I'm using an old Dell charger to provide power to a number of Arduinos over ethernet. The charger puts out a healthy 19v (rated at 6A), it's plugged into this 8 port PoE injector and I want to use a buck converter at the other end to supply the Arduino Mega with a clean 5V.
Aside from Dell chargers being a cheap way of getting nicely spec'd power supplies, using 19V means I get reduced loss of power over the line, and the buck converter on the other end is efficient and doesn't overheat (critical in my application as Arduino is mounted in the wall).
I'm testing with a variable output buck converter, but currently I have to push in around 6.5V into the Arduino's Vin before I get a stable 5V out of the Arduino... yes yes, this is exactly what the Arduino spec says, 7-12V ideal.
Now instead of buying a dedicated 7V buck converter only for the Arduino to regulate that down to 5V, I'd like to supply the Mega with a known-good 5V supply from my buck converter circuit. (Or use one of these.) This would minimise further power loss.
My question:
I can't find a way to power the Arduino directly, bypassing its regulator. Is this correct? Did I miss something?
I had always assumed the regulator is connected to the power jack, but not to the Vin pin.
Grumpy_Mike:
Yes the 5V pin, put it in there. Just as a precaution also connect the 5V to the Vin to stop reverse biasing the regulator.
Aha! My lack of understanding about the architecture of the Arduino ... didn't know I could use that to power it.
Two more questions in that case:
1/ Within what kind of voltage tolerances do I have to work if not using the onboard reg? e.g. would I damage the AVR if I accidentally put 5.2V through? Or 6V? etc.
2/ I'm guessing if I do what you suggested (also connect 5V to Vin) then I'm okay to also connect the Arduino's USB to my laptop?
The datasheet of the ATMEGA368P (which is used on several Arduino boards) says: 1.8V to 5.5V
The minimal voltage of 1.8V should only work with 4MHz.
Most Arduino boards work with 16MHz so you have:
a minimum voltage of about 4.5V
a maximum voltage of 5.5V
So 5.2V is within the range, 6V is not (it may not burn your controller immediately but...)
JohnLincoln:
If you are using a mains powered supply which gives out plenty of current, and you have a variable buck regulator, why not just turn the output voltage up so that it is in the recommended range, and let the onboard regulator take care of supplying the correct voltage?
It seems to me that you are trying to complicate things by trying to adjust the output voltage of your regulator to exactly 5V, when you already have a 5V regulator on your Arduino.
Could do, but I'm installing a lot of units around the home inside walls and
a) want to minimise efficiency loss, since they will be powered on 24/7 (the linear regulator is far less efficient)
b) want to minimise heat generation from the board as they are mounted in walls. The onboard reg seems paltry to me. Of course, depends on the load, but I haven't finished designing it yet so don't know what that will be.
In reality the losses may not be that huge, and the heat may not be a problem at all. But given I can achieve reliable output from buck converter it made sense to cut the onboard regulator out altogether...
Any thoughts on my last question - can I connect it to USB port of my computer whilst it's being powered externally to the 5v line which will be tied to Vin?
16 MHz will work within spec all the way down to ~3.8V.
If Vin is >3.3V/2, Vin will be used and will drive the regulator.
Otherwise, Vusb will be used if available, and be from 4.74V to 5.25V less any drop across the PTC fuse, for up to 500mA.
If you supply 5V on the 5V pin, connect diode from 5V (anode) to Vin (cathode) per sheet 10 of the regulator datasheet.
Would desoldering the onboard regulator and connecting your Buck regulator with flying leads be a possibility? Just a thought if you were planning to use 19Vdc as your input voltage. Then you could treat it exactly as you would an unmodified board in terms of connecting USB at the same time?