I have a old computer and I see it has a computer power supply. (no kidding) I read the out put values and I see it says 5 volts 30 amps. Now i know if i touch 5 volts and ground on the Arduino, nothing happens don't even fell it. what happens if I touch 5 volts 30 amps? would I feel it because of the amps? and also I it where to short it would it cause dangerous sparks? any information about computer power supplies is greatly appreciated.
In essence, 5v @ 30 amps is the max it will deliver on demand, it never forces 30 amps out of the 5v line(s)
If you short it, the over-current protection circuit should shut down that output (or the whole PSU). Like a bullet-proof vest, it's best not to test it by asking it to do what it's meant to do. Likely worst case is you'll toast the PSU or blow the input fuse. I'm sure you'll get a spark. So, you know, turn off any nearby gas appliances and oxygen tanks if you really have to see it for yourself.
Touching it isn't likely to do anything. It's 30 amps potential current. It doesn't mean 30 amps is flowing through it. The circuit has to be low enough resistance to allow 5v to flow at 30 amps, and I imagine the resistance of human skin is just a bit higher than that. Do you notice anything when you touch the top of a 9v battery? (Maybe don't lick the PSU for now, though.) I'm sure the Wikipedia article on electrocution would have some insight on this.
If you were hoping to build an electric chair, you might have to go for higher voltage. It would probably make a great lab PSU though. Someone here asked about that recently and found some good articles on the topic.
The circuit has to be low enough resistance to allow 5v to flow at 30 amps, and I imagine the resistance of human skin is just a bit higher than that.
Right... It's just about impossible to get dangerious current through your body at 5V. A car battery is capable of putting-out hundreds of amps, but you can't feel anything when you touch the battery terminals.*
I was trying to find-out what voltage is considered safe to touch buy the regulatory agencies... I didn't find a solid answer, but you can get "wall wart" power supplies up to 48VDC, and obviously there is nothing to stop you from touching the output connector. Studio microphones also use 48V "phantom power" over the audio cable, and again there is nothing to prevent someone from touching a live male connector. (Remember that AC has a peak that's about 50% higher than than the RMS value, so 48VAC may not be as safe.)
The relationship between voltage, resistance, and current is determined by [u]Ohm's law[/u]. In order to get high current, you either need high voltage and/or low resistance. Since your body has high resistance, it requires higher-voltage before you can get enough current to feel anything.
Most of the time, we are working with "constant voltage", which means current depends on resistance. But, if you try to get-out too much current, something "gives", (or something burns-up! 
) For example, Ohm's Law says that if you put a 1 Ohm resistor across a 9V battery, you'll get 9 Amps. But if you tried it, you'd find that a regular-little 9V battery can't put-out 9 Amps, and you'd find that the battery is no-longer putting-out 9V with a 1 ohm "load" across it (at least not for long) . Ohm's Law is still true, even though you don't get the "calculated" results.
- You should be careful when connecting/disconnecting a car battery, because there are inductors in cars, and inductors can generate high voltages (like for a spark plug) when current is flowing and you suddenly disconnect it.
DVDdoug:
A car battery is capable of putting-out hundreds of amps, but you can't feel anything when you touch the battery terminals.
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In order to get high current, you either need high voltage and/or low resistance. Since your body has high resistance, it requires higher-voltage before you can get enough current to feel anything.
Having worked often enough on cars around the battery area, I know for a fact that, if you lower your skin's resistance enough (say, by sweating in the summertime while working on the car), you can easily feel a good size tingle from a car battery (leaning against the vehicle, reconnecting the positive cable - not a fun feeling; not as bad as mains AC, nor the HV of a spark coil - but still a bit of a surprise when it happens).
Does the fan spin? Otherwise it's probably not on.
First of all: THINK ABOUT YOUR SAFETY!!
But You can (depending on the model?) turn on a old PSU, by cutting the green line and connect it to a ground-line.