Slightly OT Electrical Question

[Anthony Bowles]

Not DIY because I wouldn't even consider doing much in the way of electrics in the house but I'm hoping one of the electrical gurus out there might be able to shed some light on this.

Basically I'm wondering if I've got a problem with the electrics or just

2 "appliances" at the same time. The other day the DVD player was only transmitting sound via it's coax connection (to the amp). No picture (or sound) was coming out of the scart. After much fiddling with various connections etc I touched the case of the DVD player and could feel a humming through my finger tips. If ran my fingers across the case they didn't so much as "glide" across it as "juddered". Anyway this seemed to cure the problem and the DVD came back on. The only thing I could think of was either it's becoming faulty or it was some build up of static (however I didn't get a static shock).

Anyway, it's happened again. I don't think I've used the player in a week and it's gone off. Touching it doesn't help this time but it's got that same humming feeling.

Now this is where I start to think "hold on what's going on" because the same thing is now happening when I put my Palm T3 (metal case not plastic) into it's cradle. When I touch it in the cradle it's humming and feels "sticky". When it's out of the cradle it smooth. I've had it a year and never noticed that before.

I haven't noticed it on anything else but I'm not about to go round the house touching everything with a metal case!

In all honesty my other half thinks I'm daft cos she can't see what I'm going on about (beyond the fact the DVD player's gone funny).

Is it possible something's wrong with the electrics (in which case I'll get an electrician out) or just coincidence?

Cheers

Anth

[G&M]

Is there a full moon ?

Failing that have you just had a new carpet installed ?

[John Rumm]

I have experienced exactly the same phenomenon. It s probably the mains getting capacitively coupled to the case work, possibly in combination with a poor earth (for devices that are earthed).

Is power to your property fed by overhead cables? Have you tried plugging it into a different mains socket (and all the things that it is connected to like the TV, amp etc?

The feeling you get is unlikely to be dangerous in itself, but may serve as a warning that the earth at that socket position is not as good as it could be.

[Mike Harrison]

This is normal for any non-earthed mains operated device with a metal case - it's capacitive coupling of the mains through the transformer. Very low current and not harmful.

[Stefek Zaba]

aye, more or less;

if it's through the transformer it's scarcely capacitive, is it It is usually capacitive coupling, either through interference-suppression capacitors in the mains input filter (but in that case the case *should* damn well be earthed, so you won't feel any juddering/humming/whatevering: and if you can, it could indicate a broken earth conductor in the supply lead (bad) or at the wall socket (worse); or, in the case of Class-II ("double insulated") kit, designed not to need an earth connection, capacitive coupling through stray capacitances as mains wires or switched-mode PSU wiring comes close to the case. VCRs and TVs additionally may have small-value capacitors which couple the mains feed to the outer core ("earth") of their aerial connectors - for these, you can sometimes get rid of the tingle by reversing the mains input lead if it's one of those figure-of-8 leads.

. Very low current and not harmful.

Not harmful by itself, no, but in some cases an indication of the case not being earthed when it should be. In the OP's case, it's *not* that for his Palm 3, but possible for the DVD player, depending on whether it takes a 2-wire feed from the mains or a 3-wire one...

Stefek

[N. Thornton]

this is normal with unearthed goods. There is a small amount of capacitance from L to case, resulting in miniscule current flow. This causes that feeling when you stroke the metal gently. Normal and harmless on unearthed items. If it happens on earthed goods its another matter entirely.

NT

[Mike Harrison]

Yes - transformers have capacitance between their primary and secondary windings.

[Stefek Zaba]

A miniscule amount, mayhap: 99.999% of the energy transferred is transferred *inductively*, though (and 99.999% of the losses are inductive, too ;-)

[The Natural Philosopher]

I missed the original post but all steel cases will vibrate at 100hz in the presence of a mains frequency transformer due to magnetic field leakage.

Its even possible to induce quite large hum currents in them - you should never for exaple earth circuits to more than one part of them, as they will be at different potentials.

[The Natural Philosopher]

neither of which is normally connected to the case.

Unless the case is earthed it is EXTREMELY bad practice to even use an RFI filter connected to the case.

However it is done, and the net result is a case at 110vAC with respect to neutral. albeit at fairly low potential.

I suspect they assume it will be connected to some other properly grounded kit.

[The Natural Philosopher]

There is enough capacitative couling for better trasnformers to be wound with an (earthed) screen winding between primary and secondary layers to reduce the transfer of energy to approximately zero.

[Stefek Zaba]

Thanks for that nugget - you learn something every day here on Usenet!

[Andy Wade]

A nonsensical statement, Sir. 100% of losses are resistive :-). Pure inductance, whether of the self or the mutual flavour, can only store (and retrieve) energy. If losses you want, resistance you need.

Losses in transformers come in three sorts, (i) I^2 * R loss in the windings (cooper loss), (ii) eddy current loss in the core - which is just another sort of I^2 * R loss, and (iii) hysteresis loss in the core, proportional to the area of the B-H loop being traversed and the frequency, and representable by an equivalent shunt resistance.

Imperfect magnetic coupling between the windings will cause an additional voltage drop, compared with ideal 100% coupling, but this is not a dissipative loss. In equivalent circuit terms it appears as the voltage drop across an inductance - the leakage inductance.

In small transformers of the type likely to be found in domestic equipment the impedance of the transformer is dominated by the winding resistances; leakage reactance is negligible. Most of the loss is copper loss (except off-load).

[Stefek Zaba]

Argh! Uncle! Uncle! I give up!

You and the NatPhis are both right to take me to task on the sloppy stuff I wrote. What I was kinda trying to get across is that the energy transfer and energy losses are all to do with electromagnetism, rather'n capacitive effects. And I was more or less wrong on both counts (tail between legs) - there is an (unwanted) capacitive component to the transfer, and as you detail the underlying mechanism for the energy loss (inefficiency, giving rise to heating) is resistive - with actual "ordinary" resistance dominating in small domestic transformers, while the eddy currents (which I was thinking, loosely, about) are more accurately considered as resistive.

We now return you to your normal broadcasts...

Stefek

[The Natural Philosopher]

Actually the iron losses whilst they may appear as resistive in an idealised model, are not strictly resistive in nature...

hmm.

[jim.]

Hiya Anth. Get a neon screwdriver, check the cases involved if it lights then contact a qualified electrician. A newsgroup's great for tips on the odd diy project, but this one's possibly very dangerous. best left to peole who know what they're talking about. (imo)

jim.

[Andrew McKay]

A neon screwdriver is considered to be potentially very dangerous (as in lethal) to those in the profession.

Andrew

[Dave Plowman (News)]

A neon screwdriver will by its nature show a positive result on some things which are perfectly safe.

[Christian McArdle]

If I had a neon screwdriver, it would be in the bin. Terrible things, full of false positives and false negatives.

Christian.

[Bob Mannix]

You're just too picky! All tools should be used within their limitations. A neon screwdriver is a useful screwdriver that has the added benefit of being able to tell which of two wires is live. I can live with false positives and I wouldn't trust my safety to a negative - I'm getting to old to flick live wires with my finger to check them :o). Mind you, I think we've already had this conversation.