y2 capacitor. It might be that it is "cheap" design that creates the need for a Y2 capacitor to be fitted.

No, it's a fundamental of interwinding capacitance and high switching frequency. You get EMC issues no matter what the design. You can't filter the output because the signal is referenced to the mains side.

 

[bernardgreen]

You get EMC issues no matter what the design

I have to dis-agree from my experience of switch mode power supplies used to provide power to telephone exchange racks. The supply had to be "silent" to avoid introducing noise onto the comversatiom They provided +5 for logic and +/-15 for the TDM (time division multiplex ) audio bus circuitry.

it's a fundamental of interwinding capacitance and high switching frequency.

Interwinding capacity can be reduced by the use of two bobbins on different legs of the core. Resonant switching reduces EMC by driving the transformer with a wave form that is not square edged.

 

[JBR]

You are presumably talking about Class II equipment since, if it were Class I, the metal casing would be required to be earthed.

Assuming it is Class II, then does it not really come down to the standard argument - that creating any extra bits of touchable earthed metal increases hazards? In other words, whilst it may well get rid of tingles, it would slightly increase the chances of 'real' (potentially injuring or lethal) electric shocks.

Kind Regards, John

OK. The DVD recorders are both Panasonic, so perhaps I can assume they are correctly constructed and are safe.

As for earthing though, as I mentioned, after I had re-attached the aerial lead there was no further 'tingling'. I'm still not sure whether the aerial cable screening is an earthed component, but if so I assume that in attaching the plug to the recorder caused its casing to become earthed.

Perhaps I'm barking up the wrong tree!

 

[JohnW2]

As for earthing though, as I mentioned, after I had re-attached the aerial lead there was no further 'tingling'. I'm still not sure whether the aerial cable screening is an earthed component, but if so I assume that in attaching the plug to the recorder caused its casing to become earthed. ... Perhaps I'm barking up the wrong tree!

No, I think you're probably barking up the right tree!

If, as is quite likely, attaching the aerial connector has the effect of earthing the cases of the DVD recorders, then one would expect it to get rid of the 'tingling' and, unless you took steps to introduce isolation into teh aerial feed, you would not be able to do anything to prevent that 'incidental earthing' happening.

However, if there were no such 'incidental earthing', then to deliberately install earthing of the cases (just to get rid of the 'tingles') would represent creating more 'unnecessarily earthed metal' in your house which, at least in theory, increases hazards (since if you happen to touch something live, this increases your chances of simultaneously touching something earthed). In an ideal world, all buildings with electrical installations would be 'earth-free environments' but, in practice, that would be essentially unattainable.

Kind Regards, John

 

[JBR]

No, I think you're probably barking up the right tree!

If, as is quite likely, attaching the aerial connector has the effect of earthing the cases of the DVD recorders, then one would expect it to get rid of the 'tingling' and, unless you took steps to introduce isolation into teh aerial feed, you would not be able to do anything to prevent that 'incidental earthing' happening.

However, if there were no such 'incidental earthing', then to deliberately install earthing of the cases (just to get rid of the 'tingles') would represent creating more 'unnecessarily earthed metal' in your house which, at least in theory, increases hazards (since if you happen to touch something live, this increases your chances of simultaneously touching something earthed). In an ideal world, all buildings with electrical installations would be 'earth-free environments' but, in practice, that would be essentially unattainable.

Kind Regards, John

Thanks, John.

To be honest, I'm not really sure whether the screening of the aerial cable is earthed. I assume it just travels up to the aerial mast on my chimney and is connected to some part of the aerial itself, so how it could be earthed I don't really know. Perhaps there is an earth connection in the wall socket somewhere. I haven't removed it to have a look.

Perhaps I can connect my multimeter to both the aerial plug and an earth pin in a socket to see if there is any potential difference between the two.

 

[JohnW2]

Thanks, John. To be honest, I'm not really sure whether the screening of the aerial cable is earthed. I assume it just travels up to the aerial mast on my chimney and is connected to some part of the aerial itself, so how it could be earthed I don't really know.

Agreed. I suppose if all the brickwork, from chimney to ground, were very wet, there would be some sort of connection to earth.

It might not take all that good a connection to earth to get rid of 'tingles'. Indeed, even without any real connection to earth, a long run of the outer of the cable might have sufficient capacitive couple to earth to reduce or eliminate the tingles.

Perhaps I can connect my multimeter to both the aerial plug and an earth pin in a socket to see if there is any potential difference between the two.

I think that would have the potential to be misleading, since, if the outer of the coax is 'floating', a high-impedance voltmeter could well show quite a voltage due to capacitive effects.

Kind Regards, John

 

[JBR]

Thanks. This is all very interesting for one who is not particularly well versed in electrickery.

 

[JohnW2]

Thanks. This is all very interesting for one who is not particularly well versed in electrickery.

You're welcome.

Kind Regards, John

 

[aptsys]

I have to dis-agree from my experience of switch mode power supplies used to provide power to telephone exchange racks. The supply had to be "silent" to avoid introducing noise onto the comversatiom They provided +5 for logic and +/-15 for the TDM (time division multiplex ) audio bus circuitry.



Interwinding capacity can be reduced by the use of two bobbins on different legs of the core. Resonant switching reduces EMC by driving the transformer with a wave form that is not square edged.

Resonant switching doesn't improve anything, think of it like your RF transmission line - you have your switching frequency (and harmonics if you don't do valley switching) which is coupled across the transformer. You've now got everything on the secondary side of the transformer acting as one half of the dipole. The other half is on the primary side, so you cannot attenuate the signal without reference to the primary side.

Resonant switching improves your efficiency but only relieves some EMC complaints. Interwinding capacitance is traded off for other transformer losses, and there is a steep slope of efficiency loss when you move the coils away from each other.

The supplies at BT have never been floating output, so they are not relevant

 

[bernardgreen]

The supplies at BT have never been floating output, so they are not relevant

The power to the audio and logic processing circuits may or may not be ground referenced, the nominal 48 volt lime supply has it's positive side connected to Earth

It was T&N research and development where low EMC switch mode power supplies were used n te racks.