Abstract:
A method of controlling a supply of electric power to an electric load and a circuit thereof are disclosed. The circuit includes a control voltage generating part means for generating a control voltage which decreases over time, an amassing part for amassing the control voltage upon a voltage signal increasing over time and discharging the amassed voltage nearby a zero crossing point of AC voltage from the AC source in order to generate a saw-tooth wave, a trigger signal generating part for generating trigger signals based on the saw-tooth wave, wherein the phase angle of the trigger signals varies depending on the saw-tooth wave, and a switch part for supplying electric energy from the AC power source to the electric load in response to the trigger signals. Therefore, electric power being supplied to an electric load fades out.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of controlling a supply of electric power to an electric load and a circuit thereof. More particularly, the present invention relates to a method which controls a phase of AC voltage being supplied to the electric load at every cycle of the AC voltage in order to fade out the power consumption of electric energy of the load for a predetermined time period, and a circuit which performs the method. 
     2. Prior Art 
     Generally, an electric device, such as an electric lamp, an electric heater or the like, is powered on or off by an on/off switch. In case of the electric lamp, a variable resistor is used for controlling the brightness of lamp, and a power switch is used for turning on the lamp, such that a user can reduce the brightness of the lamp or turn off the lamp before sleeping. 
     On the other hand, the facility and utility of lamps have been improved as a lamp having a function of a timer. The function of timer of the lamp may be used in sleeping. That is, when a user sets a turn-off time of the lamp, the needless lighting of the lamp is prevented. 
     FIG. 1 is a view for showing a circuit of a conventional power control circuit for an electric lamp. 
     Referring to FIG. 1, the circuit includes a time setting part  71 , a reference voltage generation part  72 , a comparing part  70 , and a driving part  73 . The time setting part  71  includes a resistor and a capacitor for setting a time period of supplying power to a lamp LP. The time period is achieved by a time constant of the resistor and the capacitor. The reference voltage generation part  72  generates a reference voltage which is used for determining whether a discharging voltage of the capacitor is greater than the reference voltage. The comparing part  70  compares the discharging voltage with the reference voltage, and outputs a voltage. The driving part  73  turns on or off the lamp LP according to the voltage based on the comparison result of the comparing part  70 . 
     Further, the reference voltage generation part  72  includes resistors R 2  and R 3  and a diode D. 
     The time setting part  71  includes the resistor R 1 , the capacitor C 1 , and a variable resistor VR. The time constant of the time setting part  71  is adjusted by the variable resistor VR. 
     The comparing part  70  includes an operational amplifier OP and two resistors R 4  and R 5  for comparing the reference voltage with the discharging voltage. 
     The driving part  73  is a switching device such as a triac or the like. 
     Hereinafter, the operation of the circuit will be described. 
     When the driving part  73  is turned off, the AC source AC is not applied to the lamp LP. When the switch SW is switched on by a user, the capacitor is charged with electric energy which is supplied through the resistor R 1  from the DC source Vcc. 
     At the same time, the operational amplifier OP outputs a high level of voltage to the triac T of the driving part  73  because of a voltage difference between the two terminals of resistor R 2 , such that the AC power is supplied from the AC source to the lamp LP. 
     When the switch SW is switched off, the capacitor starts to discharge the charged energy thereof to the variable resistor VR. However, although the switch SW is switched off, the operational amplifier OP continuously outputs the voltage of high level until the value of the discharging voltage is less than that of the reference voltage. 
     As the discharging voltage of the capacitor C is gradually decreased, the difference between the reference and discharging voltages is decreased, too. Therefore, by varying the resistance value of the variable resistor VR, the time constant of the time setting part  71  is determined, such that the light time of the lamp LP is adjustable. 
     As the voltage of the non-inverting terminal is decreased, the voltage value is less than that of the reference voltage, such that the operational amplifier OP outputs a low level voltage. As the result of outputting the low level voltage, the gate voltage of the triac T drops down to zero, such that the lamp LP is turned off. 
     Therefore, the circuit controls the power supply of the lamp as one example of electric loads by directly either connecting or disconnecting the AC source with the lamp, or turning off the lamp when a time set by a timer passes since the lamp is turned on. 
     However, the control circuit has problems that the lamp is abruptly turned off so that the eye of a user may be stimulated by the abrupt variation of the brightness, and the power consumption of the circuit is much more. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method of controlling electric power which can fade out electric power being supplied to an electric load. 
     It is an other object of the present invention to provide a circuit for controlling electric power which can fade out an electric power being supplied to an electric load. 
     In order to achieve the above objects, a circuit according to one aspect of the present invention includes means for generating a control voltage which decrease with time; means for accumulating a charge a the control voltage upon a voltage signal increasing with time and discharging an accumulating charge nearby a zero crossing point of AC voltage from the AC source in order to generate a saw-tooth wave; means for generating trigger signals based on the saw-tooth wave, wherein the phase angle of the trigger signals varies dependent on the saw-tooth wave; and means for supplying an electric energy from the AC power source to the electric load in response to the trigger signals. 
     A method according to another aspect of the present invention includes the steps of: a) making a control voltage which decreases with time; b) accumulating a charge from the control voltage upon a voltage signal increasing with time and discharging an accumulated charge nearby a zero crossing point of AC voltage from the AC source in order to generate a saw-tooth wave; c) generating trigger signals based on the saw-tooth wave, wherein the phase angle of the trigger signals varies dependent on the saw-tooth wave; and d) supplying an electric energy from the AC power source to the electric load in response to the trigger signals. 
     According to the present invention, there are provided a circuit and a method for controlling electric power which can fade out electric power being supplied to an electric load. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     This invention will be better understood and its various objects and advantages will be more fully appreciated from the following description taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a view for showing a circuit of a conventional power control circuit for an incandescent electric lamp; 
     FIG. 2 is a block diagram for illustrating the functions of the present invention; 
     FIG. 3 is a view of showing a circuit according to one example of the block diagram depicted in FIG. 2; 
     FIG. 4 is a view of showing a circuit according to another example of the block diagram depicted in FIG. 2; 
     FIG. 5 is a flowchart for illustrating the operation of the present invention; 
     FIG. 6 is a waveform for illustrating the circuit depicted in FIG. 3; 
     FIG. 7 is a waveform for illustrating the circuit depicted in FIG. 4; 
     FIG. 8A is a view for showing a circuit according to one example of the voltage dividing part; 
     FIG. 8B is a wave form for illustrating the circuit of the FIG. 8A; 
     FIG. 9A is a view for showing another example of the voltage dividing part; 
     FIG. 9B is a wave form view for illustrating the circuit of the FIG. 9A; 
     FIG. 10 is a view for showing an example of the switching part; and 
     FIGS. 11 through 13 are views for showing various embodiments of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Preferred embodiments of the present invention will be illustrated below with reference to the accompanying drawings. 
     FIG. 2 is a block diagram for illustrating the functions of the present invention. FIGS. 3 and 4 are views showing a circuit of the block diagram depicted in FIG.  2 . FIG. 5 is a flowchart for illustrating the operation of the present invention. FIG. 3 is a circuit in which a silicon controlled rectifier (hereinafter, referred to as SCR) is used as a switching device of a driving part. FIG. 4 is a circuit in which a triac is used as the switching device of the driving part. 
     Referring to FIG. 2, the switch circuit for fading out a supply of electric power includes the driving part F, a DC power source part G, a switch part B, a voltage dividing part C, a control part D, and a trigger signal generating part E. 
     The driving part F controls the supply of an AC source to a lamp L. The DC power source part G rectifies an AC voltage of the AC source to generate a DC voltage. The switch part B controls each electric charging of the first and second charging parts A 1  and A 2 . The voltage dividing part C divides the discharging voltage of the second charging part A 2 . The control part D generates a control signal corresponding to a divided voltage from the voltage dividing part C. The trigger signal generating part E generates a trigger signal in response to the control signal. 
     The driving part F includes a diode bridge for full-rectifying the AC voltage from the AC source, the SCR, a diode D 5 , and a pull-down resistor R 9 . The anode and cathode of the SCR are connected with the output terminals of the diode bridge. The gate of the SCR is connected with the diode D 5  and the pull-down resistor R 9 . 
     The DC power source part G includes a resistor R 11  and a zener diode DZ 1  for providing a regulated voltage to each part of the switch circuit. The control part includes an electric charging circuit  10  and an electric discharging circuit  20 . The electric charging circuit  10  includes a transistor Q 1 , a capacitor C 3 , and two resistors R 4  and R 5 . A charging time of the capacitor C 3  is determined by the voltage from the voltage dividing part C and the DC power source G. The electric discharging circuit  20  includes two transistors Q 2  and Q 3  and a plurality of resistors R 6  through R 8  for discharging the electric charged energy of the capacitor C 3  before charging the capacitor C 3 . 
     The trigger signal generating part G includes a resistor R 10 , a transistor Q 4 , and a diode D 6 . The trigger signal generating part G generates a trigger voltage according to the electric charged voltage of the electric charging circuit  10  and provides the trigger voltage to the gate of the SCR. The voltage dividing part C outputs a divided voltage according to the resistance ratio of the series resistors R 1  and R 2 . 
     The first charging part A 1  is a capacitor C 1  for charging the voltage from the DC power source G, and the second charging part A 2  is a capacitor C 2  for charging the voltage from the DC power source G or the capacitor C 1  of the first charging part A 1  through the switch part B. 
     Hereinafter, the operation of the present invention will be described. 
     The diode bridge rectifies the AC voltage to generate a full rectified voltage. The full rectified voltage is provided to the zener diode DZ 1  through the resistor R 11  of the DC power source part G, such that the DC power source part G outputs the regulated voltage. 
     The resistance value of the circuit including the resistors R 10  and R 11  is greater than that of the lamp L, such that the lamp L is turned off. While the lamp L is turned off, the capacitor C 1  of the first charging part A 1  is charged by the DC voltage through the diode D 7  (S 1 , S 2 ). 
     When the switch part B is switched on, the capacitor C 2  is charged by the DC voltage inputted through the switch part B (S 3 , S 4 ). When the switch part B is switched off, the capacitor C 2  starts to discharge the charged energy to the resistors R 2  and R 1  (S 5 ). 
     That is, when the switch SW of the switch part B is switched on the ON terminal thereof, the lamp L is turned on. If a user wants to fade out the power consumption of the lamp L, the user causes the switch SW to be switched on the F/O terminal of the switch SW. While the switch is tuned on, the charging operation of the capacitor C 2  is completed. As soon as the switch is turned off again, the capacitor C 2  starts to discharge the charged energy to the resistors R 1  and R 2 . When the discharging voltage of the capacitor C 2  is greater than a predetermined voltage, the capacitor C 3  of the first charging circuit  10  is superposition-charged (S 6 , S 7 ). 
     The base voltage of the transistor Q 3  starts to increase passing to the zero point of transfer rectified signal, as shown in ( 2 ) of FIG.  6 . When the base voltage of transistor Q 3  is less than the turn-on voltage V ON3 , the collector of the transistor is open. However, since a voltage of (R 7 +R 8 )/R 8  times as much as the base voltage of the transistor Q 3  is inputted to the base of the transistor Q 2  through resistor R 6 , the transistor Q 2  is turned on faster than the transistor Q 3 , such that the capacitor C 3  is first discharged before being charged. 
     As the base voltage V BE3  of the transistor Q 3  is increased, the base voltage V BE3  of the transistor Q 3  reaches at the turn-on voltage V ON3 , such that the transistor Q 3  is turned on. When the transistor is turned on, the base voltage of the transistor Q 2  drops down to a low level, such that the transistor Q 2  is turned off and the capacitor C 3  starts to be charged. 
     When the transistor Q 1  is turned on by the voltage, V C2 ×R 1 /(R 1 +R 2 ), applied to the base thereof, the capacitor C 3  is charged by a current from the emitter of the transistor Q 1 . 
     The charging voltage of the capacitor C 3  charged by the current of the transistor Q 1  is referred to as V2. The voltage V2 abruptly increases during charging, and has a constant value after the charging operation is completed, as shown in ( 4 ) of FIG.  6 . 
     At the same time, the regulated voltage from the DC power source part G is applied to the capacitor C 3  through the resistor R 5 . The charging voltage V1 charged to the capacitor C 3  via the resistor R 5  has a saw-tooth wave form as shown in ( 3 ) of FIG.  6 . The velocity difference of charging the capacitor C 3  by the voltages V1 and V2 is adjustable by decreasing the resistance ratio of the resistors R 4  and R 5 . When the resistance value of the resistor R 4  varies, the voltage V2 varies. Therefore, the trigger starting point of the SCR can be vary. 
     By adjusting the time constant of the capacitor C 3  and resistor R 5 , the voltage V1 is less than each of the turn-on voltage V BE4  between the base and emitter of the transistor Q 4 , the turn on voltage of the diode D 5 , and the trigger voltage of the gate of the SCR, thereby preventing the SCR from being triggered by the maximum value of the voltage V1. That is, the voltage V1 is as follows: 
      V1 max &lt;(V BEA +V D5 +V GT ) 
     Therefore, the charging voltage of the capacitor C 3  is V1+V2, and the capacitor C 3  is superposition-charged by the voltages V1 and V2. 
     In step  8 , whether the charging voltage of the capacitor C 3  is sufficient to trigger the SCR after the capacitor C is charged fully (S 8 ) is determined. 
     When the charging voltage of capacitor C 3  is less than the trigger voltage of the SCR, in step  9 , it is determined whether the half cycle of AC voltage is passed or not (ST 9 ). 
     During the half cycle of the AC voltage, the capacitor C 3  is continuously charged. As soon as the charging operation of the capacitor C 3  is completed, the capacitor C 3  starts to discharge the charged energy (S 11 ). 
     When the charging voltage of the capacitor C 3  is greater than the trigger voltage, the SCR is turned on, such that the lamp L is turned on. That is, the charging voltage of the capacitor C 3  is applied to the base of the transistor Q 4  of the trigger signal generation part E, such that the gate voltage of the SCR declines to a pull-up voltage, such that the SCR is triggered by the pull-up voltage applied to the gate thereof. The lamp L is turned on for the half cycle of AC voltage (S 10 ). This operation is depicted in ( 5 ) of FIG.  6 . 
     For example, when a great voltage is applied from the voltage dividing part C to the base of the transistor Q 1 , one charging voltage V2 of the capacitor C 3  has a wave form  1  depicted in ( 4 ) of FIG.  6 . Therefore, the charging voltage of the capacitor C 3  is the voltage V1+V2, depicted with a dotted line in ( 5 ) of FIG.  6 . 
     The points  1 ,  2 ,  3 , and  4 , depicted in ( 5 ) of FIG. 6 indicate the trigger time points of the SCR with the charging time. In the time point  1 , the SCR is turned on. The voltage between the anode and cathode of the SCR declines to the turn-on voltage of the SCR, as shown in ( 6 ) of FIG.  6 . Therefore, the AC voltage is inputted from time point  1  of half cycle of AC voltage to lamp L, as shown in ( 7 ) of FIG.  6 . 
     When the SCR is turned on, the capacitor C 3  discharges the charged voltage to the resistors R 5 , R 7 , and R 8 . As the capacitor discharges the charged voltage, the base voltage of the transistor Q 3  declines to be turned off. 
     When the transistor Q 3  is turned off, the turn-on voltage of the SCR is applied to the base of the transistor Q 2 , such that the transistor Q 2  is turned on. Therefore, the capacitor C 3  is discharged abruptly through the transistor Q 2 , and the transistor Q 4  is turned off. The gate voltage of the SCR declines to zero voltage, but the SCR remains on. 
     When the half cycle of AC voltage passes, that is, the zero crossing point of AC voltage is inputted to the SCR, the SCR is turned off by the zero voltage of the AC voltage, such that the power supply to the lamp L is stopped (S 11 , S 12 , S 13 ). 
     After the lamp L is turned off, the steps  6  through  13  are repeated again until the discharging voltage of the capacitor C 2  drops down to the turn-off voltage of the transistor Q 1 . 
     Further, the trigger point of the SCR is gradually delayed, such that the power being supplied to the lamp L fades out, as shown in ( 7 ) of FIG.  6 . 
     The time period for fading out the lamp L is determined by the time constant of the second charging part A 2 , as shown in ( 5 ) of FIG.  6 . Therefore, if a variable resistor VR 1  which is substituted for the resistor R 1  varies, the time period for fading out the lamp can vary by varying the resistance value of the variable resistor VR 1 . 
     The operation of fading out the brightness of the lamp will be described with reference to ( 4 ), ( 5 ), and ( 7 ) of FIG.  6 . 
     As the capacitor C 2  is discharged, the discharging voltage V C2  of the capacitor C 2  decreases with time. The discharging voltage V C2  of the capacitor C 2  is provided to the base of the transistor Q 1  through the voltage dividing part C. The charging voltage V2 which is applied from the transistor Q 1  to the capacitor C 3  varies from  1  to  2  as shown in ( 4 ) of FIG.  6 . Since the voltages V1 and V2 are superposed on the capacitor C 2 , the charging voltage of the capacitor C 2  decreases as shown in ( 5 ) of FIG.  6 . 
     As the charging voltage of the capacitor C 3  gradually decreases, the trigger point of the SCR is shifted from  1  to  4 . This shifting operation of the trigger point reduces the duty cycle of AC voltage applied to the lamp L, such that the lighting time of the lamp is gradually decreased. As the lighting time of the lamp is reduced, the brightness of the lamp L is reduced, too. Therefore, the brightness of the lamp L fades out during the time constant of the capacitor C 2 . 
     According to another embodiment depicted in FIG. 4, the first anode A 1  and the second anode of the triac of the driving part F are respectively connected with both output terminals of AC power source, and the gate of the triac is connected with the resistors R 11  and R 12 . 
     The control part D includes the same charging circuit  10  as that of FIG.  3  and the discharging circuit  20  including transistors Q 2  and Q 3 , diodes D 1  and D 2 , and a plurality of resistors R 6  through R 8 . The trigger signal generating part E includes two resistors R 9  and R 10  and two transistors Q 4  and Q 5  for providing a trigger signal to the gate of the triac according to the charged voltage of the charging circuit  10 . 
     The switch part B and the voltage dividing part C are the same as those of FIG.  3 . The DC power source part G rectifies AC voltage from the AC power source and regulates a half-rectified voltage. The DC power source part G includes two resistors R 13  and R 14 , a capacitor C 4 , and two zener diodes DZ 1  and DZ 2 . 
     Hereinafter, the operation of the circuit depicted in FIG. 4 will be described with reference to the flowchart depicted FIG.  5 . 
     By steps  1  and  2 , the capacitor C 1  is charged. The AC voltage of the AC power source is half-rectified by the resistor R 13 , the capacitor C 5 , the diode D 3 , and the zener diode DZ 2 . The half-rectified voltage applied to the capacitor C 4  is regulated by the resistor R 14  and the zener diode DZ 1 . Since the impedance of the capacitor C 5  and the resistor R 13  is much greater than that of the lamp L, the lamp L is turned off when the triac is not triggered. 
     The capacitor C 1  of the first charging part A 1  is charged with the regulated voltage. After the steps  3  through  7  in turn occur, as illustrated in FIG. 3, the base voltage of the transistor Q 3  increases by passing the positive zero crossing point of AC voltage, as shown in ( 1 ) of FIG.  7 . The transistor Q 3  is not turned on until the base voltage of the transistor Q 3  attains the turn-on voltage V ON3 . Therefore, the transistor Q 2  is turned on, as shown in ( 2 ) of FIG. 7, such that the capacitor C 3  is discharged. 
     When the base voltage of the transistor Q 3  attains the turn-on voltage V ON3 , the transistor Q 3  is turned on as shown in ( 1 ) of FIG.  7 . when the base voltage of the transistor Q 2  is a low level, as shown in ( 2 ) of FIG. 7, the transistor Q 2  is turned off, such that the capacitor C 3  is charged. 
     The charged voltage of the capacitor C 2  is applied to the base of the transistor Q 1  through the resistors R 1  and R 2 . The transistor Q 1  is turned on by the base voltage thereof, V C2 ×R 1 /(R 1 +R 2 ). At the same time, the capacitor C 3  is charged by a current outputted from the emitter of the transistor Q 1 . 
     When the charged voltage of the capacitor C 3  is referred to as V2, the charged voltage V2 has a waveform of an initial slant and a constant voltage level. 
     At the same time, the regulated voltage of the DC power source part G is also applied to the capacitor C 3  through the resistor R 5 . The voltage V1 charged in the capacitor C 3  by the regulated voltage has a saw-tooth waveform, as shown in ( 3 ) of FIG.  7 . the difference of charging velocity between the voltages V1 and V2 is proportional to the resistance ratio of the resistors R 4  and R 5 . When the capacitor C 3  is discharged fully, the time constant of the capacitor C 3  and the resistor R 3  is adjusted to prevent the triac from being triggered. That is, the maximum voltage of the capacitor C 3  which is charged during the one cycle of the AC voltage is less than the turned-on voltage V ON4  of the transistor Q 4 . The maximum voltage Vlmax of the capacitor C 3  is as follows: 
     
       
         V1max&lt;V ON4 . 
       
     
     Therefore, the charging voltage VC 3  of the capacitor C 3  is V1+V2. That is, the voltages V1 and V2 are superposed on the capacitor C 3 . 
     The capacitor C 3  is charged during one half cycle of AC voltage, but during the other half cycle of AC voltage, the capacitor C 3  is discharged (S 11 ). When the charged voltage of the capacitor C 3  attains the turned-on voltage V ON4 , the transistor Q 4  of the trigger signal generating part E is turned on and the charged current of the capacitor C 3  is provided to the gate of the triac, such that the triac is triggered (S 8 ). Once the lamp L is turned on, the lamp L maintains in the turned-on state (S 10 ), as shown in ( 5 ) of FIG.  7 . 
     When a high voltage is applied from the voltage dividing part C to the base of the transistor Q 4 , the charged voltage V2 of the capacitor C 3  has a waveform  1  depicted in ( 4 ) of FIG.  7 . Therefore, the voltage V1+V2 applied to the capacitor has a waveform indicated by a dotted line in ( 5 ) of FIG.  7 . The dotted line from  1  to  2  indicates the trigger level of the triac. When the triac is turned on at the time point  1 , the AC voltage is applied to the lamp L from the time point  1  to the zero crossing point of the AC voltage, as shown in ( 6 ) of FIG.  7 . 
     When the triac is turned on, the capacitor C 3  starts discharging (S 11 ). As the capacitor discharges the charged voltage, the base voltage V BE3  of the transistor Q 3  declines to be turned off and the transistor Q 2  is turned on. 
     When the transistor Q 2  is turned on, the capacitor C 3  is discharged abruptly through the transistor Q 2 , and the transistor Q 4  is turned off. The gate voltage of the triac drops down to the zero voltage, but the triac remains turned on. 
     When the half cycle of AC voltage passes, that is, the zero crossing point of AC voltage is inputted to the triac, the triac is turned off by the zero voltage of the AC voltage, such that the power supply to the lamp L is stopped (S 11 , S 12 , S 13 ). 
     After the lamp L is turned off, steps  6  through  13  are repeated again until the discharging voltage of the capacitor C 2  declines the turn-off voltage of the transistor Q 1 . 
     Since the operation of the circuit depicted in FIG. 4 for the positive half cycle of AC voltage is the same as that of FIG. 3, the detailed illustration of the operation of the circuit for the positive half cycle is omitted, and the operation for the other half cycle will be illustrated below. 
     During the negative half cycle of the AC voltage, since the base voltage V BE3  of the transistor Q 3  is zero as shown in ( 1 ) of FIG. 7, the transistor Q 3  is turned off and the transistor Q 2  is turned on. The negative voltage during the negative half cycle is applied to the base of the transistor Q 2 , such the base voltage V BE2  of the transistor Q 2  is varied as indicated by the dotted line in ( 2 ) of FIG.  7 . 
     As the negative voltage of the AC voltage is increased, the base voltage V BE2  of the transistor Q 2  is varied. The transistor Q 2  is not turned on until the base voltage V BE2  of the transistor Q 2  declines the turned-on voltage V ON2 . When the transistor Q 2  is turned off, the voltages V1 and V2 are superposed upon the capacitor C 3 , such that the charged voltage of the capacitor C 3  is increased to the turned-on voltage V ON4  of the transistor Q 4 . When the transistor Q 4  is turned on, the triac is turned on too. The base voltage V BE2  of the transistor Q 2  is increased to the turned-on voltage of the transistor Q 2 , such that the capacitor C 3  discharges the charged energy thereof. Therefore, during the negative half cycle of the AC voltage, the transistor Q 3  is turned on and the turned-off state of the transistor Q 2  maintains as shown in ( 2 ) of FIG.  7 . 
     As the anode voltage of the triac approximately reaches the zero crossing point, the capacitor C 3  discharges the energy charged therein, and the triac is disabled. When the anode voltage of the triac is zero, the triac is turned off. Those operations are repeated every cycle of the AC voltage while the discharging of the capacitor C 2  is completed. The time points on each of which the triac is triggered are varied with time. Each of the time points for triggering the triac is determined by the discharging voltage V C2  of the capacitor C 2 . The reference numerals  1  through  4  of FIG. 7 indicate the variation of the discharging voltage V C2  of the capacitor C 2 . The time constant of the capacitor C 2  can be varied by substituting a variable resistor for the resistor R 1 . 
     As the discharging voltage V C2  of the capacitor C 2  is decreased, the turned-on time of the triac is delayed. Therefore, the brightness of the lamp L fades out. 
     The discharging voltage V C2  of the capacitor C 2  is decreased with time. The discharging voltage V C2  of the capacitor C 2  is provided to the base of the transistor Q 1  through the voltage dividing part C. The charging voltage V2 which is applied from the transistor Q 1  to the capacitor C 3  is varied from  1  to  4  as shown in ( 4 ) of FIG.  7 . Since the voltages V1 and V2 are superposed on the capacitor C 3 , the charging voltage of the capacitor C 3  is decreased as shown in ( 5 ) of FIG.  7 . 
     As the discharging voltage of the capacitor C 2  is gradually decreased, the trigger point of the triac is shifted from ? to ?. This shifting operation of the trigger point reduces the duty of AC voltage applied to the lamp L, such that the lighting time period of the lamp is gradually decreased. As the lighting time period of the lamp is reduced, the brightness of the lamp L is reduced too. Therefore, the brightness of the lamp L fades out during the time constant of the capacitor C 2 . 
     Further, when the resistor R 1  is substituted with the variable resistor VR 1  as shown in FIG. 8A, the brightness of the lamp L is variable as shown in FIG.  8 B. 
     When the turn-off of the lamp L is not needed, incomplete turn-off is achieved by the charging circuit  10 . If the resistance value of the resistor R 5  is reduced, the charging voltage of the capacitor C 3  through the resistor R 5  shown in FIG. 3B is increased, such that the lamp L is not turned off. That is, although the transistor Q 1  is turned off, the capacitor C 3  is sufficiently charged only by the current flowing from the resistor R 5 . Therefore, the lamp L remains turned-on, and the brightness of the lamp L is determined by the resistance value of the resistor R 5 . 
     Hereinafter, the maintaining operation of the turned-on the lamp L will be described with reference to FIGS. 9A and 9B. 
     FIG. 9A is a view for showing one exemplary circuit with which the resistor R 5  can be substituted. FIG. 9B is a view of showing an output voltage VR 2  of the circuit depicted in FIG.  9 A. The circuit includes a variable resistor VR 2  and a resistor R 0  in series. As illustrated above, the SCR is not triggered at the maximum value of the complex resistance of the variable resistor VR 2  and resistor R 0 . The resistance value of the resistor R 0  is set to be able to cause the lamp to light with a predetermined brightness. Therefore, when the variable resistor has the minimum resistance value, the lamp L maintains the lighting state. If a user would like to control the lamp L to be either turned on or off, the turned-on/off control of the lamp L can be achieved by causing the resistance of the variable resistor VR 2  to be the minimum value. When the resistance of the variable resistor VR 2  is the minimum value, the current flowing through the variable resistor VR 2  is maximum, such that the trigger point of the SCR approximates the zero crossing point of AC voltage, as indicated by  1  in ( 1 ) of FIG.  9 B. Therefore, since the SCR is turned on at the time point  1 , as shown in ( 2 ) of FIG. 9B, the brightness of the lamp L is fixed. When the discharging voltage V C2  of the capacitor C 2  is zero, the brightness of the lamp L is controlled by the variable resistor VR 2 . When the resistance value of the variable resistor VR 2  is increased by the increase of the time constant of the capacitor C 3 , the charging time of the capacitor C 3  is delayed such that the trigger time point of the SCR is delayed, as shown in ( 1 ) FIG.  9 B. When the trigger time point of the SCR is delayed, the duty time of the AC voltage applied to the lamp L is reduced, such that the brightness of the lamp L is reduced as much as the turned-on time point of the SCR is delayed. Therefore, when the resistance value of the variable resistor VR 2  is gradually reduced, the trigger point of the SCR is shifted from  1  to  4 , as shown in ( 1 ) and ( 2 ) of FIG. 9B, such that the brightness of the lamp L is decreased in proportion to the increase of the resistance value of the variable resistor VR 2 . That is, when the resistance value of the variable resistor VR 2  is maximum, the current flowing through the variable resistor VR 2  is minimum. 
     When the current of the variable resistor VR 2  has the minimum value, the SCR is not triggered. Further, when the resistance value of the resistor R 3  is increased, the charging velocity of the capacitor C 2  is decreased, such that the brightness of the lamp L does not fade out but fades in. Since the fade-out time is controlled by varying each value of the capacitor C 2 , the resistor R 3 , and the voltage V C1 , the circuit according to the present invention performs the same functions as those of a conventional timer. 
     As shown in FIG. 10, the switch SW of the switch part B is substituted with a sensor for sensing the human body. When the sensor SEN senses the human body, by the switching operation of the sensor SEN the capacitor C 2  is charged. The sensor SEN for sensing the human body may be replaced with a supersonic sensor, a thermal sensor, an infrared sensor or the like. 
     FIGS. 11 through 13 are views for showing various examples of the discharging circuit  20 . The circuits depicted in FIGS. 11 through 13 perform the same functions as those of FIGS. 3 and 4. 
     Referring to FIG. 11, the emitter of transistor Q 3  is connected with the output terminal of the DC power source part G. The collector of the transistor Q 3  is connected with one terminal of the resistor R 6 , and the other terminal of the resistor R 6  is connected with the base of the transistor Q 2 . One terminal of the resistor R 6 ′ is connected with the base of the transistor Q 2  and the other terminal of the resistor R 6 ′ is connected with the emitter of the transistor Q 2 . The base of the transistor Q 3  is connected with the anode of the SCR through the resistor R 7 . 
     When the base voltage of the transistor Q 3  is lower than the emitter voltage of the transistor Q 3 , the transistor Q 3  is turned on, such that the transistor Q 2  is also turned on. When the transistor Q 2  is turned on, the capacitor C 3  discharges the charged energy through the transistor Q 2 . As the regulated voltage from the DC power source part G is increased, the transistor Q 3  is turned off. While the transistor is turned off, the capacitor C 3  is charged. 
     Referring to FIG. 12, the discharging circuit  20  includes a diode D 8  and two resistors R 7  and R 8 . The anode of the diode D 8  is connected with the emitter of the transistor Q 1  and the base of the transistor Q 4 , respectively. The cathode of the diode D 8  is connected with a connecting node k of the resistors R 7  and R 8 . 
     According to the circuit depicted in FIG. 12, when the AC voltage reaches the zero crossing point, the voltage of the node k is zero, such that the capacitor C 3  discharges the charged energy through the diode D 8  and the resistor R 8 . When the voltage of the node k is higher than the maximum charged voltage of the capacitor C 3 , the capacitor C 3  is charged. 
     The operation of the circuit depicted in FIG. 13 is the same as that of FIG. 3 except that the types of transistors Q 1 , Q 2 , and Q 4  are changed into the PNP type. 
     In FIG. 13, the resistance values of the resistors R 7  and R 8  are set to cause the base voltage of the transistor Q 2  to be higher than the rectified voltage from the DC power source part. The transistor Q 2  is not turned off until the rectified voltage is lower than a voltage set by the resistors R 7  and R 8 . Therefore, the capacitor C 3  is charged while the transistor is turned off. 
     The circuit according to the present invention may be used in a place such as a passage way, an entrance, or the like. Further, the circuit is used for brightening indoors with a constant intensity of illumination. Further, the circuit fades out the brightness of the lamp after the lamp is turned on. 
     According to the present invention, a push button is provided as a method of controlling electric power, and a circuit is provided which can fade out electric power being supplied to an electric load. 
     While this invention has been particularly shown and described with reference to particular embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be effected therein without departing from the spirit and scope of the invention as defined by the appended claims.