Hi, I wonder how much current can we draw from the 5V pin on the UNO Q? The official pinouts pdf didn't specify this
I'd like to power a small 5V cooling fan that draws 0.14A (140mA), can I simply connect the fan to the +5V and GND pins on the UNOQ directly?
Or, is there a better safer way (without using many other electrical components)
For reference, The fan is here (look at the smallest one, 5V and 0.14A)
Many thanks in advance! :)
Hi @kurakuradave.
The 5V pin is connected directly to the power supply on the board's USB-C socket. So you must make sure that power supply is capable of handling the current draw of the UNO Q board and the current draw of any peripherals such as your fan.
Another consideration is that the electrical connection between the USB socket and the 5V pin on the UNO Q's circuit board are thin strips of copper. They have a limited current rating. That said, the trace is plenty sufficient for 140 mA.
So the question really comes down to the capabilities of the power supply you are using, not the UNO Q itself.
It is definitely good to be careful to not exceed the 60 C recommended maximum environmental operating temperature. However, you should note that the UNO Q is very efficient, and thus passive cooling of the stock board will be plenty sufficient for many applications.
In applications/conditions where you do find that passive cooling of the stock board is not quite sufficient, you could try using the commonly available adhesive heat sinks.
Thanks @ptillisch, I understand a bit more now 
I'm trying to power it with a 100W USB-C PD powerbank for a wearable project that will be in (almost) constant movement
The UNO Q will be encased (to protect it from accidental bumps during movement), so I thought active cooling would be needed
Do I need to worry about "flyback current" and use a diode?
Does "flyback current" really apply here, because, motor fans only spin in one direction (unlike motors for wheels which can reverse direction at any moment).
Is it correct that because the motor only ever spins one direction, there won't be any "flyback current" ?
Thanks!
Where possible, supply devices like fans and relays from off-board suppies to protect the main board.
Use a board pin to control a transistor and let the transistor do the switching.
Check if you need to use a diode, but it's generally good practice.
If you do the controlling through the board, it opens the possiblity of using temperature control and reducing unnecessary power usage.
A simple rule is: never connect anything with a coil directly to an Arduino pin. Arduino pins are signal-level pins, not power output pins. Small LEDs at low current are usually OK.
Look at the processor datasheet carefully. There is a limit to how much current a single pin can supply or sink, but even more important, there is also a maximum total current limit for the entire chip. That total includes the processor itself plus all loads connected to it.
Which part gets hot and how hot.
Maybe a stick-on heatsink could solve the problem, assuming there actually is a problem.
Chips are perfectly happy running at 65C or even above.
Old trick: if you can keep your finger on it for a long time, it's fine.
Powering a fan seems counter-productive.
The power has to come from somewhare, which could make that part even hotter.
Leo..
@gilshultz please note @kurakuradave's question:
This is not about powering the fan from an I/O pin.
I know, it will work, but the best choice is not to use the Arduino as a power supply.
It is better to use a power splitter like this:
