reference will now be made in detail to the embodiments , examples of which are illustrated in the accompanying drawings , wherein like reference numerals refer to like elements throughout . fig1 illustrates a laser printer that is an image forming apparatus according to an embodiment . an image forming apparatus 102 may be provided with a power cable 104 for receiving power . a plug 106 may be provided at one end of the power cable 104 . the plug 106 may be inserted into a socket 108 of a plug receptacle or a power strip . thereby , commercial alternating current ( ac ) power supplied from an external power source can be supplied to the image forming apparatus 102 . the image forming apparatus 102 may be supplied with power through the plug 106 inserted into the socket 108 and the power cable 104 , and performs an operation associated with image processing . inserting the plug 106 into the socket 108 may be referred to as plug - on , and separating the plug 106 inserted into the socket 108 from the socket 108 may be referred to as plug - off . fig2 illustrates an exemplary control system of a image forming apparatus , for example , as illustrated in fig1 . as illustrated in fig2 , a control unit 240 for controlling overall operations of the image forming apparatus 102 may be electrically connected , for example , to a paper feed unit 230 , a paper eject unit 220 , an image transfer unit 250 , a fixing unit 270 , a display 206 , and a speaker 208 so as to be able to conduct communication . a power supply ( e . g ., switching mode power supply ( smps )) 202 generates system direct current ( dc ) powers of 5 v and 24 v by ac - dc conversion , and supplies the generated power to the control unit 240 , the image transfer unit 250 , for example . the dc power of 5 v may be supplied to the control unit 240 made up of a microprocessor , circuit elements , etc . the dc power of 24 v may be supplied to the fixing unit 270 . the system dc powers of 5 v and 24 v output from the power supply 202 may be selectively supplied to other components of the image forming apparatus 102 . the power supply 202 may supply the input commercial power to varied components ( e . g ., a fixing heater of the fixing unit 270 ). the paper feed unit 230 feeds a printing medium ( e . g ., paper ) stacked in a paper feed cassette to the image transfer unit 250 . the image transfer unit 250 forms a predetermined image in response to an image signal , and transfers the formed image to an image plane of the printing medium . the fixing unit 270 fixes , e . g ., semi - permanently fixes the image transferred to the printing medium . the paper eject unit 220 ejects the printing medium to which the image is fixed by the fixing unit 270 to the outside . the control unit 240 controls the operation , e . g ., overall operations of the image forming apparatus 102 and may be electrically connected to a plurality of sensors for detecting conditions of the components of the image forming apparatus 102 so as to be able to conduct communication . the display 206 displays guide messages for informing a user of information about operations and / or conditions of the image forming apparatus 102 . the speaker 208 outputs a guide sound and / or a warning sound generated during the operation of the image forming apparatus 102 . fig3 illustrates an embodiment of a power supply 202 , for example , as illustrated in fig2 . the power supply 202 illustrated in fig3 includes a discharge circuit unit 302 , an electromagnetic interference ( emi ) filter 304 , a rectifier 306 , and a transformer 308 . the discharge circuit unit 302 discharges an x - cap ( first capacitor ) of the emi filter 304 . the emi filter 304 may be a line filter made up of a coil and a capacitor in order to remove various noises included in the power , e . g ., commercial power ( ac power ) supplied through the power cable 104 . the rectifier 306 converts ac power to dc power , or converts a phase of the ac power to another desired phase . the transformer 308 lowers a voltage of the dc power rectified by the rectifier 306 to a desired level of dc voltage . as illustrated in fig3 , the x - cap of the emi filter 304 may be charged in a plug - on state by commercial power input to the power supply 202 , and discharged by an action of the discharge circuit unit 302 when the plug - on state is changed to a plug - off state . if the x - cap is not discharged after being charged , the charged voltage may be applied to opposite metal terminals of the plug 106 . as such , it may be necessary to rapidly discharge the x - cap for safety . fig4 illustrates a configuration of a discharge circuit unit 302 , for example , as illustrated in fig3 . as illustrated in fig4 , the discharge circuit unit 302 according to the embodiment includes a first discharge circuit unit 312 powered to detect power , for example , for a positive (+) half period of the ac power , and a second discharge circuit unit 322 powered to detect power , for example , for a negative (−) half period of the ac power . the first and second discharge circuit units 312 and 322 may be connected in series between opposite ends live and neutral of an ac power input line , and may be disposed so as to have a symmetrical structure with respect to a node ( first node ) n 1 . a discharge resistor r 11 and an n - channel enhancement type metal oxide semiconductor field effect transistor ( mosfet ) q 11 and another n - channel enhancement type mosfet q 21 and another discharge resistor r 21 may be connected in series with the node n 1 centered therebetween . sources of the two mosfets q 11 and q 21 may be connected to the node n 1 . the two discharge resistors r 11 and r 21 discharge the x - cap in the event of plug - off . a current - limiting resistor r 12 and a pnp type bipolar transistor q 12 and another pnp type bipolar transistor q 22 and another current - limiting resistor r 22 may be connected in series between the opposite ends live and neutral of the ac power input line , and are connected in parallel with the discharge resistors r 11 and r 21 . an emitter of the pnp type bipolar transistor q 12 may be connected to a gate of the mosfet q 11 , and an emitter of the other pnp type bipolar transistor q 22 may be connected to a gate of the other mosfet q 21 . collectors of the two bipolar transistors q 12 and q 22 may be connected to the node n 1 . a resistor r 13 may be connected between the emitter and a base of the bipolar transistor q 12 , and a resistor r 14 may be connected between the base and the collector of the bipolar transistor q 12 . a resistor r 23 may be connected between the emitter and a base of the other bipolar transistor q 22 , and a resistor r 24 may be connected between the base and the collector of the other bipolar transistor q 22 . an ac coupling capacitor c 11 may be connected to opposite ends of the resistor r 14 . an ac coupling capacitor c 21 may be connected to opposite ends of the resistor r 24 . according to an embodiment , the first discharge circuit unit 312 and the second discharge circuit unit 322 are symmetrically connected in series between the opposite ends live and neutral of the ac power input line with the node n 1 centered therebetween . due to this symmetrical serial connection structure , no current flows through the mosfets q 11 and q 21 or the bipolar transistors q 12 and q 22 for the positive and negative half periods of the ac power in the plug - on state , and only a very small amount of standby current flows through the resistors r 12 and r 22 having a relatively very great resistance value . in the discharge circuit unit 302 of fig4 , the resistance value of the resistor r 12 may be relatively greater than that of the resistor r 11 ( e . g ., about five times ). the resistance value of the resistor r 22 may be relatively greater than that of the resistor r 21 ( e . g ., about five times ). when the plug 106 is in the plug - on state , the current flows through the resistor r 12 or r 22 having the relatively greater resistance value . thereby , power consumption caused by the standby current when the plug 106 is in the plug - on state is minimized . in the event of the plug - off , the voltage charged in the x - cap should be reduced , for example , to a level harmless to a human body within a very short time ( e . g ., within one second ). to do so , the x - cap should be able to be rapidly discharged . in the event of the plug - off , the electric charges of the x - cap should be discharged within as short a time as possible ( e . g ., within one second ). in consideration of this , values of the resistor r 14 and the capacitor c 11 and values of the resistor r 24 and the capacitor c 21 are determined . assuming , for example , that turn - on voltage of the bipolar transistor q 22 is 0 . 7 v and that input voltage is rectified dc voltage , magnitudes of the resistor r 24 and the capacitor c 11 are set so that vt = vt × 0 . 9 × ( 1 - ⅇ - 1 r ⁢ ⁢ 24 ⁢ ⁢ c ⁢ ⁢ 11 ) and a time constant r 24 c 11 is less than 8 . 45 , and thereby the x - cap has only to be set to be discharged for a shortest time while the standby power is minimized . since a discharge time of the x - cap relates to the electric charges charged by the unrectified ac power , the resistance values of the resistors r 11 and r 21 which meets the time constant rc & lt ; 1 ( where r is r 11 or r 21 , and c is x - cap ) have only to be set . the discharge circuit unit 302 illustrated in fig4 may be divided into a discharge circuit and a detection circuit , in addition a division into the first discharge circuit 312 and the second discharge circuit 322 . in the discharge circuit unit 302 of fig4 , the discharge circuit may be a circuit including the resistors r 11 and r 21 and the mosfets q 11 and q 21 , and the detection circuit may be a circuit including the resistors r 12 , r 13 , r 14 , r 22 , r 23 , and r 24 and the capacitors c 11 and c 21 , and the bipolar transistors q 12 and q 22 . in the configuration of the discharge circuit unit 302 of fig4 , the other components excluding the resistors r 11 and r 21 for the discharge and the resistors r 12 and r 22 for limiting the current may be packaged into one semiconductor chip . alternatively , in the configuration of the discharge circuit unit 302 of fig4 , the other components ( including the resistors r 12 and r 22 ) excluding the resistors r 11 and r 21 for the discharge may be packaged into one semiconductor chip . due to this packaged configuration , the discharge circuit unit can be simply configured by designing only the magnitudes of the resistors r 11 and r 21 or only the magnitudes of the resistors r 11 , r 21 , r 12 , and r 22 . fig5 a and 5b illustrate an operation of the discharge circuit unit for the positive (+) half period of the ac power supplied to the image forming apparatus according to an embodiment . before the ac power is supplied ( prior to the plug - on ), no power is supplied , and thus the two mosfets q 11 and q 21 and the two bipolar transistors q 12 and q 22 are in a turn - off state . in this state , when the ac power begins to be supplied by the plug - on , only a very small amount of current flows through the resistor r 12 whose resistance value is relatively greater than that of the resistor r 11 . a flow of the current is equal to a path indicated by an arrow of fig5 a . as illustrated in fig5 a , a small amount of standby current ( detection current ) flowing through the resistor r 12 flows through : the resistor r 13 , the resistor r 14 , the mosfet q 21 having a diode function , and the resistor r 21 ; the resistors r 13 , r 14 , r 24 , r 23 , and r 22 ; or the capacitors c 11 and c 21 and the resistors r 23 and r 22 . an amount of the current flowing along this path may be sufficiently restricted by the resistor r 12 having a great resistance value , and thus is very small . the resistance values of the resistors r 12 and r 22 may be determined in consideration of standby power regulations . as illustrated in fig5 b , since the capacitors c 11 and c 21 are virtually electrically short - circuited when the ac is input , a current of the base of the bipolar transistor q 12 flows to the end neutral through the resistor r 14 and the capacitor c 11 . thus , pnp type bipolar transistor q 12 is turned on . when the bipolar transistor q 12 is turned on , a voltage of the emitter of the bipolar transistor q 12 , i . e . the gate of the mosfet q 11 , electrically becomes a low level , and the n - channel enhancement type mosfet q 11 is continuously maintained in a turn - off state . when the plug 106 is in the plug - on state , the current is caused to flow the resistors r 12 and r 22 having the relatively greater resistance value , and thereby the standby power of the image forming apparatus 102 is greatly reduced . fig6 a and 6b illustrate an operation of the discharge circuit unit for the negative (−) half period of the ac power supplied to the image forming apparatus according to an embodiment . when the ac power is converted from the positive (+) half period to the negative (−) half period , only a very small amount of current flows through the resistor r 22 whose resistance value is relatively greater than that of the resistor r 21 . in this case , a flow of the current is equal to a path indicated by an arrow of fig6 a . as illustrated in fig6 a , a small amount of standby current ( detection current ) flowing through the resistor r 22 flows through : the resistor r 23 , the resistor r 24 , the mosfet q 11 having a diode function , and the resistor r 11 ; the resistors r 23 , r 24 , r 14 , r 13 , and r 12 ; or the capacitors c 21 and c 11 and the resistors r 22 and r 23 . an amount of the current flowing along this path is sufficiently restricted by the resistor r 22 having a great resistance value , and thus is very small . the resistance values of the resistors r 12 and r 22 may be preferably determined in consideration of the standby power regulations . as illustrated in fig6 b , since the capacitors c 21 and c 11 are virtually electrically short - circuited when the ac is input , a current of the base of the bipolar transistor q 22 flows to the end live through the resistor r 24 and the capacitor c 21 . for this reason , the pnp type bipolar transistor q 22 is turned on . when the bipolar transistor q 22 is turned on , a voltage of the emitter of the bipolar transistor q 22 , i . e . the gate of the mosfet q 21 , electrically becomes a low level , and thus the n - channel enhancement type mosfet q 21 is continuously maintained in a turn - off state . in this way , when the plug 106 is in the plug - on state , the current is caused to flow the resistors r 22 and r 12 having the relatively greater resistance value , and thereby the standby power of the image forming apparatus 102 is greatly reduced . as illustrated in fig5 a , 5b , 6a , and 6b , the x - cap is charged while the ac power is supplied . when the x - cap is charged , as illustrated in fig7 a and 7b , the side of the end live may be positive (+) polarity , and the side of the end neutral may be negative (−) polarity . as illustrated in fig8 a and 8b , the side of the end live may be negative (−) polarity , and the side of the end neutral may be positive (+) polarity . in this way , in the state in which the side of the end live is the positive (+) polarity , and the side of the end neutral is the negative (−) polarity , the plug 106 undergoes plug - off , and the x - cap is discharged , which is defined as positive (+) discharge . in the state in which the side of the end live is the negative (−) polarity , and the side of the end neutral is the positive (+) polarity , the plug 106 undergoes plug - off , and the x - cap is discharged , which may be defined as negative (−) discharge . fig7 a and 7b illustrate positive (+) discharge of the x - cap in the discharge circuit unit in an event of the plug - off of an image forming apparatus according to an embodiment . as illustrated in fig7 a , in the state in which the side of the end live of the x - cap is the positive (+) polarity , and the side of the end neutral of the x - cap is the negative (−) polarity , when the plug 106 undergoes plug - off , the electric charges charged in the x - cap begin to undergo positive (+) discharge . the mosfet q 11 is turned off , and the bipolar transistor q 12 is turned on . as such , the x - cap is discharged through the resistor r 12 , the turned - on bipolar transistor q 12 , the mosfet q 22 having a diode function , and the resistor r 21 . the x - cap is discharged through the resistors r 12 , r 13 , r 14 , r 24 , r 23 , and r 22 . the capacitor c 11 is charged by discharge current of the x - cap . when the capacitor c 11 is charged , the flow of the current of the base of the bipolar transistor q 12 is interrupted . thus , as illustrated in fig7 b , the bipolar transistor q 12 is turned off , and the voltage of the emitter of the bipolar transistor q 12 is electrically converted to a high level . accordingly , the voltage of the gate of the n - channel enhancement type mosfet q 11 is also converted to a high level , and the mosfet q 11 is turned on . when the mosfet q 11 is turned on , the x - cap is discharged through the resistor r 11 having a relatively small resistance value , the turned - on mosfet q 11 , the mosfet q 21 having a diode function , and the resistor r 21 . due to r 11 & lt ; r 12 , and r 21 & lt ; r 22 , the x - cap can be rapidly discharged through the resistors r 11 and r 21 having a relatively small resistance value . fig8 a and 8b illustrate negative (−) discharge of the x - cap in the discharge circuit unit in the event of the plug - off of the image forming apparatus according to the embodiment . as illustrated in fig8 a , in the state in which the side of the end live is the negative (−) polarity , and the side of the end neutral is the positive (+) polarity , when the plug 106 undergoes plug - off , the electric charges charged in the x - cap begin to undergo negative (−) discharge . the mosfet q 21 is turned off , and the bipolar transistor q 22 is turned on . as such , the x - cap is discharged through the resistor r 22 , the turned - on bipolar transistor q 22 , the mosfet q 12 having a diode function , and the resistor r 21 . the x - cap is discharged through the resistors r 22 , r 23 , r 24 , r 14 , r 13 , and r 12 . the capacitor c 21 is charged by discharge current of the x - cap . when the capacitor c 21 is charged , the flow of the current of the base of the bipolar transistor q 22 is interrupted . thus , as illustrated in fig8 b , the bipolar transistor q 22 is turned off , and the voltage of the emitter of the bipolar transistor q 22 is electrically converted to a high level . accordingly , the voltage of the gate of the n - channel enhancement type mosfet q 21 is also converted to a high level , and the mosfet q 21 is turned on . when the mosfet q 21 is turned on , the x - cap is discharged through the resistor r 21 having a relatively small resistance value , the turned - on mosfet q 21 , the mosfet q 11 having a diode function , and the resistor r 21 . due to r 11 & lt ; r 12 , and r 21 & lt ; r 22 , the x - cap can be rapidly discharged through the resistors r 11 and r 21 having a relatively small resistance value . an exemplary embodiment can be applied to various electric appliances operated by the power supplied through the plug inserted into the socket and the power cable connected to the plug . for example , an exemplary appliance can be applied to various industrial apparatuses , office automation apparatuses , household electric appliances , etc . using the power as an energy source . the discharge circuit unit and the image forming apparatus having the same in accordance with an embodiment can minimize standby power occurring in a standby mode of an electric product . the discharge circuit unit may be configured to be connected in series between opposite ends of an ac power input line . thereby , it is possible to reduce the number of elements , to be easily commercialized , and to reduce consumption of standby power ( detection power ) while ac power is supplied . ac blocking is realized using mosfet having a diode function , and thereby a separate blocking diode is not required . although a few embodiments have been shown and described , it would be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention , the scope of which is defined in the claims and their equivalents .