Hazardous voltages

[Stopp,Gene]

I can think of two dangers to look out for when building your own power
supply for the first time:

1. Coming into personal contact with the mains voltage.

This you just need to be careful about. The thing about the mains is
that it is has a high enough voltage potential *plus* the current needed
to break down impedance blockages and make its own path through a
moisture-laden organic entity (such as ourselves). The power line coming
into a house has a source impedance that (for our soft-bodied purposes)
is unlimited. As for current versus voltage, I believe that I've heard
that it is the voltage that is lethal, in that it will screw up the
nervous impulses that control heart muscle and/or breathing diaphram
contraction. You may well survive the tissue burns, but pass out from
oxygen deprivation and die unless somebody restores your muscle actions
via CPR.

Eeeewww - sorry about the gruesome post but electricity *can* be scary
sometimes!

2. Successfully avoiding shocking yourself until the mistake that you
made in wiring causes the circuit to blow up at which point you freak
out and touch the hot wires.

Don't laugh, this really happens. If you mis-wire a transformer winding
and plug it in, you may see some nasty fireworks. The power cord might
jump, the lights will dim, and a loud 60 (or 50) hertz buzzing noise
will be heard until something fries or the breaker pops. You may get the
panic-driven urge to try to disconnect something while the power is
still on. Or, you may have the transformer wired up okay, but there may
be a reversed-polarity electrolytic cap after the rectifier. For a
'lytic this big, it will most likely explode, and this will not be a
minor event.

So, here is some advice:

1. Use some kind of power switch to enable and disable your circuit
under test, and keep you hand on it.
2. Wear eye protection and stand back.
3. Use a GFI outlet, as mentioned in a previous post.
4. Design your circuit using some kind of verified schematic, and make
sure your polarities are correct.

Regarding the GFI outlet, my own personal butt has been saved at least a
couple times with these. Once I was standing in a puddle of water in
bare feet when an extension cord fell into the puddle (Duh!). The GFI
popped, I felt nothing. Another time I was sawing up some lumber with a
circular saw and actually cut through the power cord (double Duh!). This
event blew up the GFI - it gave its life for me. Unfortunately removing
standard outlets and replacing them with GFI's involves working with
potentially live mains wires, so they can't protect you until you've put
them in already!

Whew - I guess this all sounds scary, and maybe intimidating to people
just starting out with power circuits, but really it's not much more
dangerous than many of the things we all do everyday. Ever think about
how much energy you expose yourself to while driving in your car,
surrounded by fast-moving strangers?

 - Gene