Feeding 5V (or more?) to Arduino and friends

Hi everyone,

I'm working on a data acquisition system based around an arduino and two CD4052 multiplexers. It gives me 16 (+2) analog inputs and I'll also be using a couple of digital inputs as frequency counting.

Here's the thing : my pressure sensors require a +5V to give their very precious reading.
But it seems the +5V of a usb port is somewhat unprecise (ranging from 4,5V to 5,2V from what I measured) so I'd like to use a simple 7805 to drop down a +12V that I'll have from a battery.

Thing is : the arduino will always be connected to a PC for logging and visualization. So should I use the VIN port to give it a +5V as well and ensure that I have the right scale on the ADC of the arduino?
Also, should I feed the arduino a +7V or higher so that it does himself the regulation?

Sorry if I'm being a complete noob but I can't find what I want by searching so here I am :slight_smile:

Firstly if you supply power to the Arduino via the barrel jack or Vin then
it has an on-board 5V regulator that might be enough for your sensors
(but how much current do they need and how many are they?).

You are right, USB power is awful quality (can be better from a powered USB hub).

You have an option of feeding a reference voltage to the AREF pin and using
analogReference(EXTERNAL) too.

I agree with MarkT, using the 5V output of the Arduino board is just as good as the 7805.
You can supply 7 to 12V to the barrel jack and keep that connected.

When you want to connect it to the computer you plug in the USB. The Arduino 5V output should be stable at 5.0V.

Hello everyone,

Thanks for your help, I might go with the barrel jack then. But as soon as I plug it in, it will draw it's power from there and not the USB, right?

Also, I have 16 pressure sensors that I measured to be drawing at max 2mA so that'll be 30-40mA for these at max. Plus a couple of others but it should be in this ballpark.

I read that the Uno can supply 200mA, that's through the 5V pin?

thanks for your help,

Marc

marc426:
I read that the Uno can supply 200mA, that's through the 5V pin?

Maybe.

The heat of the regulator depends on how many volts it has to drop. A 12V input doesn't have the same limits as a 9V input. The only true test is to build it and put your finger on the regulator.

If you find your Arduino regulator burns your finger when its running, switch to an external regulator with a heatsink.

The Arduino has an auto-voltage switch that will make DC barrel jack voltage override USB voltage. So you can safely have it plugged into the computer and plug a 12V battery into DC barrel jack and it will use that as its power source.

This will allow you more current than USB, but keep in mind there is still a max current output per pin and for +5V pin. I have head the +5V pin can actually produce slightly more than 200mA but have never confirmed this, I always use 200mA as a max current draw, if I want any more current I use an external power source entirely independent of the Arduino. Definitely test your power source though and keep an eye on your heat generated. The differential between 12V and 5V means it has to dissipate more heat. The individual pins can supply a max current of 40mA and the 3.3 volt pin can supply a max current of 50mA.

I would do some testing though with your usb power quality. If you have any spare usb cables, cut the non computer end off and strip the rubber and internal wires. Then plug it into a USB port and use a multimeter to measure your voltage output. Because it could be that your specific computer has great usb power and is supplying like 5.001V in which case, no need for the 12V battery.