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

Alvin Goats agoats at compuserve.com
Sun Apr 13 21:55:15 EST 2003

I was just browsing through this thread and need to make a few comments:

It is actually the amount of electrical energy in joules (aka
'watt-seconds') that can kill you. Low voltage with enough amperage will
kill you just a dead as high voltage. Too many people are injured each
year due to the "ground loop", an essentially zero voltage many ampere
current handling lead. 

Capacitors store charge, which when discharged, has a 'time constant'.
Whether it is an LC circuit (inductor/coil/choke and capacitor) or an RC
circuit (resistor and capacitor), there is a time that it takes to
charge or discharge. This time sets the 'seconds' part of the joule. The
amount of charge on the capacitor depends on the dielectric and size of
the parallel plates, as well as the voltage applied to the capacitor
during charging. A small, dime sized 0.01 microfarad capacitor will hold
less charge than an oil filled gallon can sized, sealed 0.01 microfarad
capacitor. The ceramic one will typically burn holes in your fingers,
where as the oil filled monster will take a one inch diamter galvanized
steel pipe and vaporize the section that is shorted between the
capacitor's leads (been there, done that, left a beautiful multicolored
'mineral deposit on glass' effect to the painted walls). The amount of
energy stored has to due with the number of electrons collected on the
plate and not really the voltage, voltage only helps speed up the
charging of the capacitor as long as the dielectric's voltage limit
isn't exceeded.

I will note for the uninitiated: high energy capacitors explode with the
energy of dynamite. Shorting through the dieclectric lets the full
charge flow between the plates, superheating the  dielectric, plates and
package. The superheating occurs so instantly as to be the same as any
explosive: solids or liquids instantly going to a gas state and taking
more volume than the solid or liquid did. 

And capacitors can self charge from the static elecricity in the air.
That's why the high energy capacitors are sold with a shorting bar
across the leads. If they are removed from use, the leads should be
shorted to prevent self charging.

DC power supplies taken from AC lines have one large filter capacitor to
smooth the ripple from the AC line. Rectifiers take the sine wave AC and
'flips' the bottom half of the sine wave up on top, so you have
repeating 'hills'. These hills are still AC, it just doesn't go negative
anymore. To smooth the ripples out, you add a capacitor which takes time
to charge and discharge. The amount of time should be very long and can
be lengthened by adding a coil (inductor or choke, aliases for the same
thing). The smoother the ripple pattern, the larger the capacitance of
the capacitor. Most capacitors for DC circuits are in the 1,000
microfarad range; precision supplies may have capacitors in the 200,000
microfarad size. The voltage on the capacitor is typically 1.25x the
maximum AC voltage of the rectified AC voltage (it has to do with the
dielectric, too high a voltage will punch through the dielectric
shorting the capacitor and doing severe damage to things as mentioned
above). So while the voltage may be low, the capacitance is very high.
Being so high, there is still a major danger due to the energy
discharge: joules.

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. Since it
requires high voltage for them to work (they send electrons towards a
charged wire grid where the electrons are absorbed by phosphor on the
back of the glass on the screen, which then glows giving you the image).
The grid and plate in the monitor form the capacitor. High voltage and a
large plate allows for a lot of electrons to gather, the more electrons
the deadlier it becomes. 

High voltage DC with very little current behind it can kill. Low voltage
DC with high enough current can kill as well, note the favourite
electricution methods of terrorists and torturers with 12 volt car
batteries and wet cloth. Done briefly, it is extremely painful, done
more than briefly, it is lethal. Again, the issue is in joules. Low
voltage high current generally requires sweat or water to help conduct
electricity better, however, if there is enough current available it
will "carbonize" the tissue which reduces the resistance of the tissue,
thus allowing more current to flow.

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. Isolation transformers add another bit of safety
in that they BLOCK DC currents. You can have a 100 volt DC supply on an
AC line which means any DC operating equipment sees a 100 VDC with the
AC voltage as a 'ripple'. Isolation transformers will block the DC
portion, leaving the AC voltage as the only thing being passed. This is
a trick that is used to supply power and receive signals from down hole
oil field equipment on a single conductor coaxial line, and it is used
for 'inline amplifiers' as used on TV antennas. The isolation
transformer removes the DC power supply from the AC line, otherwise you
would blow your VCR, stereo and/or TV.

230 V center tapped isolation transformers are set up to deliver 230
volt and two 110 volt lines. Using the two ends yields the 230 VAC,
using the center tap and one leg gets 110 VAC, the center tap and the
other leg gives the other 110 VAC. To 'load balance' the transformer so
that you don't burn out the secondary windings on the transformer, you
would use both lines. Often, the AC electrical outlet that is attached
to an isolation transformer used by construction has one socket wired to
one leg and the other socket wired to the opposite leg just to force
load balancing (current sharing). 

My worst experiences:

100 VDC battery rated at 4amp-hour: accidentally touched both ends with
the same hand. Hand clenched into a fist, curled my wrist and folded my
arm at the elbow for about 10 minutes. Hurt like heck.

15 kVolt AC, 60 cycle: hurt like made and jumped about 4 inches to get
me. Watch out for the humidity in the air, it reduces the dielectric of
air and lengthens the distance for high voltage to jump.

400 Volt AC, 400 cycle: read the 100VDC after my hand quit spasming.
Spasms lasted about 3 minutes and took about 5 minutes to start the hand
clench to fist etc pattern. Higher frequency AC acts more like DC to the
human body due to the time constant issue. Low frequency of 50 to 60
hertz has the effects everyone is expecting for AC; at about 300 cycles,
it starts to act more like DC. 

I deal in energy, lots of it on occaision. Highest current: 100,000 Amps
at 230 Volts. Highest Voltage: 500,000 Volts, 0.25 Amps. Highest
Energy:  150,000 Volts at 10,000 Amps. Largest ground loop current I've
encountered: 150 Amps, 'zero' voltage (yes, current flow through
anything with some resistance will generate a voltage, but if the
resistance is so small as to be measured in conductance instead, the
voltage can be so low as to not register, i.e. 0.00001 Volts; and if you
ever deal with superconductors, the resistance really can be zero, hence
0 volts).

No matter how good you are or how careful you are, you're still going to
get 'bit' by electricity.


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