Danger of powering breadboard with cellphone charger

[gilperon]

Hi, instead of using small batteries (which I know are really safe) I want to use my old cellphone charger. It says it provides 5v and 1150mA. I know 50mA can kill a person, so my question is: if I use this charger (cut wires and connect them to my breadboard and the outlet of a wall) and if I make some mistake and short the wires + and - in my hand will I get killed? Ohm law says that U = RxI so a small R (which would be my finger R) would provide a high I that would kill me (as I believe).

I know I wont be killed cause I see many people using this kind of power outlet of wall as a power supply to their breadboard so why is it not dangerous? 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.

 

[Audioguru]

Don't you know Ohm's Law?
How on earth can 5V deliver a current of 50mA?? Only when it is connected to a piece of carbon or metal that has a resistance of only 100 ohms. For 1150mA the resistance must be 4.35 ohms.

Just now I measured the resistance of my skin from one hand to the other hand. It is 1,000,000 (1M) ohms. Then with 5V the current is only 0.005mA (5uA) and I do not feel anything.

Your skin resistance drops at higher voltages then the current is much higher.
Read about it in Google where is says people have been killed with 110VAC and 42VDC.

If you short your charger with a copper wire then it might smoke or burn out.

 

[bigdogguru]

It says it provides 5v and 1150mA.

The power supplies stated specifics indicate the maximum current the power supply in question CAN deliver at 5V, i.e., maximum power VI.

Ohm's law still applies and therefore dictates the delivered current based on the characteristics of the circuit to which the power supply is delivering power.

For example the power supply in question would be expected to deliver only 5mA @5v across a 1KΩ resistor.

In the similar way a circuit breaker doesn't dictate current flow to a device, it only limits the maximum current that can be supplied at a particular voltage.

BigDog

- - - Updated - - -

Oops, Audioguru beat me to a reply.

 

[dick_freebird]

I would beware the solid-state chargers, they can have
isolation faults (see iPhone). Old school heavy iron wall
warts, you'd have to really try to get in trouble with.

Current kills, but only when voltage has its back.

 

[gilperon]

Hi @Audioguru. Thank you so much for your experiment! I didnt know my hand resistance would be so big!

As you say, if I touch both wires (+ and -) of a 5V that provides 1150mA I will not get electrocuted? Are you sure of that? I know you measured the body resistance and it looks very high so I will probably not die. But I am worried cause I will be shorting 2 wires that comes directly from the outlet. If I dont die, what do you think is gonna happen if I touch both wires? Will my house's light blink because of that? Will my wall wires get too hot because of the 1150mA going throgut it?

Also would like to say thank you to @bigdogguru that gave me the idea to use a circuit breaker to avoid me getting electrocuted. But I still have a question for you about ohm's law. Why does my charge states it provides 1050mA? It should only provide the voltage, isnt? Cause the current will depend on the resistance I will be using in my system. If I use a tiny resistance the current would have to be veeery high, isnt? So why does the charger states the current?

 

[bigdogguru]

Why does my charge states it provides 1050mA? It should only provide the voltage, isnt? Cause the current will depend on the resistance I will be using in my system. If I use a tiny resistance the current would have to be veeery high, isnt? So why does the charger states the current?

As I previously indicated, the charger states the Maximum Current that can be supplied at 5V under any load conditions for the charger to remain within spec and function as expected.

Using your example of a "tiny" resistance, this resistance of a purely resistive load must not be less than 4.35Ω for the supplied current to remain below the stated maximum of 1150mA, otherwise if the delivered current should exceed the stated maximum of 1150mA, the charger will be out of spec and may cease to function as expected, i.e., the charger or load could be damaged, fuse blown, etc.

It should also be mentioned, operating a power supply or any component for that matter either close to or at their maximum specifications is typically not prudent, as there is always a question as to the accuracy of these stated maximums, other specs and the stability of the load conditions.


BigDog

 

[Audioguru]

What outlet? Your electricity outlet is 120VAC in North America or is 230VAC overseas. It will kill you and burn you. Instead you said you will use the 5VDC output from your charger.

5V from your charger is almost nothing and you will not feel it. The MAXIMUM allowed current from your charger is 1150mA. Use Ohm's Law to calculate the amount of current the 5V will provide into a resistance.

Your house does not use 5V for its electricity. Instead it uses 120VAC or 230VAC. Its maximum allowed current is 100A or 200A.
Since the maximum output power of your charger is 5V x 1.15A= 5.75W and it might heat with 4W then it draws almost 10W from your electricity that can supply 120V x 15A= 1800W from one outlet. Do you think the lights will blink and your wall wires will get too hot when you connect 10W to a source of 1800W?? The wires in the wall are designed to carry 1800W and if there is an overload then a safety circuit breaker will detect it and disconnect it.

Of course a tiny resistance is like a dead short and will overload the charger unless it has a circuit to prevent overload damage.

Since you do not understand anything about simple electricity then please keep away from it.

 

[gilperon]

Hi,

Maybe I explained me wrong. I have a charger that I plug into the wall (my outlet in Brazil provide 220v) and in the charger specs says it will provide 5v and 1050mA.

You all were very clear, very clear and helped me a lot but maybe I could not make myself easy to understand.

@bigdogguru explained very well that if I use more current than my charger is designed to it may burn, have the fuse activated... so I will try to keep my current under the specs to avoid what @bigdogguru said. But I want to know this: I have a LED. I have no idea what is it resistance so how do I know what value of resistance I should use to avoid using more than 1050mA of my charger? My LED is a simple LED of .5 cents. How do I know my LED resistance so I can check if the current provided by my charger will not exceed the specs?

@Audioguru you are right, I am trying to stay away from the outlet while I cant understand it and be safe with it. That's nice to know that my wall wires can carry up to 1800W with no problem! So I can be sure that I will not burn my wires shorting them in the 5v connect to the outlet. But I have a final question to you: I always see many circuits and people always connect the + and - from the battery in the circuit. Why when I plug my negative pole from battery to a led I also need to plug the positive pole? When I try to plug the positive pole to my hand wet or to the ground it does not work? Why do I always need to connect the positive pole to the battery?

 

[bigdogguru]

@bigdogguru explained very well that if I use more current than my charger is designed to it may burn, have the fuse activated... so I will try to keep my current under the specs to avoid what @bigdogguru said. But I want to know this: I have a LED. I have no idea what is it resistance so how do I know what value of resistance I should use to avoid using more than 1050mA of my charger? My LED is a simple LED of .5 cents. How do I know my LED resistance so I can check if the current provided by my charger will not exceed the specs?

Actually, you are asking the wrong question. You need to examine the specs of the LED to determine the maximum amount of current the LED can handle without damaging it. Also, another important specification when dealing with LEDs is its Forward Voltage (V_F) rating.

Once these two specification values are determined, an appropriate current limiting resistor can be calculated which limits the current flowing through the LED and supplied by the power supply (charger).

LED Current Limiting Resistors



BigDog

 

[Audioguru]

An ordinary LED is usually 5mm in diameter and has its voltage and brightness specified on its datasheet when it has a current of 20mA.
Here is a photo of some diffused red 5mm LEDs but they are also made with a clear plastic. The short wire is the negative terminal (cathode).
The forward voltage of most red LEDs is about 1.8V. White, blue and bright green are usually 3.2V. But the actual voltage of an LED could be plus or minus 0.3V.

You have a 5V charger and the red LED is in series with a current-limiting resistor. The voltage across the resistor is about 5V - 1.8V= 3.2V. Use Ohm's Law to calculate the resistor value. 3.2V/20mA= 160 ohms.

 

[gilperon]

Thank you so much! Now I understand it all! I read a lot of materials and nowhere I could find that a LED allows all current to go. I thought an LED had a resistor inside (which caused the light) but as you said it has not, it will allow all current to go in it. It will only keep the voltage.

The 2 links you provided helped me a lot! The formula R = (Vpower_supply - Vled)/Iled helped me understand it all!

I will also use a 160 ohm resistor as @Audioguru told. I got much more excited cause now I understand how a circuit with an LED works!

 

[Audioguru]

I thought an LED had a resistor inside (which caused the light)

Hee, hee. When I had my first job working for Philips in 1965 I saw the first LED made by Philips. I joked that it was red hot diode!

Lately I was given a bag of red Philips Lumileds Luxeon Superflux LEDs from a member on one of these forums for helping him. The SuperFlux LEDs are extremely bright since they have a square case (0.3" x 0.3") with 4 pins for cooling and their maximum current rating is 70mA. Their light beam is very wide.
I made two night lights with them and some are in solar garden lights.

One night light has 36 LEDs, each with about 42.5mA. They are on a stripboard mounted in the case for a compact audio tape cassette with many cooling holes in it. Boy oh boy is it ever blindingly bright and it gets HOT! It lights up the entire neighbourhood. I power it from a 12VDC/500mA wall-wart.
Four of the 2.5V LEDs are in series and in series with a 47 ohm resistor.

 

[gilperon]

Nice really nice! I have never seen a LED like the one you showed. Mine is not too bright and not even light up my room! I am impressed your LEDs lighted your neighborhood!

I will be looking for a LED like the one you said! I would like to have a powerful LED!

 

[Audioguru]

My square LED has a maximum rated current of 70mA (2.5V x 70mA= 0.175W) but many LEDs are available with much higher current ratings.
Instead of using a huge chip at a very high current the high power LEDs actually have many smaller chips inside that share the current.
Look here:

 

[darkgnat]

As long as you don't take it apart and jam your finger into the mains (120V/240V) side of the charger, it's not going to kill you. If you short it out for a long time on the breadboard and the charger catches on fire you can burn your house down, though. A good practice is to unplug a project when you're not actively using and monitoring it.