JP2010025668A - Constant resistance control circuit - Google Patents

Abstract

A constant resistance control circuit capable of rapidly increasing to a target temperature and having low loss power is provided.
A resistance value of a series fixed resistor R2 and a pair of parallel fixed resistors R3 and R4 constituting a bridge circuit 2 is set so that a heater resistance Rh is in an equilibrium state when the heater temperature is at a target temperature. An FET Q1 is provided between the bridge circuit 2 and the constant voltage source 3. The comparison circuit 4 turns on the FET Q1 when the first voltage Vh falls below the voltage obtained by subtracting the hysteresis voltage Vhys from the second voltage Vm, and the first voltage Vh sets the voltage obtained by adding the hysteresis voltage Vhys to the second voltage Vm. It is provided to turn off the FET Q1 when exceeding.
[Selection] Figure 2

Description

  The present invention relates to a constant resistance control circuit, and in particular, a heater resistor whose resistance value varies according to temperature, a series fixed resistor connected in series to the heater resistor, and a parallel connection to the heater resistor and the series fixed resistor And a pair of parallel fixed resistors connected in series with each other, and the resistance values of the series fixed resistor and the parallel fixed resistor are set so that the heater resistance is in an equilibrium state at a target temperature. The present invention relates to a constant resistance control circuit having a bridge circuit.

  In various sensors such as an oxygen sensor and a gas sensor, there is a method of heating at a constant temperature using a heater resistance. In such a sensor, it is necessary to control the heater resistance to be heated at a constant temperature. As an apparatus for heating the heater resistance at a constant temperature, for example, it is composed of a heater resistance and three fixed resistors, and three fixed resistors are provided so that the heater resistor is in a parallel state when it is at a target temperature. There has been proposed a constant resistance control circuit that includes a bridge circuit and a differential amplifier for obtaining an output of the bridge circuit, and inputs the output of the differential amplifier to the bridge circuit (for example, Patent Document 1).

According to this constant resistance control circuit, a current can be supplied to the heater resistance so that the bridge circuit is in an equilibrium state, that is, the resistance value of the heater resistance is constant and constant at the target temperature. However, the above-described constant resistance control circuit has a problem that when the bridge circuit is close to an equilibrium state, the current flowing through the heater is also small and cannot easily be raised to the target temperature.
Japanese Patent Laid-Open No. 9-318584

  In view of the above, the present invention focuses on the above-described problems, and an object thereof is to provide a constant resistance control circuit capable of quickly increasing to a target temperature.

  The invention according to claim 1, which has been made to solve the above problem, includes a heater resistor whose resistance value varies according to temperature, a series fixed resistor connected in series to the heater resistor, the heater resistor, and the series fixed. A pair of parallel fixed resistors connected in parallel with each other and in series with each other, and the series fixed resistors and the parallel fixed resistors are arranged so as to be in an equilibrium state when the heater resistance is at a target temperature. In a constant resistance control circuit having a bridge circuit in which a resistance value is set, a constant voltage source for supplying a constant voltage to the bridge circuit, a switching element provided between the constant voltage source and the bridge circuit, and the heater resistor And the first voltage generated between the series fixed resistors and the second voltage generated between the pair of parallel fixed resistors, the first voltage is less than the second voltage. A constant resistance control circuit, comprising: a comparison circuit provided to turn on the switch element sometimes and to turn off the switch element when the first voltage exceeds the second voltage Exist.

  According to a second aspect of the present invention, the comparison circuit turns on the switch element when the first voltage falls below a voltage obtained by subtracting a predetermined hysteresis voltage from the second voltage, and the first voltage is the first voltage. The constant resistance control circuit according to claim 1, wherein the switch element is provided to be turned off when a voltage obtained by adding the hysteresis voltage to two voltages is exceeded.

  According to a third aspect of the present invention, the comparison circuit outputs a voltage obtained by adding a voltage obtained by multiplying a difference between the first voltage and the second voltage by a gain and a reference voltage, and A first comparator for comparing an output of the differential amplifier and a voltage obtained by multiplying the hysteresis voltage by the gain and the reference voltage; and an output of the differential amplifier and the hysteresis from the reference voltage The constant resistance control circuit according to claim 2, further comprising: a second comparator that compares a voltage obtained by subtracting a voltage obtained by multiplying the gain by the gain.

  As described above, according to the first aspect of the present invention, the comparison circuit controls on / off of the switch element provided between the bridge circuit and the constant voltage source, and the heater resistance has a resistance value corresponding to the target temperature. Is controlled to be constant. Therefore, even if the heater resistance falls below the target temperature, the constant voltage supplied from the constant voltage source can be supplied to the bridge circuit and can be quickly raised to the target temperature.

  According to the second aspect of the present invention, when the on / off control of the switch element is simply performed by comparing the first voltage and the second voltage, the on / off cycle becomes fine, and the current is always supplied to the switch element. Therefore, the loss power in the switch element is increased, but by providing hysteresis in the comparison between the first voltage and the second voltage, it is possible to provide an off period in which the on / off period is long and no current flows in the switch element. Therefore, the loss power at the switch element can be reduced.

  According to invention of Claim 3, a hysteresis can be provided in the comparison with a 1st voltage and a 2nd voltage by simple structure.

First Embodiment A constant resistance control circuit according to a first embodiment of the present invention will be described below with reference to FIG. As shown in FIG. 1, the constant resistance control circuit 1 includes a bridge circuit 2, a constant voltage source 3, an FET Q <b> 1 as a switch element, and a comparison circuit 4. The bridge circuit 2 includes a heater resistor Rh, a series fixed resistor R2, and a pair of parallel fixed resistors R3 and R4. The heater resistance Rh is made of, for example, platinum, and the resistance value increases as the temperature rises. The series fixed resistor R2 is provided in series with the heater resistor Rh between a constant voltage source 3 (to be described later) and the ground. A pair of parallel fixed resistors R3 and R4 are provided in parallel with the heater resistor Rh and the series fixed resistor R2 between a constant voltage source 3 (to be described later) and the ground. The pair of parallel fixed resistors R3 and R4 are connected in series with each other.

  The resistance values of the series fixed resistor R2 and the pair of parallel fixed resistors R3 and R4 are such that the bridge circuit 2 is in an equilibrium state (ie, the first voltage Vh = the second voltage Vm) when the heater resistance Rh is at the target temperature. Is set to The first voltage Vh is a voltage generated between the heater resistance Rh and the series fixed resistance R2. The second voltage Vm is a voltage generated between the pair of parallel fixed resistors R3 and R4.

For example, the target temperature of the heater resistance Rh is 400 ° C., and the resistance value Rh _{400 °} C. of the heater resistance Rh at the target temperature 400 ° _C. is 13.3333Ω. At this time, if the resistance values of the series fixed resistor R2 and the pair of parallel fixed resistors R3 and R4 are set so as to satisfy the following expression (1), the bridge circuit 2 is in an equilibrium state when the heater resistance Rh is the target temperature. It becomes.
R2: Rh _{400 ° C.} = R3: R4
Rh _{400 ° C} x R3 = R2 x R4
13.3333Ω × R3 = R2 × R4 (1)

  Therefore, for example, the resistance value of the series fixed resistor R2 is set to 2.5Ω, the resistance value of the parallel fixed resistor R3 is set to 10Ω, and the resistance value of the parallel fixed resistor R4 is set to 53.33Ω. Further, the capacitor C is connected in parallel to the bridge circuit 2 having the above-described configuration. The capacitor C supplies the voltage across the capacitor C to the bridge circuit 2 even when the FET Q1 described later is turned off, and the bridge circuit 2 outputs the first voltage Vh and the second voltage Vm.

  The constant voltage source 3 is a power source that supplies a constant voltage Vb of, for example, 12V to the bridge circuit 2. The FET Q <b> 1 is a p-channel field effect transistor provided between the constant voltage source 3 and the bridge circuit 2. The FET Q1 has a source S connected to the constant voltage source 3 and a drain D connected to the bridge circuit 2. In the FET Q1, the source S and the gate G are connected via the resistor Rb.

  The comparison circuit 4 includes an inverter amplifier 41 and an inverter amplifier 42. In the inverter amplifier 41, the first voltage Vh is input to the negative input terminal, and the second voltage Vm is input to the positive input terminal. The inverter amplifier 41 compares the first voltage Vh and the second voltage Vm, and outputs a Hi level signal when the first voltage Vh falls below the second voltage Vm (Vm> Vh). When Vh exceeds the second voltage Vm (Vm <Vh), a Lo level signal is output. The inverter amplifier 42 inverts the output from the inverter amplifier 41 and inputs it to the gate of the FET Q1.

  That is, when the first voltage Vh falls below the second voltage Vm (Vm> Vh), the inverter amplifier 42 inputs a Lo level signal to the gate of the FET Q1. As a result, the FET Q1 is turned on, and the constant voltage Vb is supplied to the bridge circuit 2. In contrast, when the first voltage Vh exceeds the second voltage Vm (Vm <Vh), the inverter amplifier 42 inputs a Hi level signal to the gate of the FET Q1. Thereby, the FET Q1 is turned off, and the constant voltage Vb supplied to the bridge circuit 2 is cut off.

Next, the operation of the constant resistance control circuit 1 configured as described above will be described below. First, before explaining the operation, it is assumed that the resistance value Rh of the heater resistance Rh at _{400 ° C.} Rh _{400 ° C.} = 13.3333Ω, the resistance value temperature coefficient of the heater resistance Rh is 3000 ppm / ° C., and the heater at 0 ° C. The resistance value Rhz of the resistor Rh is calculated from the following equation.
Rhz + Rhz × 3000 × 10 ⁻⁶ × 400 = 13.3333Ω
Rhz (1 + 1.2) = 13.3333
Rhz = 6.0606Ω

Next, it is assumed that the constant resistance control circuit 1 is turned on at room temperature of 25 ° C., and the resistance value Rh _{25 °} C. of the heater resistance Rh at _{25 ° C.} is obtained from the following equation.
Rh _{25 ° C.} = Rhz + Rhz × 3000 × 10 ⁻⁶ × 25
= 6.0606 + 6.0606 × 3000 × 10 ⁻⁶ × 25
= 6.5151Ω

  When the power is turned on, the voltage across the capacitor C is supplied to the bridge circuit 2, and the first voltage Vh and the second voltage Vm are output from the bridge circuit 2. For example, when the power is turned on with the heater resistance Rh at room temperature of 25 ° C. and the resistance value of 6.5151Ω, the first voltage Vh becomes lower than the second voltage Vm (Vm <Vh). Therefore, the inverter amplifier 41 supplies the Hi level signal to the inverter amplifier 42, the inverter amplifier 42 supplies the Lo level signal to the gate of the FET Q1, and the FET Q1 is turned on. As a result, the constant voltage Vb is supplied to the bridge circuit 2 and the temperature of the heater resistor Rh increases. When the temperature of the heater resistance Rh increases, the resistance value of platinum having a positive resistance value temperature coefficient increases.

The resistance value of the heater resistance Rh increases and tries to exceed Rh _{400 ° C.} = 13.3333Ω. That is, when the first voltage Vh exceeds the second voltage Vm (Vh> Vm), the inverter amplifier 41 supplies the Lo level signal to the inverter amplifier 42, and the inverter amplifier 42 supplies the Hi level signal to the gate of the FET Q1. Then, the FET Q1 is turned off. As a result, the supply of the constant voltage Vb to the bridge circuit 2 is interrupted, the temperature of the heater resistor Rh decreases, and the first voltage Vh becomes lower than the second voltage Vm. By repeating the above operation, the resistance value of the heater resistor Rh can be controlled to be constant, that is, the heater resistor Rh can be controlled to be constant at the target temperature.

  According to the constant resistance control circuit 1 described above, the comparison circuit 4 controls the on / off of the FET Q1 provided between the bridge circuit 2 and the constant voltage source 3, and the heater resistance Rh has a resistance value corresponding to the target temperature. It is controlled to be constant.

Second Embodiment Next, a second embodiment of the constant resistance control circuit 1 of the present invention will be described below with reference to FIG. In this figure, the same reference numerals are given to the same parts as those in FIG. 1 already described in the first embodiment, and the detailed description thereof is omitted. As shown in the figure, the constant resistance control circuit 1 includes a bridge circuit 2, a constant voltage source 3, an FET Q 1, and a comparison circuit 4. The bridge circuit 2, the constant voltage source 3, and the FET Q1 are the same as those in the first embodiment described above, and thus detailed description thereof is omitted here. A significant difference between the first embodiment and the second embodiment is the configuration of the comparison circuit 4.

  In the first embodiment described above, the comparison circuit 4 compares the first voltage Vh and the second voltage Vm, and turns on the FET Q1 when the first voltage Vh falls below the second voltage Vm, and the first voltage Vh. Is provided to turn off the FET Q1 when the voltage exceeds the second voltage Vm. However, in the second embodiment, the comparison circuit 4 compares the first voltage Vh and the second voltage Vm, and the first voltage Vh is a voltage obtained by subtracting a predetermined hysteresis voltage Vhys (for example, 5 mV) from the second voltage Vm. When (Vh <Vm−Vhys) falls below (Vm−Vhys), the FET Q1 is turned on, and when the first voltage Vh exceeds the voltage (Vm + Vhys) obtained by adding the hysteresis voltage Vhys = 5 mV to the second voltage Vm ( Vh> Vm + Vhys) is provided to turn off the FET Q1.

Specifically, the comparison circuit 4 includes a differential amplifier 43, a power supply circuit 44, an amplifier 45, a first comparator 46, a second comparator 47, and a logic circuit 48. The differential amplifier 43 is supplied with a first voltage Vh and a reference voltage Vr, which will be described later, at the + input terminal, and with a second voltage Vm at the − input terminal. Therefore, the differential amplifier 43 outputs an output voltage Vo as shown in the following equation (2) and inputs the output voltage Vo to first and second comparators 46 and 47 described later.
Vo = (Vh−Vm) × G + Vr (2)
G is the gain of the differential amplifier 43.

  The power supply circuit 44 supplies the reference voltage Vr to the + input terminal of the differential amplifier 43. The power supply circuit 44 supplies the first comparator 46 with a voltage (Vr + G × Vhys) obtained by adding the reference voltage Vr to the voltage obtained by multiplying the hysteresis voltage Vhys by the gain G. The power supply circuit 44 supplies the second comparator 47 with a voltage (Vr−G × Vhys) obtained by subtracting a voltage obtained by multiplying the hysteresis voltage Vhys by the gain G from the reference voltage Vr.

  The amplifier 45 is provided between the power supply circuit 44 and the + input terminal of the differential amplifier 43. The first comparator 46 receives a voltage Vo at its − input terminal and a voltage (Vr + G × Vhys) at its + input terminal. The first comparator 46 compares the voltage Vo with the voltage (Vr + G × Vhys), and outputs a Hi level signal when the voltage Vo falls below the voltage (Vr + G × Vhys). The voltage Vo is the voltage (Vr + G × Vhys). ), A Lo level signal is output. In other words, as shown in FIG. 3B, the first comparator 46 outputs a Hi level signal when the first voltage Vh falls below Vm + Vhys, and the Lo level when the first voltage exceeds Vm + Vhys. The signal is output.

  The second comparator 47 receives a voltage Vo at its + input terminal and a voltage (Vr−G × Vhys) at its − input terminal. The second comparator 47 compares the voltage Vo with the voltage (Vr−G × Vhys), and outputs a Hi level signal when the voltage Vo exceeds the voltage (Vr−G × Vhys). When it falls below (Vr−G × Vhys), a Lo level signal is output. In other words, as shown in FIG. 3C, the second comparator 47 outputs a Hi level signal when the first voltage Vh exceeds Vm−Vhys, and the first voltage Vh falls below Vm−Vhys. Output a Lo level signal.

  The logic circuit 48 includes a NOR circuit 481 and a NAND circuit 482. In the NOR circuit 481, the output of the first comparator 46 is input to one input terminal, and the output of a NAND circuit 482 described later is input to the other input terminal. In the NAND circuit 482, the output of the second comparator 47 is input to one input terminal, and the output of the NOR circuit 481 is supplied to the other input terminal. Therefore, as shown in FIGS. 3D and 3E, the NOR circuit 481 becomes Lo level when the first voltage Vh falls below the voltage (Vm−Vhys), turns on the FET Q1, and turns on the first voltage Vh. When the voltage exceeds the voltage (Vm + Vhys), it becomes Hi level and turns off the FET Q1.

  Next, the operation of the constant resistance control circuit 1 configured as described above will be described with reference to FIG. When the power is turned on, the voltage across the capacitor C is supplied to the bridge circuit 2, and the first voltage Vh and the second voltage Vm are output from the bridge circuit 2. For example, when the power is turned on with the heater resistance Rh at room temperature of 25 ° C. and the resistance value of 6.5151Ω, the first voltage Vh becomes lower than the second voltage Vm (Vm <Vh). Therefore, the first comparator 46 supplies a Hi level signal to the logic circuit 48, and the second comparator 47 outputs a Lo level signal to the logic circuit 48. Thus, an L level signal is output from the NOR circuit 481 of the logic circuit 48, and the FET Q1 is turned on. When the FET Q1 is turned on, the constant voltage Vb is supplied to the bridge circuit 2 and the temperature of the heater resistor Rh increases. When the temperature of the heater resistance Rh increases, the resistance value of platinum having a positive resistance value temperature coefficient increases.

  When the resistance value of the heater resistor Rh increases and the first voltage Vh exceeds the voltage (Vm + Vhys) obtained by adding the hysteresis voltage Vhys to the second voltage Vm, the output of the first comparator 46 changes from the Hi level to the Lo level. In response to this, the NOR circuit 481 of the logic circuit 48 outputs a Hi level signal and turns off the FET Q1. As the FET Q1 is turned off, the constant voltage Vb supplied to the bridge circuit 2 is cut off, the temperature of the heater resistor Rh is lowered, and the resistance value is also lowered. As a result of the decrease in the resistance value, the NOR circuit 481 maintains the output of the Hi level signal even when the first voltage Vh falls below the voltage (Vm + Vhys) and the output of the first comparator 46 returns from the Lo level to the Hi level. Then, the FET Q1 is kept off.

  When the resistance value of the heater resistor Rh continues to decrease and the first voltage Vh falls below the voltage (Vm−Vhyz) obtained by subtracting the hysteresis voltage Vhyz from the second voltage Vm, the output of the second comparator 47 changes from the Hi level to the Lo level. Become. In response to this, the NOR circuit 481 outputs a Lo level signal to turn on the FET Q1. When the FET Q1 is turned on, the constant voltage Vb is supplied to the bridge circuit 2 and the temperature of the heater resistor Rh increases. As a result of the increase in the resistance value, even if the first voltage Vh exceeds the voltage (Vm−Vhys) and the output of the second comparator 47 returns from the Lo level to the Hi level, the NOR circuit 481 outputs the Lo level signal. To keep the FET Q1 on. By repeating the above operation, the resistance value of the heater resistor Rh can be controlled to be constant, that is, the heater resistor Rh can be controlled to be constant at the target temperature.

  By the way, in the first embodiment, when the constant voltage Vb = 12V, R2 = 2.5Ω, Rh = 13.333Ω, R3 = 10Ω, and R4 = 53.333Ω, the stability in units of microseconds and milliseconds. The first voltage Vh also remains constant at approximately 4V. That is, since the comparison circuit 4 of the first embodiment simply compares the first voltage Vh and the second voltage Vm to perform the on / off control of the FET Q1, the on / off cycle becomes fine, and FIG. As shown, a current always flows through the FET Q1. On the other hand, in the second embodiment, by allowing the first voltage Vh to vary by ± hysteresis voltage Vhys = ± 5 mV, current is intermittently supplied to the FET Q1 as shown in FIG. An off period during which no current flows through the FET Q1 can be provided.

  In other words, the constant resistance control circuit 1 according to the second embodiment is effective when used for a device or application that allows ± 5 mV fluctuations at a reasonably fast frequency. This is because, for example, in a sensing device that requires heating by the heater resistance Rh, a high-speed minute voltage change of the heater resistance Rh does not affect the sensor output, or the sensor output is delayed by integration or the like. It can be applied to applications where such is possible.

Next, the on-duty of the FET Q1 in the second embodiment described above will be obtained. As shown in FIGS. 3A and 3E, the on-time t _{ON of the} FET Q1 is a time for the first voltage Vh to rise by 10 mV, which is twice the hysteresis voltage Vhys, that is, the voltage across the capacitor C is 10 mV. It is equal to a charging period that only rises. On the other hand, the off time t _{OFF of the} FET Q1 is equal to the time for the first voltage Vh to decrease by 10 mV, which is twice the hysteresis voltage Vhys, that is, the discharge time for the voltage across the capacitor C to decrease by 10 mV. The on-time t _ON may be related to the supply requirement of the constant voltage Vb, and is assumed to be 10 μS here. On the other hand, when the capacitance of the capacitor C is 1200 μF, the off time t _OFF is 40 μS as shown in the following equation (3).
10mV = 0.3A × td / 1200μ
td = 40 μS (3)

In the second embodiment, the gain G of the differential amplifier 43 is 10 times, and the comparison voltage of the first comparator 46 and the second comparator 47 is the reference voltage Vr ± 50 mV. The change is ± 50 mV / 10 = ± 5 mV. Therefore, as described above, assuming that the _ON time t _ON is 10 μS and t _OFF is 40 μS, the period T = t _ON + t _OFF = 50 μS and the oscillation period = 1 / period = 20 kHz are assumed. The FET Q1 is switched based on the above principle. Therefore, the FET Q1 is turned on only during the on-time t _ON , and the on-duty of the FET Q1 is 10 μS / 50 μS = 1/5.

Next, the loss power in the FET Q1 of the constant resistance control circuit 1 of the first embodiment and the second embodiment will be compared. First, consider the loss power of the constant resistance control circuit 1 in the first embodiment shown in FIG. In order to fix the resistance value of the heater resistor Rh in the resistance value Rh _{400 ℃} = 13.3333Ω, i.e., requires the temperature of the heater resistance Rh of the resistance value Rh _{400 ℃} = 13.3333Ω to constant at 400 ° C. Suppose that the direct current is 0.3A. In the first embodiment described above, the comparison circuit 4 simply compares the first voltage Vh and the second voltage Vm to perform the on / off control of the FET Q1, so that the on / off cycle becomes fine. For this reason, since the FET Q1 is turned off and turned on before the current flowing through the FET Q1 becomes zero, as shown in FIG. 4A, the FET Q1 is almost the same as a state where a DC current of 0.3 A is supplied. It becomes a state. Therefore, the voltage Vhh between the FET Q1 and the bridge circuit 2 is 4.75 V as shown in the following formula (4) when the resistance value of the series fixed resistor R2 is 2.5Ω.
Vhh = 13.3333 × 0.3A + 2.5 × 0.3A
= 4.75V (4)

Assuming that it is used as an in-vehicle device and assuming that Vb = 12V, a voltage difference of Vb−Vhh occurs between the source S and drain D of the FET Q1. Further, it is considered that a current of 0.3 A, which is substantially equal to the current flowing through the heater resistor Rh, flows through the FET Q1. Therefore, the loss power P _L1 generated between the source S and the drain D of the FET Q1 is 2.175 watts as shown in the following formula (5).
P _L1 = (Vb−Vhh) × ih
= (12-4.75) x 0.3
= 2.175 watt (5)

When the FET Q1 is simply turned on, the current ih flowing through the heater resistor Rh is 0.72 A as shown in the following equation (6). However, in the first embodiment, the on / off of the FET Q1 is controlled so that the current ih flowing through the heater resistor Rh is 0.3A.
ih≈Vb / (Ron + R2 + Rh)
= 12 / (1 + 2.5 + 13.3333)
= 0.72A (6)
In Expression (6), Ron is ignored because it is larger than the on-resistance of the FET Q1, and the parallel fixed resistors R3 and R4 are larger than the series fixed resistor R1 and the heater resistor Rh. Therefore, the amount suppressed from 0.72 A to 0.3 A appears as the loss power P _L1 between the drain D and the source S of the FET Q1.

Next, the loss power of the constant resistance control circuit 1 in the second embodiment shown in FIG. 2 will be considered. In the second embodiment described above, the comparison circuit 4 increases the ON / OFF cycle of the FET Q1 by allowing the first voltage Vh to vary with ± hysteresis voltage = ± 5 mV, as shown in FIG. , FET Q1 is intermittently supplied with a current to provide an off period. The on-duty of the FET Q1 is controlled so that an average current of 0.3 A flows through the heater resistor Rh. Therefore, the loss power P _L2 generated between the source Q and the drain of the FET Q1 is only the amount generated by the above 0.3A flowing through the on-resistance Ron = 1Ω of the FET Q1, as shown in the following formula (7). .
P _L2 = 1 × 0.3A = 0.3watt
As is apparent from the above, the loss power can be suppressed to a very small value compared to the first embodiment.

Here, also in the case of the second embodiment, when the average energy applied to the heater resistance Rh is the same value as that in the case of the first embodiment, the first and second embodiments have the same resistance to the heater resistance Rh. The average supply power is not changed. In the case of the first embodiment, the current × time product supplied during the period of 50 μS is supplied to the ON time t _{ON of} 1/5 in the case of the second embodiment. As long as the average power consumption of the body Rh is not changed, it does not change regardless of the first and second embodiments. However, considering the power supplied from the constant voltage Vb, the energy consumption of the 2.175 watt heat generation calculated to always be lost in the first embodiment is improved by the amount of heat generation reduced in the second embodiment. Is done.

  In the second embodiment described above, the comparison circuit 4 includes the differential amplifier 43, the first comparator 46, the second comparator 47, and the logic circuit 48. However, the present invention is not limited to this. As the comparison circuit 4 of the second embodiment, the first voltage Vh is compared with the second voltage Vm, and the first voltage Vh is obtained by subtracting a predetermined hysteresis voltage Vhys (for example, 5 mV) from the second voltage Vm (Vm− Vhys) (Vh <Vm−Vhys), the FET Q1 is turned on, and when the first voltage Vh exceeds the voltage (Vm + Vhys) obtained by adding the hysteresis voltage Vhys = 5 mV to the second voltage Vm (Vh> Vm + Vhys) ) Other configurations may be used as long as the FET Q1 is provided to be turned off.

  Further, the above-described embodiments are merely representative forms of the present invention, and the present invention is not limited to the embodiments. That is, various modifications can be made without departing from the scope of the present invention.

It is a circuit diagram which shows the constant resistance control circuit of this invention in 1st Embodiment. It is a circuit diagram which shows the constant resistance control circuit of this invention in 2nd Embodiment. (A)-(E) are time charts showing the first voltage, the output of the first comparator, the output of the second comparator, the output of the NOR circuit, and the on / off state of the FET in the constant resistance control circuit shown in FIG. (A) is a time chart which shows the electric current which flows into FET in 1st Embodiment, (B) shows the electric current which flows into FET in 2nd Embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Constant resistance control circuit 2 Bridge circuit 3 Constant voltage source 4 Comparison circuit 43 Differential amplifier 46 1st comparator 47 2nd comparator Rh Heater resistance R2 Series fixed resistance R3 Parallel fixed resistance R4 Parallel fixed resistance Q1 FET (switch element)
Vh 1st voltage Vm 2nd voltage

Claims (3)

A heater resistor whose resistance value varies according to temperature, a series fixed resistor connected in series to the heater resistor, and a pair of parallel fixed resistors connected in series to the heater resistor and the series fixed resistor and connected to each other in series And a constant resistance control circuit having a bridge circuit in which resistance values of the series fixed resistance and the parallel fixed resistance are set so that the heater resistance is in an equilibrium state at a target temperature,
A constant voltage source for supplying a constant voltage to the bridge circuit;
A switching element provided between the constant voltage source and the bridge circuit;
The first voltage generated between the heater resistor and the series fixed resistor and the second voltage generated between the pair of parallel fixed resistors are compared, and when the first voltage falls below the second voltage, A comparison circuit provided to turn on a switch element and to turn off the switch element when the first voltage exceeds the second voltage;
A constant resistance control circuit comprising: The comparison circuit turns on the switch element when the first voltage falls below a voltage obtained by subtracting a predetermined hysteresis voltage from the second voltage, and the first voltage adds the hysteresis voltage to the second voltage. 2. The constant resistance control circuit according to claim 1, wherein the constant resistance control circuit is provided so as to turn off the switch element when a voltage exceeds a predetermined voltage.   A differential amplifier that outputs a voltage obtained by adding a voltage obtained by multiplying a difference between the first voltage and the second voltage by a gain and a reference voltage; an output of the differential amplifier; and A first comparator that compares a voltage obtained by multiplying the hysteresis voltage by the gain and a voltage obtained by adding the reference voltage; an output of the differential amplifier; and a voltage obtained by multiplying the hysteresis voltage from the reference voltage by the gain. The constant resistance control circuit according to claim 2, further comprising: a second comparator that compares a voltage obtained by subtracting.

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