Hi Martin
Yes, like you I am pretty much clued up on this sort of thing. :) My personal
guess is that one device has a higher potential to ground than the other one
does. That's where you get the ground loop and that is why there's a tingle
sensation.
Gordon
On 1 Jul 2012, at 14:51, Martin McCormick <mar...@x.it.okstate.edu> wrote:
What you are describing when talking about the tingle
you get from touching the MacBook Pro and Braille display may be
what's called a ground loop. You may already know all about
this, but it boils down to Earth is all relative. Some Earth
terminals are better Earthed than others and so there is a
potential difference between two things that really should be at
exactly the same potential. It happens in all kinds of
situations and even if the Earth and electrical systems are all
in tip-top condition but it is worse if anything about the
Earthing system is dodgy.
We used to have a modem interface on the telephones that
the university rented to customers. The phone system is digital
and the modem interface was a blinding 9600-baud connection
which may make you laugh now, but which was sought after back in
1987 when we got this system.
You connected your P.C.'s serial port to this thing and
used it just like your normal dial-up modem of the day. We had
enough Earth potential difference between these serial modems
and the customer's computers that the display lights frequently
flashed or glowed as if data were being handled even when
nothing was going on.
I remember asking several people if they ever got
shocked off of this setup as that would have been potentially
dangerous as in life, safety and the possibility to ruin
equipment, but nobody ever reported being hurt but that is the
consequence of ground loops.
Also, remember when we were discussing hooking up mixers
and audio gear? Ground loops are the bain of our existence,
especially if we are working with unbalanced lines such as the
RCA phono plug type cables.
If you really want to feel something that you will get
away from fast, touch the terminals of a 70-volt audio system
with your fingers.
Better yet, don't and take my word for it. Public
address amplifiers such as for big buildings and auditoriums
typically use a 600-ohm output transformer from the amplifier,
itself and then you supply a 600-ohm to 4 or 8-ohm transformer
at each speaker installation. That means the audio right out of
the amplifier can hit 70 to 100 volts during peaks and it really
tickles but painfully so if that makes sense.
So what is positive feedback? That's when you have a
microphone on while your hand is in contact with the 70-volt
system. You say something, shock your fingers and yell or swear.
The louder you yell, the more shock you get and that's a
positive feedback loop.
The loop breaks when your lungs run out of air and,
well, you get the idea.
Gordon Smith writes:
> Hi Martin
>
> I know that my Braille display and MacBook Pro are two examples of this.
> If I put them together and touch them both together, I get an electrical
> tingle through my fingers. Also if you run your finger along the top of
> the display, you can sort of feel the tingling sensation to a lesser
> extent.
>
Okay. I truly don't claim to be an expert, but I think I
know why that happens, also. The Braille display may have a
mains power transformer inside or connected nearby. While the
transformer is most likely perfectly safe and in good working
order, there is capacitance between the insulated wire in the
windings and the frame or chassis of the display. I have been
curious about this effect, also as I have equipment that does
this and what is most likely happening is that this capacitance
is electrostatically coupling some of the mains current to the
case or exposed metal parts and there is no Earth there to bleed
it away. Some mains transformers seem to do this more than
others, but it is not usually hazardous unless it is a real
shock and then it needs to be investigated.
Usually, if the transformer has shorted to its frame,
smoke and blown fuses result.
> Also if you put the terminals of a 9 volt battery to your tongue, you get
> a nasty tingle. It doesn't hurt exactly, but it isn't comfortable. I
> certainly wouldn't be comfortable with the idea of having a sensor
> attached to my tongue. I really don't think that is a workable option.
No. I hope you haven't found all of this too shocking
for the list. This topic has potential.
Martin
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