pin 5V/3.3V safety

Hello,

I've just bought ARDUINO UNO R3 and I heard a lot that it's was safe because of low voltage, I having a hard time trying to figure out why !

According to Arduino doc, the 3.3V pin can deliver up to 40mA and the 5V up to 400mA, or above 10mA it can be dangerous and 50mA is deadly. So, where is the mistake ?

And does it always deliver 400mA when I use the 5V pin ?

Thanks a lot,
bye

It can deliver that. Assuming you're talking about risk to the body, you need to divide the voltage by the resistance of your body to get to the actual current that flows through your body; so below milliAmps (probably microAmps).

I heard a lot that it's was safe because of low voltage,

Well considering a toy train set uses 12V on exposed rails then an Arduino at 5V posses even less of a danger to people. Just don't drop it in a glass of water.

According to Arduino doc, the 3.3V pin can deliver up to 40mA

That was an early figure based on the USB / Serial converter chip, I believe it is more like 140mA these days.

and the 5V up to 400mA

That is if the power comes from a USB cable based on the 500mA USB current limit.

or above 10mA it can be dangerous and 50mA is deadly. So, where is the mistake ?

The mistake is that those figures are for the current sourced from a single digital output pin. The actual figure is more than 40mA causes damage to the processor chip.

And does it always deliver 400mA when I use the 5V pin

No it depends where the voltage is coming from. If it comes from the power jack then a lot depends on what voltage you are putting in there, the limit comes from the overheating of the internal voltage regulator.
The actual current you get from the 5V pin depends on the resistance of the load you are using, 400mA is just the maximum for a USB connector.

Of course !! I forgot about body resistance !

Meaning that if I touch with both hands 5V wire and 0V wire I can calculate tu current flow through my body using Ohm law right ?

If I take body_R = 1kohm it makes

I = 5/1000 = 5mA

So it is safe ?

Thanks to both of you !

klownerz:
Of course !! I forgot about body resistance !

Meaning that if I touch with both hands 5V wire and 0V wire I can calculate tu current flow through my body using Ohm law right ?

If I take body_R = 1kohm it makes

I = 5/1000 = 5mA

So it is safe ?

Thanks to both of you !

Under typical conditions, the resistance the skin + body is way higher than that.

Google says around 100,000 ohms dry for average person falling to about 1000ohms when dripping wet ( eg in a bath).

It’s voltage generating the current that kills , also the current needs to pass through the heart to kill ( not that I’m any sort of expert in this area).

Suggest you google “ extra safe low voltage” ,” Tens machine” etc oh and of course “ohms law” , “ basic electric circuits”

I’ve just bought ARDUINO UNO R3 and I heard a lot that it’s was safe because of low voltage, I having a hard time trying to figure out why !

As far as safety to humans, anything below 50V is generally considered safe.

Most regulations require anything above 50V to be insulated/isolated so you can’t touch it. (That’s for “consumer products”… For example, you can buy a high voltage power supply with exposed terminals but in the “final design” you’re supposed to enclose the thing and insulate the wires/connections.)

That’s voltage… Current (Amps or milliamps) depends on voltage and resistance ([u]Ohm’s Law[/u]).

Resistance means the “resistance to the flow of current”.

Your body has very-high resistance so for example, you can touch a 12V car battery that’s capable of hundreds of amps and it’s safe. (An inductive load or spark plug coil in a car can generate enough voltage to shock you.) With nothing connected to the battery, no current flows. If you connect a regular little LED to a car battery with the appropriate current-limiting resistor, you’ll get 10 or 20mA. Turn on your headlights and a lot more current flows (though the lights).

Same thing with the Arduino’s 3.3V connection. With nothing connected no current flows out. If you connect an LED (with the proper resistor) you’ll get 10 or 20mA. If you connect 10 LEDs, you’ll get excess current… The voltage will drop and you might damage the Arduino.

If I take body_R = 1kohm it makes

If your body measures 1K your meter is broken! :wink:

or above 10mA it can be dangerous and 50mA is deadly. So, where is the mistake ?

The spec actually says 50mA maximum so above 50mA it could be “deadly” to the Arduino, but NOT to you! (because that current isn’t flowing through your body.)

P.S.
The most “dangerous” thing about touching an Arduino (or any PC board) is a[u]static discharge[/u] from your body. …You know when you touch something like and get a spark and sometimes a little shock? If you have a charge on your body and you touch a circuit board you can kill it! …Sometimes the discharge is too small to feel, but then your chip dies “for no reason”.

Thanks a lot everyone, brilliant answers ! :slight_smile:

DVDdoug:
If your body measures 1K your meter is broken! :wink:

Depends whether or not you have ECG electrode gel on your hands, :grinning: :grinning: :grinning:

The best way to kill yourself with 5V is to connect each wire to a thumb tack ( US ) or drawing pin ( UK ) and stick one in each wrist.

Hmmm, if you implying that sticking a thumbtack in a vein will result in fatal haemorrhage, I doubt it. And hitting an artery is even more difficult. :roll_eyes:

No it reduces the resistance and so lets 5V get enough current to kill you.