JPS6338560Y2 - - Google Patents

Description

[Detailed description of the invention] (a) Industrial application field The present invention relates to a thermistor disconnection detection circuit for an electromagnetic cooker.

(b) Conventional technology Unlike other heating cookers, electromagnetic cookers do not generate flames and are therefore generally safe, but when heating cooking oil for tempura, etc., if the temperature reaches 350°C or higher, it may spontaneously ignite. There is a risk of fire.

Therefore, a thermistor that detects the load temperature is used under the top plate to detect the temperature.

However, the thermistor may break during product transportation,
If the thermistor is destroyed for some reason, such as by the impact of placing a load such as a pot on the top plate, the user will not notice the temperature rise until it becomes unable to detect the temperature and spontaneously ignites, which can be extremely dangerous. It was hot.

(c) Purpose The purpose of this invention is to prevent the load from heating up and causing a fire hazard when the thermistor is destroyed for some reason and becomes open, with the temperature detection function stopped. .

(d) Structure of the invention The invention consists of an inverter circuit that inductively heats a load, a bridge circuit that has a thermistor with a negative characteristic on one side that detects the temperature of the load and outputs a signal based on the detected temperature, and the bridge circuit. an open collector type comparator that connects an output terminal on one side to a negative input terminal and connects an output terminal on the other side to a positive input terminal, and sends out a signal for controlling oscillation of the inverter circuit by comparing both inputs; A transistor whose base is connected to the input side of the thermistor and whose collector is connected to the output side of the comparator via a resistor, and which forcibly sets the output of the comparator to a low level to stop the oscillation of the inverter circuit when the thermistor is disconnected. It is equipped with the following.

(E) Embodiment An embodiment of the present invention will be described below based on the attached drawings. FIG. 1 is a block diagram showing the circuit of the electromagnetic cooker of the present invention, in which 1 is an inverter circuit (not shown) that supplies high-frequency power to a load to inductively heat the load, and 2 is an oscillation operation of the inverter circuit 1. 3 is a starting circuit that generates a starting pulse (high level pulse) that causes the inverter circuit 1 to perform an initial operation; 4 is a starting circuit that detects the oscillation operation of the inverter circuit 1 and starts the oscillation of the inverter circuit 1; An oscillation feedback circuit 5 sends out an oscillation signal to continue the operation, and a thermistor disconnection 5 stops the operation of the inverter circuit 1 when the load temperature reaches a preset upper limit temperature or when a thermistor 10 (described later) is disconnected. This temperature detection circuit 5 is a temperature detection circuit that also serves as a detection circuit.The temperature detection circuit 5 will be explained based on _FIG . It is grounded via diodes 8 and 9 as means, a thermistor 10 with a negative characteristic as a temperature detection element for detecting the temperature of the load, and a resistor 11, and the other side is connected to a DC power supply V _DD in advance. Resistors 12 and 13 for sending a signal at a predetermined level corresponding to the set upper limit temperature of the load
It consists of. Reference numeral 14 has a negative input terminal connected to the connection point a between the thermistor 10 and the resistor 11 to input a signal based on the temperature of the load detected by the thermistor 10, and a positive input terminal connected to the connection point a between the thermistor 10 and the resistor 11.
an open collector type comparator connected to the connection point b between 2 and 13 and inputting a signal at a predetermined level corresponding to a preset upper limit temperature of the load;
15 connects the base to the diode 9 and thermistor 1
0, the collector is connected to the output terminal of the comparator 14 via a resistor 16, and the emitter is connected to the DC power supply _VDD.The voltage level of the base of the transistor 15 is equal to that of the collector. The emitter-base current flows through the thermistor 10 with the voltage level lowered by the voltage drop caused by the diodes 8 and 9, thereby making it conductive. Furthermore, the emitter-collector current flowing due to conduction of the transistor 15 is caused by the resistor 1
6 to the output terminal of the comparator 14 as a bias power supply. 6 is the oscillation feedback circuit 4
This is an AND circuit which receives a signal from the temperature detection circuit 5 and sends an H level signal to operate the inverter circuit 1 via the drive circuit 2 when both signals are high level (hereinafter referred to as H level) signals.

Next, to explain the operation in the above-mentioned configuration, first, the starting pulse (H level pulse) output from the starting circuit 3 is passed through the oscillation feedback circuit 4.
Input to AND circuit 6. In addition, the temperature detection circuit 5
Since the load is not heated, its temperature is low, the resistance value of the thermistor 10 is large, and the voltage level at point a is lower than the voltage level at point b, so the comparator 14 outputs an H level signal as an operating signal. At this time, the transistor 15 is conductive, so that an emitter-collector current flows and is supplied to the comparator 14 as a bias power source, so that an H level signal is input to the AND circuit 6.
Therefore, the AND circuit 6 sends out the starting pulse output from the starting circuit 5 as it is, and outputs this pulse to the drive circuit 2, which causes the inverter circuit 1 to start oscillating. Then, in order for the oscillation feedback circuit 4 to detect the oscillation operation of the inverter circuit 1 and send out an oscillation signal, a self-excited oscillation loop is formed to oscillate the inverter circuit 1 via the AND circuit 6 and the drive circuit 2 again, and its operation is controlled. By continuing, the load is heated by induction.

After that, when the temperature of the load rises and the resistance value of the thermistor 10 decreases, and the voltage level at point a rises higher than the voltage level corresponding to the upper limit temperature output from point b, the output terminal of the comparator 14 becomes low as a stop signal. It becomes a level (hereinafter referred to as L level) signal, and the bias power supplied by the conduction of the transistor 15 is grounded from the output terminal of the comparator 14. Therefore, the output of the temperature detection circuit 5 becomes an L level signal as a stop signal, so the AND
Circuit 6 sends an L level signal to inverter circuit 1
The oscillation operation of the load is stopped and the heating operation of the load is stopped.

Next, a case where the thermistor 10 is disconnected will be explained. If the thermistor 10 is disconnected, a
Since the voltage level at point B becomes zero level and the voltage level at point b is always higher, the output terminal of comparator 14 becomes an H level signal. The bias power supply that was being supplied to the output terminal of 14 is stopped. Therefore,
Regardless of the level of the signal output from the output terminal of the comparator 14, the output of the comparator 14 becomes an L level signal as a stop signal.
The AND circuit 6 can always send an L level signal to stop the operation of the inverter circuit 1.

(f) Effect As described above, the present invention is designed to stop the operation of the inverter circuit and stop heating the load when the thermistor that detects the temperature of the load is disconnected. It is possible to provide an extremely safe electromagnetic cooker that can prevent ignition, abnormal rise in load temperature, etc.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an electric circuit showing an embodiment of the electromagnetic cooking device of the present invention, and FIG. 2 is a circuit diagram showing an embodiment of the temperature detection circuit shown in FIG. 1... Inverter circuit, 5... Temperature detection circuit (thermistor disconnection detection circuit), 10... Thermistor, 14... Comparator, 15... Transistor.

Claims (1)

[Scope of utility model registration request] An inverter circuit that inductively heats a load, a bridge circuit that has a thermistor with negative characteristics on one side that detects the temperature of the load and outputs a signal based on the detected temperature, and the output end of one side of the bridge circuit is connected to a negative input terminal. an open collector type converter whose output terminal on the other side is connected to a positive input terminal and which sends out a signal for controlling the oscillation of the inverter circuit by comparing both inputs; an electromagnetic transistor comprising: a transistor whose collector is connected to the output side of the converter via a resistor, and which forcibly sets the output of the converter to a low level to stop oscillation of the inverter circuit when the thermistor is disconnected; Cooker thermistor disconnection detection circuit.