Powering Arduino Uno from 5V pin, what exactly is the voltage range/tolerance? I want to bypass the onboard regulator and use the 5V pin to feed arduino with external, regulated source.

Everybody just keeps saying regulated 5V, but how precise does the regulation need to be exactly? Can you state a voltage range and what makes you think so?

10 ways to destroy arduino, Method #5: Apply >5V to the 5V Connector Pin mentions "applying more than 6V", can anyone elaborate how it can be calculated?

Is there a higher spike that arduino can handle for a short period of time?
My concern is that the current draw from arduino might be under 60mA (right after switching power on and before the program initilizes and starts running), which is the lowest draw at which my source guarantees the 5V regulation --- when unloaded it gives out 6.5V. How could I resolve this better than putting a 60mA dummy load between the regulator and arduino itself?
Unfortunately there is no one "clear cut" answer for all you ask.

There are some hard limits, though, which you can get from the main chip's datasheet:


  Operating Voltage: 1.8 - 5.5V


But that's not the whole story. The minimum voltage depends on the clock speed of the chip, as shown in this graph:



But wait, there's more. The Arduino also contains a 3.3V regulator which is fed from the 5V supply. That will have a minimum voltage above 3.3V that it needs to maintain a 3.3V output. So if you want to use the 3.3V regulator then, according to the datasheet for the regulator (LP2985-33DVBR):


  3 Description
  The LP2985-N low noise linear (LDO) regulator delivers up to 150mA output current and only requires 300mV dropout voltage of input to output.


So a minimum of 3.3 + 0.3V, or 3.6V.

That gives you a usable range, clock frequency permitting, of 3.6V to 5.5V.

As to how to protect the Arduino from a power-on spike like you describe - you could:


Add your own extra regulation in the form of a 5.1V zener diode.
Add a dummy load which the Arduino can switch out of the circuit once booted up
Change your source to something with better regulation
Ignore it - the regulation with even a light load will be considerably better than open circuit.  Add a resistor across your supply that would draw much less than 60mA and measure the voltage. See how bad the regulation at low load is.