I want to power my RPi-3 using a 4A 5V USBcharger. I connect a hard drive, wireless keyboard adapter, and a pendrive usually and I get a yellow coloured lightning bolt on the upper right side corner of my monitor? Why is this so? Can I use the above mentioned charger as a psu? What is the max power input for RPi-3 and safe voltage and current levels?
i get a yellow coloured lightning bolt on the upper right side corner of my monitor? Why is this so?
It means the voltage is dropping when too much current is being drawn. Although this is obviously well within the limits of the charger, since it is sold for charging it may not be great at dealing with a fluctuating demand for arbitrary amounts of current, which charging will not do. Recharging a battery is not a super voltage sensitive scenario, especially since the batteries in phones etc. are usually much less than 5V anyway (more like 3.4 - 3.7, I think).
I suspect, however, this is a multiport charger and you are using a microUSB cable that is the real culprit. If so, try with a few other cables. I have a bunch of phone chargers, someone of them small and of dubious quality, but they all actually work with Pis -- however, I have a few (maybe 1 in 5 or 10) microUSB cables that don't.
charger- you need a proper power supply. See Raspberry Pi Power Limitations