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Topic: Power from external 5V regulated supply? (Read 9044 times) previous topic - next topic

Andy Brown

Is it true that you can provide a regulated 5V supply to the Arduino through the 5V pin that you would normally draw power out from? I've seen a few references on the internet that indicate you can do this but nothing from anyone that's actually doing it.

My Mega1280-based project is powered by a 12V battery that goes straight into a TS2940 regulator to provide 5V to the external components. If I can also power the arduino this way then I will because bypassing the onboard regulator will save some power and extend battery life.
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davekw7x

#1
Jul 14, 2010, 01:13 am Last Edit: Jul 14, 2010, 07:31 pm by davekw7x Reason: 1
Some voltage regulator chips are very touchy about applying power to the output pin with zero volts on the input.  I am not familiar with your particular device, but unless I saw a specification from the manufacturer that it can tolerate reverse voltage. I would probably put a diode (1N4001 or some such thing) across the chip.  Anode on the output, cathode on the input.  If you don't know what this means, or if you have even the slightest doubt, then DON"T DO IT.  If you install the diode the wrong-way around, you very well may destroy everything that the +5 Volt output feeds if you ever apply the normal voltage to the input of the regulator.  See Footnote.

Regards,

Dave

Footnote:
In power supply design, the expression "smoke test" is more than a figure of speech.  It may have been mentioned on this forum somewhere:  All components are manufactured so as to contain a certain amount of smoke.  If you let all of the smoke out, the device will no longer work.

My recommendation: Get information and advice from the Internet, but don't just try it and see if you can get lucky.  Evaluate and make sure you understand completely before applying that information in a way that it affects any physical device (and that can affect your wallet).

RuggedCircuits

If anything there is the potential to damage the MC33269D voltage regulator:

http://www.onsemi.com/pub_link/Collateral/MC33269-D.PDF

Looking through the datasheet I do not see any information on whether backfeeding the output will damage it, so I am going to assume that it WILL. Otherwise, they would have listed this kind of protection as a feature.

I suppose you could remove this regulator from the board if you never plan to power it from the external DC jack or the USB port. Then everything could be powered from the 5V pin without damage.

Haven't tried, so don't hold me responsible if I'm wrong.  :)

http://www.ruggedcircuits.com

retrolefty

The way I like to supply regulated external +5vdc to a standard Arduino board is to take a spare USB cable, lop off the PC end, locate the positive and minus power wires and wire them to the supply. That way the built in 500ma thermofuse is in play and it no way interferes with the on board auto-voltage components included the +5vdc regulator chip.

Lefty

RuggedCircuits

Good point...the regulator can't be damaged else it wouldn't work when the board would normally be plugged into the USB port.

Looks like it's just a "got away with it" situation on the regulator design :)  Now I'm curious to see how much current it is allowing to flow into the output pin.

http://www.ruggedcircuits.com

digitalman2112

Nicely timed question as I'm about to do the same thing with a Gator+ board (power from external regulated power supply) - should I use the USB connector method vs. using the 5v pad then?


RuggedCircuits

I think the 5V pad will work as the circuitry is rugged enough to handle the backfeed (it was mostly designed to prevent DCIN backfeed to USB power), but the USB method is preferred.

zoomkat

My arduino has a bad external power regulator, so soon I'll be powering it from another 7805 chip connected to the +5v bus. The onboard external regulator goes thru a diode and connects to the +5v bus.
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Andy Brown

Lots of very good information here. Thanks to all for replying.

Quote
You have not revealed WHICH Arduino board you are asking about.

It's a mega clone from ebay: this one. It claims to be the reference design but people claim lots of things on ebay so, based on the cautions given by people here, I'm not going to do it.

Quote
The way I like to supply regulated external +5vdc to a standard Arduino board is to take a spare USB cable, lop off the PC end, locate the positive and minus power wires and wire them to the supply. That way the built in 500ma thermofuse is in play and it no way interferes with the on board auto-voltage components included the +5vdc regulator chip.


I like this idea. Do you tie the data wires to ground or leave them floating?
Home of the Nokia QVGA TFT LCD hacks: http://andybrown.me.uk

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