JPH073353B2 - Heater control circuit - Google Patents

Description

TECHNICAL FIELD The present invention relates to a heater control circuit for controlling the temperature of a heater resistance.

[Conventional technology]

FIG. 6 is a circuit diagram showing a conventional heater control circuit. In the figure, RH is a heater resistance, RR is a reference resistance, RA is a first resistance, RB is a second resistance, NI is a first node, N2 is a second node, N3 is a third node, 1 Is an operational amplifier and E is ground.

In FIG. 6, the output voltage of the operational amplifier 1 changes so that the voltages at the second node N2 and the third node N3 become equal. The heater resistance RH, the reference resistance RR, the first resistance RA, and the second resistance RB are the resistance values at 20 ° C. of RH ₂₀ , RR ₂₀ , and R, respectively.
A ₂₀ , RB ₂₀ and temperature coefficients αH, αR, α
If A, αB, the heater temperature is TH, and the ambient temperature is Ta and the resistance values are rH, rR, rA, and rB, respectively, then rH = RH ₂₀ {1+ (TH-20) αH} (1) rR = RR ₂₀ {1+ (Ta-20) αR} (2) rA = RA ₂₀ {1+ (Ta-20) αA} (3) rB = RB ₂₀ {1+ (Ta-20) αB} (4) It holds. Since rH · rB = rR · rA holds in the circuit stable state, the difference DT between the heater temperature TH and the ambient temperature Ta is DT = (Ta−20) (K−1) + (K−1) / αH ... (5 ). However, K = (RR ₂₀・ RA ₂₀ ) / (RH ₂₀・ R
B ₂₀ ), αA = αB, and αH = αR. Since αH is in the order of 10 ⁻³ , the first term of equation (5) can be almost ignored,
DT is constant.

[Problems to be Solved by the Invention]

As described above, in the conventional heater control circuit, the difference DT between the heater temperature TH and the ambient temperature Ta is constant. For this reason, in a sensor for flow rate measurement in which an output is obtained by transfer of heat from the heater resistance RH, the output sensitivity of the sensor is determined by the flow path, so in order to measure a wide range of flow rate from low flow rate to high flow rate. Requires multiple sensors and channels.

[Means for Solving the Problems]

In order to solve such a problem, according to the present invention, a heater resistance that generates heat by Joule heat, a reference resistance for ambient temperature measurement connected to the heater resistance at a first node, and one of them is a second node. A first resistor connected to the heater resistor and the other connected to the ground, a second resistor connected to the reference resistor at one of the third nodes and a second resistor connected to the ground at the third node, and the first output terminal Connected to the node of the second non-inverting input terminal
And an operational amplifier having an inverting input terminal connected to a third node, and at least one of a reference resistor, a first resistor and a second resistor, and a switch and a resistor are connected in series. A plurality of series-connected bodies are connected in parallel to form a resistance group.

[Action]

The heater control circuit according to the present invention can measure a wide range of flow rates from a low flow rate to a high flow rate without requiring a plurality of sensors and flow paths.

〔Example〕

From the formula (5) described in the section of the related art, the heater temperature TH
The difference DT between the ambient temperature Ta and the ambient temperature Ta is a function of αH and K, and αH is a basic characteristic of the heater resistance. Therefore, the value of DT is changed by changing the value of K. K = (RR ₂₀・ RA ₂₀ ) / (RH
₂₀ · RB ₂₀ ), the value of K can be changed by changing the value of RA ₂₀ , RB ₂₀ , or RR ₂₀ using a switch. This is shown in FIG. In Fig. 1, reference resistance
RR, the first resistor RA or the second resistor RB is a resistor group in which a plurality of series-connected bodies in which a switch and a resistor are connected in series are connected in parallel, thereby setting the value of K to the switch. It can be changed by turning on and off.

FIG. 2 shows a first heater control circuit according to the present invention.
3 is a circuit diagram showing an embodiment of FIG. In FIG. 2, the second resistor forms a resistor group consisting of resistors RB1 to RBn, and the value of K can be changed by turning on / off the switches connected in series. Only one switch may be turned on, and 2
You may combine more than one piece. By combining them, finer control becomes possible.

FIG. 3 is a graph showing the relationship between the flow rate and the sensor output, that is, the output of the operational amplifier 1, when DT changes. If the DT values of the curves S1 to S5 are DT1 to DT5, then DT5>DT4>DT3> D
T2> DT1. As can be seen from the graph, when the DT value is high, the output sensitivity of the sensor is high, but when the flow rate increases, it is saturated immediately. Therefore, by changing the DT value according to the flow rate, the sensitivity of the sensor output is changed and the measurement range is expanded.

FIGS. 4 and 5 are circuit diagrams showing second and third embodiments of the present invention. In the circuit of FIG. 4, the first resistor RA of FIG. 1 is used as a resistor group of resistors RA1 to RAn. None, the circuit of FIG. 5 has the reference resistor RR of FIG. 1 as a resistor group of resistors RR1 to RRn.

In the above embodiment, one of the resistors RA, RB, and RR is set as a resistance group, but two or more of these resistors are set as a resistance group, and a plurality of resistance groups may be combined. Good.
Also, various resistance networks may be used as a method of changing each resistance value.

〔The invention's effect〕

As described above, according to the present invention, at least one of the reference resistance, the first resistance, and the second resistance is a resistance group in which a plurality of series connection bodies in which a switch and a resistance are connected in series are connected in parallel. This has the effect of being able to measure a wide range of flow rates from low flow rates to high flow rates without the need for multiple sensors and flow paths, and therefore to reduce the size and cost of the flow measurement device. There is an effect that can be.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an outline of the present invention, FIG. 2 is a circuit diagram showing a first embodiment of a heater control circuit according to the present invention, and FIG. 3 is a graph showing the relationship between flow rate and sensor output,
FIGS. 4 and 5 are circuit diagrams showing second and third embodiments of the present invention, and FIG. 6 is a circuit diagram showing a conventional heater control circuit. RH ... Heater resistance, RR ... Reference resistance, RA ... First resistance, RB ...
Second resistance, N1 to N3 ... First to third nodes, E ... Ground,
1 ... Operational amplifier.

Claims (1)

[Claims] 1. A heater control circuit for a hot wire type flow meter, wherein a heater resistance that generates heat by Joule heat, a reference resistance for measuring ambient temperature connected to the heater resistance at a first node, and one of them is a second resistance. A first resistor connected to the heater resistor at its node and the other connected to the ground; one connected to the reference resistor at the third node and the other connected to the ground at the other node; An output terminal connected to the first node, a non-inverting input terminal connected to the second node, and an inverting input terminal connected to the third node; and the reference resistor and the first resistor. And at least one of the second resistors is a heater control circuit that is a resistor group in which a plurality of series-connected bodies in which a switch and a resistor are connected in series are connected in parallel.