Abstract:
A condensation control system to remove or prevent condensation on a surface, such as a mirror. The condensation control system has a first terminal that is adapted to be directly connected to an AC power source and a second terminal. In one embodiment, the condensation control system includes a heating element, a power regulation device having a trigger, a voltage divider having an output, and a humidity sensor. The heating element is electrically coupled in series with the power regulation device between the first and second terminals. The voltage divider is electrically coupled in series between the first and second terninals and in parallel with the heating element and the power regulation device. The humidity sensor is electrically coupled between the output of the voltage divider and the trigger of the power regulation device. The humidity sensor senses an amount of humidity and triggers the power regulation device to activate the heating element when the amount of humidity sensed by the humidity sensor is greater than a predetermined humidity threshold set point.

Description:
This application claims the benefit of provisional application Ser. No. 60/137,279 filed Jun. 3, 1999. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to preventing and/or removing condensation from smooth surfaces, and more particularly, to a system for automatically preventing and/or removing condensation from at least a portion of a glass or mirrored surface. 
     DESCRIPTION OF THE RELATED ART 
     Many people are familiar with the problems associated with using a bathroom mirror shortly after taking a shower or bath, or while the shower or bath is otherwise on. The moisture in the air from the hot water of the shower or bath condenses on the mirror surface and impairs visibility. Although many residential bathrooms have an exhaust fan that accelerates the removal of moisture from the bathroom, most bathroom fans require an appreciable amount of time before condensation on the mirror surface is dissipated. 
     U.S. Pat. No. 4,956,542 to Prosser discloses a mirror defogger assembly that uses a heating element to prevent condensation from forming on the surface of a mirror. The mirror defogger assembly includes a humidistat, a control module/transformer, and a heating element that is attached to the surface of the mirror. The humidistat sends an electronic signal to the control module/transformer when the humidity level reaches a predetermined point. The electronic signal, when received by the control module/transformer, activates the heating element, thereby warning the mirror surface and preventing condensation. Although mirror defogging systems that incorporate a heating element are known in the art, such systems are limited due to cost, reliability, as well as a number of other factors. For example, in the mirror defogger assembly of Prosser, the use of a transformer to convert household AC power to a level suitable for use with the heating element adds both cost and bulk to the assembly. 
     SUMMARY OF THE INVENTION 
     The present invention provides a reliable and inexpensive condensation prevention and/or removal system that can be attached to any surface upon which condensation may form. Advantageously, embodiments of the present invention can be directly connected to standard household wiring without the use of special power supplies or transformers. Moreover, embodiments of the present invention automatically turn on and off to control condensation formation based on the humidity of the ambient environment and without any user intervention. This automatic operation keeps the condensation prevention and/or removal system from being left in an on condition, thereby enhancing safety and reducing operating costs. 
     According to one embodiment of the present invention, a condensation control system is provided having a first terminal that is adapted to be directly connected to an AC power source and a second terminal. The condensation control system includes a heating element, a power regulation device having a trigger, a voltage divider having an output, and a humidity sensor. The heating element is electrically coupled in series with the power regulation device between the first and second terminals. The voltage divider is electrically coupled in series between the first and second terminals and in parallel with the heating element and the power regulation device. The humidity sensor is electrically coupled between the output of the voltage divider and the trigger of the power regulation device. The humidity sensor senses an amount of humidity and triggers the power regulation device to activate the heating element when the amount of humidity sensed by the humidity sensor is greater than a predetermined humidity threshold set point. 
     According to another embodiment of the present invention, a condensation control system is provided having a first terminal that is adapted to be directly connected to an AC power source and a second terminal. The condensation control system includes a heating element, a power regulation device having a trigger, first, second, and third resistors, and a humidity sensor. The heating element is electrically coupled in series with the power regulation device between the first and second terminals. The first resistor and the second resistor are electrically coupled in series between the first and second terminals and in parallel with the heating element and the power regulation device. The third resistor is electrically coupled between the trigger of the power regulation device and the second terminal. The humidity sensor is electrically coupled in series between the first resistor and the second resistor. The humidity sensor has a first terminal that is electrically coupled to the first resistor and a second terminal that is electrically coupled to the second resistor and the trigger of the power regulation device. The humidity sensor senses an amount of humidity and triggers the power regulation device to activate the heating element when the amount of humidity sensed by the humidity sensor is greater than a predetermined humidity threshold set point. 
     According to further aspects of the present invention, the power regulation device may include a triac, a thyristor, or other types of SCRs. Moreover, embodiments of the present invention may be used with different types of humidity sensors, such as variable resistance humidity sensors and variable capacitance humidity sensors, and may be used to activate the heating element to remove or prevent condensation over one half of an AC cycle, or over the full AC cycle. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Illustrative, non-limiting embodiments of the present invention are described by way of example with reference to the accompanying drawings, in which: 
     FIG. 1 is a plan view of a condensation prevention and/or removal system for a mirror according to one embodiment of the present invention; 
     FIG. 2 is a schematic diagram of an exemplary implementation of a condensation prevention and/or removal system according to one embodiment of the present invention; 
     FIG. 3 is a schematic diagram of another exemplary implementation of a condensation prevention and/or removal system according to another embodiment of the present invention; 
     FIG. 4 is a schematic diagram of another implementation of a condensation prevention and/or removal system according to a further embodiment of the present invention; 
     FIG. 5 is a schematic diagram of yet. another. implementation of a condensation prevention and/or removal system according to an embodiment of the present invention; and 
     FIG. 6 is a schematic diagram of another implementation of a condensation prevention and/or removal system according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention will be understood more completely through the following detailed description which should be read in conjunction with the attached drawings in which similar reference numbers indicate similar structures. 
     FIG. 1 represents an illustrative implementation of a condensation prevention and/or removal system  100  that is attached to a surface of a mirror  130 . As shown in FIG. 1, the condensation prevention and/or removal system  100  includes a controller  110  and a heating element  120 . The entire system  100  (including the controller  110  and the heating element) can be attached to either the front surface or the rear surface of the mirror  130 . Alternatively, controller  110  can be attached to the front surface of the mirror  130  and heating element  120  can be attached to the rear surface of the mirror  130 , or vice versa. Preferably, the controller  110  is attached to the front surface of the mirror  130  so that changes in humidity are quickly detected, with the heating element  120  being attached to the rear surface of the mirror  130 , to provide a clearer field of view. For increased aesthetic appeal, damage prevention, etc., the controller  110  may be covered by a small circular mirror, a decorative cover plate, etc. 
     As shown in FIG. 1, heating element  120  is disposed over a central portion of the mirror  130  in a serpentine manner. However, it should be appreciated that the heating element  120  can be disposed over a different portion of the mirror, or can be disposed over the entire surface of the mirror. Further, the geometry in which the heating element is disposed on the mirror may vary, depending on the shape of the mirror, as well as other factors. In one embodiment of the present invention, heating element  120  is disposed to form a company logo or to form letters, words, or numerals. For example, the heating element can be arranged in the shape of a hotel&#39;s corporate logo. It should also be appreciated that multiple systems  100  can be used, with a first being disposed to provide a clear viewing area, and the second to display a logo, etc. Alternatively, multiple heating elements can be used (e.g., one to provide a clear viewing area and another to display a logo) with only a single controller  110 . 
     As shown in FIG. 1, the system  100  can be directly connected to standard electrical wiring  115  with the use of wire-nuts  125 . Other types of electrical connectors such as crimp connectors, etc. may alternatively be used. To reduce the potential of injury, the system  100  may be connected to a circuit that is protected by a ground fault interruption type of circuit breaker or other type of protective circuit breaker. Advantageously, the system  100  can be connected to new or existing wiring without the use of a step-down transformer as discussed. in detail further below. 
     FIG. 2 is circuit schematic diagram of a condensation prevention and/or removal system according to one exemplary embodiment of the present invention. As shown the condensation prevention and/or removal system  200  includes a heating element  120 , a power regulation device  220 , a humidity sensor  230 , a pair of fixed resistors  250 ,  260  that form a voltage divider, and a potentiometer or variable resistor  240 . The system may be connected to a conventional source of power  210 , such as an ordinary 120 volt or 220 volt AC power line. Advantageously, the condensation prevention and/or removal system  200  can be directly connected to a source of AC power  210  without the use of a transformer or specially designed power supply. This reduces the cost and bulk associated with the system  200 , and reduces component count, thereby enhancing reliability. 
     In the embodiment of FIG. 2, the power regulation device  220  is depicted as a triac. However, it should be appreciated that other types of power regulation devices may alternatively be used, including SCR&#39;s, thyristors, etc. An advantage to the use of a triac in FIG. 2 is that it permits continuous operation during the full AC cycle without the use of additional components, such as a full wave rectifier. 
     Humidity sensor  230  is a variable resistance humidity sensor in which a resistance of the sensor  230  decreases with an increase in relative humidity. For example, in one embodiment of the present invention, a Scimarec HS15 humidity sensor is used, although other types of humidity sensors may alternatively be used, as the present invention is not limited to a particular type of humidity sensor. As the resistance of sensor  230  decreases, the amount of current provided to the trigger of the power regulation device  220  is increased above the triggering threshold of the power regulation device  220 . The triggering of the power regulation device  220  permits current to flow through heating element  120 , thereby increasing the temperature of the surface to which the heating element is attached to prevent or dissipate condensation. 
     The humidity threshold set point (i.e., the level of relative humidity at which the current provided to the trigger of the power regulation device  220  is sufficient to turn on the power regulation device  220 ) can be adjusted for different humidity environments by changing the value of resistance of potentiometer  240 . For example, by increasing the resistance of potentiometer  240 , the humidity threshold set point is increased, and by decreasing the resistance of potentiometer  240 , the humidity threshold set point is decreased. Although not depicted in FIG. 1, potentiometer  240  can be disposed so that the potentiometer  240  is accessible to a user and can be adjusted by a user, should this be desired. 
     Preferably, the resistance of the heating element  120  is controlled so that the power density of the heating element  120  does not exceed 35 W/ft 2  (389 W/m 2 ). In a typical ambient environment, this value of power density allows the heating element  120  to ramp up rapidly in temperature to prevent or dissipate condensation, while remaining below a temperature of approximately 150° F. (66° C.). Empirical testing has demonstrated that by preventing the maximum temperature of the heating element  120  from rising above approximately 150° F. (66° C.), thermally induced stresses on the mirror  130  (in FIG. 1 ) are reduced, thereby lessening the potential for breakage or shattering. In addition, the risk of personal injury due to contact with the heated surface of the mirror  130  is reduced. For higher temperature ambient environments, a lower power density heating element can be used to prevent the heating element going beyond 150° F. (66° C.). Of course, it should be appreciated that the values listed herein are provided merely for illustrative purposes, as the present invention is not so limited. 
     It should be appreciated that the condensation prevention and/or removal system  200  of FIG. 2 automatically turns on the heating element  120  when the relative humidity level detected by the humidity sensor  230  rises above the predetermined threshold necessary to trigger the power regulation device  220 . In addition, when the relative humidity level of the environment in which the system  200  is used falls below the predetermined threshold, such as when a bathroom door is opened, or when a bathroom fan finally removes a sufficient amount of the humid air, the condensation prevention and/or removal system  200  automatically shuts off the heating element  120 . This helps to prevent the mirror from heating up to a high level. For additional safety, a thermally activated switch can be used (not shown) to shut off the heating element when it reaches a certain temperature, as known to those skilled in the art. In the event that the humidity of the environment in which the condensation prevention and/or removal system  200  is used never reaches the predetermined threshold of humidity, then the system remains in a standby state in which the heating element  120  is off. 
     FIG. 3 is circuit schematic diagram of a condensation prevention and/or removal system according to another exemplary embodiment of the present invention. In contrast to the embodiment described with respect to FIG. 2, the circuit of FIG. 3 uses only one half of the AC cycle. Condensation prevention and/or removal system  300  includes a heating element  120 , a power regulation device  320 , a humidity sensor  330 , a pair of fixed resistors  350 ,  360 , a diode  370 , and a potentiometer or variable resistor  340 . As in the condensation prevention and/or removal system of FIG. 2, the condensation prevention and/or removal system  300  may be connected to a conventional source of power  310 , such as a 120 volt or 220 volt AC power line. As in the embodiment of FIG. 2, condensation prevention and/or removal system  300  can be directly connected to the source of power  310  without the use of a transformer or specially designed power supply. 
     In the embodiment of FIG. 3, the power regulation device  320  is depicted as cathode gate thyristor. However, it should be appreciated that other types of power regulation devices can alternatively be used, as the present invention is not limited to a particular type of power regulation device. By using a thryistor as the power regulation device  320  instead of a triac, a lower triggering current may be used to trigger the power regulation. device than in the embodiment of FIG.  2 . This, in turn, enhances the reliability of the system  300 , as the humidity sensor  330  is not required to handle as large a current. Humidity sensor  330  is again a variable resistance humidity sensor in which the resistance of the humidity sensor  330  decreases with an increase in relative. humidity. As the resistance. of humidity sensor  330  decreases, the amount of current provided to the trigger of the power regulation device  320  is increased above the triggering threshold of the device  320 , in a manner analogous to FIG.  2 . The triggering of the power regulation device  320  permits current to flow through heating element  120 , thereby increasing the temperature of the surface to which the heating element  120  is attached to prevent or dissipate condensation. 
     The series connection of resistors  350  and  360  form a voltage divider and are valued to coarsely set the humidity threshold set point of the condensation prevention and/or removal system  300 . However, as in the embodiment of FIG. 2, the humidity threshold set point of condensation prevention and/or removal system  300  can be adjusted for different humidity environments by changing the resistance value of potentiometer  340 . Because the embodiment of FIG. 3 operates only during the positive half wave cycle, the circuit of FIG. 3 uses less energy than the circuit of FIG.  2 . In addition, because the heating element  120  only conducts current during the positive half wave cycle, the possibility of excessive heating of heating. element  120  is less than the embodiment of FIG.  2 . Nonetheless, the power density. of the heating element  120  can be controlled in a manner similar to that described above with respect to FIG. 2 to further ensure safe operation. Alternatively, or in addition to controlling the power density of the heating element  120 , an electrical fuse (not shown) may be inserted in series with the heating element  120  and the power regulation device  320  to provide fail safe operation, or a thermally activated switch (not shown) may be used instead. 
     It should be appreciated that the condensation prevention and/or removal system  300  of FIG. 3 automatically turns the heating element  120  on and off when the relative humidity level detected by the sensor  330  rises above or falls below a predetermined threshold of humidity in a manner similar to that described with respect to the embodiment of FIG.  2 . 
     FIG. 4 is circuit schematic diagram of a condensation prevention and/or removal system according to another exemplary embodiment of the present invention. Condensation prevention and/or removal system  400  includes a heating element  120 , a power regulation device  420 , a humidity sensor  430 , a pair of fixed resistors  450 ,  460 , a potentiometer or variable resistor  440 , and a diode bridge rectifier  470 . The embodiment of FIG. 4 operates in a manner similar to the embodiment described above with respect to FIG. 3, only condensation prevention/removal is performed over the full AC cycle. Activation of the heating element  120  over the full AC cycle permits condensation prevention and/or removal to be effected more quickly than the embodiment of FIG.  3 . As in the embodiments of FIGS. 2 and 3, condensation prevention and/or removal system  400  can be directly connected to a source of AC power  410  without the use of a transformer or a specially designed power supply. As in the embodiments of FIGS. 2 and 3, the power density of the heating element  120  can be controlled to ensure safe operation, or a thermally activated switch or fuse may be used. As the operation of the condensation prevention and/or removal system  400  is similar to that described above with respect to FIG. 3, a detailed discussion of the operation of condensation prevention and/or removal system  400  is omitted herein. 
     FIG. 5 is circuit schematic diagram of a condensation prevention and/or removal system according to yet another exemplary embodiment of the present invention. The condensation prevention and/or removal system  500  is substantially similar to that described above with respect to FIG. 3, in that the heating element  120 , when activated, uses only one half of the AC cycle. Like the embodiment described above with respect to FIG. 3, the condensation prevention and/or removal system  500  includes a heating element  120 , a power regulation device  520  in the form of a thyristor, a pair of fixed resistors  550 ,  560 , a diode  570 , a potentiometer or variable resistor  540 , and a humidity sensor  530 . Moreover, the condensation prevention and/or removal system  500 , like the System  300  of FIG. 3, may be connected to a conventional source of power  510 , such as a 120 volt or 220 volt AC power line. 
     However, in contrast to the embodiment Of FIG. 3, the humidity sensor  530  used in condensation prevention and/or removal system  500  is a variable capacitance humidity sensor, rather than a variable resistance humidity sensor. Specifically, the capacitance of humidity sensor  530  increases with an increase in relative humidity. As the capacitance of the humidity sensor  530  increases, the impedance of the humidity sensor  530  decreases, thereby providing a sufficient current to trigger the power regulation device  520  into conduction. The conduction of the power regulation device  520 , in turn, activates the heating element  120  to prevent and/or dissipate condensation. Although variable capacitance humidity sensors are generally more variable in terms of capacitance from one sensor to another, their small size and low cost, in comparison to a variable resistance humidity sensor, makes them attractive for applications in which size and cost are important design parameters. As in the embodiment of FIG. 3, potentiometer  540  permits the humidity threshold set point. to be adjusted to suit a particular environment or the preferences of a particular user. In particular, by increasing the resistance of potentiometer  540 , the humidity threshold set point is decreased, and by decreasing the resistance of potentiometer  540 , the humidity threshold set point is increased. 
     FIG. 6 is a circuit schematic diagram of a condensation prevention and/or removal system according to a further embodiment of the present invention. The condensation prevention and/or removal system  600  is again substantially similar to the system  300  described above with respect to FIG. 3, in that the heating element  120 , when activated, uses only one half of the AC cycle. The condensation prevention and/or removal system  600 , like the system  300  of FIG. 3, may be connected to a conventional source of power  610 , such as a 120 volt or 220 volt AC power line. Furthermore, condensation prevention and/or removal system  600  also includes a heating element  120 , a power regulation device  620 , a pair of fixed resistors  650 ,  660 , a diode  670 , a potentiometer or variable resistor  640 , and a variable resistance humidity sensor  630 , like the embodiment of FIG.  3 . However, in contrast to the embodiment of FIG.  3 . the power regulation device  620  of the condensation prevention and/or removal system  600  includes two distinct power regulation devices  621  and  622 . Power regulation device  621  is a cathode gate thyristor with a sensitive (i.e., low triggering current) gate, while power regulation device  622  is a high power cathode gate thyristor or other high power triggerable device that can conduct large values of current without destruction. The embodiment of FIG. 6 is particularly well suited for preventing or removing condensation from larger surface that utilize larger heating elements, for example to prevent and/or remove condensation on surface that is a square foot or more (0.09 square meters) in area. The embodiment of FIG. 6 may also be used with multiple heating elements, for example, a first heating element to provide a clear viewing area, and a second heating element to display a logo, etc. Advantageously, the embodiment of FIG. 6 permits larger heating elements to be used while avoiding the conduction of large triggering currents through the variable resistance humidity sensor  630 , which can impact reliability. 
     As described above, the condensation prevention and/or removal systems of embodiments of the present invention provide automatic condensation prevention and/or removal whenever the relative humidity detected by the humidity sensor rises above a predetermined threshold. Each of the described embodiments also automatically turn off when the relative humidity falls below the predetermined threshold. For this reason, embodiments of the present invention cannot accidentally be left in the on condition. 
     It should be appreciated that a number of alterations made be made to any of the above described embodiments of FIGS. 1-6 that are considered to be within the spirit and scope of the present invention. For example, in each of the embodiments of FIGS. 2-6, a visual indicator may be connected in parallel with the heating element  120 , to provide a visual indication of heating element operation. Such a visual indicator provides the user with positive feedback that the condensation prevention and/or removal system is operational. It also provides feedback that allows the user to adjust the relative humidity level at which the heating element  120  of the condensation prevention and/or removal system turns on and off. Further, the embodiments described with respect to FIGS. 1-4 and  6  may be modified to utilize a variable capacitance humidity sensor, rather than a variable resistance humidity sensor, and each of the embodiments described with respect to FIGS. 2-5 may be adapted to utilize a power regulation device, such as that described with respect to FIG.  6 . 
     It should be appreciated that embodiments of the present invention provide an inexpensive and reliable mirror defogging system that uses few components and can be directly connected to standard home wiring in new or existing construction. Moreover, embodiments of the present invention consume a relatively small amount of power in a standby mode (i.e., when the relative humidity is below a level capable of triggering the power regulation device), approximately 0.2W at 120V. 
     Having described several embodiments of the invention in detail, various modifications and improvements will readily occur to those skilled in the art. Such modifications and improvements are intended to be within the spirit and scope of the invention. Accordingly, the foregoing. description is by way of example only, and is not intended as limiting. The invention is limited only as defined by the following claims and the equivalents thereto.