Abstract:
A protection circuit limits operating power supplied to an electrical device from an electrical power source below a predetermined power rating. The protection circuit comprises a switching device for adjusting electrical power supplied from the power source to the electrical device; a sensor sensing a parameter relating to the operating power; and a comparator comparing the parameter sensed by the sensor and, in response to identifying an undesirable operating condition, controlling the switching device to adjust electrical power supplied to the electrical device to below a predetermined power rating.

Description:
The present invention relates to a protection circuit for limiting the operating power of an electrical device and, in particular but not exclusively, to lighting devices. 
     BACKGROUND OF THE INVENTION 
     To protect the environment, people worldwide are becoming increasingly cautious and have generally accepted that the use of fossil fuels should be minimized, and in particular electricity from the mains/household power. 
     Lighting is one area where energy saving can readily be achieved by the general public. For example, light bulbs of relatively lower power ratings are recommended for general lighting purposes for which lighting intensity is often not critical. In certain jurisdictions, regulations have been made for constraining the operation of lighting apparatus, etc. to below a specific wattage rating, e.g. 190 W for light kits on ceiling fans. 
     However, due to negligence or lightheartedness, some people may still use over-power light bulbs for brighter illumination, and this will cause unnecessary wastage of energy and can be dangerous. 
     The invention seeks to eliminate or at least to mitigate such a problem or shortcoming by providing a new or otherwise improved protection circuit for limiting the operating power of an electrical device and method thereof. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the invention, there is provided a protection circuit for limiting operating power of an electrical device in a circuit connected to an electrical power source below a predetermined power rating, the protection circuit comprising:
         a switching device for use in connection with the electrical device for adjusting electrical power supplied from the power source to the electrical device;   a sensor for sensing a parameter relating to the operating power of the electrical device; and   a comparator for comparing the parameter sensed by the sensor to identify an undesirable operating condition in which the electrical device operates at a power exceeding the predetermined power rating and for, in response to identifying such an undesirable operating condition, providing an output signal for controlling the switching device to adjust electrical power supplied to the electrical device to below the predetermined power rating, while maintaining an electrical current flow through the electrical device.       

     Preferably, the sensor comprises a voltage sensing device for sensing an operating voltage of said electrical device, the operating voltage being said parameter. 
     More preferably, the voltage sensing device comprises a first resistor for connection in series between said electrical device and said power source, the resistor having one end on the side of said electrical device for sensing of the operating voltage of said electrical device. 
     Further more preferably, the comparator comprises a voltage comparator having a first input connected via a second resistor to the end of the first resistor to input the operating voltage of said electrical device and a second input connected to the other end of the first resistor on the side of said power source to input the voltage appearing at the other end of the first resistor as a reference for comparison. 
     Yet further more preferably, the other end of the first resistor is for direct connection to said power source such that the voltage of said power source is taken as the reference. 
     It is preferred that the sensor comprises a current sensing device having a resistor for connection in series between said electrical device and said power source to sense an operating current of said electrical device, the operating current being said parameter. 
     It is further preferred that the comparator comprises a voltage comparator having two inputs to which opposite ends of the resistor are connected respectively for comparing the potential difference developed across the resistor by said operating current to identify said undesirable operating condition. 
     In a preferred embodiment, the switching device comprises a relay arranged to switch between at least two first and second circuits, with the first circuit being for normal connection of said electrical device to said power source, and with the second circuit being for connection of said electrical device to said power source upon identifying of said undesirable condition. 
     More preferably, the first circuit is of relatively low resistance for normally connecting said electrical device to said power source, and the second circuit is of relatively large resistance for connecting said electrical device to said power source upon identifying of said undesirable condition, thereby reducing electrical power supplied to said electrical device to below the predetermined power rating. 
     More preferably, the switching device includes a solid state switch for connecting said electrical device to said power source, the solid state switch having a control terminal switchable by the relay between the first and the second circuits of different resistances to control duty cycle of conduction of the solid state switch. 
     Further more preferably, the solid state switch comprises a triac whose control terminal is controlled by a capacitor charging via one of the first and second circuits as switched in by the relay. 
     More preferably, the protection circuit includes a latching unit connected between the comparator and the relay for latching said output signal of the comparator to the relay such that the relay will stay activated. 
     More preferably, the relay comprises an electromagnetic relay having a switch and an electromagnet for operating the switch. 
     In a preferred embodiment, the switching device comprises a solid state switch for connecting said electrical device to said power source, the solid state switch having a control terminal, and a trigger unit connected to the control terminal for in response to said output signal of the comparator providing a series of timed trigger signals to turn on the solid state switch at progressively different firing angles, each over a predetermined time interval, thereby controlling duty cycle of conduction of the solid state switch. 
     More preferably, the protection circuit includes a controller connected between the comparator and the trigger unit for controlling the timing and firing angle of the trigger signals of the trigger unit. 
     Further more preferably, the controller comprises a counter for providing a series of control signals at said predetermined time intervals to control the timing of the trigger signals provided by the trigger unit. 
     Yet further more preferably, the trigger unit comprises a capacitor and a plurality of resistance paths connected to the control terminal of the solid state switch, each of the resistance paths being of a different resistance for progressively adjusting charging time of the capacitor and in turn the firing angle of the trigger signals provided by the trigger unit, the resistance paths being alternatively activated by respective control signals of the counter. 
     Yet yet further more preferably, the counter has a plurality of outputs for outputting respective control signals, and each of the resistance paths comprises a resistor and a solid state switch having a control terminal connected to a respective output of the counter for activation thereby. 
     Yet further more preferably, the controller includes a timer connected to the trigger unit for progressively adjusting the firing angle of the trigger signals of the trigger unit, in response to each control signal provided by the counter. 
     Yet yet further more preferably, the counter has at least one output connected to the timer for outputting the series of control signals to the timer, the control signals being binary signals. 
     It is preferred that the switching device is arranged to deliver electrical power from said power source to said electrical device initially at maximum power, and subsequently to reduce said electrical power to below the predetermined power rating. 
     It is preferred that the controller operatively increases the firing angle of the trigger signals of the trigger unit to reduce the duty cycle of conduction of the solid state switch so as to reduce said electrical power to below the predetermined power rating. 
     It is preferred that the controller operatively reduces the firing angle of the trigger signals of the trigger unit to increase the duty cycle of conduction of the solid state switch so as to increase said electrical power until it exceeds the predetermined power rating, and subsequently increases the firing angle of the trigger signals of the trigger unit to reduce the duty cycle of conduction of the solid state switch so as to reduce said electrical power to below the predetermined power rating. 
     It is preferred that the predetermined power rating is 190 watts. 
     According to a second aspect of the invention, there is provided a method of limiting operating power of an electrical device in a circuit connected to an electrical power source below a predetermined power rating, the method comprising the steps of:
     (a) providing and connecting a switching device with said electrical device for adjusting electrical power supplied from said power source to said electrical device;   (b) sensing a parameter relating to the operating power of said electrical device;   (c) using the parameter sensed by the sensor to identify an undesirable operating condition in which said electrical device operates at a power exceeding the predetermined power rating; and   (d) in response to identifying such an undesirable operating condition, controlling the switching device to adjust electrical power supplied to said electrical device to below the predetermined power rating, while maintaining an electrical current flow through said electrical device.   

     Preferably, step (a) includes using a relay as the switching device and arranging the relay to switch between at least two first and second circuits, with the first circuit being for normal connection of said electrical device to said power source, and with the second circuit being for connection of said electrical device to said power source upon identifying of said undesirable condition. 
     More preferably, step (a) includes providing the first circuit with relatively low resistance for normally connecting said electrical device to said power source, and providing the second circuit with relatively large resistance for connecting said electrical device to said power source upon identifying of said undesirable condition to thereby reduce electrical power supplied to said electrical device to below the predetermined power rating. 
     More preferably, step (a) includes using a solid state switch as the switching device, the solid state switch having a control terminal, and using the relay to switch between the first and the second circuits of different resistances to control duty cycle of conduction of the solid state switch. 
     Further more preferably, step (a) includes using a triac having a control terminal as the solid state switch and controlling the control terminal by a capacitor charging via one of the first and second circuits as switched in by the relay. 
     It is preferred that step (d) comprises controlling the switching device to reduce electrical power supplied to said electrical device to below the predetermined power rating. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The invention will now be more particularly described, by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1  is a bottom perspective view of an electric ceiling fan including a light kit that incorporates a protection circuit in accordance with the invention; 
         FIG. 2  is a top perspective view of the ceiling fan of  FIG. 1 , showing the protection circuit which is housed in a case located at an upper end of a mounting shaft of the ceiling fan; 
         FIG. 3  is an enlarged perspective view of the upper end of the mounting shaft of  FIG. 2 , showing the case more clearly; 
         FIG. 4  is an exposed perspective view of the case of  FIG. 3 , revealing the protection circuit inside; 
         FIG. 5  is a schematic circuit diagram of a first embodiment of the protection circuit of  FIG. 4 ; 
         FIG. 6  is a schematic circuit diagram of a second embodiment of the protection circuit of  FIG. 4 ; 
         FIG. 7  is a flow chart illustrating the operation of the protection circuits of  FIGS. 5 and 6 ; 
         FIG. 8  is a schematic circuit diagram of a third embodiment of the protection circuit of  FIG. 4 ; 
         FIG. 9  is a flow chart illustrating the operation of the protection circuit of  FIG. 8 ; 
         FIG. 10  is a schematic circuit diagram of a fourth embodiment of the protection circuit of  FIG. 4 ; 
         FIG. 11  is a flow chart illustrating the operation of the protection circuit of  FIG. 10 ; 
         FIG. 12  is a schematic circuit diagram of a fifth embodiment of the protection circuit of  FIG. 4 ; 
         FIG. 13  is a flow chart illustrating the operation of the protection circuit of  FIG. 12 ; 
         FIG. 14  is a schematic circuit diagram of a sixth embodiment of the protection circuit of  FIG. 4 ; 
         FIG. 15  is a flow chart illustrating the operation of the protection circuit of  FIG. 14 ; 
         FIGS. 16A to 16N  are waveform diagrams of the operating voltage of the light kit, at progressively reducing duty cycles resulting from phase control implemented by the protection circuit of  FIG. 4 ; and 
         FIGS. 17 and 18  are tables showing the relationship between the duty cycle of the operating voltage of the light kit and the power consumed thereby. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring first to  FIGS. 1 to 5  and  7  of the drawings, there is shown an electrical device i.e. electric ceiling fan  10  which incorporates a protection circuit  100  embodying the invention. The ceiling fan  10  is of the typical construction including a motor housing  11  that is suspended from the ceiling by a vertical mounting shaft  12 , a fan motor  13  in the housing  11  and driving four or five horizontal fan blades  15 , and a switch cup  14  mounted below the motor  13 . 
     The ceiling fan  10  of the type concerned includes at its bottom a light kit L for adding ambient light to the room, which is mounted on the switch cup  14 . As is generally known in the art, the light kit L is formed by four (for example) lamp sockets  4  each for holding an incandescent light bulb  5 . A pair of electric cables  2  connects the lamp sockets  4  to the mains AC power source M via the subject protection circuit  100  and an ON/OFF switch (not shown) on the switch cup  14  for turning on/off the light kit L. 
     For environmental protection or safety reasons or to meet certain safety regulations, there is a limit in the power rating or operating wattage for the light kit L, i.e. all the light bulbs  5  together, an overload limit that one should observe. The wattage limit is, for example, 190 W as set by the relevant authority, and this translates into a maximum load current of about 1.65 A for a mains voltage of 115V (110-120V). To meet this requirement, standard 40 W bulbs may be used as the light bulbs  5 . 
     Due to negligence or lighthearted violation, some people may use light bulbs of an excessive wattage (each at e.g. 60 W or even 100 W) for brighter lighting. This is not only a breach of regulations but can indeed be hazardous. The protection circuit  100  serves to automatically restrict the operation of the light kit L to below the aforesaid 190 W wattage rating. 
     In the circuit diagram of  FIG. 5 , as from the mains power source M to the light kit L, the protection circuit  100  comprises a DC power unit  110 , a load voltage sampling unit  120 , a voltage comparator  130 , a signal latching unit  140  and a light dimmer  150 . 
     The DC power unit  110  is connected across the live and neutral of the mains power source M. It may be implemented by a diode bridge with appropriate filtering capacitors as is generally known in the art, and it provides 24V DC for the light dimmer  150  and 12V DC for the other components, or any other DV voltage(s) as appropriate. 
     The load voltage sampling circuit  120  samples the load voltage by utilizing a very small resistor R 1  (e.g. 0.04Ω) connected in series between the mains live and the load (i.e. the light kit L) as a current sensing device to monitor the load current, and a resistor R 2  and variable resistor R 3  in series to provide the sampled load voltage. 
     The potential difference developed across the resistor R 1  is dependent upon the load current, which in turn reflects the prevailing operating power of the light kit L. 
     The load voltage across the light kit L at zero (or near-zero) load current is practically the mains voltage, and the latter is thus used as a reference voltage. By comparing the sampled load voltage with the mains voltage (i.e. the reference voltage) using the voltage comparator  130 , the loading condition (i.e. the operating power) of the light kit L can be determined and monitored. 
     The voltage comparator  130  has a first input connected to the resistor R 2 /R 3  for sampling the load voltage and a second input connected to the mains live for obtaining the reference mains voltage. By comparing the load voltage with the mains voltage, the voltage comparator  130  continuously monitors the operating power of the light kit L. 
     The voltage comparator  130  is tuned by adjusting the variable resistor R 3  such that it will provide a high output signal (e.g. rising from logic low to logic high like the rising edge of a pulse signal) immediately upon the load current exceeding the limit of 1.65 A (i.e. the light kit L operating at a power above the limit of 190 W), for as long as such a condition prevails. This is an undesirable condition in which the light bulbs  5  draw too large a load current (because of their excessive wattage), whereupon the light dimmer  150  is tripped to mitigate possible dangers. 
     Looking at the current sensing resistor R 1  specifically, its opposite ends are connected to the two inputs of the voltage comparator  230  respectively, whereby the comparator  230  compares the potential difference developed across the resistor R 1  by the load current to monitor and identify the undesirable over-power operating condition by reference to current as the parameter for consideration. 
     The light dimmer  150  is implemented by a relay  151  and a resistor R 4 . The relay  151  is of the electromagnetic type, though it can be of the solid state type. It has a SPDT (single-pole double-throw) switch  152  which is connected in series with the resistor R 4  across two circuit nodes N 1  and N 2  in the live circuit between the load voltage sampling unit  120  and the light kit L. The switch  152  is operated by an electromagnet  153  of the relay  150  for, in a normal position, shorting the circuit between the light kit L and the mains power source M and, in a tripped position, inserting the resistor R 4  into the said circuit. 
     The signal latching unit  140  has an output connected to the electromagnet  153  for passing onto it a said output signal of the voltage comparator  130  so as to toggle the switch  152  into the tripped position. Such a switching action will be performed almost immediately upon the voltage comparator  130  detecting the light kit L operating at a power level exceeding 190 W. 
     In the tripped position of the switch  152  in comparison with its normal condition, the electrical current through the light kit L is considerably restricted by the resistor R 4  connected in series, whereby the light bulbs  5  can only run at a much reduced power level below 190 W. 
     The signal latching unit  140  serves to latch and hold the comparator&#39;s output signal once it appears, and this ensures that the relay  151  will stay activated and in turn the light dimmer  150  and hence the overall protection circuit  100  will stay tripped once it has been tripped, even after the load current has been considerably reduced. 
     User actions should be taken afterwards, i.e. replacing the light bulbs  5  with lower power ones and then re-switching on the power to the light kit L, whereupon the protection circuit  100  will be reset and then allow normal operation of the light kit L below 190 W. 
     The operation described above is summarized by the flow chart of  FIG. 7 . From start (step  101 ) in about 0.5 s after power has been switched on (long enough for the incandescent load inrush current to settle, usually in about 0.3 s), the voltage comparator  130  determines whether the light bulbs  5  are operating at a power above 190 W (step  102 ). If this is not the situation, no control on the light bulbs  5  is required and exercised and their light intensity remains unchanged (step  103 ). 
     In case the operating power exceeds 190 W, the signal latching unit  140  holds a high output signal “ON” from the voltage comparator  130  (step  104 ), and the light dimmer  150  kicks in and restricts the operating power of the light kit L, thereby lowering the intensity of the light bulbs  5  (step  105 ). 
     The light dimmer  150  operates based on resistance. To avoid excessive rise in the temperature of the resistor R 4 , its resistance should be large enough (e.g. 100 kΩ) to adequately limit the load current. In this embodiment, the load current will be reduced to such a low level that the light bulbs  5 , though still running, will emit light that is barely noticeable, especially in a not-too-dark ambiance. 
     It is worth noting that the light bulbs  5  will not be switched off or disconnected, nor are they intended to be so, at the end of operation of the protection circuit  100 . A small, non-zero electrical current flow through the light bulbs  5  will be maintained, for example to provide a light indication that the protection circuit  100  has tripped or the bulbs  5  are of a power rating that is too high. 
     This situation is less than ideal because the light kit L may seem malfunctioning (i.e. not working at all) in the tripped condition in which the light bulbs  5  appear unlit, though the truth of the matter is that they are merely unfit for use i.e. too high power. 
     A second protection circuit  200  of  FIG. 6  embodying the invention is free from such a shortcoming. This protection circuit  200  has a similar circuit design and operation as the first protection circuit  100 , with the majority of equivalent components designated by the same reference numerals increased by 100, except for reference signs comprising alphabets that remain the same. 
     The only major difference lies in the light dimmer  250 , which operates based on phase control over the AC power waveform instead of resistance. Apart from the relay  251  and resistor R 4  (4.7 kΩ), the light dimmer  250  includes another resistor R 5  (190 kΩ), a triac  254 , a diac  255  and a capacitor C 1 . The triac  254  and diac  255  are both solid state switching devices. 
     The switch  252  of the relay  251  is connected with its COM terminal to the circuit node N 1  and with its L 1  and L 2  terminals to the other circuit node N 2  via the two resistors R 4  and R 5  respectively and then the capacitor C 1 . The triac  254  is connected across the circuit nodes N 1  and N 2  (in the live circuit between the load voltage sampling unit  220  and the light kit L), with its gate electrode (i.e. control terminal) connected via the diac  255  and the capacitor C 1  to the circuit node N 2 . 
     The triac  254  controls by connecting and disconnecting the live circuit between the power source M (via the load voltage sampling unit  220 ) and the light kit L. It conducts load current in either direction after it has been triggered (turned on) by either a positive or a negative trigger voltage pulse being applied to its gate electrode. Once triggered, the triac  254  will continue to conduct until the current through it drops below a certain threshold value, i.e. at the end of each half cycle of the AC power. 
     The diac  255  serves to apply the positive/negative trigger pulse to the gate electrode of the triac  254  based on the voltage developed across the capacitor C 1 . In operation, the capacitor C 1  will be charged (for applying the said positive/negative trigger pulse) and then discharge (rapidly via the gate electrode of the triac  254 ) repeatedly in synchronism with, and during each of, the half cycles of the AC voltage. 
     The capacitor C 1  charges up via either one of the resistors R 4  and R 5 , depending on the switching position of the relay  251 . The diac  255  will conduct to apply a trigger or firing pulse upon the capacitor voltage exceeding the diac&#39;s breakdown voltage, whereby the triac  254  is turned on to conduct the load current through to the light kit L, until the relevant half cycle of the AC power ends. 
     It is understood that a triac with a built-in diac at its gate may be employed in place of the triac  254  and diac  255  for simplicity. 
     In the normal position of the relay switch  252 , the resistor R 4  is connected (as shown), whose resistance (4.7 kΩ) is sufficiently small for the capacitor C 1  to charge up rapidly to the breakdown voltage of the diac  255 . This causes the triac  254  to begin conduction almost at start of each half cycle, whereby the triac  254  conducts practically continuously (i.e. at 100% duty cycle of conduction) and the light kit L operates at maximum power. 
     In case the light bulbs  5  are over-powered and together consume power that exceeds the predetermined limit of 190 W, this will immediately be detected by the voltage comparator  230  based on sampled data from the load voltage sampling unit  220 . The voltage comparator  230  will then generate an output signal, maintained by the signal latching unit  240 , for the light dimmer  250 . Upon the light dimmer  250  being triggered, the relay switch  252  changes to the tripped position thereby switching in or connecting the resistor R 5 , in replace of the resistor R 4 , to the capacitor C 1 . 
     Because the resistor R 5  is of a considerably larger resistance (190 kΩ), the capacitor C 1  will take much longer to charge up to the breakdown voltage of the diac  255 . This causes the triac  254  to begin conduction with a delay from the start of each half cycle, whereby the triac  254  conducts only over part of the time in each half cycle (e.g. at 25% duty cycle of conduction) and hence the light kit L operates at a reduced power (i.e. 9.09% of the maximum power). 
     The resistance of the resistor R 5  is chosen to give the light kit L a reduced power based on two criteria. First, the reduction in power should be adequate to allow for the highest power light bulbs available on the market that might be used. Second, the light bulbs  5  in question would still appear lit or glow in the normal ambient lighting condition for the use of the light kit L, as a noticeable indication that the light bulbs  5  are unfit (i.e. too high powered) for use and should therefore be replaced. 
     The operation of the protection circuit  200  described above is summarized in the flow chart of  FIG. 7 , as discussed in relation to the first protection circuit  100 , with equivalent steps designated by the same reference numerals increased by 100 i.e. steps  201  to  205 . Description is not repeated here for clarity. 
     By selecting the resistance of the resistor R 5 , a firing pulse can be applied at a certain point in each AC cycle (i.e. phase control) of the AC power. This allows one to predetermine the percentage of current that flows via the triac  254  through to the light bulbs  5  in case of an over-power but tripped condition, thereby providing a noticeable dimmed light indication to call for bulb replacement. 
     However, the user is unable to control or maximize the intensity of the dimmed light indication, for example to suit the prevailing ambient lighting condition. Although it is possible to employ manual means e.g. a variable resistor for the resistor R 5 , adjustment can be cumbersome to put in place or to use. 
     Reference is now made to  FIGS. 8 and 9 , showing a third protection circuit  300  embodying the invention, in which the intensity of the dimmed light signal for indicating use of over-power light bulbs is self controllable. This protection circuit  300  has a similar design and operation as the second protection circuit  200 , with the majority of equivalent components designated by the same reference numerals increased by 100, except for the reference signs comprising alphabets that remain the same. 
     There are a couple of principal differences. The first difference resides in the light dimmer  350 , which although operates likewise based on the principle of phase control it is capable of finer adjustment. The other difference is the use of a counter  360  as a controller that automatically adjusts the light dimmer  350 , and this necessitates the use of a pulse generating unit  340  in place of the aforesaid signal latching unit for pulse triggering, in succession if appropriate, the counter  360  in a timed manner. 
     The light dimmer  350  includes similar triac  354 , diac  355  and capacitor C 1 , likewise connected in the same way relative to the circuit nodes N 1  and N 2  as the second embodiment  200 . The previous relay and associated resistors are replaced by four triacs  356 . 1  to  356 . 4  (collectively  356 ) with respective resistors R 4  to R 7  connected in series. The triacs  356  with respective resistors R 4  to R 7  are connected in parallel from the mains live to the capacitor C 1  to provide four different resistance paths which are selectable as alternatives for the charging of the capacitor C 1  (to the breakdown voltage of the diac  355 ). 
     Each of these four resistance paths is controlled by means of the gate electrode of the corresponding triac  356 , and the four gate electrodes act as separate control inputs IN 0  to IN 3  of the light dimmer  350 . By applying an appropriate pulse signal at one of the control inputs, the corresponding triac  356  will be turned on to render the associated path conductive, thereby switching in the relevant resistor R 4 /R 5 /R 6 /R 7  to allow the capacitor C 1  to charge up via that resistor. 
     The time it will take for the capacitor C 1  to charge up during each half cycle of the AC power to the breakdown voltage of the diac  355  is dependent upon the resistance (i.e. 4.7 kΩ/40 kΩ/125 kΩ/190 kΩ) of the resistor R 4 /R 5 /R 6 /R 7  that is connected for capacitor charging. Upon breakdown of the diac  355 , the capacitor C 1  applies a firing pulse to the triac  354  to turn on the same. 
     Thus, by switching the resistors successively from R 4  to R 7 , the capacitor C 1  will take a progressively longer period of time to charge up to cause breakdown of the diac  355  during each half cycle of the AC power, thereby providing corresponding timed firing pulses for the triac  354 . The triac  354  will then be conducting for a complementary progressively shorter period of time over each half cycle of the AC power, whereby the light lit L will consume less power and its intensity reduced accordingly. 
     The shortening periods of time in each half cycle of the AC power during which the triac  354  conducts are illustrated in  FIGS. 16A ,  16 E,  16 I and  16 M, from 100%, 75%, 50% to 25% duty cycle of conduction, with relationship between such duty cycles and the power consumption listed in  FIG. 17 . 
     The counter  360  may be an IC decade counter chip which has a trigger input (i.e. clock pin) and four outputs Q 0  to Q 3  (i.e. first four of ten output pins being used). By having its outputs Q 0  to Q 3  connected to the inputs IN 0  to IN 3  of the light dimmer  350  respectively, each via a respective series circuit of a diode D 0 /D 1 /D 2 /D 3  and a resistor R 8 /R 9 /R 10 /R 11  (820Ω), the counter  360  controls the triacs  356 . 1  to  356 . 4  and hence corresponding 1 st  to 4 th  resistance paths for the charging of the capacitor C 1 . 
     The counter  360  is designed, each time upon receiving a trigger pulse at its trigger input, to shift logic high from one of its outputs Q 0  to Q 3  to the next, i.e. counting. Such a counting operation will start with the first output Q 0  (i.e. the initial state) to the second output Q 1 , then to the third output Q 2  and finally to the fourth output Q 3 , whereupon counting will be terminated, e.g. further counting disabled, by means of a disable pin (negation of enable as shown in  FIG. 8 ) of the counter chip. 
     The pulse generating unit  340  is configured to provide a series of the aforesaid trigger pulses to the counter  360 , at predetermined regular time intervals each of half a second (0.5 s), upon or while receiving an output signal from the voltage comparator  330  detecting that the light kit L consumes more than 190 W power, for as long as such an operating condition persists. 
     As explained earlier, this occurs immediately when the voltage comparator  330  determines, based on the load current/voltage detected by the sampling unit  320 , that the light kit L operates at a power level above 190 W or draws a current larger than 1.65 A. If such an overload condition persists, the pulse generating unit  340  will provide a further trigger pulse every subsequent half-a-second time interval until the overload condition is rectified. 
     At the outset, the counter&#39;s first output Q 0  is at logic high and this activates or turns on the first triac  356 . 1  (i.e. the 1 st  resistance path) to switch in the resistor R 4  (4.7 kΩ) for rapid charging up of the capacitor C 1  to the breakdown voltage of the diac  355 . This causes the triac  354  to begin conduction almost at start of each half cycle, whereby the triac  354  conducts practically at 100% duty cycle of the AC power ( FIG. 16A ) and the light kit L operates at maximum power. 
     In the event that the light bulbs  5  are too powerful and draw excessive current (larger than 1.65 A) or power (higher than 190 W), this is detected by the voltage comparator  330 , which in turn triggers the pulse generating unit  340  to provide a first trigger pulse for the counter  360  to adjust the light dimmer  350 . The counter  360  advances one count and shifts the logic high from its first output Q 0  to its second output Q 1 , thereby turning on the second triac  356 . 2  of the light dimmer  350  (i.e. the 2 nd  resistance path) instead to switch in the resistor R 5  (40 kΩ) for slower charging of the capacitor C 1 . This causes the triac  354  to start conduction with a delay of 25% of the time over each half cycle, whereby the triac  354  conducts at 75% duty cycle of the AC power ( FIG. 16E ), and the light kit L operates at a lower power reduced by one step (i.e. 90.92% of the maximum power). 
     The voltage comparator  330  continuously monitors the load voltage/current. If the load current still exceeds 1.65 A, the pulse generating unit  340  will generate a second trigger pulse for the counter  360  half a second after the first. The counter  360  then advances another one count and shifts the logic high to its third output Q 2 , thereby activating the third triac  356 . 3  (i.e. the 3 rd  resistance path) to switch in the resistor R 6  (125 kΩ) for further slower charging up of the capacitor C 1 . This causes the triac  354  to begin conduction with a delay of 50% of the time over each half cycle, whereby the triac  354  conducts at only 50% duty cycle of the AC power ( FIG. 16I ) and the light kit L consumes 50% of the maximum power, at a power reduced by another step. 
     The aforesaid operations will be repeated once more, for the last time, a further 0.5 s later if the load current still exceeds 1.65 A. Thus, the counter  360  will shift the logic high to its fourth output Q 3  to turn on the fourth triac  356 . 4  (i.e. the 4 th  resistance path), thereby switching in the largest resistor R 7  (190 kΩ) to further slow down charging of the capacitor C 1 . The triac  354  will begin conduction with a delay of 75% of the time over each half cycle, such that the triac  354  will conduct at only 25% duty cycle of the AC power ( FIG. 16M ) and the light kit L operates at minimum power (i.e. reduced by yet another step to 9.09% of the maximum power). 
     Overall, the light dimmer  350  will gradually reduce the intensity of the light bulbs  5  in steps, every half a second. Depending on the original power rating of the light bulbs  5 , the load current may drop below the 1.65 A limit after the counter  360  has advanced only once (current reduced to 75% duty cycle), or twice (to 50% duty cycle), without the need for the current to further reduce down to 25% duty cycle. 
     Under the control of the counter  360  the light dimmer  350  gradually kicks in to reduce the load current in steps, and the interim load condition is then analyzed to determine if a further operation of the light dimmer  350  is required. This is done to avoid unnecessarily over reducing the load current after the protection circuit  300  has tripped, with a view to ensuring that the dimmed light of the light bulbs  5  will be noticeable (i.e. sufficiently lit or glowing) as a signal calling for bulb replacement. 
     The operation described above is summarized by the flow chart of  FIG. 9 . From start (step  301 ) in about 0.5 s after power has been switched on (long enough for incandescent load inrush current to settle, usually in about 0.3 s), the voltage comparator  330  determines whether the light bulbs  5  are operating at a power above 190 W (step  302 ). If this is not the situation, no control on the light bulbs  5  is required and their light intensity remains unchanged (step  303 ). In case the operating power exceeds 190 W, the pulse generating unit  340  outputs a trigger pulse (step  304 ) for the counter  360 , which thus advances one count or adds one to a stored value (step  305 ). The light dimmer  350  then kicks in and reduces the light intensity (step  306 ) by shortening the duty cycle of the AC power by 25%. The operation will subsequently loop back (via path  307 ) and be repeated from start (step  301 ) to check whether the operating power still exceeds 190 W (step  302 ), and so forth. One or more loops may be necessary to bring the operating power down below 190 W. 
     To maximize the load current after the protection circuit has tripped for a brightest possible dimmed light signal calling for bulb replacement, a more delicate circuit and control are required. 
     Reference is now made to  FIGS. 10 and 11 , showing a fourth protection circuit  400  embodying the invention, in which the intensity of the dimmed light signal to indicate use of over-power light bulbs can be maximized. This protection circuit  400  has a similar design and operation as the third protection circuit  300 , with the majority of equivalent components designated by the same reference numerals increased by 100, except for the reference signs comprising alphabets that remain the same. 
     Although the operation of the light dimmer  450  is also based on the principle of phase control over the conduction of triac  454 , the phase control in this embodiment does not rely on the charging/discharging of a capacitor in an analogue manner, but instead it is performed digitally by means of a single IC (integrated circuit) controller chip  470 . 
     The light dimmer  450  is implemented by the triac  454 , whose gate electrode is connected to the output of the controller chip  470  via diode D 1  and resistor R 4  for direct control by the controller chip  470 . The triac  454  connects and applies the mains live appearing downstream of the load voltage sampling unit  420  to the light kit K, with phase control over the applied load voltage. 
     The controller chip  470  is fabricated to include the pulse generating unit  440  and the counter  460 , both of which find equivalent counterparts in the third protection circuit  300  though the earlier ones are discrete components. The pulse generating unit  440  similarly, upon detection by the voltage comparator  430  that the light kit L consumes more than 190 W power or draws a current larger than 1.65 A (i.e. an overload condition), provides a series of trigger pulses for the counter  460  at predetermined regular time intervals (i.e. 0.1 s), for as long as such an operating condition persists. The time intervals in this case are however much shorter i.e. 0.1 s, as digital components act a lot faster than analogue ones. The counter  460  is a binary counter having four output pins Q 0  to Q 3 . 
     The controller chip  470  further includes a pulse output unit  471  at its output for providing firing pulses to turn on the triac  454 , and a timer  472  connected to the pulse output unit  471  for determining the precise moment when each firing pulse from zero crossing is outputted. Also included is a zero crossing detector  473  for resetting the pulse output unit  471  and the timer  472 . The zero crossing detector  473  has an input connected to the mains live for detecting zero crossings in the live/load voltage (i.e. start of every half cycle) and, in response, outputting a reset signal to reset both the pulse output unit  471  and the timer  472  at each zero crossing. 
     The binary counter  460  is programmed to advance one count and output the resultant count at its output pins Q 0  to Q 3 , each time when it receives a trigger pulse from the pulse generating unit  440 , i.e. every 0.1 s since detection of an overload condition. 
     The control signal is a four-digit binary number with a value ranging from “0000” (i.e. the initial state) to “1111”. There are sixteen possible values in total, each representing a respective firing angle in each half cycle (180°) of the AC power, at which the triac  454  should be turned on under the control of the controller chip  470  to deliver a corresponding level of power. 
     The timer  472  determines the firing angle according to the control signal (i.e. its value) by computing the delay in time from zero crossing in each half cycle (180° ) of the AC power when a firing pulse is to be issued to turn on the triac  454 . The firing angles are arranged sequentially at regular intervals apart, progressively increasing from 0° (i.e. the initial state at maximum power), 11.25°, 22.5°, . . . , 112.5°, 123.75° 0  to 135° (see  FIGS. 16A to 16M ), in steps of 11.25°. The incremental steps are constant i.e. 11.25°, and this is equivalent to 6.25% in the duty cycle of the AC power. 
     Taking 60 Hz as the utility/mains frequency for example, the period of each cycle (360°) of the AC power is 1/60 second. Each step of 11.25° in the phase angle thus occupies a time interval of 0.5208 ms (millisecond), which is to be adopted as one unit of delay in time from zero crossing (multiple units anticipated) before issuing a thus timed firing angle for the triac  454 . 
     In this embodiment, the largest firing angle intended for use is 135° as this corresponds to a 25% duty cycle of the AC power, which represents a power reduction to 9.09% and this is already sufficiently low to allow for use of highest possible power light bulbs in the light kit L. Thus, only the first thirteen firing angles are being made use of. 
     In the default initial state, the binary counter  460  presents “0000” as the control signal that calls for a firing angle of 0°, at which the pulse output unit  471  issues a firing pulse to turn on the triac  454  of the light dimmer  450  almost at zero crossing i.e. start of each half cycle of the AC power. The triac  454  thus conducts practically at 100% duty cycle of the AC power ( FIG. 16A ), and the light kit L operates at maximum power. 
     In the event that the light bulbs  5  are too powerful and draw an excessive current (larger than 1.65 A) or power (higher than 190 W), this is detected by the voltage comparator  430 , which in turn triggers the pulse generating unit  440  to issue a first trigger pulse for the binary counter  460  to advance one count to “0001”. This count represents the next firing angle that is increased by one step from the earlier angle i.e. to i.e. 11.25°, under the timing control of the timer  472 . The pulse output unit  471  then turns on the triac  454  at 93.75% (reduced by 6.25%) duty cycle of the AC power ( FIG. 16B ), and the light kit L operates at a correspondingly reduced power. 
     If the load current/power remains excessive over (or at the end of) the predetermined 0.1 s time interval, the operation described in the preceding paragraph is repeated. Accordingly, the pulse generating unit  440  issues another trigger pulse for the binary counter  460  to advance another count to “0010”. This count represents the next firing angle that is increased by one step from the earlier angle i.e. to 22.5°, again under the timing control of the timer  472 . The pulse output unit  471  then turns on the triac  454  at 87.5% (reduced by another 6.25%) duty cycle of the AC power ( FIG. 16C ), and the light kit L operates at a correspondingly further reduced power. 
     The operation will be repeated again a further period of 0.1 s later, or every 0.1 s time interval if repeated more than once, until the power consumption of the light kit L reduces down to below 190 W. When the power consumption comes within limit, the protection circuit  400  maintains the prevailing operating condition. More specifically, the voltage comparator  430  issues no further trigger for the controller chip  470 , which then continues to operate with no further changes and in particular maintaining the firing angle for the triac  454  to keep the existing duty cycle of the AC power at which the light kit L operates. 
     The whole process of operation of the protection circuit  400  will be performed each time the light kit L is switched on. The light bulbs  5 , upon being switched on, are permitted to lit up to maximum intensity. In about half a second after the power has been switched on (long enough for incandescent load inrush current to settle, usually in about 0.3 s), if the light bulbs  5  draw excessive power/current, they will be dimmed gradually from maximum intensity in one or more steps (each 0.1 s) over a short period of time of say a few tenths of a second (depending on how much the original operating power is in excess), until their power consumption is reduced to within limit. The whole process will usually take no longer than a second or so. 
     The use of digital means, and the timer  472  and pulse output unit  471  in particular, allows more delicate and much finer and swifter control over the firing angle for the light dimmer  450  and in turn the power consumption (and hence brightness) of the light bulbs  5 . Compared with the earlier embodiments, considerably more power levels are available for gradually adjusting the operating power of the light bulbs  5 , and at a much faster pace, such that their brightness can be maximized, and in a shorter period of time, when their operating power is brought down to within the limit of 190 W. 
     At the end of the operation of the protection circuit  400  (in say one to two seconds from switch-on), the light bulbs  5  will be operating at a power not-too-far below the limit of 190 W, such that they would be sufficiently bright for illumination purpose. In this case, there would be no need at all to replace the light bulbs  5 . The dimming of the light bulbs  5  immediately following switch-on would be noticeable as a signal that the bulbs  5  are of a power rating that is too high, though the signal can be disregarded because power consumption has automatically been capped and the reduced brightness would still be adequate. 
     The operation described above is summarized by the flow chart of  FIG. 11 . From start (step  401 ) in about 0.5 s after power has been switched on, the voltage comparator  430  determines whether the light bulbs  5  are operating at a power above 190 W (step  402 ). If this is not the situation, no control on the light bulbs  5  is required and their light intensity remains unchanged (step  403 ). In case the operating power exceeds 190 W, the pulse generating unit  440  outputs a trigger pulse (step  404 ) for the counter  460 , which thus advances one count or adds one to a stored value (step  405 ). 
     The new count causes the timer  472  to add one unit of delay in time (i.e. 0.5208 ms) corresponding to one step of increment in the firing angle (i.e. 11.25°) for the triac  454  of the light dimmer  450  (step  406 ). Upon detection of each zero crossing in the AC power (i.e. start of each half cycle) by the zero crossing detector  473 , the timer  472  runs the total delay in time, at the expiration of which the timer  472  triggers the pulse output unit  471  (step  407 ) to issue a firing pulse and turn on the triac  454  for conduction over the remainder of the relevant AC power half cycle. The light dimmer  450  thus reduces the intensity of the light bulbs  5  by shortening the duty cycle of the AC power by one step of 6.25%. 
     The operation will subsequently loop back (step  408 ) in 0.1 s and be repeated from start (step  401 ) to check whether the operating power still exceeds 190 W (step  402 ), and so forth. One or more loops, each over the predetermined time interval of 0.1 s, may be necessary to bring the operating power gradually down to below 190 W. 
     Because the power and hence intensity of the light bulbs  5  is gradually reduced in finer steps (by increasing the triac firing angle incrementally), the light bulbs  5  will remain sufficiently bright after the operating power (or load current) has been capped. This avoids the need to replace the light bulbs  5 . 
     The whole process of operation of the protection circuit  300 / 400  will be performed each time the light kit L is switched on. The light bulbs  5  will lit up to maximum intensity upon being switched on. If the light bulbs  5  exceed the designated power limit, they will be dimmed from maximum intensity gradually in one or more steps (each 0.5 s/0.1 s) over a short period from half a second to a couple of seconds at most, until they consume power below the limit. 
     The two protection circuits  300 / 400  operate by reducing gradually, from maximum power, the power consumption by the light kit L in steps to bring it down to below the power limit. Conversely, it is possible to adjust (increase) the power consumption gradually from minimum power, as is the case with the fifth and sixth protection circuits  500  and  600  also embodying the invention. 
     The fifth protection circuit  500  is shown in  FIGS. 12 and 13 , and it has a similar design similar to that of the third protection circuit  300 , with the majority of equivalent components designated by the same reference numerals increased by 200, except for the reference signs comprising alphabets that remain the same. 
     As one major difference, the protection circuit  500  includes a “one” pulse generating unit  541  which is connected between the voltage comparator  530  and the counter  560 , in parallel with the pulse generating unit  540 . Both units  540  and  541  are controlled by a common same output of the voltage comparator  530 , but the latter operates in a different manner. 
     The voltage comparator  530  is arranged to provide a reversed, low output signal (i.e. falling from logic high to logic low like the falling edge of a pulse signal) upon detecting the light kit L operating at a power below the limit of 190 W, for as long as such an under-power operating condition prevails. The pulse generating unit  540  is activated in response to such a low output signal. 
     When the light kit L changes to operate above 190 W, the voltage comparator  530  will then provide a high output signal (i.e. rising from logic low to logic high like the rising edge of a pulse signal). The “one” pulse generating unit  541  is activated in response to such a high output signal. 
     Another major difference lies in the arrangement of resistances in the four resistance paths of the light dimmer  550 , in that the resistors R 4 , R 5 , R 6  and R 7  in the 1 st  to 4 th  resistance paths are arranged to have reducing resistances from 190 kΩ, 125 kΩ, 40 kΩ to 4.7 kΩ respectively. This is directly opposite to the increasing resistance arrangement of the 1 st  to 4 th  resistance paths in the third protection circuit  300 . 
     Upon switch-on of the light kit L, the counter  560  has its first output pin Q 0  at logic high (i.e. the initial state) to activate the 1 st  resistance path (including the resistor R 4  of 190 kΩ) of the light dimmer  550 , whereby the light kit L starts operation at minimum power, i.e. 25% duty cycle of the AC power (step  501 ). While the light kit L operates below 190 W as detected by the voltage comparator  530  (step  502 ), the pulse generating unit  540  is activated. 
     The counter  560  will then count up one count for one time, or for more than one time i.e. repeated every 0.5 s if necessary (step  507 ), by successively shifting its logic high signal to the next output pin Q 1 /Q 2 /Q 3  to increase the operating power of the light kit L to 50%/75%/100% duty cycle, thereby progressively increasing its lighting intensity (steps  504  to  506 ). This operation will stop as soon as the power consumption of the light kit L exceeds 190 W, in case the light bulbs  5  have a power rating that is too high. 
     Accordingly, the counter  560  starts operation in the same manner as the earlier counterpart under the control of the pulse generating unit  540 , but it adjusts (increases) the power consumption gradually from minimum power until the light kit L consumes more than 190 W. 
     When this occurs (step  502 ), the “one” pulse generating unit  541  is then activated instead to trigger the counter  560  to count down one count once by shifting its logic high signal one output pin back, to the immediately preceding output pin (steps  503  and  508 ). The operating power of the light kit L is thus adjusted back (i.e. reduced) by 25% to bring it down to within one step below 190 W, whereby the lighting intensity is reduced by one step (step  509 ), and the operation finally ends (step  511 ). 
     Referring finally to the sixth protection circuit  600  shown in  FIGS. 14 and 15 , it has a similar design similar to that of the fourth protection circuit  400 , with the majority of equivalent components designated by the same reference numerals increased by 200, except for the reference signs comprising alphabets that remain the same. 
     As one major difference, the protection circuit  600  includes a “one” pulse generating unit  641  which is connected between the voltage comparator  630  and the counter  660 , in parallel with the pulse generating unit  640 . Both units  640  and  641  are controlled by a common output of the voltage comparator  630 , but the latter operates in a different manner. 
     The voltage comparator  630  is arranged to provide a reversed, low output signal (i.e. falling from logic high to logic low like the falling edge of a pulse signal) upon detecting the light kit L operating at a power below the limit of 190 W, for as long as such an under-power operating condition prevails. The pulse generating unit  640  is activated in response to such a low output signal. 
     When the light kit L changes to operate above 190 W, the voltage comparator  630  will then provide a high output signal (i.e. rising from logic low to logic high like the rising edge of a pulse signal). The “one” pulse generating unit  641  is activated in response to such a high output signal. 
     The binary counter  660  is programmed to advance one count and output the resultant count at its output pins Q 0  to Q 3 , each time (i.e. every 0.1 s) when it receives a trigger pulse from the pulse generating unit  640  while the light kit L operates below 190 W. Upon receiving a trigger pulse from the “one” pulse generating unit  641  when the light kit L changes to operate above 190 W, the counter  660  will finally reduce its count by one. 
     Another major difference lies in the operation of the timer  672 , which is programmed not only to increase the delay in time from zero crossing when a firing pulse should be issued (i.e. the firing angle) for the triac  654 , but also to reduce the same. The timer  672  has a default initial setting corresponding to a firing angle of 135°, which represents the minimum power at which the light kit L is to operate and to start operation. 
     Upon switch-on (step  601 ), the light kit L operates below 190 W as detected by the voltage comparator  630  (step  602 ). Under the control of the pulse generating unit  640  (step  604 ) via the counter  660  (step  605 ), the timer  672  reduces the firing angle by one step of 11.25° (step  606 ) from 135° (i.e. the initial firing angle). The triac  654  will then be turned on at a reduced firing angle (step  607 ), thereby increasing the intensity of the light kit L by one step (step  609 ). 
     The counter  660  will then count up one count for one time, or for more than one time i.e. repeated every 0.1 s if necessary (step  608 ), to cause the timer  672  to progressively reduce the firing angle from 123.75°, 112.5°, . . . , 22.5°, 11.25° to 0° (see  FIGS. 16M to 16A ) in steps of 11.25°, thereby increasing the operating power and hence intensity of the light kit L. This operation will stop as soon as the power consumption of the light kit L exceeds 190 W, in case the light bulbs  5  have a power rating that is too high. 
     Accordingly, the counter  660  starts operation in the same manner as the earlier counterpart under the control of the pulse generating unit  640 , but it adjusts (increases) the power consumption gradually from minimum power until the light kit L consumes more than 190 W. 
     When this occurs (step  602 ), the “one” pulse generating unit  641  is then activated instead to trigger (step  611 ) the counter  660  to count down one count (step  615 ), thereby causing the timer  672  to increase the firing angle by one step of 11.25° (step  616 ). This restores the firing angle for the triac  654  (step  617 ) to the immediately preceding value at which the light kit L operates just below 190 W, whereby the lighting intensity is reduced by one step back (step  618 ), and the operation finally ends (step  619 ). 
     The whole process of operation of the protection circuit  500 / 600  will be performed each time the light kit L is switched on. The light bulbs  5  will gradually lit up, progressively in steps (each 0.5 s/0.1 s), in one or two seconds from being switched on. If the light bulbs  5  exceed the designated power limit, they will be dimmed by one step back from the brightest intensity just beyond the limit, so that the power consumption is contained within the limit. 
     The invention has been given by way of example only, and various modifications of and/or alterations to the described embodiments may be made by persons skilled in the art without departing from the scope of the invention as specified in the appended claims.