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

[Tom Wiltshire]

Just to add a little to what Harry and Dave have said;

On 6 Feb 2009, at 00:11, Horton wrote:

> Unfortunately, the book is so anti-EVER plugging ANYTHING you make  
> into ANYTHING
> wall-powered under risk of DEATH that it's made me afraid of
> electricity.

Understanding is a good antidote to fear. I build my own AC power  
supplies, which is theoretically about as high risk as it gets, but  
in practice the circuits are simple and if you check what you've done  
carefully then even this is safe. After all, that's all anyone does  
commercially.
One of the best things you can do is make sure you have good mains  
circuit breakers installed. These can cut the electricity fast enough  
to save your life even if you do get zapped. Our dog once chewed  
through a mains extension cable and was saved like this (240V AC,  
dog's wet mouth, it made her yelp...).

> 1. I see people with home-built and circuit-bent stuff running into
> huge marshall stacks frequently. Why are these guys not dying?

Why should they? If the amp is well grounded and safe, you can plug  
what you like into it.

> Why is the book so emphatic that you should NEVER EVER do this?

To make sure that some idiot doesn't try to sue them for saying it  
was ok when they do something stupid.

> 2. How could you get electrocuted by plugging in something that's only
> spitting out an output, anyway?

An output is still an electrical connection, so once it's plugged in,  
the two things become one electrical circuit. Hence a fault in your  
amp can make your electric guitar really "electric" and zap you.

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

Again, why should you? I think the USB bus only provides +5V power,  
which you could probably lick safely although I wouldn't recommend  
the experiment. So the only way you'd get zapped would be some  
horrible fault in the computer with the USB ports whereby AC power  
got past the AC isolation and into the low voltage circuits. You'd  
probably know this had happened from the acrid smell of burning!

The most dangerous thing I've worked on is an old 1970s tube amp.  
This has a 400V DC power rail inside it, and you definitely **don't**  
want to brush against that when it's live. I avoided ever having it  
switched on whilst the case was open. For these, I really would say  
that you can't be too careful.
Nowadays I work mostly with +/-15V supplies for audio circuits and  
+5V or +3.3V microprocessors circuits. These are really pretty safe,  
and as long as your power supply is good, safe, and well grounded,  
you won't come to any harm poking your fingers into them.

Be informed and be careful, but don't be fearful. If you're not sure,  
don't. If you are sure, check.

T.