Kettle controller

The present invention provides apparatuses and methods for boiling a liquid such as water. The liquid is heated at full power when the measured temperature of the liquid is below a first predetermined threshold. When the temperature is above the first predetermined threshold and below a second predetermined threshold, partial power, which may be based on the duty cycle, is applied to the heater. When the measured temperature of the liquid reaches the second predetermined threshold, power is removed from the heater after a predetermined time. When the increase of the measured temperature is less than a predetermined temperature change during a predetermined time duration, it is determined that the liquid is boiling and power is removed to the heater. If the level is too high or too low, an alarm may be activated and/or power removed from the heater.

BACKGROUND OF THE INVENTION

Electric kettles have long been used to heat liquids such as water. Generally, electric kettles include a temperature sensor to measure the temperature of the liquid being heated. Many kettles include a time sensor as well.

One form of thermostat in kettles cuts power to the heating element after the water in the chamber has been brought to boil. A conduit, typically in the form of a copper tube, is provided from a location above the surface of the water in a filled kettle to a location adjacent a thermostat which is adapted to cut power to the element when it senses the high temperature due to the steam.

However, with prior art kettles, the contained liquid may fast boil the liquid when the temperature of the liquid is sufficiently. Consequently, the liquid may spurt out of the kettle, causing injury to the user. It is desirable to provide an efficient way of boiling a liquid that also provides enhanced safety to the user.

SUMMARY OF THE INVENTION

The present invention provides methods and apparatuses for boiling a liquid such as water.

With an aspect of the invention, a liquid is heated at full power when the measured temperature of the liquid is below a first predetermined threshold. When the temperature is above the first predetermined threshold and below a second predetermined threshold, partial power is applied to the heater. When the measured temperature of the liquid reaches the second predetermined threshold, power is removed from the heater after a predetermined time.

With another aspect of the invention, the boiling temperature of the liquid is determined. The first predetermined temperature differs from the boiling temperature by a first temperature difference, and the second predetermined temperature differs from the boiling temperature by a second temperature difference.

With another aspect of the invention, the boiling temperature of the liquid is determined from the altitude of the apparatus and the barometric pressure.

With another aspect of the invention, when an increase of the measured temperature is less than a predetermined temperature change during a predetermined time duration, it is determined that the liquid is boiling and power is removed to the heater.

With another aspect of the invention, the level of the heated liquid is determined. If the level is too high or too low, an alarm is activated and/or power removed from the heater.

With another aspect of the invention, partial power applied to a heater is based on a duty cycle of the applied signal to the heater.

DETAILED DESCRIPTION

FIG. 1shows system100for boiling liquid103according to an embodiment of the invention. Heater is provided electrical energy from a signal through inputs107a,107b. Container (kettle)101, which holds liquid103, is in close proximity to heater105so that heat is transferred to liquid103.

Liquid103may comprise different mixtures, e.g., water, a combination of water and coffee, or other chemical compounds. Also, whileFIG. 1shows an alternating current power input (107a,107b), embodiments of the invention may support different types of power inputs, e.g., three-phase alternating current and direct current.

Controller111adjusts the power applied to heater105through switch109by measuring the temperature of liquid103from temperature sensor113. With an embodiment of the invention, controller111controls switch109by adjusting the duty cycle of the applied signal201as will be discussed.

As will be discussed, controller111determines whether the water level is too high or too low from an indicator provided by temperature sensor113.

FIG. 2shows an exemplary signal201providing power to heater105according to an embodiment of the invention. Signal201is applied to heater105during time duration251(one cycle) while signal is removed during subsequent time duration253(two cycles). Heater105is provided with partial power, where the duty cycle is ⅓. Because the duty cycle is ⅓, the average power supplied to heater105is 1/9 of full power.

Each cycle of signal201is approximately 16.6 mS for 60 Hz (US) and 20 ms for 50 Hz (Europe, China). According to an embodiment of the invention, the average voltage Vsapplied to heater is:
Vs=NVf(EQ. 1)
the full power (Pf) is:
Pf=Vf2/R(EQ. 2)
and the slow boiling power Psis equal to:
Ps=N2Pf(EQ. 3)
where N is the duty cycle (on time/(on time+off time), Vfis the input voltage, and R is the resistance equivalent of heater105.

With an embodiment of the invention, the slow boiling power (partial power) ranges from 300-1000 W (corresponding to a duty cycle (N) between ⅓ and ½) when boiling water. The partial power rate is determined by water level and kettle characteristics e.g., low water level or narrow hatch kettle may use a high rate power 700-1200 W but high water level or big hatch kettle may use a low rate power 300-600 W. A range from 500 to 1000 W is a typical slow boiling power range.

FIG. 3shows apparatus300for boiling a liquid in a container (not shown) according to an embodiment of the invention. Heater301is provided power by a signal through inputs303a,303band switching circuit305. Switching circuit305may be implemented in a number of ways. For example, switching circuit305may comprise a triac that is turned on and off by main control circuit307in accordance with a desired duty cycle. (A triac is a three terminal semiconductor for controlling current in either direction.) With full power being applied to heater301, switching circuit305continuously conducts (i.e., the duty cycle is 1). If the duty cycle is ⅓ (corresponding to partial power), then switching circuit305may repetitively turn on for one cycle and turn off for two cycles as shown inFIG. 2. If the duty cycle is ½, then switching circuit305may turn on for one cycle and turn off for one cycle.

As will be further discussed, main control circuit307determines the duty cycle from temperature sensor309and synchronizes switching circuit305turning on and off with the zero crossings of the voltage of input signal201. Main control circuit307determines the zero crossings from zero crossing/frequency detector311. Detector311is able to detect zero crossings even though the frequency of input signal201may vary, e.g., 60 Hz for the US and 50 Hz for Europe.

Voltage detection circuit315measures the voltage level of input signal201so that main control circuit307can determine the full power (Pf) in accordance with EQ. 2. The value of the full power is dependent on the characteristics of the kettle. As an example, typically 2 kW to 3 kW is applied to heater301when main control circuit307determines that full power should be applied, typically when the temperature of the liquid, as measured by temperature sensor309, is below a predetermined temperature. The predetermined temperature is dependent on the characteristics of the liquid and on environmental factors (e.g., altitude and barometric pressure).

If full power were applied to heater301when the liquid temperature is above the predetermined temperature (e.g., 95° C. at sea level), the water may bubble excessively and spurt out of the kettle. Consequently, when the liquid temperature is above the predetermined temperature, main control circuit307reduces the applied power to heater301to partial power (Pf), as given by EQ. 3. Typically, N is equal to ⅓ to ½. With partial power, bubbling of the water is reduced, preventing the water from spurting out and harming the user.

Level sensor313provides an indicator of the level of the liquid to main control307. If the liquid level is too low, the kettle may overheat, causing damage to the kettle and possibly starting a fire. If the liquid level is too high, the liquid may overflow the kettle and cause hot vapor to injure the user. Consequently, when main control circuit307detects the liquid level as being too low or too high, main control circuit307removes power from heater301and/or activates an alarm.

Capacitor317filters noise from switching circuit305in order to reduce electromagnetic interference. For example, capacitor317may comprise a X2 capacitor to filter triac switching noise in order to pass electromagnetic compatibility tests. (An X capacitor is a RFI capacitor used in positions where if failed would not be hazardous to anyone who touches the case of the equipment. The X capacitors are connected across the line conductors. There are three sub-classes of X capacitors: X1, X2 and X3. The most common is X2 sub-class, used for IEC-664 Installation Category II. The X2 capacitors are rated for peak pulse voltage in service of less or equal to 2.5 KV.) The value of capacitor317may be determined by:
Cx2=0.03˜0.055I(uF)  (EQ. 5)
where I is the full power current. For example, when full power is 3000 W/240V, I=12.5 A and Cx2=0.375(0.47 uF) ˜0.68 uF.

FIG. 4shows apparatus400for boiling a liquid according to an embodiment of the invention. Heater401heats liquid in a container (not shown) when power is applied by a input signal passing through switching circuit403. Processor405determines duty cycle N from information provided by temperature sensor409, altitude sensor413, and barometric pressure sensor415. (In contrast to apparatus300, apparatus400directly determines the boiling temperature of the heated liquid.) Consequently, the boiling temperature (Tb) of the liquid, which is a function of the altitude and the barometric pressure, may be determined by processor405from a look-up table or from a mathematical relationship. The table that is later presented shows some examples of the boiling temperature of water as a function, of the altitude and barometric pressure.

As previously discussed, when the measured liquid temperature is below a predetermined temperature, switching circuit403(as instructed by processor405) applies full power to heater401. When the measured liquid temperature is above or equal to the predetermined temperature, partial power is applied to heater401.

Processor405instructs switching circuit403to switch at zero crossings as determined by zero crossing detector407in accordance with the determined duty cycle. As previously discussed, processor405instructs switching circuit403to remove applied power to heater401when the level of the liquid in the container is too low or too high as determined by level sensor411.

FIG. 5shows flow diagram500in which a liquid is boiled according to an embodiment of the invention. While embodiments of the invention can boil different types of liquids, flow diagram500illustrates the boiling of water. If user requests to boil a liquid in step501, processor307determines whether the liquid level is too high or too low in step503. If so, an alarm is activated to inform the user in step505. In addition, power may be removed from heater301.

In step507, apparatus300applies full power to heater301or401in synchronism with the zero crossings of input signal201. Because apparatuses300and400can operate at different altitudes, the boiling temperature of water may vary (e.g., 100° C. at sea level, 95° C. at 5000 feet, and 89° C. at 10000 feet). Consequently, the heated water can boil at a temperature substantially below the boiling temperature at sea level (100° C.).

In step509, processor307determines if the measured temperature of the heated water is 88° C. or greater. If so, the water may boil at a temperature less than 100° C. depending on the altitude and the barometric pressure. (For example, distilled water in Denver, Colo. boils at 93° C. at a barometric pressure of 29 inches HG. At higher elevations (e.g. La Paz, Bolivia), the boiling temperature is even lower.) In step511, if the measured temperature of the heated water rises less than a predetermined temperature change (e.g., 2° C.) during a predetermined time duration (e.g., at least 10 seconds), then process500deems that the water is boiling and turns off the heater in step517. (A liquid generally cannot be heated to a temperature above its boiling point. Upon reaching the boiling temperature, a phase transition takes place and all energy is utilized to convert the liquid into gas rather than to heat the liquid.)

If apparatus300deems that the heated water is not boiling and that the measured temperature is above 95° C. (first predetermined temperature) in step513, then partial power is applied to heater301in step515. In the exemplary embodiment, the partial power is 1/9 of full power because the duty cycle is ⅓.) Heater301continues to heat the water with partial power until the measured temperature reaches a second predetermined temperature (e.g., 98° C. for water). When the measured temperature is approximately equal to the second predetermined temperature, heater301is turned off after a predetermined time (e.g., 10 seconds) so that the liquid reaches a boiling temperature. Experimental results suggest that the water temperature typically increases from 98° C. increase to 100° C. within a 10-second duration.

FIG. 6shows flow diagram600in which a liquid is boiled by apparatus400according to an embodiment of the invention. In step601, the boiling temperature (Tb) of the liquid is determined from the type of liquid (e.g., distilled water), the altitude, and the barometric pressure of apparatus400. The following table provides examples of the boiling temperature of water as a function of the altitude and barometric pressure.

In step603, the liquid is heated at full power until the measured temperature is within 5° C. of boiling temperature of the liquid as determined by step605. (For example, if the liquid is water at sea level, the corresponding temperature is 95° C.) Subsequently, partial power is applied to heater401in step607until the measured temperature of the liquid is within 2° C. of the boiling temperature as determined by step609. (For example, if the liquid is water at sea level, the corresponding temperature is 98° C.) Subsequently, the heater is turned off after a predetermined time (e.g., 10 seconds).

As can be appreciated by one skilled in the art, a computer system with an associated computer-readable medium containing instructions for controlling the computer system can be utilized to implement the exemplary embodiments that are disclosed herein. The computer system may include at least one computer such as a microprocessor, digital signal processor, and associated peripheral electronic circuitry.