1.  Field of the Invention
The invention relates to a rotatable plug, more particularly to a rotatable plug that can be stored conveniently when not in use and a power supply device having the rotatable plug. 
2.  Description of the Related Art
A plug of a conventional power supply device includes a plug body and two conductive terminals protruding outwardly from the plug body.  When the power supply device is not in use, because the conductive terminals protrude from the plug body, they are likely to be hit by an external force that may result in the bending and even breakage thereof.  Further, it is difficult to store the conventional power supply device.  Therefore, devices with rotatable plugs have been currently developed to resolve the aforesaid issues. 
Referring to FIGS.  1, 2 and 3, a conventional power supply device having a rotatable plug includes a housing 11, a rotatable plug 12 and a fixing element 13.  The housing 11 includes a housing body 111, a holding seat 112 formed on an end of the housing body 111, and a receiving hole 113 extending through the holding seat 112. 
The rotatable plug 12 includes a rotating support body 121 inserted into the receiving hole 113, two conductive prongs 122 fixed on the rotating support body 121, two metal pivot studs 123 protruding outwardly and respectively from two opposite ends of the rotating support body 121, two conductive plates 124 respectively pivoted to the pivot studs 123, and three projections 125, 126 projecting from the rotating support body 121 opposite to the conductive prongs 122.  Two conductive wires (not shown) may be respectively inserted through the conductive plates 124 to connect electrically with a circuit board (not shown). 
The fixing element 13 fixes the rotatable plug 12 to the holding seat 112, and includes a fixing main body 131, two stop bosses 132 disposed on one side of the fixing main body 131 and extending towards the housing body 111, a stepped portion formed in the middle of the fixing main body 131 and having a first part 133 and a second part 134, and two spaced-apart slots 135 formed in the fixing main body 131.  The conductive plates 124 extend respectively through the slots 135. 
The housing 11 (see FIG.  1) is removed in FIGS.  2 and 3 to facilitate description of the rotating mechanism of the rotatable plug 12.  With reference to FIG.  2, when the conductive prongs 122 of the rotatable plug 12 are in a use or extended position, the projection 125 abuts against the first and second parts 133, 134 of the stepped portion and is positioned thereat.  When a force is applied on the conductive prongs 122 to rotate the same in a direction of an arrow (R1), through the resiliency of the stepped portion, the projection 125 can slide over the second part 134.  As such, the conductive prongs 122 can drive the rotating support body 121 to rotate relative to the holding seat (see FIG.  1).  As shown in FIG.  3, the projection 126 is blocked by the stop boss 132 and is positioned thereat.  At this state, the conductive prongs 122 are disposed in a non-use or folded position.  By applying a force again on the conductive prongs 122 to rotate the same in a direction of an arrow (R2), the conductive prongs 122 are restored to their use position. 
However, the above-mentioned device is not modular so that a new mold must be opened in respond to different sizes of housing 11.  Moreover, three projections 125, 126 are needed to position the rotatable plug 12 in the use and non-use positions.  Finally, because the rotatable plug 12 is connected to the fixing element 13 through insertion of the conductive plates 124 into the slots 135, the conductive plates 124 are likely to separate from the fixing element 13 due to frequent use which may cause poor electrical contact.