# Am I GUARANTEED not to ruin arduino circuit if I use this trick???????

Hello I have an arduino uno which outputs 5 volts and recommends a maximum current of 20mA
If I use a 250ohm resistor in parallel(flag, should have put series) with any circuit am I GUARANTEED not to blow the arduino uno because V = IR and (V / R) = I and (5 / R) < 20mA therefore R needs to be greater than 250ohms and then I am GUARANTEED not to go over 20mA

Are you considering powering something from an Arduino GPIO pin ? If so then stop right now

If you put any value of resistor (or its equivalent) in parallel with a 250 ohm resistor then the effective resistance will be less that 250 ohms which is not what you want

UKHeliBob:
Are you considering powering something from an Arduino GPIO pin ? If so then stop right now

If you put any value of resistor (or its equivalent) in parallel with a 250 ohm resistor then the effective resistance will be less that 250 ohms which is not what you want

Sorry I meant to say "series" I fixed it in the post as well. Was that the only thing wrong, or are you still saying that if I am "considering powering something from an Arduino GPIO pin ? If so then stop right now" ?

You seem not to know the difference between parallel and series connections.

If you put a 250 Ohm resistor IN PARALLEL with another circuit the resistor alone will take 20mA and the other circuit will take whatever extra current it needs. And bang goes the Uno.

If you put a 250 Ohm resistor IN SERIES with another circuit then the current will be limited. But of course the "other circuit" will probably not work because the resistor is stealing most of the voltage it needs.

Steve

slipstick:
You seem not to know the difference between parallel and series connections.

If you put a 250 Ohm resistor IN PARALLEL with another circuit the resistor alone will take 20mA and the other circuit will take whatever extra current it needs. And bang goes the Uno.

If you put a 250 Ohm resistor IN SERIES with another circuit then the current will be limited. But of course the "other circuit" will probably not work because the resistor is stealing most of the voltage it needs.

Steve

I know, I did not mean to write "parallel" I meant to write "series". Oh ya I could see the resistor would limit the voltage as there would be a voltage drop at that resistor of I(total current of entire circuit) multiplied by 250 ohms. So are you saying that the best method would be to calculate the current for each circuit I create and make sure that the current does not exceed 20mA and if it does then I should add an appropriate resistor in series with my circuit.

If your circuit requires more than 20 mA and you limit it to 20 mA, your circuit will not work.

sterretje:
If your circuit requires more than 20 mA and you limit it to 20 mA, your circuit will not work.

Okay yes I see what your'e saying that the cicuit may not work and I can adjust the resistance accordingly, but as far as keeping the arduino board safe, my trick should work right? in theory I could connect my arduino to power a 2000 Watt RMS subwoofer with no damage to my arduino board as long as I add that 250 ohm resistor in series with the added circuit

This is something of an “XY Problem”.

It is true that a 220 Ohm resistor will limit the current to a safe level connected to any other device which is not itself powered. You might however have problems if that device is inductive as switching it off suddenly can produce not a current but a voltage surge.

So there is no “general case” and the question is in itself, not helpful. You need to explain what you actually want to do.

Sorry I meant to say "series" I fixed it in the post as well

That was the wrong thing to do without flagging the change in the post as it now makes nonsense of my post referring to it. Please don't ever do that again

in theory I could connect my arduino to power a 2000 Watt RMS subwoofer

Powering a device and providing a signal to a device are two different things and you would be wise to differentiate between them

The Arduino output is safe (using the 250 Ohm resistor) as long as you do not connect it to anything that has lower voltage than Arduino GND and higher than Arduino "5V". Note that the "5V" = GND when Arduino is not powered.

UKHeliBob:
That was the wrong thing to do without flagging the change in the post as it now makes nonsense of my post referring to it. Please don’t ever do that again

Sorry about that, won’t happen again

UKHeliBob:
Powering a device and providing a signal to a device are two different things and you would be wise to differentiate between them

Sorry I should have been more specific, when I said “in theory I could connect my arduino to power a 2000 Watt RMS subwoofer” I meant that the arduino output would be directly connected to the subwoofer and no amplifier would be used.

I know you can power and send a signal to an amp which in turn is connected to the speaker so they are different in that sense, but if we are just using a speaker and no amp then the signal would also be the power if I am not mistaken

Paul__B:
This is something of an "XY Problem".

It is true that a 220 Ohm resistor will limit the current to a safe level connected to any other device which is not itself powered. You might however have problems if that device is inductive as switching it off suddenly can produce not a current but a voltage surge.

So there is no "general case" and the question is in itself, not helpful. You need to explain what you actually want to do.

Yes I see what you are saying that it would be better to calculate based on my specific circuit and act accordingly, but I was just trying to make a method that I could use as a safeguard, before doing the calculations.

"which is not itself powered" thanks, I will make sure to look out for that

why is it that it will produce a "voltage surge but not a current surge", if the voltage increases, wouldn't the current increase as well, sorry I am not very familiar with inductive devices

Where are you getting this 20mA number from?

The Arduino Uno has a 5V pin which is an output from the on board regulator.

5V: This is a regulated output from the on board voltage regulator. This power will come from either the USB or DC input jack. This is fed into the on board 5V voltage regulator. The output from the regulator is connected to this pin. You use this pin to provide 5V to power components connected to the Arduino board. The maximum current draw is approx 400mA on usb and higher if using the DC power jack.
3.3V: This is a regulated output from the on board voltage regulator. The output from the 3.3V regulator is connected to this pin. You use this pin to provide 3.3V to power components connected to the Arduino board. The maximum current draw is 50mA

The I/O pins can supply up to 40 mA if you are referring to an I/O pin but the 5V pin labeled 5V on an Arduino Uno board can supply 400 or more mA depending on how the board is powered. The actual 5V regulator features Current Limit, Safe Operating and Thermal Shutdown Protection so you can literally short the 5V pin to Ground and about all that will happen is the uC will shut down.

So again where did you get the 20 mA number from and which pin specifically are you referring to because I assume the 5V pin?

Corrected where I had 44 mA verse 400 mA. No clue as I generally at least proof read my post. Then I neglected to choose receive emails and alerts.

Ron

Ron_Blain:
The I/O pins can supply up to 40 mA if you are referring to an I/O pin but the 5V pin labeled 5V on an Arduino Uno board can summply 44 or more mA depending on how the

Not safely, they can't. It is an absolute maximum rating. That is where the 20mA comes from, it's a more reasonable nominal value.

CJTheDJ:
why is it that it will produce a "voltage surge but not a current surge", if the voltage increases, wouldn't the current increase as well, sorry I am not very familiar with inductive devices

This article may help.

CJTheDJ:
in theory I could connect my arduino to power a 2000 Watt RMS subwoofer with no damage to my arduino board as long as I add that 250 ohm resistor in series with the added circuit

If you tell us exactly what you will be connecting, you may get a more useful answer. This whole thread
may be xyproblem.

aarg:
Not safely, they can't. It is an absolute maximum rating. That is where the 20mA comes from, it's a more reasonable nominal value.

I agree it is a Max value but I don't see much sense in adding a series resistor will will drop a voltage based on current. Driving a MOSFET yes and I still am not sure which 5V pin the thread starter is referring to? I assumed, likely wrongly, the 5V labeled pin.

Ron

Ron_Blain:
I agree it is a Max value but I don't see much sense in adding a series resistor will will drop a voltage based on current. Driving a MOSFET yes and I still am not sure which 5V pin the thread starter is referring to? I assumed, likely wrongly, the 5V labeled pin.

Ron

I was referring to the input/output pins, but also wondering if the same idea would work with the 5v output pin, I would just have to adjust the resistor since that has a higher current limit. I get this isn't the most optimal method, but I was just wondering if I could use this for safety and then after I could do the calculations. When I ran into this idea I was actually considering driving a large speaker with the I/O pins of the arduino but I was getting confused with the calculations as the speaker has an impedance and not a resistance.