View Single Post
  #5  
Old March 30th 17, 08:24 PM posted to uk.tech.digital-tv
NY
external usenet poster
 
Posts: 1,232
Default TV socket charge up

"Johnny B Good" wrote in message
...
On Thu, 30 Mar 2017 17:59:03 +0100, Brian-Gaff wrote:

I was plugging the aerial back in the back of a reasonably modern telly
last week and got a bit of a shock, well quite a big one actually off
the socket and the plug. the plug goes straight to an aerial, no amps or
anything so the whole thing has to be electrostatic I guess. I had
assumed once crts went that none of this sort of charging would happen
but it seems not.


I doubt it's static build up, Brian. More likely simply the quite common
effect of EMC filtering used by double insulated mains kit where the
chassis or 'common ground' is left floating at half mains voltage
courtesy of the high impedance capacitive voltage divider effect provided
by the EMC filtering circuit.


Been there, had the shock. In my case it happened when I unplugged the TV
aerial to my PC's DVB-T card, holding the metal screen of the aerial plug
with one hand and with the other hand on the earthed case of the PC. It was
quite a belt!

The screen of the aerial was at about 120 V wrt mains earth (the PC case).
After unplugging everything else that was plugged into the aerial (eg TV,
VCR) or was connected to the TV (eg via phono audio cables), I identified
the TV as the culprit.

I measured my body resistance (about 300 kilohms) and found resistors to
this value, and then connected them between TV aerial and mains earth, as if
it were my body (I didn't want to repeat the shock - it was that bad) and
measured the voltage again. With a British Standard human body in place, the
voltage was about 80 V - less than the high-resistance-voltmeter reading but
still quite noticeable.

Amazing really that TVs are allowed to have their aerial/phono screen at
half mains voltage, even via a high resistance.