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Author Topic: External common voltage supply to Arduino  (Read 1899 times)
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When using a large external breadboard setup an external 5v supply might be used, with a common Gnd to the Arduino.
If the USB is not in use, there are now two supplies, one for the Arduino which cannot be 5v because of the differential with the on board regulator.
Would it not be useful to have a link to disable/disconnect the on board regulator and be able to supply the 5v from the external breadboard?

This could of course be hacked but not that easily.
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I have found it useful and include a jumper on a number of my boards (http://www.wiblocks.com). The jumper lets you select between the onboard
regulated voltage or an external regulated voltage. The onboard regulator
is powered by diode or'ing the USB 5V supply and the voltage from a
2.1mm jack.

(* jcl *)

http://www.wiblocks.com


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Yes indeed. I don't believe that there is a jumper on the Arduino.
Does anybody know of any impediment to connecting 5v direct through a re-wired USB plub?
My thinking is that perhaps the USB cable could be modified to isolate the 5v from the computer and provide it to the plug at the Arduino end of the cable from an external supply (the same one as the breadboard). Then when a sketch has been downloaded the computer end can be unplugged but the power would remain.
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Just don't mixup the cables smiley-wink

I would use or'ing diodes for protection.

(* jcl *)

http://www.wiblocks.com
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That sounds like good advice and also no known obstacles. Thank you.
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I don't believe that there is a jumper on the Arduino.
You are wrong. On the earlier arduinos there was a jumper. But now there is a circuit that automatically switches to the external supply if one is present.
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You are wrong. On the earlier arduinos there was a jumper. But now there is a circuit that automatically switches to the external supply if one is present.

Unfortunately you did not read line two of my original post. I agree that you are correct regarding the socket external input, but it is quite obvious that I have explained why I am not referring to that.
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Couldn't you just connect your external +5v supply to the +5v pin of the Arduino?
 
Don
« Last Edit: October 15, 2010, 10:08:32 am by floresta » Logged

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Couldn't you just connect your external +5v supply to the +5v pin of the Arduino?
I am afraid not, it doesn't work like that and it would likely cause a problem with the built in regulator, current equalising if the total from both is greater than twice the regulator capability, the same applies to dissimilar batteries.
I will try splitting a USB cable. I do not want multiple power supplies if I need more than a USB can supply.
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I am afraid not, it doesn't work like that and it would likely cause a problem with the built in regulator, current equalising if the total from both is greater than twice the regulator capability, the same applies to dissimilar batteries.
If you connect the USB cable but have no input from PWRIN or VIN then the +5V from the USB source is backfeeding the regulator VO.

If you connect the PWRIN or VIN but have no input from the USB then the +5V from the regulator is backfeeding the MOSFET output.

So it appears that there is no problem backfeeding either of these devices - which is what would happen if you had no USB source and no PWRIN or VIN source but were feeding +5V from your external +5v supply into the +5V pin.

Don

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Thanks for the suggestion, which on the surface appears OK however...
What happens when I have this setup and then connect the USB cable to upload?
It would appear then that I have to have to disconnect my external 5v supply, fit the USB, upload, unplug USB then connect my 5v and hope I don't forget on one occasion.
Also ORing via diodes will drop 0.7v producing 4.3v which is hardly better than plugging 5v into the external input.
It would have been so much easier to have a link that isolates all board and USB supplies and permits a common 5v supply. This after all is a suggestion forum. I was not saying that there is no solution.

You are possibly right, but I am not familiar with MosFets and value my Uno too much to 'toss the coin', of having both external as you suggest and USB together. Has anybody done it? Rephrase that ...and still has a good Uno.
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You might want to look into a Bare Bones Board from Modern Device http://www.moderndevice.com/products/bbb-bub-cable-package.  It's got a versatile power scheme using good old jumpers.

Don
« Last Edit: October 16, 2010, 02:27:25 pm by floresta » Logged

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I use ftdi cables for programming and I have made a small extension lead with a switch in the 5v cable. It should be easy enough to do with a couple of usb male/female connectors.
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Thank you both for the suggestions. Will look into them.
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@floresta: I have looked at the power options from your link. That is exactly what I was suggesting would be useful on Uno. The Uno is not easily modified to achieve the same, warranty, small tracks etc. so it was for future consideration. I see from the link that I was not alone.
Maybe the team will consider it.
Maybe somebody has already done similar to a Uno board and will share the technique.
Vin is an unregulated input that feeds the onboard regulator so that is out.
The only possible input to run everything from an external regulated 5v (from say a breadboard) would seem to be the 5v output pin. Doing so relies upon the FET not passing the voltage back to the USB and thus the computer (two 5v sources connected). Can anybody say please whether the FET is up to this scenario? (A link would be better)
« Last Edit: October 18, 2010, 02:56:37 am by donone » Logged

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