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Topic: External Power Supply (Read 123 times) previous topic - next topic

Slow-Learner

I note that the external supply may be between 6 and 12V dc. I assume (but have been unable to find a reference that confirms this) that the centre electrode is +ve. Neither have I been able to find out what the power dissipation limit of the on board regulator is as installed. Can anyone point me to refernce sources that will help with either of these questions? I am concerned that running Arduino Uno plus Ethernet off a 12 volt 2A wall wart will over heat the regulator in constatnt use, and would therefore like to design my own lower voltage source that will ensure that the on board chip stays within its Arduino Uno board mounting recommended power dissipation limit.

groundFungus

The power dissipation limit that I have seen for the 5V regulator on an Uno is 1 Watt (see this post).  That means that with a 12V input the current limit out of the regulator is about 140mA.  The Uno itself uses about 50mA of that.

wvmarle

An external power supply may be 5V, or 6.5-12V. 6V is a bit low but may still work.

5V is ideal (if regulated - like you get out of a spare mobile phone charger) as you can just connected it to the 5V pin.
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

Paul__B

I note that the external supply may be between 6 and 12V dc.
7 and 11 V more likely.  :smiley-roll:

I assume (but have been unable to find a reference that confirms this) that the centre electrode is +ve.
It is.  The circuit diagram or PCB pattern would tell your this

Neither have I been able to find out what the power dissipation limit of the on board regulator is as installed. Can anyone point me to reference sources that will help with either of these questions?
It is pretty irrelevant.  If you are running anything other than the bare UNO and a couple of indicator LEDs, you require a proper 5 V regulated power supply; you cannot use the "Barrel Jack" or "Vin".

I am concerned that running Arduino UNO plus Ethernet off a 12 volt 2A wall wart will over heat the regulator in constant use,
Correct.  :smiley-eek:  Promptly.

and would therefore like to design my own lower voltage source that will ensure that the on board chip stays within its Arduino UNO board mounting recommended power dissipation limit.
You use a 5 V "buck" regulator module to feed power to the 5 V terminal (and always with a common ground); you do not use the "Barrel Jack" or "Vin".  :smiley-lol:

Slow-Learner

Many thanks. I'll have to adapt my power supply design to do this as I'd intended to supply 8V to Vin, but can easily fit a 5V device instead and re-route the output to the 5V.

My only concern then  would be having a USB connected at the same time to use the monitor, and therefore potentially two power sources connected in parallel.

Paul__B

My only concern then  would be having a USB connected at the same time to use the monitor, and therefore potentially two power sources connected in parallel.
OK, let's look at that.  It's a UNO.  There is firstly, absolutely no suggestion that this can damage the UNO itself; transfer of power beyond 500 mA in either direction might however trip the polyfuse.

The concern is that if the external 5 V is higher than the USB 5 V, you might "back-feed" the USB port on the PC or laptop.  This is in itself, a valid concern as there seem to be reports of USB ports being damaged in this fashion.

The point against this being likely however, is that many or most powered USB hubs do in fact, connect the accessory 5 V supply rated at something like 2.5 A directly to the 5 V terminal on all outputs and the input because without the accessory 5 V supply, the input must feed those outputs and you do not want to insert any device - such as a diode - which will cause a voltage drop.

So given the relative number of Arduino UNOs (some even genuine) and powered USB hubs, I feel if this were a frequent problem, it would be announced to a far greater audience than this forum!

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