Abstract:
Disclosed herein is a dimmer switch having a broad range of control with a relatively simple circuit having a number of safety features. The circuit employs a triac and diac to selectively conduct an AC power wave. A variable phase shift network having an improved range of control governs the firing angle of the diac. The variable phase shift network includes a first and second series R-C circuits coupled by a bridge resistor.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a completion application of co-pending U.S. Provisional Application No. 60/906,061 filed on Mar. 9, 2007, for “Dimmer Switch Assembly,” the entire disclosure of which is hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a dimmer switch for controlling the delivery of electrical voltage from a source to load which is especially useful in dimming lamps. More specifically, the invention relates to a dimmer switch having a broader range of control with a relatively simple circuit having a number of safety features. 
     2. Prior Art 
     Modern control circuits for dimmer switches commonly use a thyristor, most commonly a triac, to chop the AC power wave from an AC voltage source to the load, such as an incandescent light. A gate controls current flow through the triac, which is signaled to conduct by current flow through the gate. The switch that provides current varies depending on design; examples of devices that conduct current to the gate include an integrated circuit (IC) and another thyristor. However a lower cost dimmer switch may be provided by using a thyristor instead of an IC. 
     However, inductive loads are a concern when using a thyristor to control switching the power wave of from an AC power source. With inductive loads, dV/dt may become too large, and the sudden rise in voltage can cause the device to be triggered when not desired. Some have solved the problem by introducing “soft start” circuits into the dimming network. However, these circuits add cost and complexity to the switch. 
     Another concern in using a thyristor is that excessive current can cause damage to the thyristor when the thyristor fires and the potentiometer of a phase shift network, which governs the firing angle of the switch, is set too low. This problem is commonly solved by adding a protective resistor in series with the potentiometer. However, the addition of this additional resistor affects the firing angle of the phase shift network. 
     Thus, it is desirable to provide a simplified dimmer switch having a broad range of control while having desirable safety features. 
     SUMMARY OF THE INVENTION 
     The present invention provides a dimmer switch for controlling the illumination level of a light source or light or lamp by controlling the delivery of alternating current electrical voltage from a source to the light source. The dimmer switch comprises a strap mounted to a housing. The strap supports an on/off switch for selectively applying power to the load and an electrical circuit mounted to said strap. The strap may also serve the function of a heat sink for the electrical circuit. 
     The electrical circuit comprises a switch connected in series with the light for selectively conducting a portion of the AC current from the source to the light. The switch includes a gate for initiating the selective conduction of current from the source to the lamp. A trigger is coupled to the switch gate for causing the switch to conduct current when a trigger limit voltage is exceeded. 
     A variable phase shift network for adjusting the firing angle of the switch has a first phase shift circuit across the switch, the first phase shift circuit includes a leading capacitor and a first resistor. The variable phase shift network has a second phase shift circuit across the switch. The second phase shift circuit includes a surge protection resistor, a selectively variable resistor and a firing capacitor, the firing capacitor being coupled to the trigger. The variable phase shift network also has a bridge resistor disposed between the first phase shift circuit and the second phase shift circuit. The bridge resistor couples the leading capacitor to the firing capacitor. 
     A user selectable dimmer control switch is supported by the strap. The dimmer control switch is coupled to the variable resistor for adjusting the resistance of the variable resistor between about zero resistance and full resistance, whereby the firing angle of the switch is adjusted to control the illumination level of the light. 
     A snubber circuit is connected across said switch for preventing misfiring of the switch. 
     Further objects, features and advantages of the present invention will become apparent to those skilled in the art from analysis of the following written description, the accompanying drawings and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front view of the dimmer switch according to the principles of the present invention, revealing the on-off button and dimmer control; 
         FIG. 2  is a perspective view of the dimmer switch of  FIG. 1 , revealing a housing and connection wires; 
         FIG. 3  is a schematic representation of the electrical circuit of the present invention; 
         FIG. 4  is an alternate schematic representation of the electrical circuit of the present invention, modified for a three way switch. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to  FIG. 1 , a front view of the dimmer switch  1  according to the principles of the present invention for controlling the illumination level of a lamp or light by controlling the delivery of alternating current electrical voltage from a source to the load reveals a strap supporting an on/off switch  4  and dimmer control switch  5 . The on/off switch  4  enables a user to selectively conduct power to the load while the dimmer control switch  5  is coupled to a variable resistor, enabling a user to adjust the resistance of the variable resistor, or potentiometer, as it is referred to herein. 
     Referring now also to  FIG. 2 , a perspective view of the dimmer switch  1  of  FIG. 1  reveals the strap  3  mounted on a housing  2  and connection wires  6 , 7 , 8  leading from the housing  2 . The housing  2  contains an electrical circuit which is mounted on the strap  3 , which also functions as a heat sink for the circuit. 
     Referring now to  FIG. 3 , a schematic representation of the electrical circuit  10  of the present invention is shown. The circuit  10  interconnects a power source  11  which is used to power a lamp  12 . The load  12  is, preferably, a lamp in which case the circuit  10  is used for dimming. The circuit  10  includes a switch  13  which, in the present embodiment, is a single pole, single throw switch for selectively opening or closing the circuit  10 . 
     A switch  20 , which in the present embodiment is a triac Q 1 , is connected in series with the light  12  and the source  11 . The switch  20  includes terminals  21  and  22  and a gate  23 . The switch  20  selectively conducts a portion of the AC current from the source  11  to the light  12 . When current is provided at the gate  23 , terminals  21  and  22  will conduct current to the lamp  12 . The application of current at the gate  23  initiates selective conduction of current to the lamp  12 . Before firing, the triac Q 1  is an open switch, and no voltage is applied across the load  12 . After the triac Q 1  fires, all of the voltage of the power source  11  is applied across the load  12 . 
     A trigger  24 , such as a diac B 1 , is connected to the gate  23 . The trigger  24  provides a current to the gate  23  when a trigger limit voltage is exceeded. The diac B 1  prevents any current to the gate  23  until the triggering voltage has reached a certain level in either direction. Therefore, the firing point of the diac B 1  is every half cycle. In the preferred embodiment a diac is employed because of characteristics including a consistent firing point and a symmetrical waveform above and below its centerline. 
     The firing point of the trigger  24  is adjusted by a variable phase shift network  19 . Network  19  has a first phase shift circuit  14  connected across switch  20 , circuit  14  includes a first resistor  31  such as R 1  and a leading capacitor  42  such as C 2 . The first end of first resistor R 1  is connected in series with lead capacitor C 2  by a node N 2  and the second end is connected to the network  19  at node N 1 . In the preferred embodiment, R 1  has a resistance of 68K ohms and C 2  has a capacitance of 104 microfarads. A second phase shift circuit  16  is connected across switch  20 ; circuit  16  includes a surge protection resistor  33  such as R 3 , a selectively variable resistor  35  such as potentiometer VR 1 , and a firing capacitor  43  such as C 3 . The first end of resistor R 3  is connected in series with the second end of variable resistor VR 1 . The first end of variable resistor VR 1  is connected in series with firing capacitor C 3  by node N 3 . Firing capacitor C 3  is connected to diac B 1 . In the preferred embodiment, R 3  has a resistance of 1K ohms, VR 1  has a resistance of between about 0 and 250K ohms and C 3  has a capacitance of 104 microfarads. 
     The second end of resistor R 3  is connected to the network  19  at node N 4 , as there is no element between N 4  and N 1 , R 3  and R 1  are connected at their respective second ends. Leading capacitor C 2  and firing capacitor C 3  are connected at their respective first ends. A bridge resistor R 4  connects first resistor R 1  at N 2  and variable resistor VR 1  at N 3 , as such R 4  is disposed between first phase shift circuit  14  and second phase shift circuit  14 , coupling leading capacitor C 2  to firing capacitor C 3  at their respective second ends. As a result, C 3  fills through C 2 , adding to the phase shift of the network  19 . 
     In operation, the selectively variable resistor VR 1  affects the charging rate of the firing capacitor C 3 . C 3  charges through R 3  and VR 1 . R 3  is present in the event VR 1  is set too low to avoid excessive current flow. As VR 1  increases, C 3  will fill more slowly. Conversely, with a lower VR 1  value, C 3  will fill more quickly. When VR 1  is set to a higher resistance, it takes longer for the capacitor C 3  to charge to the firing voltage of the diac B 1 . As such, the longer it takes for the capacitor C 3  to fire, the smaller the conductance phase angle will be, and as such, the triac Q 1  will conduct over a smaller portion of the AC power wave form. The position of VR 1  will determine the power supplied to the lamp  12  by controlling the charging rate of the capacitor C 3 . 
     The present invention employs a double phase shift firing circuit. The double phase shift firing circuit employs the first series R-C circuit  14  that is coupled across the triac Q 1 , and a second series R-C circuit  16  coupled across the triac Q 1 . Firing capacitor C 3  is coupled to the R-C circuit by means of resistor R 4  and to the diac B 1  coupled to the gate terminal of the triac by means of potentiometer VR 1 . Since the leading capacitor C 2  is connected to the firing capacitor C 3  through the resistor R 4 , the R 4  resistor and C 3  capacitor provide a second time constant that gives a greater range of control and improved symmetry, both of which are matters of importance in a dimming switch. 
     Those skilled in the art will recognize the advantages that the R 1 , R 4 , VR 1 , R 3  arrangement provides a divider where the voltage is balanced between nodes N 1 -N 2  and N 2 -N 4 . This arrangement enables VR 1  to fine tune dimming, resulting in better control. 
     Additionally, dimmer  1  may include an optional radio frequency interference filter (not shown). A filter is designed to slow high speed switch transients produced by dimmer  1 . 
     The resistor R 2  and capacitor C 1  in series across the triac Q 1  is commonly referred to as a snubber  18 . The snubber  18  prevents dV/dt from becoming too large with inductive loads which may trigger the triac Q 1 . The snubber circuit  18  suppresses the sudden voltage rises appearing across the triac Q 1  when the triac Q 1  opens due to inductive loads. Snubber  18  also suppresses fast voltage rises across triac Q 1  when Q 1  turns off, which may cause false (out of sync) triggering of Q 1 . 
     Referring now to  FIG. 4 , an alternate schematic representation of the electrical circuit  10  of the present invention is shown, modified for a three way switch. The circuit  10  includes a single pole double throw switch  15 . 
     The foregoing discussion discloses and describes the preferred structure and control system for the present invention. However, one skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims, that various changes, modifications and variations can be made therein without departing from the true spirit and fair scope of the invention as defined in the following claims.