Abstract:
An externally powered temperature calibration device includes a system that provides a warning of high temperatures within the device after the device has been disconnected from the external power. The warning system includes a capacitor that provides power to a light-emitting diode (“LED”) after the calibration device has been disconnected from the external power. A temperature sensor monitors the temperature of an internal component. An output signal from the sensor is used to control a switch that connects the capacitor to one of several resistors having different resistances. The switch therefore controls the discharge rate of the capacitor based on the sensed temperature at the time the calibration device was disconnected from the external power. As a result, the period during which the capacitor powers the LED can be commensurate with the time required for the internal component to cool from its initial temperature.

Description:
TECHNICAL FIELD 
       [0001]    This invention relates to electrically powered devices, and, more particularly, to electrically powered devices with the potential to cause injury after electrical power has been disconnected from the device. 
       BACKGROUND OF THE INVENTION 
       [0002]    A variety of electrically powered heating devices are in existence to provide a wide variety of functions. For example, electric stoves, frying pans and clothes irons are commonly used in homes. Soldering irons, temperature test chambers, and temperature calibration devices are commonly used in industry. 
         [0003]    It is sometimes difficult to determine if an electrically heated surface is hot enough to cause injury. This is particularly true after the heated surface is no longer being heated. Such surfaces can remain very hot for a considerable period after heating power has been terminated. A variety of techniques have been developed to address this problem. One approach is to coat the heated surface with a material that changes color with temperature. While this is feasible in some cases, heated surfaces can sometimes be too large to make this approach practical. Also, temperature indicating materials cannot provide an indication of whether a heated surface that can be touched but not seen is too hot to touch. For example, this approach cannot provide an indication whether a heated surface inside a device is too hot to touch before one begins to disassemble the device. 
         [0004]    Another approach to providing an indication that an electrically heated surface is too hot to touch is to use an electrical temperature sensor coupled to a warning light or the like. For example, electric stoves having a glass cooktop commonly include a warning light that is readily visible when the temperature of the cooktop is too hot to touch. This high temperature warning system can be very useful since, it is not readily apparent that the cooktop is at a high temperature after the underlying burner is no longer receiving electrical power. Furthermore, a temperature sensor and indicator can provide a warning that an internal surface, such as a cooking oven, is too hot. Also, this approach works well regardless of the temperature to which the surface was heated or the amount of time required for the surface to cool sufficiently that it is safe to touch. Unfortunately, the use of a temperature sensor and indicator is only practical if, after the surface is no longer being heated, electrical power continues to be applied to the device since the operation of the temperature sensor and indicator requires a continued supply of electrical power. 
         [0005]    One class of electrically heated devices that presents a particular challenge to providing an indication of dangerous temperatures are temperature calibration devices or “dry well” calibrators which are used in calibrating temperature probes and sensors. Conventional dry well calibrators include removable inserts having bores therein that receive the temperature probes that are to be calibrated. These inserts are often changed with inserts of varying hole sizes to accommodate different temperature probe diameters. Heating elements thermally coupled to the insert heat the probes to a temperature that is set by a user. The insert and surfaces surrounding its opening as well as internal components of these dry well calibrators can become very hot while calibrating temperature probes at high temperatures. When a user is done with a probe calibration he might unplug the drywell calibrator from it&#39;s AC power source and leave it unattended while the calibrator is still very hot. A second user may remove the hot insert to set up the calibrator for another test, thereby causing an injury if touched. Unfortunately, if a dry well calibrator is disconnected from external power before the next user changes the insert for a different size, there is no warning to him of an unsafe temperature condition. 
         [0006]    The above-described techniques for warning of excessive temperatures do not lend themselves well to warning of excessive temperatures of the internal components of dry well calibrators. The use of a temperature indication material is impractical because of the large amount of surface area that can be at a high temperature. Also, it would not be possible to see the temperature indicating material until the internal component or outer case was removed from the dry well calibrator, thereby potentially exposing the heated surfaces to inadvertent contact. 
         [0007]    The other approach described above, i.e., using a temperature sensor and indicating light, would provide an indication that some internal surfaces are too hot to touch, but it would provide this indication only while the dry well calibrator was plugged into an AC power receptacle. Unfortunately, because of the relatively light weight and small size of conventional dry well calibrators, they are frequently unplugged and moved to different locations. For example, a dry well calibrator may be unplugged and moved from a calibration facility to a repair facility. Therefore, as a practical matter, the use of a temperature sensor and indicating lamp is not likely to be effective in providing adequate high temperature warnings. 
         [0008]    Accidental burn injuries may also occur with other types of devices that are electrically heated by external power that may be disconnected from the devices. For example, clothes irons, curling irons, soldering irons and other similar devices can be inadvertently touched by users after they have been unplugged yet while they are still sufficiently hot to cause injury. 
         [0009]    Similar safety problems can also exist with other types of electrically powered devices that can cause injury after power has been disconnected from the device. For example, hydraulic devices may include a pressure pump that raises the pressure of hydraulic fluid to a very high level. After power has been disconnected from the hydraulic device, the high pressure of the hydraulic fluid may remain present in the device. However, the presence of the high pressure may not be apparent, and injury may result if the pressure is inadvertently released. 
         [0010]    There is therefore a need for a system and method that can provide an externally visible indication of dangerous internal temperatures and other unsafe conditions in electrically powered devices such as dry well calibrators even after external power has been removed from such devices. 
       SUMMARY OF THE INVENTION 
       [0011]    A warning system and method for an electrical device powered by external electric power can warn of an unsafe condition even after the electrical device has been disconnected from the external electrical power source. A capacitor or other energy storage device within the electrical device stores electrical energy from the external electrical power. As a result, the energy storage device can provide electrical power after the electrical device has been disconnected from the external electrical power. A property that may result in the unsafe condition is monitored by a sensor and used to set a rate at which the stored electrical energy is depleted from the energy storage device. The electrical energy stored in the energy storage device is used to supply power to a warning device. The warning device therefore provides warning of the unsafe condition until the stored electrical energy has been depleted below a predetermined level. The energy storage device is therefore used as both a source of electrical power and a timing element to set the duration of the warning based on the nature of the sensed property when power was removed from the electrical device. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is an exploded isometric view of some of the internal components of a temperature calibration device that includes a high temperature warning system according to various embodiments of the invention. 
           [0013]      FIG. 2  is a cross-sectional view of the internal components of the temperature calibration device shown in  FIG. 1 . 
           [0014]      FIG. 3  is an exploded isometric view of the temperature calibration device shown in  FIG. 1 . 
           [0015]      FIG. 4  is a front elevational view of the temperature calibration device of  FIG. 1 . 
           [0016]      FIG. 5  is a block diagram showing one example of a control system for the temperature calibration device of  FIGS. 1-4  that includes a high temperature warning system according to one example of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    Embodiments of the present invention are directed to systems for warning of unsafe conditions in electrically powered devices that can cause injury after the devices have been disconnected from the electrical power. Certain details are set forth below to provide a sufficient understanding of the invention. However, it will be clear to one skilled in the art that the invention may be practiced without these particular details. In other instances, well-known circuits, control signals, and timing protocols have not been shown in detail in order to avoid unnecessarily obscuring the invention. 
         [0018]    The internal components of a dry well calibrator heating block assembly  10  according to one example of the invention are shown in  FIG. 1 . The dry well calibrator  10  includes a cylindrical adapter insert  14  having one or more cylindrical bores  16   a,b,c  sized to receive temperature probes “P” having corresponding dimensions. The insert  14  is typically manufactured from a thermally conductive metal. 
         [0019]    The insert  14  fits into a cylindrical bore  18  formed in a heated block  20  of a suitable material, such as a metal with good thermal conduction properties. The block  20  has a configuration that is rectangular in both vertical and horizontal cross-section, although, of course, it may also have a square, round or other configuration. The inside diameter of the bore  18  is only slightly larger than the outside diameter of the insert  14  to ensure good heat conduction from the block  20  to the insert  14 . 
         [0020]    With further reference to  FIG. 2 , a pair of upper heating elements  30 ,  32  and a pair of lower heating elements  36 ,  38  are placed in respective bores  40 ,  42 ,  46 ,  48  in the block  20 . 
         [0021]    With reference also to  FIG. 3 , the above-described components of the dry well calibrator heating block  10  are surrounded by an outer case  80  formed by case sections  80   a,b,c,d . The case section  80   d  contains circuitry  82  that is connected to the heating elements  30 ,  32 ,  36 ,  38  for supplying power to the heating elements  30 ,  32 ,  36 ,  38 . A fan assembly  84  containing a fan  86  is positioned inside the case section  80   a  so that the fan  86  is behind a grill  88 . The case  80  is separated from the block  20  by insulation (not shown) and an insulating space, and the fan  86  provides airflow through this insulating space to remove heat and maintain the circuitry  82  at a sufficiently low temperature. 
         [0022]    As best shown in  FIG. 4 , a keypad  90  mounted on a panel  92  of the case section  80   a  is connected to the circuitry  82  in the case section  80   d  ( FIG. 3 ) to control the operation of the dry well calibrator heating block  10 . A display  94 , which is also connected to the circuitry  82  in the case section  80   d  ( FIG. 3 ), provides information about the operation of the dry well calibrator  10 , such as the temperature of the block  20 . 
         [0023]    In operation, the keypad  90  ( FIG. 4 ) is used to set the temperature of the block  20  as well as the rate at which the temperature of the block  20  is changed to reach the desired set temperature. Once the temperature of the block  20  has stabilized, the temperature probe P ( FIG. 1 ) is inserted into a corresponding sized bore  16  of the insert  14 . The probe P is then calibrated by ensuring that a readout device (not shown) connected to the probe P indicates the temperature of the probe P is within an acceptable tolerance or equal to the set temperature of the dry well calibrator  10 . 
         [0024]    One embodiment of a system  100  for controlling the operation of the temperature calibration device  10  shown in  FIGS. 1-4  is shown in  FIG. 5 . The system  100  also includes a system  102  for warning of an unsafe condition in the temperature calibration device  10 . The control system  100  includes a temperature sensor  104  mounted on a surface to be monitored, such as the block  20  ( FIGS. 1-3 ). The temperature sensor  104  provides an analog signal indicative of the temperature of the block  20 . This analog signal is applied to an analog-to-digital (“A/D”) converter  106 , which outputs a plurality of bits on a bus  108  indicative of the temperature of the block  20 . These bits are applied to a controller  110 , which may be implemented by conventional means such as a properly programmed microprocessor. The controller  110  receives user commands from the keypad  90  ( FIG. 4 ) and applies signals to the display  94  for providing information to the user, as explained above. The controller  110  also outputs a temperature control signal to a driver  114 , which, in turn, outputs a temperature control voltage V TC  to the heating elements  30 ,  32 ,  36 ,  38  ( FIGS. 1 and 2 ). The above described components are powered by a supply voltage V + , which is generated by a power supply  120  from an AC supply voltage. 
         [0025]    In normal operation, the user enters commands through the keypad  90 , thereby causing the controller  110  to apply the temperature control voltage V TC  to the heating elements  30 ,  32 ,  36 ,  38  through the driver  114 . During these keypad entries, the controller  110  can apply the appropriate signals to the display  94  to assist the user in operating the control system  100 . The temperature of the block  20  will then increase or decrease depending on the polarity of the temperature control voltage V TC . As the block  20  is heated, the temperature of the block  20  is monitored by the temperature sensor  104  to provide feedback to the controller  110 . The controller  110  can then regulate the temperature control voltage V TC  to ensure that the temperature of the block  20  reaches the temperature set by the user using the keypad  90 . The control system  100  may also be capable of controlling the rate that the temperature of the block  20  increases or decreases to the set temperature as well as the rate that the temperature of the block  20  returns to an ambient temperature. 
         [0026]    After the temperature calibration device  10  has been used to calibrate a temperature probe P ( FIG. 1 ), it may be disconnected from the source of AC power. However, the temperature of the block  20  and other components internal to the calibration device  10  may remain at a high temperature for a substantial period. The duration of this period will, of course, vary with the temperature of the block  20  at the time power was removed from the device  10 . However, the warning system  102  provides a warning to a user of this high temperature condition even after AC power has been removed from the system  100 . 
         [0027]    The warning system  102  includes a large capacitor  130  receiving the supply voltage V +  from the power supply  120  through a diode  134 . When the power supply  120  is disconnected from AC power, the diode  134  isolates the capacitor  130  from the power supply  120 . However, the capacitor  130  continues to supply a voltage V CAP  for a period that is determined by the capacitance of the capacitor  130  and the rate at which current is drawn from the capacitor  130 . 
         [0028]    The voltage V CAP  from the capacitor  130  is applied to a switch  140  that is controlled by the controller  110 . The controller  110  causes the switch  140  to couple the voltage V CAP  to one of four resistors  142 ,  144 ,  146 ,  148 . The resistance of the four resistors  142 - 148  are different from each other so that the capacitor  130  is discharged at different rates depending upon which resistor  142 - 148  is coupled to the capacitor  130  after the power supply  120  is no longer receiving AC power. The switch  140  is powered by the voltage V CAP  so that it continues to couple the capacitor  130  to one of the resistors  142 - 148  after AC power has been removed from the power supply  120 . 
         [0029]    In operation, the discharge rate of the capacitor  130  is determined by the controller  110  during the operation of the system  100  when power is still being applied to the power supply  120 . The discharge rate is set by the controller  110  as a function of the current temperature of the block  20 . If the block  20  is very hot, the controller  110  may cause the switch  140  to couple the capacitor  130  to the resistor  148  having the highest resistance, thereby minimizing the discharge rate of the capacitor  130 . If the temperature of the block  20  is below a predetermined temperature, the controller  110  may cause the switch  140  to couple the capacitor  130  to the resistor  142  having the lowest resistance, thereby maximizing the discharge rate of the capacitor  130 . Intermediate temperatures of the block  20  cause the switch  140  to couple the capacitor  130  to one of the other resistors  144 ,  146 . 
         [0030]    The high temperature warning system  102  also includes an oscillator powered by the voltage V CAP  from the capacitor  130 . When the oscillator  150  is enabled by a low enables signal from the controller  110 , it periodically drives a cathode of a light-emitting diode  160  low. The anode of the light-emitting diode also receives the voltage V CAP  from the capacitor  130 . Therefore, during normal operation of the system  100  when the oscillator  150  is enabled by the controller  110 , the light-emitting diode  160  periodically emits light to warn a user that the block  20  and other internal components are too hot to touch. As shown in  FIG. 4 , this light-emitting diode  160  is mounted on the same panel  92  on which the keypad  90  and display  94  are mounted. 
         [0031]    When the power supply  120  is disconnected from the source of AC power, the controller  110  no longer receives the supply voltage V +  so that the controller  100  applies a low enables signal to the oscillator  150 . Insofar as the oscillator  150  is still powered by the voltage V CAP  from the capacitor  130 , the oscillator  150  continues to periodically drive a cathode of the light-emitting diode  160  low. Also, since the anode of the light-emitting diode  160  is powered by the voltage V CAP  from the capacitor  130 , the light-emitting diode  160  continues to periodically emit light. The light-emitting diode  160  continues to periodically emit light as long as the voltage V CAP  from the capacitor  130  is above a predetermined voltage. The duration of this period is, in turn, determined by the discharge rate of the capacitor  130 . As explained above, the discharge rate is determined by the temperature of the block  20  when AC power was removed from the power supply  120 . Therefore, the duration of the period during which the light-emitting diode  150  periodically emits light is determined by the temperature of the block  20  when the system  100  is disconnected from AC power. If the block  20  is very hot when AC power is removed from the system  100 , the light-emitting diode  160  will continue to blink for a long period commensurate with the time required for the block  20  to cool to a sufficiently low temperature. If the temperature of the block  20  is below a predetermined temperature value when AC power is removed, the light-emitting diode  160  will blink for a much shorter period of time commensurate with the time required for the block  20  to cool to a sufficiently low temperature. Intermediate temperatures of the block  20  cause the light-emitting diode  160  to blink for periods of intermediate durations. Therefore, the capacitor  130  is used not only as an energy storage device to apply power to the light-emitting diode  160  when AC power has been removed from the system  100 , but it is also used as a timing element to control the duration during which the light-emitting diode  160  is periodically illuminated. 
         [0032]    While the warning system  102  according to the present invention has been described in the context of a system for warning of a high temperature in a specific temperature calibration device, it can be used to warn of other unsafe temperature conditions in other devices. The warning system  102  can also be used to provide a high temperature warning in devices such as soldering irons, clothes irons, curling irons, electric fry pans and other similar devices. The warning system can also be used to provide warnings of unsafe conditions other than high temperature. In such case, the temperature sensor  104  ( FIG. 5 ) would be replaced by a sensor capable of monitoring the condition that may be unsafe. For example, in a system for warning of high hydraulic pressures, the sensor might be a pressure sensor. Other applications of the warning system  102  will be apparent to one skilled in the art. 
         [0033]    Although the present invention has been described with reference to the disclosed embodiments, persons skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. For example, although the warning provided by the system described herein is a visual warning provided by the light-emitting diode  160 , it will be understood that a different type of warning may be provided, such as an audible warning. Further, although the capacitor  130  is used to store energy from the externally applied AC power, it will be understood that other types of energy storage devices may be used in place of the capacitor  130 . Such modifications are well within the skill of those ordinarily skilled in the art. Accordingly, the invention is not limited except as by the appended claims.