JPS6244611B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a sensor control circuit for a high frequency heating device.

Many of the recent high-frequency heating devices have come to have an automatic food cooking function using a microcomputer-based control circuit. Therefore, the importance of stable sensor circuits for detecting the doneness of food products is becoming more and more important. The humidity sensor was born against this background.The humidity sensor, which detects the humidity of food and changes its resistance value, consists of a sensor part and a heater part on the surface of the sensor part. , its usage is indirect heat (AC voltage), reflex (DC voltage),
voltage) are always required. Indirect heating is performed to prevent dew condensation on the sensor, and refreshing is performed to remove foreign matter adhering to the sensor surface. Since this refresh must be performed frequently, it is very bothersome when the switching sound of relays, etc. can be heard outside. Therefore, it is necessary to use a relay with low noise.

A conventional example is shown in FIG. 1 is an indirect heating/refresh switching relay, and the position of the relay contact in the drawing indicates the indirect heating position. During indirect heating, the battery voltage is applied to the humidity sensor 5 through the NC contact of the relay 1 and the resistor 3 (alternating current). During refreshing, a DC constant voltage is applied to the humidity sensor from the NO contact of relay 1, smoothed by diode 2 and capacitor 8, passed through transistor 4, and resistor 12. As you can see from the diagram, relay 1 is inserted into the main circuit, so a relay with a somewhat large capacity is required (500
~600mA or more). Therefore, the switching sound of the relay can be easily heard from outside and is very unpleasant to the ears. Also, relays require both NC and NO contacts.

The present invention was made in view of the above circumstances, and its purpose is to construct a refresh circuit using a relay with a small capacity and very low switching noise, and which is composed only of NO contacts.

This will be explained using a specific example. (Fig. 2) The basic configuration is to control the base of the constant voltage transistor 20 to control the refresh and keep the indirect heat of the sensor energized. The operation will be explained below.

13 is a rectifying diode, and 14 is a smoothing capacitor, which converts alternating current into direct current and applies it to the collector of transistor 20. Now, when refreshing is not performed, the voltage applied to the collector passes through the resistor 16 and is applied to one end of the terminal of the reed relay 17, but since the contact of this relay is open, it is not applied to the base of the transistor 20. Therefore, no voltage is output to the emitter of transistor 20. During refresh, the contacts of reed relay 17 are closed and voltage is applied to the base of transistor 20. Therefore, a constant voltage determined by constant voltage diode 18 and diode 19 is outputted to the emitter of transistor 20. This voltage is connected to diode 24 and resistor 2
5 and is applied to the sensor. Because a large current does not flow through the relay 17 due to its circuit configuration, a small capacity relay can be used (approximately 2 mA). Also, the resistor 21 is a resistor to ensure that the transistor 20 is turned off when the relay 17 is turned off, and the capacitor 2
2 is a capacitor for preventing excessive voltage from being output to the output 20. In other words, when there is no capacitor 22 and the relay 17 is off, the load on the transformer becomes light and the capacitor 14
A high voltage is applied across both ends of the If the relay 17 is turned on in this state, a high voltage may be applied to the output of the transistor 20 at that moment (until the voltage regulator diode 18 and diode 19 stabilize). This may cause the sensor heater to become spot heated (a state in which only a portion of the heater is abnormally heated). To this end, the rise of the transistor 20 is slightly blunted by the capacitor 22 to prevent overvoltage from being output. This capacitor also serves as a ripple absorber.

Indirect heat is always applied through the resistor 23, but as shown in the sensor equivalent circuit in FIG. It doesn't appear. Also, transistor 2 with diode 24
There is no problem because the reverse bias voltage applied to 0 is cut off.

25 is a resistor for current limiting, and 15 is a resistor for discharging the charge charged in the smoothing capacitor 14 when the power is turned off.

By employing the circuit of the present invention, the current capacity of the refresh signal relay can be reduced, and the switching noise of the relay can be suppressed to a practically negligible level. Also, the relay only requires one NO contact circuit. It also has the effect of suppressing spot heating of the sensor.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a conventional sensor control circuit, FIG. 2 is a circuit diagram of a sensor control circuit according to an embodiment of the present invention, and FIG. 3 is an equivalent circuit diagram of the sensor. 4... Constant voltage transistor, 5, 28... Humidity sensor, 24... Reverse voltage blocking diode, 31
...humidity sensor resistance.

Claims (1)

[Claims] 1 A humidity sensor body having a resistance heating layer on the surface thereof, two terminals for feeding power to the resistance heating layer on the surface of this sensor, and a terminal for taking out a signal accompanying changes in humidity, etc. of this sensor body, Between the two terminals, there is usually an alternating current for heating the resistive heating element to a relatively low temperature, and a switch that can be opened and closed to heat the resistive heating element to a relatively high temperature at any time. A sensor control circuit characterized by supplying power by superimposing direct current.