OK, this I am guessing, might be a pretty simple (and silly) question, but... hey ho, better to be safe than sorry, eh!
I have modified an ATX PSU that provides regulated 3.3V, 5V and 12V DC output. I have an Arduino project currently running that uses this +5V supply. The UNO is powered by a separate USB power supply. I only have several digital pins on the UNO connected, which provide the input and output signals to and from PIR's, LED's and relays - which are all powered by the external PSU.
The circuit works as required.
My question: Am I ok to use this +5V supply to power the UNO by connecting it to the Vin pin on the UNO (and obviously the Gnd from the supply to one of the Gnd pins on the UNO) thus negating the need to have the UNO powered by the USB supply?
You can power the UNO with 5 volts by connecting the 5 volt power to the 5 volt pin on the UNO. Or you could power the UNO with 12 volts by connecting to the Vin pin. Vin uses the regulator on the UNO to provide 5 volts to run the board. Applying 5 volts to the 5V pin just bypasses the regulator. Be sure the 5 volts is really 5 volts, or just a bit below it.
V-in is the input of the onboard 5volt regulator.
It needs at least 6volt for the regulator to make a stable 5volt.
At >6.6volt, an onboard mosfet switches the USB supply off.
So it makes sense to power V-in with ~7volt.
More than 7volt just heats up the regulator more.
How much depends on the current draw of things powered from the 5volt an/or 3.3volt pins.
There is a diode with a drop of ~0.7volt between the DC socket and V-in.
If you power the Uno on the DC socket, add 0.7volt to the above statements.
If you power the Uno with >=9volt, it's wise to use the DC socket.
That takes 0.7volt off the supply, and makes it easier for the regulator.
It also adds reverse protection.
If you have a regulated and reliable 5volt source, you can power the Arduino on the 5volt pin.
PIR sensors can be powered from the 5volt pin of the Arduino, because they only draw 0.1mA.
Wise to power relays etc. externally.
Hope this answers most of your questions.
Leo..
Hi,
If you are using a PC power supply, the 5V will be rated at many many Amps.
Please put a fuse in the +5v lead to protect your circuit, 1.5A would be good, as that will be the same as most USB supplies.
Make sure it is a fast blow fuse, not an automotive or slo-blo fuse.
The reason is, if you have a short, even through protoboard or between leads, or component, only 1.5A will flow and fuse will pop.
Any damage will be minimized.
Without fuse, a short will mean the PC supply will put its rated current through the short, your protoboard or lead, or component which ever has the short will possibly be destroyed and some shrapnel produced.
Damaged will possibly be maximized.
Tom...
PS.I have the privileged of a variable current limited powersupply.
