sreevisakh wrote:... If I have to connect my usb harddisk to it what should I be using?
If the Pi restarts and then runs normally with the USB drive connected, it shows that the inrush current (the higher initial current) is pulling down the 5.5Volt supply momentarily, just long enough to reset the Pi.
If you can't live with that (i.e. only connect the drive when the Pi is off) then you need a bit more power.
The problem with power supplies that output 5Volts (as distinct from those that provide an over-voltage e.g. 5.2V) is that there are voltage losses due to connectors and connecting cable. So the voltage at the Pi is often less than 5V. You are likely to have problems if it falls much below 4.8V (see my post: http://captainbodgit.blogspot.co.uk/201 ... rrypi.html
The reason that some power supplies rated at 2 or 3Amps seem to work is not due to the current rating, but because the voltage is 5.2V (or maybe 5.5V) at the power supply and this provides at least 5V on the Pi itself.
I dont know what power rating you need, because I dont know the initial and running current your system is drawing from the supply. However I suspect the Northpada supply will be ok (https://www.amazon.co.uk/gp/customer-re ... B00MTX9GD8
I hope this helps.