Try this do not turn on the AC but turn on all the electrical things on the car except AC. If the rpms drop to 400 than the there is voltage sensor in the throttle body which increases idle rpms as soon as the heavy load items are turned on . But if the idle rpms increase than the your sensor is fine.
The AC compressor is just a pump it has to take cool refrigerant from one component to the other meanwhile pressurizing it. Now just thing of it if the entire system has the capacity to hold 1000gm of refrigerant and you overfill it with 1100gm refrigerant what will happen. The compressor wont to be able to pump properly and hence put load on the engine thats why rpms dropping in idle.
Well I recommend you to get your cars AC system checked by some authorised workshops like punjab car ac or ahmad car ac. Punjab is authorised dealer of denso whereas ahmad car ac are authorised dealers of senden