I have a question about ohm law and my arduino output current, its maybe a stupid question from me but I dont get it..
The +5v of my arduino uno has a maximal intesity of 50mA if I correctly understand the datasheet, right ?
But if I listen to ohm law, I = V / R, so imagine I put a simple wire between the +5v of my arduino and the GND without any others components, it will mean the R will be nearly null (let say 1 ohm for the calculation).
So we have I = 5 / 1, so the arduino should output 5 amp of current.. Not only 0.05.. I dont get it ..
Sorry if its a stupid question, but I am really beginning in electronic.
karnalta:
So we have I = 5 / 1, so the arduino should output 5 amp of current.. Not only 0.05.. I dont get it ..
Sorry if its a stupid question, but I am really beginning in electronic.
Yes, in ideal world it will be! And in ideal world if you put 0 (zero) ohm resistor you will have infinite current!
But world is real, so if you shortcut your output (or if you put a small resistor) the current will be limited by the maximum current that output can be supply.
Ohm's law is always true, so sometimes something has to "give". It's a law of nature with man-made units of measure (Volts, Amps, and Ohms).
If you connect a low resistance load to one of the Arduino's outputs, you may damage the Arduino as it "tries" to supply the required/calculated current.
If you put a zero-ohm (or nearly zero) load across a voltage source, you will no longer get 5V.
Many power supplies can supply 5 Amps and with a 1 Ohm load across a high-current-capable power supply, you will get 5 Amps. If you put a 1 Ohm resistor across a typical "wall wart" power supply, the voltage will drop and you might burn-out the device.
If you put a 1 Ohm resistor across a car battery, you will get (about) 12 Amps. (The voltage is usually a little more than the "nominal" 12V.) And, you will get ~144 Watts of heat in the resistor, so you'd better have a high-power resistor or you'll burn it up! ("Don't try this at home." ) If you put a 1 Ohm resistor across the 120/220V power in your house, you'll blow the circuit breaker (if the resistor doesn't burn-up first).
Thank you, so it mean ohm law is always limited by the device anyway.
So it mean that if I have a multi meter that can mesure intensity up to 1A, I can put it to shortcut the arduino 5v, I will not measure more than 50mA ? I wont blow it.
so it mean ohm law is always limited by the device anyway.
No not ohms law, that stil holds but you are not taking into account all the restance in the circuit. Any power supply has an output impedance, that is like a seriese resistor. Normally this is small compaied with the load and you can ignore it. However with a heavy load, low resistance, it can be much bigger tha the load and this resistance dominates the current you can get.
A battery can blow up if you draw too much current because it heats up because you are passing current through this resistance.
I am not sure where you get this 50mA from it is not a figure I recognise on the Arduino.
Intensity is the wrong word, it is current.
I can put it to shortcut the arduino 5v, I will not measure more than 50mA ?
No, exceed current ratings and you damage electronics.
DVDdoug:
Ohm's law is always true, so sometimes something has to "give". It's a law of nature with man-made units of measure (Volts, Amps, and Ohms).
Ohm's law is not "always true". It doesn't apply to a vacuum (as in thermionic valves), it does not apply to insulators, it does not apply to gasses or plasmas, it does not apply to superconductors (ie at very low temperatures). It does not apply to quantum-tunneling composites. It doesn't apply to semiconductor junctions and it doesn't apply to living tissues / cell membranes.
Its an approximation to the behaviour of solid conductors, uniform semiconductors and ionic liquids when moving charge carriers interact in random thermal manner with other things.
Furthermore Ohm's law only applies on the macroscopic level and breaks down at the nano / quantum scale.
Well in fact Ohm's law (he is a person so capital letter, it is his law so an aposphory ) to material with linear resistance.
Which is like saying it only works when it works.
In the contex that the OP asked the problem he had was he was not taking into account all the resistances in the circuit. If he had of he would have found it worked exactly.
What is hard for me to deal with, is that when you use Ohm's law to calculate a resistor, it doesn't take in parameter the maximum current capacity of your energy source.
Let say I have a device who require 5v and 10mA to work and that my power supply is 5v and 50mA. The law will give me R = 0/0.01, so 0. I won't use any resistor, but then I will throw too much current into the device ?
Or maybe I shouldn't not compare a general device like a laser pointer with a LED, maybe only LED need you to also take care of the current you throw at them and other device will only take the amount of current they need while you are respecting their voltage ?
You should include the output impedance of your power source in your calculations and then it will work.
It is like trying to calculate how much wall paper you will need to paper a wall and not taking into account the pattern repeat length.
Let say I have a device who require 5v and 10mA to work and that my power supply is 5v and 50mA. The law will give me R = 0/0.01, so 0. I won't use any resistor,
No this is not right, there are a number of wrong assumptions here.
I believe you but then take my imaginary problem as example.
I want to plug a device who require 5v and 10 mA (from the datasheet) to my Arduino +5v output, how do I calculate the resistor I need to use to prevent it to burn ?
It depends on what the device is. If it requires 5V at 10mA then you don't need any resistor to stop it from burning, just connect it to the 5V and it will take 10mA.
Ok, thank you. I am asking myself a lot of question because when advancing in my electronic learning (through the great Make : Electronic book), I see that there was a lot a false rules I was following with electricity / electronics.
When I had to replace a DC adapter from a device, I was always thinking that the current have to be respected, but in fact the only important thing to respect is the voltage (and the type of electricity of course).
If I replace a DC of 12v - 1000mA by a 12V - 2000mA won't pose any problem by example. The device will just use less current that what the DC can safely provide. Maybe replacing it with a 12V - 500mA will avoid a correct working of the device due to the lac of current.
Grumpy_Mike:
It depends on what the device is. If it requires 5V at 10mA then you don't need any resistor to stop it from burning, just connect it to the 5V and it will take 10mA.
Mike you are right. I agree with your thought. Everything it depend on its requirements. Just one need ensure that what is the requirements then he could solve any problem.
) If you put a 1 Ohm resistor across the 120/220V power in your house, you'll blow the circuit breaker (if the resistor doesn't burn-up first).