Abstract:
There is disclosed a two wire touch operated dimmer controlled by a membrane potentiometer. Adjusting the DC potential on the membrane potentiometer changes the output potential to a load. In many prior art touch operated dimmers controlled by membrane potentiometers, there is no isolation between the line power and the DC voltage on the membrane potentiometer. Thus, if the membrane is damaged, a user will be exposed to a ground current and possible shock when the membrane is touched. This invention avoids the above noted unsafe condition by employing a high frequency generator and a ground fault current limiting circuit which includes resistors.

Description:
[0001]     This application claims priority pursuant to 35 U.S.C. 119(e) from Provisional Patent Application No. 60/657,906, filed Mar. 2, 2005. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Disclosure  
         [0003]     The present invention relates generally to dimmers for controlling the voltage from a source to a load and, more specifically, to dimmers controlled by a membrane potentiometer and having a ground fault current limiting circuit.  
         [0004]     2. Description of the Related Art  
         [0005]     Dimmers are normally located between a source of power such as 120 VAC and a load such as a lamp, to control the brightness of the lamp by providing a variable output voltage to the lamp. The output voltage of a standard dimmer can be controlled by moving a mechanical device such as a slide or a rotary component attached to a potentiometer. A touch dimmer uses a membrane potentiometer to vary the output voltage. The difference between a standard dimmer and a touch dimmer is that the touch dimmer does not need a mechanically moving part.  
         [0006]     A touch dimmer can includes a touch- or pressure-operated device such as a membrane potentiometer which is activated by touching a flexible membrane to control a light dimming circuit. The output voltage of the dimmer is normally determined by the place on the membrane potentiometer where it is touched. Thus, the brightness of a lamp can be controlled by where the membrane is touched.  
         [0007]      FIG. 1  is a block diagram  100  of a prior art touch operated dimmer control circuit. The touch operated dimmer has a membrane potentiometer mounted behind a flexible film. A control circuit coupled to the membrane potentiometer is provided to control a dimming circuit in response to momentary voltage signals from the membrane potentiometer. The control circuit can include an electronically adjustable voltage divider which provides a continuous voltage, and a comparator which adjusts the electronically adjustable voltage divider to provide a continuous voltage Vc which corresponds to a momentary voltage Vm from membrane potentiometer  104 . The momentary voltage, V m , which corresponds to a desired output voltage level, V C , is obtained by applying pressure to a selectable point on the membrane potentiometer  104 . A voltage sensor  106  detects this voltage and turns on clock  108 . Clock  108  provides clock pulses which are fed to a wiper movement control pin of electronically adjustable voltage divider which incrementally moves wiper  110  along resistive element  112  to provide voltage, V C , which is fed to comparator  114  and to dimming circuit  102 . Comparator  114  compares the momentary voltage V m  with the output voltage level V C  to feed a comparator output voltage  116  to an up/down input of electronically adjustable voltage divider  120 . The up/down input determines the direction in which the wiper  110  moves when enabling clock pulses are received at the wiper movement input. As an example, when V m  is greater than V C , the comparator output  116  is high and the wiper  110  moves to increase V C  with each enabling clock pulse. Similarly, when V m  is less than V C , the comparator output  116  is low and the wiper  110  moves to decrease V C  with each enabling clock pulse. Thus, V C  is driven to approach V m . When pressure on membrane potentiometer  104  is released, voltage is removed from voltage sensor  106 , which turns clock  108  off. When clock  108  is turned off, it stops providing enabling pulses to the wiper movement control input of the electronically adjustable voltage divider and wiper  110  does not move. The voltage V C , which is provided to the dimming circuit  102 , determines the power provided to the load.  
         [0008]     In use it is possible that the membrane of the membrane potentiometer may be damaged. Due to cost and size issues, dimmers using membrane potentiometers no not normally have isolation between the source of power and the DC voltage. Therefore, if the membrane is damaged, a ground current can be present which can present a shock hazard to a person who touches the membrane. What is needed is a dimmer having a membrane potentiometer that can provide protection against the above noted unsafe condition.  
       SUMMARY OF THE DISCLOSURE  
       [0009]     There is disclosed a two wire touch operated dimmer controlled by a membrane potentiometer. Adjusting the DC potential on the membrane potentiometer changes the output potential to a load. In many prior art touch operated dimmers controlled by membrane potentiometers, there is no isolation between the line power and the DC voltage on the membrane potentiometer. Thus, if the membrane is damaged, a user will be exposed to a ground current and possible shock when the membrane is touched. This invention avoids the above noted unsafe condition by employing a high frequency generator and a ground fault current limiting circuit which includes resistors.  
         [0010]     The foregoing has outlined, rather broadly, a preferred blending feature, for example, of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention and that such other structures do not depart from the spirit and scope of the invention in its broadest form. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     Other aspects, features, and advantages of the present invention will become more fully apparent from the following detailed description, the appended claim, and the accompanying drawings in which similar elements are given similar reference numerals.  
         [0012]      FIG. 1  shows a prior art touch operated dimmer control circuit including a membrane potentiometer;  
         [0013]      FIG. 2  is a block diagram of a touch operated dimmer control circuit with a ground fault current limiting circuit in accordance with the principles of the invention;  
         [0014]      FIG. 3  is a schematic diagram of a series resonant capacitor ground fault current limiting circuit;  
         [0015]      FIG. 4  is a schematic diagram of a series resonant inductive ground fault current limiting circuit;  
         [0016]      FIGS. 5A-5B  are schematic diagrams of other embodiments of inductive ground fault current limiting circuits;  
         [0017]      FIGS. 6A-6B  are schematic diagrams of a resistor ground fault current limiting circuit; and  
         [0018]      FIG. 7  is a front perspective view of a wall plate with a touch operated dimmer. 
     
    
     DETAILED DESCRIPTION  
       [0019]     Techniques and methods are disclosed for a touch dimmer including a ground fault current limiting circuit controlled by a membrane potentiometer. A high-frequency generator can be coupled to a ground fault current limiting circuit to reduce the risk of shock injury to personnel due to a damaged membrane. The disclosure also discloses a control system for controlling the output voltage level of the dimmer.  
         [0020]     Referring to  FIG. 2 , there is shown a block diagram of a touch operated dimmer control circuit with a ground fault current limiting circuit  200 . The circuit  200  includes a dimmer switch  202  coupled to the phase conductor of a source of potential such as  120 VAC. The output of dimmer switch  202  is coupled through a load  204  such as a lamp to the neutral conductor of the source of potential. The value of the output voltage of the dimmer switch is controlled by a controller  206 . A resonant circuit  210 , coupled to receive the output signal of a high frequency generator  208 , is coupled in series with a voltage doubler  212  to form a ground fault current limiting circuit. The output of voltage doubler  212  is connected to an input of the controller  206 . The resonant circuit  210  is tuned to the same frequency as the high frequency generator  208  and the output of voltage doubler  212 , V B , controls the output of the controller  206  which, in turn, controls the output signal of dimmer switch  202 .  
         [0021]     The voltage at the output of dimmer switch  202  is controlled by the high frequency generator  208  and resonant circuit  210 . As discussed below, resonant circuit  210  includes a membrane potentiometer which varies the voltage from the high frequency generator  208  to produce an output voltage VA. Voltage VA is determined by the place where the membrane of the membrane potentiometer is touched by a user.  
         [0022]      FIG. 3  shows a schematic diagram of a touch operated dimmer ground fault current limiting circuit  300  having a capacitor resonant circuit  310  coupled in series with a voltage doubler circuit  312 . The membrane potentiometer is coupled in series with and located between capacitors C 1 , C 2 . Capacitors C 1  and C 2  and the resistor element of the membrane potentiometer provides a path for high frequency signals from the high frequency generator. Capacitors C 3 , C 4  in combination with diodes D 1 , D 4  comprise the voltage doubler  312 . The dimmer switch  202 , load  204  and controller  206  have been previously described in discussing  FIG. 2  and, therefore, in the interest of brevity, will not be repeated here.  
         [0023]     When the membrane potentiometer in the series resident circuit  310  is touched, voltage level V A  will change to a voltage which is related to the point at which the membrane potentiometer is touched. The voltage V A  is rectified to DC voltage, V B , by the voltage doubler circuit  312  and is fed to controller  206 . Controller  206  generates a corresponding control signal which sets the output voltage from the dimmer switch  202  to the load  204 .  
         [0024]      FIG. 4  is a schematic diagram of a touch operated dimmer ground fault current limiting circuit  400  having a series inductive resonant circuit  410  coupled in series with a voltage doubler circuit  412 . The series inductive resonant circuit  410  includes a capacitor CS connected in series with the primary inductance, normally the primary winding, of transformer TRX, and membrane potentiometer is connected across the secondary of transformer TRX. The resonant frequency of the circuit is set to be the same as the frequency generated by high frequency generator  208 . The dimmer switch  202 , load  204  and controller  206  have been previously described in discussing  FIG. 2  and, therefore, in the interest of brevity, will not be repeated here.  
         [0025]     When the membrane potentiometer which is connected in series with the secondary of the transformer TRX is touched, the load in the secondary winding of the transformer is reflected into the primary winding and causes the voltage VA a t the output of the resonant circuit  410  to change to a voltage which is related to the place where the membrane is touched. The voltage VA is rectified to a DC voltage V B  by the voltage doubler circuit  412  and is fed to controller  206 . Controller  206  generates a corresponding control signal which sets the output voltage from the dimmer switch  202  to the load  204 .  
         [0026]      FIGS. 5A-5B  are schematic diagrams of further embodiments of inductive ground fault current limiting circuits having resonant circuit  510   a  or  510   b , each of which includes an inductive parallel resonant circuit. Referring to  FIG. 5A , there is shown a circuit having an inductive parallel resonant circuit  510   a  having capacitor C 6  connected in parallel with the primary winding of transformer TRX. Capacitor C 6  and inductance of the primary winding of transformer TRX comprises a parallel resonance circuit having a resonant frequency which is the same as the frequency generated by high frequency generator  208 . A membrane potentiometer is connected in the secondary winding of transformer TRX. Touching the membrane of the membrane potentiometer affects the load in the secondary of transformer TRX which is reflected into the primary winding of transformer TRX and causes a voltage VA to be generated which relates to the place where the membrane is touched. As noted above, the voltage, V A , at the output of the resonant circuit  510   a  is fed to voltage doubler, the output of which is fed to a controller as described above. Referring to  FIG. 5B , the circuit shown is similar to that of  FIG. 5A  except that capacitor C 6  in  FIG. 5B  is connected across the secondary winding of transformer TRX. In all other respects, the various connections of the circuit of  FIG. 5B  is substantially the same as that of  FIG. 5A .  
         [0027]      FIGS. 6A-6B  are schematic diagrams of a resistor ground leakage current limiting circuit. Referring to  FIG. 6A , resistors R 1 , R 2  and R 3  are ground current limit resistors. The values of resistors R 1  and R 2  are selected to limit the ground current to less than 500 uA. Selecting a membrane potentiometer having a very high resistance will allow a user to sense a full range of DC voltage on the membrane potentiometer. However, a high resistance membrane potentiometer may have greater noise sensitivity than a lower resistance membrane potentiometer.  FIG. 6B  shows an alternative embodiment of a resistor ground leakage current limit circuit. In this embodiment, resistors R 1 , R 2 , R 3  and rectifiers D 1 , D 2  D 3  comprise the limit circuit. When the membrane potentiometer conducts to ground, a half cycle of the line voltage is fed to the membrane potentiometer. Therefore, a lower resistance for resistors R 1 , R 2  and R 3  can be selected as compared to the resistors of circuit of  FIG. 6A . Hence, a lower value of resistance for the membrane potentiometer can also be selected which has the advantage of reduced noise sensitivity.  
         [0028]      FIG. 7  is a front perspective view of a wall plate  700  with a touch operated dimmer  702  having a membrane potentiometer. The wall plate  700  is designed to be positioned over a standard wall box and has an opening for receiving the membrane potentiometer.  
         [0029]     While there have been shown and described and pointed out the fundamental novel features of the invention as applied to the preferred embodiments, as is presently contemplated for carrying them out, it will be understood that various omissions and substitutions and changes in the form and details of the devices described and illustrated and in their operation may be made by those skilled in the art without departing from the spirit of the invention.