Abstract:
A power limiting circuit for controlling the power of the incandescent lamp not to overrun the rated power limit includes a sampling circuit that consists of two resistors connected in parallel. A control chip compares the received sample voltage with the standard voltage then outputs control signal to drive the conduction of a SCR (silicon controlled rectifier) to change the inner states of contacts of a relay so as to further control the on/off between the input and the output. Based on this control process and technique, this invention prevents the employment of incandescent lamps having a greater power than power rating. Besides, a bi-operational amplifier circuit is used to fulfill the whole examination and control perfectly.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to the electronics field, and more particularly, to a power limiting circuit for controlling the power of incandescent lamp not to overrun the predetermined power rating. 
         [0002]    The conventional circuit for controlling or driving an incandescent lamp is commonly in an absence of the power limitation and protection. When an incandescent lamp is installed, if peripheral power that is greater than the power rating of the incandescent lamp is used by a mistake due to not knowing the type or power of the incandescent lamp, the increase of current will be induced and finally result in the burning out of lamps or wires and even the risk of fire. 
         [0003]    At present, with respect to the above problems, some users provide a new technical solution that a coil is used to control the increase of the power of incandescent lamp. When the power of the incandescent lamp increases, the voltage across the coil will increase, which is fed back to a integrated control circuit, and the circuit will output a control signal to the related peripheral circuit to control the turnoff between the wire and the load so as to protect the incandescent lamp and avoid dangerous emergencies. This technique is widely used nowadays. However, the shortcoming is that the coil can be easily influenced by temperature, when the coil is connected to power supply, the current of the coil will increase, the voltage across the coil will increase due to the rapid increase of temperature, and thus the integrated control circuit would output a false signal to control the turnoff between the output and the load. The false signal will cause the light quenching of incandescent lamp. Moreover, the circuit is lack of temperature compensation circuit and fails to work stably. 
       SUMMARY OF THE INVENTION 
       [0004]    The object of the present invention is to solve the mentioned problems and provide a power limiting circuit having the characteristics of temperature compensation and stable circuit working, wherein a bi-operational amplifier circuit is used to fulfill the whole examination and control perfectly. 
         [0005]    To achieve the above-mentioned object, a power limiting circuit comprises: 
         [0006]    a relay for controlling the on/off of a load; 
         [0007]    a silicon controlled rectifier (SCR) for controlling the switching of the relay between normally open contact (NO) and normally closed contact (NC); 
         [0008]    a sampling circuit for sampling the output loop current; 
         [0009]    a control chip for comparing the voltage value of the sampling circuit with a standard voltage value and triggering the conduction of SCR; 
         [0010]    a low-voltage direct current (DC) power supply for supplying the standard voltage to the control chip; 
         [0011]    wherein the control chip is provided with a standard voltage end and a sampling end, the sampling end is divided into two parts, one part is connected in series to a resistor R 2  and then connected to negative end of the low-voltage direct current power supply, the other part is connected to the sampling circuit that consists of two parallel connected resistors R 9  and R 10 . One end of the sampling circuit is connected in series to resistors R 11  and R 8  in sequence, and then connected to sampling end of the control chip, the other end of the sampling circuit is connected to negative end of the low-voltage direct current power supply. 
         [0012]    The resistor R 8  that is connected to the sampling circuit is a sliding rheostat. 
         [0013]    The standard voltage end of the control chip is connected between the resistors R 4  and R 17 , the other end of the resistor R 4  is connected to the third pin of a regulator U 1  in the power-voltage direct current power, the other end of resistor R 7  is connected to negative end of the low-voltage direct current power. 
         [0014]    The silicon controlled rectifier (SCR) is a single directional silicon controlled rectifier. 
         [0015]    The trigger end of the silicon controlled rectifier is connected to a reset circuit. 
         [0016]    The reset circuit is formed of a reset switch that is connected in series with a resistor and then connected in parallel with another resistor. 
         [0017]    The control chip is a bi-operational amplifier. 
         [0018]    The load is an incandescent bulb. 
         [0019]    The rated limitation power of the load is at least equal to 190 W; the voltage U 3  of the third pin of regulator U 1  is 15V, and the resistances have the following values: R 9 //R 10 =0.16Ω, R 11 +R 8 =3.375 KΩ, R 4 =18 KΩ, R 2 =10K, R 7 =240Ω. 
         [0020]    Comparing with the power limiting circuit of the prior art, the present invention is of the following advantages: 
         [0021]    1. Two resistors in parallel is used as the sampling circuit, so that the examination is only slightly influenced by temperature, and thus the abrupt change of the output voltage to sampling end that cause the control chip to output a false signal to the load could be avoided. 
         [0022]    2. One end of resistor in sampling circuit is connected in series with two protective resistors, so that not only does the sampling result appear precisely but also the current can avoid changing suddenly. It shows a good feature of temperature compensation. 
         [0023]    3. A bi-operational amplifier integrated by two comparators is used as the core control chip to drive the conduction of the single directional SCR, which possesses the merits of accurate examination and control, excellent function, lower price and stable circuit working. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    The structural and operational characteristics of the present invention will be better understood from the following description, relating to the attached drawings which show illustratively but not restrictively examples of the invention. In the drawings: 
           [0025]      FIG. 1  is a circuit diagram according to an embodiment of the present invention. 
           [0026]      FIG. 2  is a circuit principle diagram according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0027]    Referring to  FIG. 1 , a power limiting circuit is provided to control the power of the incandescent lamp not to overrun the predetermined power rating. As shown in  FIG. 1 , one end of Part  1  (alternating current input) is connected to one end of Part  6  (alternating current output) and the input end of Part  2  (direct current supply) and one end of Part  5  (control conversion/reset/action display); the other end of Part  1  (alternating current input) is connected to the input end of Part  3  (sampling); the output end of Part  2  (direct current) is connected to the input end of Part  4  (control signal processing); the output end of Part  4  (control signal processing) is connected to Part  3  (sampling) and Part  5  (control conversion/reset/action display); the output end of Part  3  (sampling) is connected to Part  5  (control conversion/reset/action display); the output end of Part  5  (controlling changeover/reset/action display) is connected to the other end of Part  6  (alternating current output). 
         [0028]    Referring to  FIG. 1  and  FIG. 2 , when the power of the load is smaller than the power rating, an alternating current is inputted by Part  1  (Alternating current input), one end of Part  1  (alternating current input) is connected through a fuse F 1  to one end (or positive end of incandescent lamp) of Part  6  (alternating current output), the other end of Part  1  (alternating current input) is connected in series to resistor R 10  and then to the other end (or negative end of the incandescent lamp) of Part  6  (alternating current output) through the normally closed contact of the relay K 1 . When the electrical power is supplied, the normally open contact of the relay K 1  is closed, and the incandescent lamp is on. 
         [0029]    Contrarily, when the power of the load is greater than power rating, an alternating current is inputted by Part  1  (alternating current input) to Part  2  (direct current supply) to output the direct current for the control chip U 2  and other circuits, and at the same time a standard voltage for comparing is provided, wherein the Part  2  (direct current supply) comprises: a protection circuit consisting of a capacitor C 1  and a resistor R 2 ; a rectification circuit consisting of diodes D 1  and D 2 ; a voltage stabilizing circuit consisting of a Zener diode D 3  and a three-pin regulator U 1 ; a filter circuit consisting of electrolytic capacitors C 2 ,C 3  and a capacitor C 4 . 
         [0030]    Meanwhile, when the power of the load is greater than the power rating, the current connected to output loop of relay K 1  increases accordingly. The current flows through Part  3  (sampling part), the voltage across the sampling circuit which consists of two parallel connected resistors R 9  and R 10  will increase. The sampling circuit is connected in series to a resistor R 11  and a sliding rheostat R 8  in sequence, and then connected to the third pin of control chip U 2 . The third pin is further connected to the resistor R 2  in series, and then connected to the other end of sampling circuit and to the negative end of direct current power. The third pin of regulator U 1  is connected in series to Resistors R 4  and R 7  in sequence and then connected to negative end of the direct current power. The second pin of the control chip U 2  is connected in series between resistors R 4  and R 7 , which provides a standard voltage to the second pin of the control chip U 2  for comparing. The control chip U 2  compares the sampling circuit voltage with the standard voltage, and when the voltage of the third pin is higher than that of the second pin, the seventh pin of the control chip U 2  will output a high level signal which travels through a diode D 4  and a resistor R 13  and then reaches the control pole of the silicon controlled rectifier U 3 . The cathode of the silicon controlled rectifier U 3  is connected to the negative end of resistor R 19 , the anode of the silicon controlled rectifier is connected to the positive end of the direct current power through a LED D 5  and a resistor R 19 . When the voltage signal acts upon the control pole, which triggers the conduction of the silicon controlled rectifier U 3  immediately, the anode of the relay K 1  is connected to the anode of the silicon controlled rectifier U 3 , when the silicon controlled rectifier U 3  becomes conductive, and a certain voltage is produced, the voltage which drives the conduction of the relay K 1  decreases, then the current which flows through the inner coil of the relay K 1  decreases correspondingly, and the electromagnetic suction force will disappear accordingly. The gag bit returns to the original position under the counterforce of the spring so as to cutoff the connection between the relay K 1  and the output or the negative end of the incandescent lamp, then the light of the incandescent lamp quenches thereby. 
         [0031]    When the power of the load recovers within the power rating, by closing a reset switch S 1  in the reset circuit by hand, the voltage in electrolytic capacitor C 16  (the control voltage of silicon controlled rectifier U 3 ) will discharge through resistor R 15 . When the voltage in control end of silicon controlled rectifier U 3  is lower than the conduction voltage of silicon controlled rectifier U 3 , the silicon controlled rectifier U 3  is cut off and the normally open contact will change to the normally closed contact to connect the input to the output, and the load will start to work normally. 
         [0032]    Several embodiments of the present invention in which the power rating is equal to 190 W are described as follow: 
         [0033]    In an embodiment of this invention, the resistors R 9 //R 10 =0.167Ω, R 11 +R 8 =3.375 KΩ, R 2 =10 KΩ, R 7 =240Ω, R 4 =18 KΩ, the voltage U 3 Of the third pin of regulator U 1  is 15V, and the voltage U 2  of the second pin of control chip U 2  is 0.2V. When the power of the load is equal to 190 W, the current of the load is equal to 1.5±0.03 A, the sampling voltage of the third pin of control chip U 2  is higher than the voltage U 2  (0.2V) of the second pin, the seventh pin (output end) of the control chip U 2  outputs a voltage signal to the silicon controlled rectifier U 3  to trigger the conduction of the silicon controlled rectifier U 3 , then the relay K 1  becomes conductive. Due to the fact that the anode of the relay K 1  is connected to the anode of the silicon controlled rectifier U 3  and the silicon controlled rectifier U 3  becomes conductive, the positive end voltage of diode D 8  which is connected in series to the relay K 1  decreases, then the voltage which drives the relay K 1  decreases accordingly, the current which flows through the inner coil of the relay K 1  decreases, so that the electromagnetic suction force disappears, and the gag bit returns to the original position under the counterforce of the spring to cut off the connection between the relay K 1  and the output or the negative end of the incandescent lamp, then the light of the incandescent lamp quenches thereby. 
         [0034]    In another embodiment of this invention, the resistors R 9 //R 10 =0.167Ω, R 11 +R 8 =7.75 KΩ, R 2 =10 KΩ, R 7 =180Ω, R 4 =18 KΩ, the voltage U 3  of the third pin of regulator U 1  is 15V, and the voltage U 2  of the second pin of control chip U 2  is 0.15V When the power of the load is equal to 190 W, the current of the load is equal to 1.5±0.03 A, the sampling voltage of the third pin of control chip U 2  is higher than the voltage U 2  (0.15V) of the second pin, the seventh pin (output end) of the control chip U 2  outputs a voltage signal to the silicon controlled rectifier U 3  to trigger the conduction of the silicon controlled rectifier U 3 , then the relay K 1  becomes conductive. Due to the fact that the anode of the relay K 1  is connected to the anode of the silicon controlled rectifier U 3  and the silicon controlled rectifier U 3  becomes conductive, the positive end voltage of diode D 8  which is connected in series to the relay K 1  decreases, then the voltage which drives the relay K 1  decreases accordingly, the current which flows through the inner coil of the relay K 1  decreases, and the electromagnetic suction force disappears, and the gag bit returns to the original position under the counterforce of the spring to cut off the connection between the relay K 1  and the output or the negative end of the incandescent lamp, then the light of the incandescent lamp quenches thereby. 
         [0035]    In another embodiment of this invention, the resistors R 9 //R 10 =0.167Ω, R 11 +R 8 =11.27 KΩ, R 2 =10 KΩ, R 7 =150Ω, R 4 =18 KΩ, the voltage U 3  of the third pin of regulator U 1  is 15V, and the voltage U 2  of the second pin of control chip U 2  is 0.124V. When the power of the load is equal to 190 W, the current of the load is equal to 1.5±0.03 A, the sampling voltage of the third pin of control chip U 2  is higher than the voltage U 2  (0.124V) of the second pin, the seventh pin (output end) of the control chip U 2  outputs a voltage signal to the silicon controlled rectifier U 3  to trigger the conduction of the silicon controlled rectifier U 3 , then the relay K 1  becomes conductive. Due to the fact that the anode of the relay K 1  is connected to the anode of the silicon controlled rectifier U 3  and the silicon controlled rectifier U 3  becomes conductive, the positive end voltage of diode D 8  which is connected in series to the relay K 1  decreases, then the voltage which drives the relay K 1  decreases accordingly, the current which flows through the inner coil of the relay K 1  decreases, and the electromagnetic suction force disappears, and the gag bit returns to the original position under the counterforce of the spring to cut off the connection between the relay K 1  and the output or the negative end of the incandescent lamp, then the light of the incandescent lamp quenches thereby. 
         [0036]    In another embodiment of this invention, the resistors R 9 //R 10 =0.167Ω, R 11 +R 8 =21.8 KΩ, R 2 =10 KΩ, R 7 =100Ω, R 4 =18 KΩ, the voltage U 3  of the third pin of regulator U 1  is 15V, and the voltage U 2  Of the second pin of control chip U 2  is 83 mV When the power of the load is equal to 190 W, the current of the load is equal to 1.5±0.03 A, the sampling voltage of the third pin of control chip U 2  is higher than the voltage U 2  (83 mV) of the second pin, the seventh pin (output end) of the control chip U 2  outputs a voltage signal to the silicon controlled rectifier U 3  to trigger the conduction of the silicon controlled rectifier U 3 , then the relay K 1  becomes conductive. Due to the fact that the anode of the relay K 1  is connected to the anode of the silicon controlled rectifier U 3  and the silicon controlled rectifier U 3  becomes conductive, the positive end voltage of diode D 8  which is connected in series to the relay K 1  decreases, then the voltage which drives the relay K 1  decreases accordingly, the current which flows through the inner coil of the relay K 1  decreases, and the electromagnetic suction force disappears, and the gag bit returns to the original position under the counterforce of the spring to cut off the connection between the relay K 1  and the output or the negative end of the incandescent lamp, then the light of the incandescent lamp quenches thereby. 
         [0037]    In another embodiment of this invention, the resistors R 9 //R 10 =0.167Ω, R 11 +R 8 =1.89 KΩ, R 2 =10 KΩ, R 7 =270Ω, R 4 =18 KΩ, the voltage U 3  of the third pin of regulator U 1  is 15V, and the voltage U 2  Of the second pin of control chip U 2  is 0.22V. When the power of the load is equal to 190 W, the current of the load is equal to 1.5±0.03 A, the sampling voltage of the third pin of control chip U 2  is higher than voltage U 2  (0.22 V) of the second pin, the seventh pin (output end) of the control chip U 2  outputs a voltage signal to the silicon controlled rectifier U 3  to trigger the conduction of the silicon controlled rectifier U 3 , then the relay K 1  becomes conductive. Due to the fact that the anode of the relay K 1  is connected to the anode of the silicon controlled rectifier U 3  and the silicon controlled rectifier U 3  becomes conductive, the positive end voltage of diode D 8  which is connected in series to the relay K 1  decreases, then the voltage which drives the relay K 1  decreases accordingly, the current which flows through the inner coil of the relay K 1  decreases, and the electromagnetic suction force disappears, and the gag bit returns to the original position under the counterforce of the spring to cut off the connection between the relay K 1  and the output or the negative end of the incandescent lamp, then the light of the incandescent lamp quenches thereby. 
         [0038]    A further embodiment of the present invention in which the power rating is equal to 380 W is described as follow: 
         [0039]    In this embodiment, the resistors R 9 //R 10 =0.167Ω, R 11 +R 8 =37.22 KΩ, R 2 =10 KΩ, R 7 =240Ω, R 4 =24 KΩ, the voltage U 3 of the third pin of regulator U 1  is 15V, and the voltage U 2 Of the second pin of control chip U 2  is 0.15V. When the power of the load is equal to 380 W, the current of the load is equal to 1.5±0.03 A, the sampling voltage of the third pin of control chip U 2  is higher than the voltage U 2 (0.15V) of the second pin, the seventh pin (output end) of the control chip U 2  outputs a voltage signal to the silicon controlled rectifier U 3  to trigger the conduction of the silicon controlled rectifier U 3 , then the relay K 1  becomes conductive. Due to the fact that the anode of the relay K 1  is connected to the anode of the silicon controlled rectifier U 3  and the silicon controlled rectifier U 3  becomes conductive, the positive end voltage of diode D 8  which is connected in series to the relay K 1  decreases, then the voltage which drives the relay K 1  decreases accordingly, the current which flows through the inner coil of the relay K 1  decreases, and the electromagnetic suction force disappears, and the gag bit returns to the original position under the counterforce of the spring to cut off the connection between the relay K 1  and the output or the negative end of the incandescent lamp, then the light of the incandescent lamp quenches thereby. 
         [0040]    A still further embodiment in which the power is equal to 100 W is described as follow: 
         [0041]    In this embodiment, the resistors R 9 //R 10 =0.167Ω, R 11 +R 8 =2.43 KΩ, R 2 =10 KΩ, R 7 =240Ω, R 4 =24 KΩ, the voltage U 3  of the third pin of regulator U 1  is 15V, and the voltage U 2  of the second pin of control chip U 2  is 0.15V. When the power of the load is equal to 100 W, the current of the load is equal to 1.5±0.03 A, the sampling voltage of the third pin of control chip U 2  is higher than the voltage U 2  (0.15V) of the second pin, the seventh pin (output end) of the control chip U 2  outputs a voltage signal to the silicon controlled rectifier U 3  to trigger the conduction of the silicon controlled rectifier U 3 , then the relay K 1  becomes conductive. Due to the fact that the anode of the relay K 1  is connected to the anode of the silicon controlled rectifier U 3  and the silicon controlled rectifier U 3  becomes conductive, the positive end voltage of diode D 8  which is connected in series to the relay K 1  decreases, then the voltage which drives the relay K 1  decreases accordingly, the current which flows through the inner coil of the relay K 1  decreases, the electromagnetic suction force disappears, and the gag bit returns to the original position under the counterforce of the spring to cut off the connection between the relay K 1  and the output or the negative end of the incandescent lamp, then the light of the incandescent lamp quenches thereby. 
         [0042]    In above mentioned embodiments, the resistors R 11  and R 8  which is connected in series can be replaced by a single resistor. However, the manufacture process of resistors may be not accurate enough, so that the resistor R 8  is preferred to be a sliding rheostat to assure the accurate sampling voltage. If the total resistance value of the resistors R 11  and R 8  is not equal to the predetermined value, the total resistance value can be adjusted by regulating the sliding rheostat R 8  so as to close to the predetermined value. Considering the cost and working stability, the control chip U 2  is preferred to be a bi-operational amplifier formed of two comparators, and the silicon controlled rectifier U 3  is preferred to be a single-directional silicon controlled rectifier. 
         [0043]    Although the present invention has been described in connection with preferred embodiment thereof, many other variations and modifications will now become apparent to those skilled in the art without departing from the scope of the invention. It is preferred, therefore, that the present invention should not be limited by the specific disclosure herein, but only by the appended claims.