Dual power rated electric heater

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.

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.

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 (.OMEGA.), the
 second resistive heating element 4 to have a resistance of approximately
 1.6 .OMEGA. 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
 .OMEGA.. When the second resistive heating element is replaced in the
 circuit, the resistance increases to 9.6 .OMEGA.. 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 .OMEGA., the
 resistance of the second resistive heating element to have a resistance of
 48 .OMEGA. 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 .OMEGA. and 9.6 .OMEGA.. This total resistance is equal
 to 8 .OMEGA.. 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 .OMEGA..
 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.