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5 volts is completely safe.
The resistance of your body (particularly across your heart which is what matters most as far as electricity safety is concerned) is about 100 kOhm. At 5 volts you'll have about 50 microamps flowing across you, which is not even enough to feel. You would probably be safe with up to 100 volts DC (not AC). If you look up Mehdi Sadaghdar on YouTube you'll see that he does a lot of experiments with electricity on his own body, and in some he shows that higher DC voltages are safe. He also points out that the human body is somewhat capacitive so AC is a lot more dangerous at the same voltage levels. In one video he holds 170 VDC to a regular light bulb with his fingers and he doesn't even feel it (or only slightly).
Just don't stab it directly into your heart and you'll be fine. 
Whilst 5v is essentially safe, mistakes @5v1150mA can certainly damage equipment, and 5Watt is enough to ignite components.
Never forget that 5volt supply is connected to mains.
100V is extremely dangerous, don't even think about it.
Above about 40V you can get a shock even with dryish skin, and once current
flows it reduces the skin's resistance causing much larger and dangerous currents
to flow. If your skin is really dry yes you can tolerate higher voltages, but the
interior of the body is much more conductive (the blood particularly), so once
your skin starts to breakdown you risk fatal currents through your heart muscle.
In water lower voltages than 40V can be life-threatening as your skin no longer
insulates well, but 5V with dry hands is perfectly OK.
5V will be painful on the tongue for instance
gilperon:
Hi (sorry for my bad english, really sorry)I have an old charger of my cellphone which uses usb port to charge it. In the charger it says 5v and 1150mA. I am thinking about cutting the wire, stripping the small wires inside and connect it to my protoboard.
What kind of plug is at the phone end? Somehow I think it's not bare pins but holes?
Hey! See if a jumper pin fits!
Or a header pin might be a tighter fit.
The problem with cheap phone chargers is that they are not always safe for other uses. There will be no over current protection on most when you short out the supply. The other thing is that the earth leckage current is going to be greater than you want and you can get "tingles" off it. That is high voltage but very high impedance so there is no shock but it is disturbing to feel.
So I would advise you not to do this but to get a proper desk top power supply. They are not too expensive.
Something like this:-
Manson Power supply
Foggiest:
Whilst 5v is essentially safe, mistakes @5v1150mA can certainly damage equipment, and 5Watt is enough to ignite components.
Never forget that 5volt supply is connected to mains.
You shouldn't run more than 200mA through an UNO nor more than ~20mA through any IO pin.
PC USB is 5V 500mA
5V 1150mA is good for external power to run shields, boards, chains of shift registers and leds, etc.
Just know if it's a regulated supply or not. For some things like charging phones it's no big deal.
I've been looking into buck and boost converters.
A regulator is like a voltage divider with a heat sink, it wastes excess power as heat, pfffft, gone.
A buck converter uses high frequency PWM filtered to flat VCC with little waste, less to much less heat.
A boost converter changes lower voltages to a higher voltage at reduced mA's. Run on one super-AAA?
The thing is that the converters output clean power efficiently. They are worth the few bucks they cost.
I have some 5V 1A USB chargers for $2.50 ea. that I am not so sure are regulated.
They would probably buck down to 3.3V ( 5V x 2/3 ) at more than 1A ( + < 1/2 ) just fine.
OTOH maybe a laptop or desktop PS could deliver all the regulated voltages I want for < $20.
gilperon:
- if a power supply (like my cellphone charger) provides 5v why does it also provide the current? Current shouldnt be a consequence of the resistance in my circuit? If I have 5v and a resistor with 100ohm the current should be calculated using U = RxI isnt? Even with 100ohm will my charger provide 1050mA according to its specs?
Your charger will allow you to draw UP TO a maximum of 1050mA from it. If you try drawing more current than that from it, it probably will destroy the charger. Drawing a lower current than 1050mA will do no harm.
Remember that Voltage is supplied and Current is drawn.
Your 100 Ohm resistor across 5 V will draw 1/20th of an Amp = 50mA or 1/4 Watt.
- why do I always need to connect the + (ground) from arduino to my circuit?
See:-
http://www.thebox.myzen.co.uk/Tutorial/Power_Supplies.html
This is an almost identical thread.
CROSS POST
Please do not do it.
Grumpy,
I hate to hijack the thread, but regarding that desktop power supply you linked, do you think it is a better/safer/more stable/add-additional-adjectives-at-will than a modified computer PSU?
After all, PC PSU has 3,3V, 5V and 12V, which are by far most common voltages a hobbyist may need when working with an Arduino. Compared to this Manson that still doesn't have a continuously adjustable range, I don't see a problem with that regard.
On the other hand PC PSU can and does supply all three voltages simultaneously, while this one appears to have only one output.
Is there something I am missing?
A PC power supply has a much higher current capacity, this might at first seem like a good thing but it is not. What this means is that say you short out something in a circuit the tracks will burn up like a fuse, or components will melt. Where as with a lower capacity supply then the internal current limit just kicks in and little or no damage occurs. Sure when you need the current go for a PC supply but for general use it is far too vicious.
After all, PC PSU has 3,3V, 5V and 12V, which are by far most common voltages a hobbyist may need when working with an Arduino.
I disagree, when powering an arduino through the external power jack, 9V is far better than 12V, as it runs so much cooler.
Hmm yes, the current limiting capability is a very appealing and I must admit that it has saved my electronics quite a few times, but 3A (Manson supply) is still plenty to burn stuff.
I usually use USB and not power jack for prototyping, then when I make the PCB, check the circuit, only then I use an external supply, but at that point Arduino is usually no longer anywhere near the project and only standalone chips are in danger.
Voltage and ground are relative to each other. If your grounds are different then what about your voltages? Keep a common ground and voltages will all be on the same scale. Might work even.
Start doing searches on electric and magnetic fields and the Faraday-Lenz law. And work from there. Learn the physics to better understand the phenomenon. Electrons have a directly replusive to other electrons leikes-repel electrical field, spring-like to a point, the higher the voltage the more densely the flow electrons are packed together, they will flow towards any lesser voltage they can. That pressure is voltage. It is the force.
When electrons move you have current and you also have magnetic field which a current thing. Making or changing a magnetic field takes or returns current flow. Initial switched-on current flow slows itself establishing a magnetic field around the flow slowing it slightly in the force of higher voltage.
Voltage from electrical field replusion makes current flow and that flow pushes the local magnetic field which resists all change but does have a measure and will fall from higher to lower energy even as it resists. It's the interaction of fields through electrons that makes electricity.
gilperon:
I connect a battery to a LED and it works but if I only connect the negative pole to the battery and the other point of the wire to my hand the LED does not light up. Why?
Because your hand is not a wire. Simple as that - do not confuse the two and do not believe any suggestion that a particular voltage is "safe" though in most cases 15 volts or less is (unless you drop the spanner across the car battery).
Hands and circuits should in almost all cases, be kept quite separate for the sake of both.
Shpaget:
Is there something I am missing?
Several things!
Good power supplies have so many benefits.
Known good start up conditions (no nasty spikes, switches on output disengaged always)
Adjustable current and volt limits, will measure and lower output to stay within limits.
Over voltage protection (will clamp supply output and risk supply to save (expensive)project)
Very clean output.
Isolated outputs.
Programmable
Sense lines for accurate volt delivery
Built like a tank to last you decades.
The list goes on and on.
A wise investment if you intend to pursue electrickery.
Note the one linked earlier (Manson 8041) is a switching supply.
TBH the second hand market is full of great lab/industry grade supplies at less than a tenth of their value.
Now is the time to buy one IMHO (I got two!)