Patent ID: 7753227

Claim:
A releasable casing comprising: a base, a lid rotatably attached on the base, a torsion spring comprising a spring body fixed on the base, a first arm and a second arm connecting with two opposite ends of the spring body, the first arm resisting against the lid, the first arm comprising a first portion connected to the first spring body, an arc-shaped second portion extending from the first portion, and an arc-shaped third portion extending from the second portion, the second portion and the third portion bending oppositely, the second arm being fixed on the base, a contacting point between the first arm and the lid is changed between the second portion and the third portion when the lid rotates relative to the base; a quadrant shape supporting portion protrudes from the lid to resist against the second portion and the third portion; the supporting portion defines a sliding channel at a bottom thereof to allow the first arm to be restricted therein and slide therealong; the supporting portion comprises a cambered surface, the contacting point between the first arm and the lid is changed between the cambered surface and the sliding channel; and the third portion of the first arm is resistibly received in the sliding channel, when the lid is located at an open position, and a conjunction of the second portion and the third portion resists against the cambered surface of the supporting portion when the lid is located at a closed position.