I have a power supply from 9v/100mA regulator.My question do i need to design a current sink circuit to reduce the current rating to 40mA?
There is nothing you need to do actually.
Why would you do that? If your regulator can supply 100mA, there's apparently no reason to lower it, because everything you will attach will draw only the necessary amount of current.
For example: 100mA PSU will provide 40mA to your device (actually is your device that "request" 40 mA from the PSU or regulator), and you still have 60mA available for something else.
thanks for the suggestion i found it extremely useful