[sdiy] Let's talk AC, DC, electrocution, death, misery, and grounding.

[harrybissell at wowway.com]

On Thu, 5 Feb 2009 18:11:35 -0600, Horton wrote

Electrocution 101:

The human body is usually between 100K ohms (dry skin) and about 500 ohms
(sweaty and standing in a pool of water :^). The nominal is 10K ohms.

Apply a voltage across this, and calculate the current. There is some debate
on what is truly fatal... but usually less than 1mA is not fatal unless the
path goes straight through your heart. 1 - 10mA can be really dangerous, you
heart can go into fibrillation (not beat in a way that pumps blood and causes
a lot of muscle damage...) higher currents may be more survivable because your
heart just stops dead, you might be revived with removal of the current and
CPR. More than that and you get into electric-hotdog-cooker territory (you get
roasted).

Battery powered gear has several benefits...
1) the voltage is usually low enough that you will never draw a harmful current.
2) Grounding is not an issue... the source is 'floating' with respect to ground.
3) total power is very limited (no hotdog cooker effect is possible)

Now if you plug into AC mains... things can get more dangerous...

I think the points of the circuit benders are this... they make connections
that may not make electrical sense from a conventional point of view. Adding
AC power to these random connections might have unintended consequences (shock
and fire hazard). Likewise the people who approach electronics from this angle
may not have the knowledge to realize that they ARE making a potentially
unsafe connection. just to keep the lawyers out the mix, they say NEVER NEVER
NEVER. Now your points inline...


> I need some straight info on all of this, please.
> 
> 1. I see people with home-built and circuit-bent stuff running into
> huge marshall stacks frequently. Why are these guys not dying? Why is
> the book so emphatic that you should NEVER EVER do this?

If you don't know what you are doing, don't do it. In many cases it can be
done safely.

> 
> 2. How could you get electrocuted by plugging in something that's 
> only spitting out an output, anyway? I mean, if I make a piezo mic, it's
> only going to be outputting a signal, not taking any voltage into it.
> Shouldn't I be able to plug that thing into anything from a tiny
> battery-powered amp to a marshall stack without worrying?

There are a few caveats. Is the Marshall amp safe? It could be broken in such
a way that it could provide lethal voltages, even on a input point. Extra
danger exists from the possibility that different sides of the AC line might
exist on the chassis (input ground) from what is truly input ground. That
might give you 120 -240 volts for your body resistance calculations :^(

Most guitar amps have a polarity switch and AC (capacitively) couple the
chassis to one side of the line. This capacitor is now in series with YOU...
it might limit the current to a safe but unpleasant level.

Older designs can have a HOT chassis that is TROUBLE no matter what you do.
I've been hit with some of these. Usually I kill them right after that to
avoid them breeding more trouble :^) 

If you make a DIY project you need some form of AC line isolation. Usually the
power supply has a transformer that stops there from being a direct path
through the "secondary" of the circuit (your part).

Some foolish designs can run directly from the AC line, and get isolation from
some other means (like an 'optocoupler' or capacitor) these are 'usually'
safe... but certainly are NOT safe when circuit bent (in some cases).

> 
> 3. I just got an Arduino, which is powered off of the USB bus. Why do
> I not get electrocuted holding it?

Voltages are too low and the computer power supply (you hope) provides the
AC line isolation. Generally you would never have a problem.

Should there be a fault in the computer that allows a direct line connection,
or through some other audio gear... and you are standing in the puddle of
water...you might still get blasted :^)

H^) harry


> 
> AH
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Harry Bissell & Nora Abdullah 4eva