JPH0634773B2 - Electric water heater - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electric water heater that keeps a cooking product at a constant temperature after boiling.

Conventional technology When keeping warm after boiling water with a conventional electric water heater,
Boiling detection is performed by detecting steam, and heat retention is performed by detecting the temperature of the container. The boiling detection means and the heat retention means are different.

Problems to be Solved by the Invention In such a conventional electric water heater, since the boiling detection means and the heat retention means are different from each other, there is no correlation between the boiling temperature and the heat retention temperature, and therefore the boiling temperature is affected by atmospheric pressure or the like. Since the heat retention temperature is constant even if it changes, even if it switches to heat retention after boiling in a place with a low atmospheric pressure, the boiling temperature is lower than the heat retention temperature, so it continues to boil during heat retention. Had.

An object of the present invention is to provide an electric water heater that solves such problems.

Means for Solving the Problems In order to solve the above problems, the present invention performs the boiling detection means and the heat retention means in the same circuit, based on the time measured by the charging time measuring means at the time of detecting the predetermined temperature, When the time measured is shorter than the reference by a certain percentage, it is determined that the time is boiling, and the heat retention temperature is set by the relative temperature from the boiling temperature.

Action According to the above configuration, since the heat retention temperature also changes according to the boiling temperature, even when the boiling temperature changes due to a change in atmospheric pressure or the like, the heat retention temperature is always set to a temperature lower than the boiling temperature by a constant temperature. It is a thing.

Embodiment One embodiment of the present invention will be described below with reference to the accompanying drawings. In FIG. 1, 1 is a commercial power source, 2 is a heater, 3 is a relay that constitutes a control element, and is constituted by a relay contact 4 and a relay coil 5. 6 is a thermistor 7 for detecting temperature, a voltage dividing circuit including a resistor 8, 9 is a resistor 10, 1
1, a charging circuit including a capacitor 12, and a transistor 13 forming a switching element, 14 a comparator forming a comparison circuit, 15 a charging time measuring means, and 16 a control means. FIG. 2 shows the waveforms at points a and b.

Next, the operation of the above configuration will be described. When the resistance value of the thermistor 7 is R ₇ , the resistance values of the resistors ₈ , ₁₀ , and ₁₁ are R ₈ , R ₁₀ , and R ₁₁ respectively, and the DC voltage is V _c , when the transistor 13 is ON, point a Potential of potential at point b Becomes Then, when the transistor 13 is ON, the temperature of the thermistor 7 is R so that V _a = V _b at 70 ° C.
By setting ₈ , R ₁₀ and R ₁₁ , the temperature of the thermistor 7 will be 7
When the temperature is 0 ° C. or lower, V _a ≦ V _b , and as a result, the output of the comparator 14 is always LOW. In this state, the control means 16 energizes the relay coil 5 to short-circuit the relay contact 4, so that the heater 2 generates heat. The heating of the heater 2, the container is heated to increase the temperature of the thermistor 7, if exceeds the 70 ° C., ON times _V a> _{V b} next to the transistor 13, as a result, the output of the comparator 15 becomes to HIGH.

At this time, if the control means 16 outputs an OFF signal to the transistor 13, the transistor 13 is turned off and the capacitor 12 is charged through the resistor 10. If V _b = V _a due to this charging, the output of the comparator 14 becomes LOW, and as a result, the transistor 13 is turned ON by the control means 16 and the electric charge of the capacitor 12 is discharged through the resistor 11. Time (charging time) during which the comparator 14 is HIGH for every fixed time (here, 1 second)
The charging time measuring means 15 measures 70 ° C.
Immediately after that, the change of the charging time per second, that is, the rate of change of the charging time is known, and the temperature change of the thermistor 7 is obtained. If this change is taken as the initial value and the change in charging time becomes 1 / n (here 1/3) of the initial value, it means that the temperature of the thermistor 7 has reached the saturation point and the water in the container boiled. It is understood that this is the boiling point. Assuming that the charging time at this point is T, if the temperature of the thermistor 7 becomes low, the resistance R
_{Since 7} becomes large, V _a becomes low and T becomes short. By utilizing this, after the boiling point, the heater 2 is energized to control the charging time to a predetermined value smaller than T (here, 25
If the temperature of the thermistor 7 is maintained at a resistance value such that Kept in. Even if the pressure is low and the boiling point is 90 ° C, the thermistor 7 Kept in.

As described above, according to the present invention, since the heat retention temperature also changes according to the boiling temperature, even if the boiling temperature changes due to a change in atmospheric pressure or the like, the heat retention temperature exceeds the boiling temperature and continues boiling. There is no problem, and the heat can be kept at a temperature close to the boiling temperature.

Also, since the rate of change of the charging time to the capacitor is not detected until the capacitor potential of the charging circuit and the divided potential of the voltage dividing circuit match, when the rate of change of the water temperature immediately after the start of energizing the heater is small. It is possible to accurately detect boiling without erroneously determining boiling and shifting to the heat retaining operation.

[Brief description of drawings]

FIG. 1 is a circuit diagram of an electric water heater showing an embodiment of the present invention, and FIG. 2 is a waveform diagram of each part in the circuit. 2 ... Heater, 3 ... Relay (control element), 6 ... Voltage dividing circuit, 7 ... Thermistor, 8 ... Resistance, 9 ... Charging circuit, 10 ... Resistance, 11 ... Resistance, 12 ... Capacitor, 13 ... Transistor (switching element), 14
...... Comparison circuit, 15 …… Charging time measuring means, 16 …… Control means.

Claims (1)

[Claims] 1. A heater for heating a liquid in a container, a control element for controlling energization of the heater, a thermistor for detecting the temperature of the liquid, and a series circuit in which a resistor and a switching element are connected in series is connected in parallel. And a charging circuit consisting of a resistor connected in series with this capacitor,
A voltage dividing circuit composed of the thermistor and a resistor, a comparing circuit for comparing the capacitor potential of the charging circuit and the voltage dividing potential of the voltage dividing circuit, and measuring the charging time of the capacitor until the compared potentials match. Charge time measuring means, and based on the output of the comparison circuit, a control element, consisting of a control means for controlling the switching element, the control means,
When a predetermined temperature is detected based on the time measured by the charging time measuring means, the control element is controlled and the heater is energized until the time measured becomes a certain percentage smaller than the reference based on the time measured at the time of detection. An electric water heater that lowers the temperature of the thermistor by controlling the energization of the heater after the temperature becomes smaller than the reference by a certain ratio.