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Topic: Why small resistance will not always produce high current? (Read 333 times) previous topic - next topic

gilperon


polymorph

It requires voltage to push current. That charger puts out a regulated 5V, and is capable of continuing to supply that voltage at up to 1150mA for an unlimited amount of time.

The resistance of your body, through the skin, is quite high. On the order of 1k to 20M and up.

Current equals Voltage divided by Resistance.

5V/1k = 1mA at the really lowest resistance that you can expect without breaking the skin. And to get that, you'd have to have very damp, thin skin, and probably also be dampened with salt water.

Your finger is -not- a small resistance. Usually, you can expect 100k and up.
Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - tinyurl.com/q7uqnvn
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Foggiest

Why if I short this circuit in my hand the 1150mA will not kill me?

Thank you so much and sorry for my english, really sorry.
With the wires in you hand unconnected, 5v will only push a tiny current through the high resistance of your skin (probably microAmps).
When the wires touch each other, 5v will push the full 1150mmA (and a bit more possibly! eek!) through the very low resistance of the touching wires, but that is not flowing through your hand.



Saying that:
Don't go touching wires together, ever.

DVDdoug

I believe the regulatory agencies consider voltages above about 50V as "dangerous".   12V or 24V is considered completely safe. 

If you are planning on having electronics as a hobby, get yourself a multimeter.    You can find a cheap one for around $10 USD (if cost is a big consideration), it will be very useful, and it can be fun to measure things like skin resistance.  ;)    The multimeter may not be sensitive enough to measure small-safe currents through your body, but it can measure the resistance.

Besides the high resistance, you won't get killed by current through one finger or through your hand.   It gets more dangerous when the current flows through your body (your heart) or your brain.   That usually means flowing form one hand to the other, or from a hand to your feet.

The resistance from one hand to the other through your body is much higher than between your finger and thumb on one hand.   (People who work with very-high voltages are taught to keep one hand in their pocket, and of course wear insulated shoes.)

Quote
Why small resistance will not always produce high current?...
 
...Ohm law says that U = RxI so a small R
Ohm's Law is a law of nature (with man-made units-of-measure) and it's ALWAYS TRUE.   So, let's say you've got 5V into 1/100th of an Ohm.    That's 500 Amps!!!!    If you hook-up your little 5V charger to such a low resistance (essentially a "short circuit"), it's obviously NOT going to supply 500 Amps...  What really happens is, the voltage drops to nearly zero and you might get something more than the rated 50mA, or it might shut-down to 0V, or it might burn-out and permanently go to zero volts.

On the other hand, a car batter CAN supply 500 Amps into a fraction of an Ohm, but at 12V it's not going to "push" dangerous currents through your body.   (You can get high voltage inductive kick-back when you disconnect a car battery, or high voltage from a spark-plug coil.)



P.S.
I'm NOT recommending that you do this, but you can "test" a 9V battery by touching it to your tongue! :D

Foggiest

I believe the regulatory agencies consider voltages above about 50V as "dangerous".   12V or 24V is considered completely safe. 

P.S.
I'm NOT recommending that you do this, but you can "test" a 9V battery by touching it to your tongue! :D
Haha!
*skips off, unfettered from legal responsibilities, laughing maniacally*
:P

polymorph

Actually, the resistance of your flesh inside the skin is low enough that 12V is potentially dangerous. Pun unintended, but there it is.

In any case, 5V is pretty safe unless you start driving probes into different parts of your body.
Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - tinyurl.com/q7uqnvn
Multitasking: forum.arduino.cc/index.php?topic=223286.0
gammon.com.au/blink - gammon.com.au/serial - gammon.com.au/interrupts

Foggiest

I work by the ghostbuster's rule:

The only time to cross or touch the wires is when under attack from a giant marshmallow man.

Worked out fine so far :)


Anyhoo PolyMorph, I was happy reading your post and now I'm sad, not sure it was an emohawk or your sig! ;)

JoeN

FACT:  A 5V charger is 100% and totally safe, until it isn't.  Note that this is not normal, something was very wrong with this charger and let mains current through to the "DC" side.

http://rt.com/news/168772-faulty-usb-electrocutes-woman/

A Filipino mother-of-two was killed while talking on a phone that was plugged into a faulty charger. The incident has been called a "wake-up call" by authorities who warned against the use of knock-offs as they pose a serious risk of electrocution.

Sheryl Aldeguer, 28, who had come to North Gosford, Australia to work as a nurse, was found dead in her apartment on April 23. Authorities have now established that she was electrocuted while speaking on a mobile phone which she had been charging. They believe charger was a faulty knock-off which released a sudden, high-voltage, electrical burst.

"The voltage seems to travel up through the faulty charger into her phone and she was wearing earplugs and also operating a laptop which was also plugged into a power point," said Fair Trading representative from New South Wales Lynelle Collins to the Sydney Morning Herald.

The electricity then went down through the earplugs into the laptop and into the power point.

"Two-hundred-and-forty volts [then] traveled up into the phone, which obviously the phone isn't designed to handle," added Collins.

Aldeguer's body was found by her friends a day after the incident. She had sustained burns to her chest and ears and it is thought that she died instantly.

Screenshot from Fairfax Media video

Friends of Alderguer believe that the victim procured the charger from a mobile accessories outlet in Sydney. Australian authorities have raided the organization and seized all of the non-standard chargers that were on sale. They have also issued a warning to Australian consumers to steer clear of the faulty products because of the serious risk of fire and electrocution.

Fair Trading Commissioner Rod Stowe has urged all consumers to discard any non-standardized chargers and bend the prongs as a precautionary measure, so they cannot be used again.

"It's a wake-up call to people who buy cheap electronic products without realizing potential hazards," said Stowe. "If the deal's too good to be true, it usually is and it can prove fatal."

He also advised people to avoid using mobile phones while they are charging because of the increased risk of electric shocks.

"It's not a good idea to actually use it while it's charging," Stowe said. "We're probably all guilty from time to time [but] our experts advise that it's not a thing we should be doing."

The crucial difference between standardized chargers and their cheaper knock-offs is the lack of insulation. Genuine products usually contain a number of layers of protection, while faulty ones will usually only have the bare minimum.

Last year a Chinese airhostess died of electrocution while talking on her iPhone while it was plugged in. A subsequent investigation into the incident revealed the charger she was using may have been a cheap knock-off that caused a sudden surge of electricity to overload the phone's battery.
I will never ask you to do anything that I wouldn't do myself.

JoeN

Also, what is the voltage of mains current in your country?  I've accidentally touched mains current several times during my life and I haven't died yet (then again, I wasn't in a tub at the time like the horror story above).  I posed a question to people in Bar Sport a while back whether more people have touched mains current or not accidentally during their lives and, in fact, it seems that most people have.  It's hard not to come by a frayed cord or improperly grounded appliance a few times during your life.   The first time for me, I was like 5 or 6 and stuck a key in an outlet to see what would happen.  :)  Then again, in the US it's 120V RMS and not 240V RMS
I will never ask you to do anything that I wouldn't do myself.

cr0sh

There's also the question of what is meant by "dangerous". As others have noted here, the danger of electrocution with such low voltage and amperage is slim-to-none.

...and while 12 or 24 volts DC isn't dangerous on its own, if your skin resistance is lowered, and/or the amperage of the supply is high enough - danger can come in a different form.

Like others have said, a car battery (or a large output SLA gel-cell or LiPo) can pump out hundreds of amp in a split second. Connect a part wrong or somehow short the battery out, and you can find yourself on the receiving end of problems.

Think fire and explosion - I've personally witnessed what a few tens of amps can do to a small wire - a bright flash, a crack like a fire-cracker, and the wire disappeared (happened in an electronics class years ago to a fellow student using a bench supply - the breadboard didn't survive, either).

You can easily weld with a couple of car batteries connected in series; having worked on Burning Man art cars in the past made from assorted golf-cart parts and such, sometimes making an impromptu welder is part of the fun. Oh - and don't let a wrench drop between terminals.

Depending on the battery chemistry (LiPo especially) - a direct short dumping all the current through a conductor - if that conductor doesn't open up quickly, the battery can off-gas, swell, burst, heat up, catch on fire, and explode. Usually in very short order. LiPo's (and most other lithium chem battery types) can easily go into "thermal runaway" by drawing too much current - this is where, even if the load is removed, the battery becomes something like internally shorted - it will swell, heat up, burn, possibly explode. People have had this happen (generally when mis-charging a battery) and burned their houses down.

Even a single cell ordinary household battery can become dangerous. One time when I was a kid, I had a AA cell lying on its side on my desk. I was sitting in my bed reading, when all of a sudden there was a loud BANG (again, like a fire cracker), and my window cracked. I was very surprised, of course; I got up to look, and I found that the battery had ruptured for some reason, and shot out the carbon rod and positive terminal across the room, hitting the window and cracking it; the casing moved backward a bit, eventually rolling off the desk. It was basically like a small cannon; had it been facing another direction (ie - towards me), I might have been injured.
I will not respond to Arduino help PM's from random forum users; if you have such a question, start a new topic thread.

Grumpy_Mike

Think about a child's train set. That has 12V across the rails and is played with quite safely.

So 12V is not dangerous until you attache it to pins and stick them in yourself. Hardly an accidental thing to do.

By the way:- DO NOT CROSS POST

JoeN

I am amazed how smart you all are! You made it very easy to understand. It was good to know that faulty usb can lead my to death so I will by a good usb cable and make sure I will use a good usb charger to avoid accidents.

What I cant understand after all your explanation is this: if the charger provides 5v why does it only provide 1050mA? If I short wire it with veeery low resistance wires, it should provide me a high current, isnt? Why it doenst?
The 1050mA is likely what it is rated at.  If you short it out, it may well provide more current for a short period of time and then either burn up, or if it is well designed, it might possibly blow out an internal fuse or a resettable fuse (PPTC) might kick in and limit the current.  If it is really badly designed, who knows, it might compromise it between the mains side and the DC side and then you have problems like the girl in the bath.  Hopefully, not.  It isn't really saying that you will never get more than 1050mA.  Here is some info about PPTCs which are becoming pretty common:

http://en.wikipedia.org/wiki/Resettable_fuse
I will never ask you to do anything that I wouldn't do myself.

Grumpy_Mike

Quote
if the charger provides 5v why does it only provide 1050mA?
That is the maximum it should be asked to supply without problems. It is not the maximum it can supply.

Any power supply has what is called an internal impedance, that acts just like a series resistance. This applies to batteries as well and limits the short circuit current. That is why you can melt your spanner ( wrench for the U.S. centric ) when you short out your car battery but not a bunch of AA batteries. The car battery has a much lower internal impedance.

I have never seen a power supply with a resettable fuse, the problem with those is that they take a long time ( electronically ) to kick in, and once they have been triggered a few times the characteristics change, they also do not like working at a high "normal" current.

Grumpy_Mike

Some processors have switchable impedance on their outputs, I wrote this about one processor, the one in the Raspberry Pi. While you can not change the output impedance of an Arduino's output pin, the current limiting principle it describes is the same for any source of power, be it a processor pin a battery or a power supply. They all have a limiting impedance.
Output impedance

Note that most devices are not happy supplying a current that is only limited by the output impedance and the rated value will be much lower. Also note as the current flows through that impedance it will generate heat and that is why shorted out power supplies can catch on fire or melt components.

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