Electrical hacking (was Re: [Techtalk] [OT] charging USB devicewithout a PC)

[Alvin Goats]

> > Too many people are injured each year due to the "ground loop", an
> > essentially zero voltage many ampere current handling lead.
> 
> Ground loops are where a big loop (or sometimes not so big) loop acts as
> an antenna and/or coil and picks up the general electronic crud that is
> flying all around us, either through magnetic induction or as an
> antenna.  This means that you get a voltage between two different ground
> connections.  The highest I've ever met this difference being is 4V, which
> was at a university with it's own power distribution system and power
> station, it caused immense quantities of havoc with all the audio
> equipment and we had to go to great lengths to sort it all out.

That's a special case. When you have multiple ground leads like in a
circuit board, the electron flow can be larger in some areas than
others, like a river that has one small stream feeding into it and a
short distance later a small stream exiting it. The small section where
they are combined is a ground loop as well, and the current in that
location is larger than the rest of the circuit board. This is why some
ground traces on PC boards get burned and components in strange areas
are fried. Localized ground loops creating more current than the local
spot can handle. 

For most powerline systems, ground loops are known but they focus on the
ones that affect their particular level of direct need or experience.
EE's and others tend to forget these things and accidentally design them
into their products and projects because they don't think about such
things. Noise and fried components abound in the prototypes. ;)

> nevertheless, as mentioned it can be a very large source of current,
> sufficient in fact to melt wires in some cases.  I've never heard of it
> directly killing, I must admit, because the low voltage is usually
> insufficient to overcome the resistance of the skin, and the current that
> passes is equal to voltage divided by the resistance.  It's perfectly
> possible though, and responsible and correct grounding is always
> neccesary.

Avoid moisture, sweating, conductive oils, fresh paint or finger nail
polish (finger nail polish is a good conformal coating, but the acetone
in it is a little conductive). These will reduce the contact resistance
of skin making it very easy to get fried by low voltage high current
sources.

> Hmm, it's usually easier to use a higher AC frequency, so that the time
> between the peaks is shorter, so the capacitor doesn't need to last so
> long.  Which means you get less ripple, and a smaller PSU.
> Good things all round :)

That's why they like to use the 4 rectifier "bridge rectifiers", instead
of lopping off one of the hilss in the AC sine wave, it does the full
flip, doubling the frequency, hence 50 hertz goes to 100, 60 hertz to
120 hertz. 


> > FYI: the picture tube of monitors and televisions are a large plate
> > capacitor. Being under vacuum, there is very little current leakage
> > through the dielectric, so they hold their charge a LONG time.
> 
> They usually have a high value resistor to discharge this over time when
> turned off, for safety, without adversely affecting the operation when
> turned on.

The key word here is 'usually', but not always. And the high voltage,
high impedence reistor can take hours to discharge the tube. The
standard 'wait 1 to 2 minutes' before working on the thing does NOT
apply here! ;)


> > Isolation transformer function is to create an AC path that is not
> > directly coupled to any power line main. Typically, they are 1:1 turns
> > ratio, but not necessarily so. Any short that can cause damage to the
> > power line main requires an isolation transformer so that the
> > transformer is damaged and not the power line grid. It doesn't always
> > work as sometimes the primary winding gets shorted as well and will take
> > the grid down anyway.
> 
> This is due to it melting under extreme overcurrent situations, and
> forming a short circuit

General failure mode is the varnish or shellac on the copper wire in the
trnasformer windings either crumbling from vibrating (current in a wire
in a magnetic field will move, if it's AC it vibrates), or by fungus
(WWII Pacific theatre learned this the hard way) or age (the stuff does
eventually 'rot'). Once the insulation is gone, it shorts, if it shorts
bad enough, the REALLY BAD THINGS HAPPEN.

> > Highest Energy:  150,000 Volts at 10,000 Amps.
> 
> my my, what a big generating station you have.

We had our own dynmo and power substation. How'd you guess?

alvin