Powering the Uno

I am new to Arduino and have just bought my first Uno. To power it I thought I'd use a 12v 2000mA poweradaptor I have lying around. Checking the voltage with a multimeter and nothing connected to the adaptor, I found it to produce 17,8v. Is this a normal thing or is the adaptor bad? Just don't want to fry the uno. With the USB cable powering the board, what should the voltage be at the 3,3v and 5v pins? Thanks!

If its a big heavy transformer type adapter, its probably not faulty, but also not regulated, so DO NOT connect it to your Arduino.

If its a switch mode PSU then its faulty, so DO NOT connect it to your Arduino.

Buy a new 12V (1A plenty) switch mode wall-wart, or take power from the USB power to start with.

The Arduino's on-board regulator can actually handle up to 20V. 12V is actually recommended because of the additional heat generated above 12V.

Most Wall-Warts only produce the rated voltage under load.

As a very quick test, you can safely attach your wall wart to your Arduino and measure the voltage at Vin or directly on the barrel jack. If the small load of the Uno is enough to put the wall-wart into regulation, you will be fine to use it. If this test still shows >12V, do not continue using this power supply with the Arduino (unless you apply a larger continuous load in addition to the Arduino.)

Again, because the Arduino's input regulator can tolerate >12V and if there is no other load, briefly applying 17V to the input should not be damaging. You should not run the Arduino long-term with an input higher than 12V.

With the USB cable powering the board, what should the voltage be at the 3,3v and 5v pins?

The 3V3 pin will be very close to 3.3V. The 5V pins will be whatever the Vusb of your PC is at the time. The USB spec allows this to be in the range of about 4.7v-5.2v (maybe a little wider than that.)

Thanks for the swift answers.

My first reaction is, there was no mention of a regulated power supply on the hardwaresite of Arduino.

This is what it says :

External (non-USB) power can come either from an AC-to-DC adapter (wall-wart) or battery. The adapter can be connected by plugging a 2.1mm center-positive plug into the board's power jack. Leads from a battery can be inserted in the Gnd and Vin pin headers of the POWER connector. The board can operate on an external supply of 6 to 20 volts. If supplied with less than 7V, however, the 5V pin may supply less than five volts and the board may be unstable. If using more than 12V, the voltage regulator may overheat and damage the board. The recommended range is 7 to 12 volts

I read from this that there is a voltage regulator on the board, and no mention of the powersupply having to be a regulated one. To be sure (because I´m not a naive english speaker) a regulated power supply delivers a stabelized power as opposite of a non regulated, that just produces a DC voltage.

Strangely enough my board, Powered through the USB cord to my computer only showes 2,5 v on the 5v pin and 2,2v on the 3,3v pin. Checking the USB poweroutput, it is 5,2v. That´s why I want to check it with an external power supply.

Powered through the USB cord to my computer only showes 2,5 v on the 5v pin and 2,2v on the 3,3v pin. Checking the USB poweroutput, it is 5,2v.

That's not good. There is a short circuit somewhere on the 5V node in your system and that is pulling the output of the 5V regulator down to 2.5V. I'm surprised the computer is not complaining about your board drawing too much current.

-- The Rugged Motor Driver: two H-bridges, more power than an L298, fully protected

I’m surprised the computer is not complaining about your board drawing too much current

Maybe the Arduino’s poly fuse is not opening completely and is dropping most of the voltage. It would keep the current draw from the PC down and would explain the low on-board voltages. Do you have another PC or a USB Hub you can attach the Arduino to? (To verify the USB port you are using can actually deliver a load.)

If nothing else is connected to the Arduino, a visual inspection of the board is in order.

Knowing this piece of information, I would NOT take my previous advice and connect your 12V 2A supply. If there is a short, you may permanently damage the Arduino.

Thanks Again for your comments.

Well, i had a bit of trouble to get my board running. I got a message after uploading that the upload was not succesfull, and it had no other info. But after a while the uploading did start to function - for no apparent reason - and did all the LED tutorials from the book. Noticing that the leds were rather dim. then I did a test with a opto-sensor (CNY70) that worked very well in another layout I use, but not on my beardboard/arduino. Troubleshooting this set-up reveiled, the current was only 2,5v.

But as we speak the board is again not uploading anymore, and still returning 2,5v (with no other components connected to it - I forgot to mention that earlier. Visulally there is nothing wrong with the board. i also tried the USB on my laptop, which is returning 5,1v, and got the same result there.

Anyone on how to work around the powerpart of the Arduino. Shall I try and put 5v on the 5v Pin, or do i make things worse. Well It can´t get any worse then this.....

Anyone on how to work around the powerpart of the Arduino. Shall I try and put 5v on the 5v Pin, or do i make things worse. Well It can´t get any worse then this…

Take two resistance measurements with your multimeter on your un-powered Uno:

  1. Measure between the 5V and GND pins? (I am seeing about 940ohms)
  2. Measure across the poly fuse, the component between the USB jack and regulator. (I measure 1ohm)

Hi James, Thanks for the help.

5v and GND = 261,8 ohms is the polyfuse the copper thingy next to the USBplug ready 500j? That is 0,09 ohms, close enough i´d guess.

What do we know now?

Yes, that is the poly fuse.

It suggests that either there is a fault on your board or your multimeter drives a different amount of current than mine. Since there are active components on the board, it is a fuzzy test.

Going back to your previous question, you should only apply 5V to the 5V pin, if you have a power supply that can limit the current. On the other hand, if you don't apply current limiting you are very likely to find the source of the problem. ;)

Do you have access to a high-resolution scanner? It might worth while to post good pictures of the top and bottom of the board.