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SCART Shock

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Amr_bekhit
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Post Number: 293
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Posted on Friday, 29 December, 2006 - 12:09 pm:   Edit Post Delete Post Print Post

Hello all!

I've been messing around again with trying to output stuff to a TV and so I've been working with the SCART interface (I found an informative website here: http://www.retroleum.co.uk/PALTVtimingandvoltages.html). However, while I was working, I could feel an electric tingle coming from the metal sleeve of the SCART connector. I hooked it up to a scope and found that there was about 30V @ 50Hz on there. Is that normal??

Incidently, I didn't get anything on the TV, but I'm not giving up yet...

--Amr
Helm PCB - PCB manufacture for HOBBYISTS.
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John_becker
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Posted on Friday, 29 December, 2006 - 04:18 pm:   Edit Post Delete Post Print Post

Amr, while I cant help directly, I feel that something may not be earthed correctly to true ground.

Years ago while at boarding school I felt similar minor tingles when in the showers, touching the metal taps. Reporting it, it turned when the electricity board investigated that there was an earthing problem which had to be fixed in case us pupils sued for premature death due to electrocution.

Check the system.

J
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Steerpike
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Posted on Friday, 29 December, 2006 - 06:06 pm:   Edit Post Delete Post Print Post

I found that to be 'normal' (or at least widespread) with most far-east a/v equipment with a switched-mode power supply. In the dark you can draw a visible spark from the earthy side of the audio or video connections, but there is very little current there. HOWEVER - it is enough of a leak to destroy the input stage of any other equipment you try to plug in while it is powered up, and since VCRs etc have no AC power off, that effectively means turning things off at the wall before making connections, for safety - your other appliances, not so much your own personal.
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Rob83
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Posted on Saturday, 30 December, 2006 - 11:25 am:   Edit Post Delete Post Print Post

I've also experienced this from a small (mid 90's) Panasonic TV.

I often get shocks from the output of small switch mode "wall-wart" PSUs. (To prove I wasn't going mad, I once lit an EL panel from an old watch from one!) upon disceting a small one recently, I noticed that there was a mains rated capacitor between the primary and secondary sides of the transformer. I imagine that this is there for EMI reduction or something. Can anyone elaborate?

(Message edited by Rob83 on 30 December, 2006)
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Epithumia
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Posted on Saturday, 30 December, 2006 - 12:22 pm:   Edit Post Delete Post Print Post

I did once get a hell of a belt off an Amstrad video modulator, even though it was otherwise working.

Rob
If you need me, Neil and me will be hanging out with the Dream King. - Tori Amos
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Zeitghost
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Posted on Saturday, 30 December, 2006 - 02:01 pm:   Edit Post Delete Post Print Post

All this stuff will be "double insulated" or whatever & will lack an earth connection.

The video I measured ran at about 100V wrt earth.

Because it's insulated, the current is very small, but as you connect more & more stuff together, it gradually adds up.

Very little consumer stuff has a real earth connection any more, and feeling a tingle on the case isn't a reliable indication of a genuine fault.
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Amr_bekhit
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Posted on Saturday, 30 December, 2006 - 03:15 pm:   Edit Post Delete Post Print Post

Thanks for the replies everyone - I'll be more careful with it just in case.

--Amr
Helm PCB - PCB manufacture for HOBBYISTS.
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Terrym
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Posted on Sunday, 31 December, 2006 - 02:54 am:   Edit Post Delete Post Print Post

As A former TV repair tech, the reason that you find such voltages on the supposedly "ground" connections - eg A/V in out sockets etc, is that most modern appliances (TV's etc) use a switch mode power supply that is not isolated from the mains. Simplified,they just rectify the incoming mains and use usually a semiconductor switching device to regulate the HT rail. This effectively makes the "ground rail" live with respect to the mains (they usually float at half mains voltage). This "ground rail" is then isolated to the outside world by the use of an isolating capacitor on things like the A/V sockets, Scart sockets, aerial sockets etc.

Many times had we been caught by somebody having done thier own repairs and removing these caps, leaving all exposed metal at lethal mains voltages. Makes quite a large spark and "splat" when connecting the aerial lead for instance.

"Double insulated" is not doubly safe and the sooner they ban this sort of thing the better.

As for the capacitor connecting primary to secondary in the plug pack, what a boge. Until you discharge that capacitor, it is effectively at mains voltage on both sides.

TM
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Armadillo
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Posted on Sunday, 31 December, 2006 - 10:34 am:   Edit Post Delete Post Print Post

FYI all electronic equipment that is 'CE' marked has to comply with safety and EMC regs. The safety aspect requires that there are two levels of protection between hazardous voltages and the operator (Note: Operator not service engineer). Either of these levels of protection should be sufficient to protect the user.
The two levels can be 'basic' (i.e. sufficient thickness, distance & electric strength) plus a safety earth - known as Class 1
Or they may be double insulated (consisting of 'basic' + 'supplementary' or 'reinforced') known as Class 2.

Class 2 is considered to be safer. The reason is that a flexible earth wire can become detached unknown to the user leaving only one layer of protection. Hence all UK electric garden tools are Class 2.
Modern TV's & VCR's are normally Class 2. (Safer). The other reason is that if you connect your VCR to your TV and they are both earthed you have the possibility of earth loops causing problems. Your computer is earthed so if you were to plug that in as well what fun all those analogue signals could have.

The switch mode PSU in TV's normally does isolate the mains from the secondaries but in common with the vast majority of SMPSU's they have 'Y' capacitors from the live & neutral to the chassis leaving the chassis floating at 1/2 mains voltage but at a current that is considered to be safe. There may well be further 'Y' capacitors between the secondary and the primary or the chassis for EMC reasons. However if the chassis is accessible to the operator anywhere there must be another capacitor to the operator.

If there is a capacitor between the secondary and the mains in a Class 2 'Wall wart' then it has to be either two single insulated capacitors (Y2) or one 'reinforced' capacitor (Y1). The capacitance must be sufficiently small to keep the current down to that defined as 'limited current' (I can't remember the figure off hand but in the close order off 3mA)

HTH

Armadillo
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The earth sucks!
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Grab
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Posted on Tuesday, 02 January, 2007 - 12:03 pm:   Edit Post Delete Post Print Post

Banning non-earthed equipment isn't something that'll ever work, sadly. Many/most other countries use two-pin mains plugs. Since no manufacturer will bother making UK-specific versions of their kit, you'll basically be banning the sale of any electronic equipment in the UK. :-/

Graham.
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John_becker
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Posted on Tuesday, 02 January, 2007 - 12:45 pm:   Edit Post Delete Post Print Post

Graham, I recemtly bought 2 new ink jet printers, an Epson and a Canon, both came with 2-pin non-earthed plugs at the unit end, but with standard (UK) 13A 3-pin plugs moulded on.

(When I edited PE, late 80s, I was one of those who campained through the mag for equipment to be supplied with moulded 13A plugs fitted as standard - thankfully they seem to be so now.)

J
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Canonman
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Posted on Tuesday, 02 January, 2007 - 01:15 pm:   Edit Post Delete Post Print Post

To quote JB:
"Graham, I recemtly bought 2 new ink jet printers, an Epson and a Canon, both came with 2-pin non-earthed plugs at the unit end, but with standard (UK) 13A 3-pin plugs moulded on."

Remember to switch them off before you plug them into the (earthed) computers, otherwise it's crackly noises from the nearby radio and sparks-a-plenty!
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John_becker
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Posted on Tuesday, 02 January, 2007 - 04:14 pm:   Edit Post Delete Post Print Post

Thanks for that Canonman - it had never occurred to me

J
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Terrym
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Posted on Friday, 12 January, 2007 - 12:52 am:   Edit Post Delete Post Print Post

Here's a better explanation of why TV's "bite":-

http://www.marcspages.co.uk/tech/4001.htm

TM
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Obiwan
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Posted on Friday, 12 January, 2007 - 01:17 am:   Edit Post Delete Post Print Post

Most of the newer printers power supply is controlled electronically, so there's nothing to "switch off". Then can't be turned off or on, until power is supplied.
Do Not Hit The Fly That Lands On The Tigers Head.
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Armadillo
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Posted on Friday, 12 January, 2007 - 08:24 am:   Edit Post Delete Post Print Post


quote:

Here's a better explanation of why TV's "bite":-

http://www.marcspages.co.uk/tech/4001.htm




I don't know if the link given is correct in saying that
"Most TVs do not have an isolating power supply like computers."


It's certainly possible. However,in my very limited experience of TV's, an isolation Tx was used, in a switch mode PSU, if only because it's a relatively easy way the derive all the voltages needed (+5, +12, +135, -12 etc). Of course almost by definition, any TV's that I look into are going to be old technology and so things may have changed.

(Message edited by armadillo on 12 January, 2007)
There's no such thing as gravity..........
The earth sucks!
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Zeitghost
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Posted on Friday, 12 January, 2007 - 08:42 am:   Edit Post Delete Post Print Post

Anything that has a scart socket must be isolated from the mains.

It may not be earthed, but it must be isolated.

The days of relying on a couple of capacitors in the aerial socket for isolation are thankfully long gone...
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Petelobus
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Posted on Friday, 12 January, 2007 - 09:40 am:   Edit Post Delete Post Print Post

Regarding CE marking as proof of safety, I'm a bit suspicious, as I'm sure that some of the far eastern stuff that comes in here (UK) has CE marks, but does not actually comply! Just bought an inspection lamp from a reasonably reputable supplier, which had loads of CE numbers on, but the only means of retaining the mains lead was the fact that it bent through 90 degrees, and was poked through a hole in the (SRB) PCB, guess that the strength of the PCB, and friction between the lead and the case MAY give enough mechanical resistance to prevent the lead being pulled out, but it struck me as not very good, especially for an inspection lamp! Hence I'd tend to worry about the quality of any capacitors used in cheap far eastern psu's. Likewise I'm concerned about the fire risk of printer and other psu's which are continuously connected to the mains. Perhaps I'm too conservative, but a DP switch in the mains circuit with a 3mm gap seems safer (but probably too expensive). Consequently I pull all the mains plugs when I'm not using the kit (and, after reading this thread, when I make any connections between items).

Ian
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John_becker
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Posted on Friday, 12 January, 2007 - 09:53 am:   Edit Post Delete Post Print Post

Ian, It might be worth speaking to your local Trading Standards Office about your lamp, and other queries re CE markings, to clarify the situation. Tell us here what they say, I for one would be most interested.

I expect your local TSO will be in the phone book.

J

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