JPS6338722B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a temperature control circuit that controls the temperature of a boiler or the like using a combustion control device.

This temperature control circuit starts combustion when the temperature of the boiler, etc. drops below a certain temperature and stops combustion when the temperature rises above a certain temperature, depending on the setting value. This is how it is controlled. That is, there is a difference (hereinafter referred to as differential) between the temperatures corresponding to the start and stop of combustion. When this differential is small, the combustion device is constantly turned on and off, allowing extremely accurate temperature control, but this poses a problem in terms of service life. Particularly in temperature control of boilers, etc., fairly accurate temperature control is often required when the set temperature is near the maximum value, but when it is set near the minimum value, it is simply to prevent the water in the boiler from freezing. Because of this, precise temperature control is not necessary. Therefore, ideally, this differential should be small when the set temperature is high and large when the set temperature is low.

This differential is usually obtained by providing hysteresis by providing a positive feedback circuit in an operational amplifier that compares the temperature signal and a reference voltage. Therefore, the circuit is configured to provide a constant hysteresis regardless of the set temperature, so depending on the characteristics of the temperature sensing element, the differential may become smaller at a lower set temperature, which is inconvenient. occurs. For this reason, expensive types of temperature control circuits are configured to change the amount of positive feedback depending on the set temperature. However, since low-priced devices cannot be constructed as described above due to price restrictions, many of them are designed with this inconvenience in mind.

The present invention makes it possible to increase the differential when the set temperature is low using an extremely simple circuit, and even with a low-priced type, when the set temperature is low, the operation interval of the combustion device is lengthened and the noise level is reduced. It is an object of the present invention to provide a temperature control circuit that can reduce the generation time and extend the life span.

An embodiment of the present invention will be described below with reference to FIG. In FIG. 1, resistors 12, 13,
A voltage dividing circuit for obtaining a temperature signal consisting of a temperature setting variable resistor 14 and a thermistor 15, a resistor 17,
A voltage dividing circuit for obtaining a reference voltage consisting of 18 is connected. The circuit made up of the resistor 16 and the capacitor 31 is a filter circuit for absorbing the temperature signal as well as noise that attempts to enter the inverting input terminal of the operational amplifier 41. A positive feedback resistor 19 is connected between the output terminal and the non-inverting input terminal of the operational amplifier 41. The output terminal of the operational amplifier 41 is connected to the base of a transistor 42 via base current limiting resistors 21 and 22. A resistor 20 connected between the base and emitter of this transistor
is the resistance for base stabilization. The base of a transistor 43 is connected to one end of the thermistor 15, and when the thermistor 15 is disconnected, the transistor 43 turns off and the output generated at its collector is sent to the combustion control device via a backflow prevention diode 47. I'm trying to send it to 60. That is, this transistor 43 is used to detect a disconnection of the thermistor 15, and when a disconnection detection accident occurs, the combustion control device 60 controls to stop all combustion. A capacitor 32 connected between the base and collector of this transistor 43 is for noise absorption. The diodes 45 and 46 are for making the voltage applied to the negative side of the reference voltage generating voltage dividing circuit higher than the voltage applied to the negative power supply terminal of the operational amplifier 41. The overheat detection circuit 50 is equipped with an operational amplifier 51 that compares the temperature signal with a reference voltage different from the above, and when a certain dangerous temperature unrelated to the set temperature is reached, and sends an output to the combustion control device 60, which acts to stop all combustion.
The transistors 52 and 53 constitute a circuit for changing the reference voltage corresponding to the dangerous temperature depending on whether the combustion state is on or off.

The thermistor 15 is arranged so as to be able to detect the temperature of a boiler or the like, and its resistance value increases as the temperature decreases. Then, since the temperature signal becomes higher than the reference voltage, the output of the operational amplifier 41 becomes [L]. Then, the transistor 42 is turned on and combustion is started by the combustion control device 60. As combustion begins and the temperature gradually increases and the resistance of the thermistor 15 decreases, the output of the operational amplifier 41 is reversed to [H]. Then, the transistor 42 is turned off and the combustion control device 60 stops combustion.

If variable resistor 14 is set to the minimum value (0 ohm), the temperature signal will be transmitted by resistors 12 and 13 and thermistor 1.
It is determined by the resistance value of 5. Therefore, thermistor 1
When the resistance value of the variable resistor 14 becomes smaller, that is, when the temperature becomes higher, it becomes equal to the reference voltage, so reducing the resistance value of the variable resistor 14 increases the set temperature. Conversely, if the resistance value of the variable resistor 14 is set to the maximum value, it will match the reference voltage when the resistance value of the thermistor 15 is large, that is, when the temperature is low. Therefore, by setting the variable resistor 14 to the maximum value, anti-freezing settings can be made.

By the way, the operational amplifier 41 to which the temperature signal is input
The collector of a transistor 42 is connected to the inverting input terminal of the transistor 42 via a resistor 23 and a diode 44. When this transistor 42 is on, the diode 44 is in a reverse bias state and is off, but when the transistor 42 is off, it is in a forward bias state and is connected to the operational amplifier 4 through the resistor 23.
This means that the inverting input terminal of No. 1 is connected to the ground potential side via the combustion control device 60. This resistance 2
The value of 3 is assumed to be relatively large. When the value of the variable resistor 14 is small, the thermistor 15 operates in a region where the resistance value is small, so connecting and separating the resistor 23 does not greatly change the voltage division ratio on the inverting input terminal side, and therefore the temperature signal does not have a major impact on However, when the value of the variable resistor 14 is increased, the thermistor 15 operates in a region where the resistance is large, so that the connection and separation of the resistor 23 has a non-negligible effect on the temperature signal. That is, when the transistor 42 is off and the resistor 23 is connected, it can be considered that the voltage dividing ratio of the voltage dividing circuit consisting of the resistors 12 and 13, the variable resistor 14, and the thermistor 15 has been changed; The reference voltage will not be reached unless the resistance becomes larger (compared to the case without the resistor 23).
That is, when combustion is off, combustion will not start unless the temperature becomes lower (compared to the case where there is no resistor 23). By connecting the circuit including the diode 44 and the resistor 23 in this way, the temperature at which combustion starts can be lowered, especially in a region where the set temperature is low.

The resistance value set by the variable resistor 14 and the combustion on/off
The relationship with the temperature at which it turns off is shown in FIG. In this figure, the solid line A shows the temperature at which combustion is turned off, the line B shows the temperature at which combustion is turned on, and the solid line C shows the temperature at which combustion is turned on when there is no circuit including the diode 44 and resistor 23. If the circuit including the diode 44 and the resistor 23 is not provided, the difference between the on temperature and the off temperature, ie, the differential, is basically determined based on the hysteresis caused by the positive feedback resistor 19 of the operational amplifier 41. That is, the difference in the vertical axis direction between curves A and C in FIG. 2 is basically a constant value determined by the positive feedback resistor 19. Note that the difference in the vertical axis direction between curves A and C in FIG. It is something that On the other hand, when a circuit consisting of the diode 44 and the resistor 23 is connected, the OFF temperature does not change (because the diode 44 is OFF when the transistor 42 is ON), but the ON temperature changes due to the change in the variable resistor 14. When the value is increased, that is, the lower the temperature, the lower the differential becomes.

In addition, a voltage division ratio changing circuit consisting of a resistor 23 and a diode 44 is connected between the signal line from the collector of the transistor 42 to the combustion control device 60 and the voltage division point of the voltage division circuit of the thermistor 15 and variable resistor 14. However, since the on/off of the transistor 42 is controlled by the output of the operational amplifier 41, variations in the output of the operational amplifier 41 are passed through the transistor 42, and therefore do not affect the voltage dividing point. This is because when the transistor 42 is on, the diode 44 is reverse biased and the resistor 23 is disconnected, and the voltage division ratio cannot be changed.
2 is off, the resistor 23 is connected, and the voltage division ratio is changed. This is because the operation of the voltage division ratio changing circuit is controlled to be on or off, so variations in the output voltage of the operational amplifier 41 do not matter at all. Therefore, it is possible to make the differential highly accurate.

As described above, by adding a simple circuit, the differential range can be increased when the set temperature becomes low. Further, by selecting the value of the resistor 23, the differential can be set arbitrarily.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram showing one embodiment of the present invention, Figure 2 is a circuit diagram showing an embodiment of the present invention.
The figure is a graph showing the relationship between the value of the variable resistor 14 and the temperature at which combustion is turned on and off to explain the operating characteristics shown in FIG. 14...Variable resistor for temperature setting, 15...Thermistor, 17, 18...Resistor for reference voltage generation, 4
1, 51... operational amplifier, 42... output transistor, 43... disconnection detection transistor, 50...
...Overheat detection circuit, 60... Combustion control device.

Claims (1)

[Claims] 1. Configure a voltage divider circuit by connecting a variable resistor for temperature setting and a temperature sensing element whose resistance changes depending on the temperature in series, and use the voltage obtained from this voltage dividing point as a temperature signal, and combine this temperature signal with a reference voltage. The output of the comparison circuit is added to the base of a transistor that turns on and off a control signal sent to a combustion control device, and the temperature is controlled by turning combustion on and off according to the temperature. In the temperature control circuit, a voltage division ratio changing circuit consisting of a diode and a resistor connected in series is connected between a signal line from the transistor to the combustion control device and a voltage division point of the voltage division circuit, and A temperature control circuit characterized in that a partial pressure ratio changing circuit changes the partial pressure ratio of the partial pressure circuit to increase the difference between a temperature at which combustion is turned on and a temperature at which combustion is turned off in a low temperature region.