Go Down

Topic: I am so confused with wall power adapters for UNO (Read 3 times) previous topic - next topic

James C4S

I fear if i rely on PCB as heat sink, i "may" damaged my UNO regulator.

Yes, that is a possibility.  However, the regulator is very robust and has both thermal and short circuit protection. 

As Mike points out, if you have a regulated 5V supply, then you just connect it to the 5V pin.  That bypasses the on-board regulator, which you don't need.
Capacitor Expert By Day, Enginerd by night.  ||  Personal Blog: www.baldengineer.com  || Electronics Tutorials for Beginners:  www.addohms.com


Feb 25, 2012, 10:45 pm Last Edit: Feb 25, 2012, 10:48 pm by paracha3 Reason: 1

The Uno uses a linear voltage regulator to condition the power supplied via the power jack. These devices are rated to deliver a specific output voltage, and up to some maximum amount of output current. However, they need a voltage differential to work. That is, a 5-volt regulator like that on the Uno must be fed more than 5 volts on the input side. This is why the specification for the Uno input voltage is 7 to 12 volts. A 5-volt input will not work.

The regulator heats up because it must dissipate an amount power which is calculated by multiplying the voltage differential times the amount of current flowing through it. So with a 12-volt input, the voltage differential is 7 volts, and if the Uno plus the LCD plus whatever else draws, say 100mA, then the regulator must dissipate 0.7W (7V x 0.1A = 0.7W). The regulator on the Uno is a small surface-mount device and there is not a lot of heat sink area, so this much power can get it pretty warm.

Alternatives would be a lower voltage input supply (e.g. 9V or 7.5V), or to use an external 5V power supply and bypass the Uno's on-board regulator by feeding the 5V directly in to the Uno's 5V pin.  CAUTION on this: Do not exceed 5 volts if using the latter method. A well regulated external supply must be used. I can't comment on whether your 5V 3A wall wart is suitable without examining it. At the very least you should measure its output voltage. If it's a decent quality regulated supply, it may be fine. If it's unregulated, it could well be supplying significantly more than the advertised 5 volts.

Hope that helps.

I checked my 5VDC adapter output and it is exactly 5.2VDC and stays there but i am little uncomfortable, injection voltage directly at the output side of the regulator. i mean i shouldn't be doing this, not to mention i dont have a male DC jack with wire on the other end etc. Sigh!

anyhow you guys make sense.

Now i need to get my hands on 9VDC 2.0+A adapter.


it is exactly 5.2VDC and stays there

That is fine, the absolute maximum operating voltage for the processor chip is 6V, so 5.2V will be fine.


Just want to post my finding so others can benefit. I have a wall adapter (5.2VDC measured output) and i inserted its output directly into "5v" and "GND" pins of arduino UNO header pins and everything is now running cool as a breeze.

FYI nothing is connected to DC jack now so i am bypassing the regulator. When i was connected 5.2VDC adapter to DC jack, the output of regular was 3.6V and things were not running.

Thanks ya all.

Go Up