Abstract:
A portable heater operates at a first power rating to rapidly heat a space and a second power rating, lower than the first, to safely maintain the temperature of the space. The heater uses a first resistive heating element and a second resistive heating element which are configured to automatically supply the first power rating during an initial heating period then step down to the second power rating for continuous operation.

Description:
This application claims the benefit of United States Provisional Patent Application Serial Number 60/038,398, filed on Feb. 14, 1997. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to electric heaters, and more particularly relates to an heater with automatic switching of series and parallel heating element connections capable of operating at a first power rating and a second power rating. 
     BACKGROUND AND DESCRIPTION OF THE PRIOR ART 
     Portable electric heaters must presently be limited to an electrical capacity of 1500 watts. This limit for continuous operation is set to reduce the risk of fire associated with continuous use heating devices operating at capacities beyond this power rating. This limit is also enforced by product certification laboratories such as Underwriters Laboratories. 
     However, in an enclosed space to be heated, the operation of a heater at 1500 watts or less results in extended heating times. Therefore, there is a need for an electric heater which operates at a higher power rating during the initial heating cycle when the space is at its coldest and then operates at a lower power rating for the remaining continuous heating operation. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an electric heater which will rapidly heat a given space without increasing the risk of fire. 
     It is another object of the present invention to provide a two-stage electric heater which operates at an initial high power level at the beginning of a heater cycle, then automatically drops to a lower power level for continuous operation. 
     In accordance with one form of the present invention, a heater is formed having two resistive heater elements and an automatic switching device. The switching device initially selects the heater elements to provide a first, reduced resistance for the initial a predetermined operating time of the heater, then selects a higher resistance for the remaining continuous operation of the heater. 
     In accordance with another form of the present invention, a heater is formed with a first resistive heating element and a second resistive heating element connected to form a series circuit. The heater further includes a thermally responsive switch coupled across the second resistive heating element. The thermally responsive switch initially presents a short circuit across the second resistive heating element, thereby reducing the total resistance of the series circuit. Upon detection of a predetermined temperature, the thermally responsive switch opens, thereby increasing the resistance of the series circuit. In the presence of a substantially constant voltage applied to the series circuit, the operation of the switch results in a first operating power and a second, lower operating power. In accordance with a preferred embodiment of the present invention, the thermally responsive switch latches after the predetermined temperature is reached and remains latched until the voltage is removed. 
     In accordance with another form of the present invention, a heater is formed with a first resistive heating element and a second resistive heating element connected to form a series circuit. The heater further includes an electrically controllable switch coupled across the second resistive heating element. The electrically controllable switch initially presents a short circuit across the second resistive heating element, thereby reducing the total resistance of the series circuit. Upon receipt of a control signal, the electrically controllable switch opens, thereby increasing the resistance of the series circuit. The heater includes a timer circuit which generates the control signal upon initial detection of a voltage applied to the series circuit. In the presence of a substantially constant voltage applied to the series circuit, the operation of the switch results in a first operating power for an initial time period and a second, lower operating power for the remaining time of operation. 
     In accordance with yet another form of the present invention, a heater is formed with a first resistive heating element and a second resistive heating element connected to form a parallel circuit. The heater further includes a thermally responsive switch coupled in series with the second resistive heating element. The thermally responsive switch initially presents a short circuit, thereby reducing the total resistance of the parallel circuit. Upon detection of a predetermined temperature, the thermally responsive switch opens, thereby increasing the resistance of the parallel circuit. In the presence of a substantially constant voltage applied to the parallel circuit, the operation of the switch results in a first operating power and a second, lower operating power. In accordance with a preferred embodiment of the present invention, the thermally responsive switch latches after the predetermined temperature is reached and remains latched until the voltage is removed. 
     In accordance with yet another form of the present invention, a heater is formed with a first resistive heating element and a second resistive heating element connected to form a parallel circuit. The heater further includes an electrically controllable switch coupled in series with the second resistive heating element. The electrically controllable switch initially presents a short circuit, thereby reducing the total resistance of the parallel circuit. Upon receipt of a control signal, the electrically controllable switch opens, thereby increasing the resistance of the parallel circuit. The heater includes a timer circuit which generates the control signal upon initial detection of a voltage applied to the parallel circuit. In the presence of a substantially constant voltage applied to the parallel circuit, the operation of the switch results in a first operating power for an initial time period and a second, lower operating power for the remaining time of operation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a heater circuit formed in accordance with the present invention. 
     FIG. 2 is a schematic diagram of a heater circuit formed in accordance with the present invention. 
     FIG. 3 is a schematic diagram of a heater circuit formed in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     When an enclosed space is to be heated, it is desirable to operate a portable space heater at the highest allowable power rating during a predetermined initial time period of the heating cycle, i.e., when the space is at its coldest temperature. To reduce the risk of fire associated with high power operation, it is also desirable to throttle back the power rating of the heater after the initial a predetermined period, to a second, reduced power rating. 
     FIG. 1 is a schematic diagram of a first embodiment of the present invention. The heater of FIG. 1 includes a series circuit formed by the connection of a first resistive heating element  2 , a second resistive heating element  4 , a thermal fuse  6  and a high-limit normally closed thermostat  8 . Terminals  10  are included to attach the series circuit to an external power source  12 . 
     The heater circuit of FIG. 1 further includes a thermal switch  14  which is connected in parallel with the second resistive heating element  4 . Thermal switch  14  operates in a normally closed (low resistance) state when the device is at a first temperature, and an open state (high resistance) upon reaching a second, higher temperature. Preferably, thermal switch  14  is a positive temperature coefficient (PTC) device which latches in the open state until power is removed from the circuit. 
     When the power source  12  is first connected to the series circuit, the thermal switch  14  is in its initial, closed state. This places a short circuit across the second resistive heating element  4 . Accordingly, the initial resistance of the series circuit is the resistance of the first resistive heating element  2 . Upon reaching a predetermined temperature (associated with a predetermined time), the thermal switch  14  opens. This replaces the second resistive heating element  4  into the series circuit and increases the total circuit resistance. The increased total circuit resistance lowers the power rating of the heater for the remaining, continuous operation of the heater. As previously discussed, the use of a latching type device for the thermal switch  14  is preferred. This prevents the heater circuit from inadvertently reverting to the high power mode. 
     FIG. 1 further illustrates the use of an optional current sensor  18 . The current sensor  18  is connected in series with the thermal switch  14 . The current sensor  18  is a low resistance device that detects current flow and activates a display element  20  to indicate the mode of operation of the heater, i.e., high power, fast heating mode or constant power mode. Alternatively, a voltage sensor may be operatively coupled across the second resistive heating element  4  to perform the mode detection function. 
     FIG. 2 illustrates an alternate embodiment of a heater circuit formed in accordance with the present invention. Referring to FIG. 2, the heater circuit includes a first resistive heating element  2 , a second resistive heating element  4 , a power source  12  and a single pole, single throw (SPST) switch  20  connected as a single series circuit. The heater further includes an electrically controllable switch  24 . The electrically controllable switch  24  includes first and second switch terminals which are electrically connected across the second heating element  4 . The electrically controllable switch  24  also includes at least a third control terminal which receives a control signal. In response to the received control signal, the switch terminals open (high resistance) or close (low resistance). The electrically controllable switch may take the form of a solid state switch or conventional relay. In this circuit configuration, when the switch terminals are closed, the second heater element  4  is bypassed (shorted) in the series circuit. This reduces the total circuit resistance and increases the power rating of the heater. As with the thermal switch  14 , it is preferable that once the electrically controllable switch  24  is opened, it remains latched in this state until the circuit is de-energized. 
     The heater circuit of FIG. 2 further includes a timer circuit  22 . The timer circuit  22  includes an input terminal which is electrically connected to the series circuit and an output terminal which is electrically connected to the control terminal of the electrically controllable switch  24 . The timer circuit  22  detects when the series circuit is energized (SPST switch  20  closed). This condition initializes the timer output terminal to a first state which closes switch  24 . Accordingly, the heater is initially in a high output mode to quickly warm the environment when the most heat is needed. After a predetermined time, the timer circuit  22  changes the state of the output terminal, thereby opening switch  24 . With switch  24  opened, the second resistive heating element  4  is replaced in the series circuit thereby reducing the power rating of the heater for the remaining heating period. Thus, the continuous duty cycle of the heater operates at the lower power rating. The timer circuit  22  may be realized by employing an appropriately configured 555 integrated circuit timer or other conventional timing circuit known in the art. 
     As an illustrative example, the heater circuit of FIGS. 1 and 2 may be constructed to provide 1800 watts of heat during the initial heating period and drop to 1500 watts of heat output for the balance of the heating period. This is achieved by selecting the first resistive heating element  2  to have a resistance of approximately 8 ohms (Ω), the second resistive heating element  4  to have a resistance of approximately 1.6 Ω and the power source to have a voltage potential of approximately 120 volts AC. Initially, when the second resistive heating element  4  is bypassed, the total resistance of the series circuit is 8 Ω. When the second resistive heating element is replaced in the circuit, the resistance increases to 9.6 Ω. As the voltage from power source  12  remains a constant 120 volts AC, this change in resistance effectively alters the power rating of the heater. 
     FIG. 3 shows an embodiment of a two-stage heater circuit formed in accordance with the present invention using a parallel arrangement of the heating elements. In this embodiment, a first resistive heating element  2  is connected in parallel with a series combination of a thermal switch  14  and a second resistive heating element  4 . An external power source  12  can be coupled across the parallel circuit by connection to terminals  10 . As with the circuits of FIGS. 1 and 2, the heater of FIG. 3 operates at an initial high power rating for a first time period, then drops to a reduced power level for continuous operation. When the thermal switch  14  is closed, the resistance of the first and second heating elements combine in parallel to form a reduced combined resistance. When the thermal switch  14  is opened, the first resistive heating element is the only resistance in the circuit, thereby increasing the total circuit resistance and reducing the operating power. It will be appreciated that the thermal switch  14  may be replaced with other automatic control means, such as the timer circuit  22  and electrically controllable switch  24  illustrated in FIG.  2 . 
     As an example of the operation of the circuit in FIG. 3, the heater circuit of the present invention may be constructed to provide an 1800 watt rating during the initial heating period and revert a 1500 watt rating for the balance of the heating period. This is achieved by selecting the first resistive heating element to have a resistance of 9.6 Ω, the resistance of the second resistive heating element to have a resistance of 48 Ω and the external power source to supply a voltage of 120 volts AC. When power is first applied to the circuit, the thermal switch  14  is closed and the total resistance of this circuit is the parallel combination of 48 Ω and 9.6 Ω. This total resistance is equal to 8 Ω. After a predetermined time, the thermal switch  14  opens and increases the resistance of the circuit to that of the first resistive heating element, or 9.6 Ω. 
     It will be appreciated by those skilled in the art, that the concept of a two-stage heating circuit as illustrated in FIGS. 1-3 can be extended to a multi-stage heater by adding additional heating elements and additional control elements. It will be further appreciated that the elements of the fuse  6 , thermostat  8 , current sensor  18  and display element  20  may also be implemented in the circuits of FIGS. 2 and 3. 
     Although illustrative embodiments of the present invention have been described herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various other changes and modifications may be effective therein by one skilled in the art without departing from the scope or spirit of the invention.