JPH02243965A - Constant voltage circuit and air flowmeter using the circuit - Google Patents

Abstract

PURPOSE:To simplify a constant voltage circuit in the manner of using Zener diode in common for a surge protecting circuit and the constant voltage circuit by using the Zener diode of the surge protecting circuit existed between a power supply and the earth to output the constant voltage. CONSTITUTION:The subject device is constituted in such a manner that at least one Zener diode 28 of the surge protecting circuit between the power supply and the earth is used and a resistor 24 is connected between the power supply and one anode of the Zener diode 28 to output the constant voltage from the midst of the anode and the earth. A terminal voltage of a resistor R1 is inputted to an operational amplifier 2 to obtain a measuring value Vout of an air flow rate and a reference voltage required for a comparison with the reference voltage Vs is outputted from the constant voltage circuit wherein the diode 28 is used. Thus, the simplification of the circuit and reduction of cost can be realized.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a constant voltage circuit that generates a constant voltage from a surge protection circuit using a battery or the like as a power source, and an air flow meter using the circuit.

[Conventional technology]

In conventional air flowmeters, the necessary reference voltage for the output amplifier was extracted from the battery power source using an operational amplifier.

FIG. 3 is a circuit diagram showing an example of a conventional hot wire air flowmeter. In FIG. 3, R and -R are resistors, 9 is a transistor, 10 is a Zener diode, 11 to 13 are operational amplifiers, H/W is a hot wire, and C/W is a cold wire. These resistors Rl, R2 and hot wire H/W
The hot wire H/W and the cold wire C/W form a bridge, and the hot wire H/W and the cold wire are placed in the air passage.

In this configuration, the hot wire H/W is heated, and the balance of the bridge is disrupted by the flow of air.By returning the output of the operational amplifier 11 to the bridge via the transistor 9, the temperature of the hot wire H/W is reduced. It becomes constant. Further, when the temperature of the air changes, the resistance value of the cold wire C/W causes the bridge to become unbalanced, and at this time, the resistance value of the hot wire H/W changes so that the bridge is balanced. That is, when the air temperature changes, if the balance conditions of the bridge are changed to keep the temperature difference between the hot wire H/W and the cold wire C/W constant, the relationship between the flow velocity and the amount of heat dissipation can also be kept constant. In this manner, the hot wire air flow meter measures the air flow rate by changing the temperature difference between the hot wire H/W and the cold wire C/W to be constant and using the terminal voltage of the resistor R as an output.

FIG. 4 is a circuit diagram showing another example of a conventional hot wire air flowmeter. In Fig. 2, RI3 to Rz+ are resistances, 22
is a transistor, 23 is a Zener diode, 24 to 27
is an operational amplifier, H/W is a hot wire, and C/W is a cold wire. This circuit is modified as shown in Figure 3, but
Similarly, a bridge is formed, and operational amplifiers 24 and 25 are used to return the wires to the bridge so that the temperature difference between the hot wire H/W and the cold wire C/W is constant.

The terminal voltage of the resistor R in Fig. 3 and the resistor R1 in Fig. 4
5 is taken out as an output, but since this voltage is low, it is amplified by an operational amplifier 12 and a Zener diode 27, and the output is used. In order to compare with a reference voltage at this time, an operational amplifier 13 and an operational amplifier 26 that output this reference voltage are required as each constant voltage circuit.

Conventional air flow meters use Zener diodes or capacitors as surge protection circuits from battery power sources. Also surge protection [! As a j route,
There is a circuit breaker using a Zener diode as described in Japanese Patent Application Laid-Open No. 119037/1983.

[Problem to be solved by the invention]

The above-mentioned conventional technology does not take into consideration the fact that the Zener diode of the surge protection circuit is used in a constant voltage generation circuit in addition to surge protection, and there is a problem that the circuit becomes complicated.

SUMMARY OF THE INVENTION An object of the present invention is to provide a constant voltage circuit in which the Zener diode of the surge protection circuit can be used in both surge protection and constant voltage generation circuits, and an air flow meter using the circuit.

[Means to solve the problem]

In order to achieve the above object, the constant voltage circuit and air flow meter of the present invention use at least one Zener diode in the surge protection circuit connected between the power supply and the ground.
A resistor is connected between the power supply and one anode of the Zener diode, and a constant voltage is output between the anode and the ground.The constant voltage output is used as the reference for the output amplifier of the air flow meter. It has a voltage configuration.

[For production]

The above constant voltage circuit and air flow meter use a Zener diode in the surge protection circuit between the power supply and ground to output a constant voltage, so the Zener diode is shared between the surge protection circuit and the constant voltage circuit. The constant voltage circuit can be simplified.

〔Example〕

An embodiment of the present invention will be described below with reference to FIGS. 1 and 2.

FIG. 1 is a circuit diagram showing an embodiment of a hot wire air flow meter according to the present invention. In Figure 1, RI+R2*R6
~R8r R23 to R26 are resistors, H/W is a hot wire, C/W is a cold wire, 9 is a transistor,
11.12 is an operational amplifier, 27.28 is a Zener diode, Vout is an output, vB is a power supply voltage, and VS is a constant voltage. This resistor RI r R2 and hot wire H/W
and the cold wire form a bridge, and the hot wire H/W and cold wire C/W are arranged in the air passage. The circuit portion of this bridge is similar to that shown in FIG.

In this configuration, the hot wire H/W is heated, and if the bridge becomes unbalanced due to air flow, the output of the operational amplifier 11 is returned to the bridge via the 1 transistor 9. H/W temperature is kept constant. Furthermore, when the temperature of the air changes, the resistance value of the cold wire C/W changes and the bridge becomes unbalanced, and at this time the resistance value of the H/W changes so that the bridge is balanced. That is, when the air temperature changes, if the balance conditions of the bridge are changed to keep the temperature difference between the hot wire H/W and the cold wire C/W constant, the relationship between the flow velocity and the amount of heat dissipation can also be kept constant. In this way, a hot wire air flow meter is a hot wire H/W
The temperature difference between the cold wire C/W and the cold wire C/W is changed to be constant, and the air flow rate is measured using the terminal voltage of the resistor R1 as an output.

The terminal voltage of the resistor R1 in FIG. 1 is taken out as an output, but since this voltage is low, it is amplified by the operational amplifier 12, and then the output voltage Vout is used as the measured value of the air flow rate. At this time, based on the input terminal of the operational amplifier 12, Wtt
A reference voltage of the operational amplifier 12 is required for comparison with the voltage, and in this embodiment, this reference voltage is output from a constant voltage circuit using a Zener diode 28 of a surge protection circuit. In other words, a surge protection circuit is constituted by the resistor 24 between the power supply and ground shown in FIG. 1 and at least one Zener diode 28, and from this power supply voltage Vs, the resistance
23 to one anode of at least one Zener diode 28, a constant voltage VS is generated between the connection point and ground. At this time, by inserting the resistor R25 and the Zener diode 27 in parallel from one anode of the Zener diode 28, the resistor R23+
The voltage VS at the connection point of R25 can be extracted as a more stable voltage. This constant voltage VS is connected to the resistor R26r
The voltage is divided by R6 and used as a reference voltage for the operational amplifier I2.

FIG. 2 is an explanatory diagram of the surge voltage of the surge protection circuit of FIG. 1. In FIG. 2, the horizontal axis is time (ms), the vertical axis is voltage (V), and the surge voltage waveform is shown. When a surge voltage as shown in FIG. 2, which is higher than the power supply focus VB, enters the power supply shown in FIG. 1, it passes through the resistor R24 and Zener diode 28 of the surge protection circuit and falls to the ground. By connecting the power supply to the anode of one of the Zener diodes 28 used in the surge protection circuit through the resistor 11, a constant voltage Vs is generated from the connection point, and in this case, the four Zener diodes 28 If so, four types of constant voltages can be generated.

According to this embodiment, the WI of the constant voltage circuit used in the hot wire air flow meter can be simplified, and a more stable reference voltage can be generated.

〔Effect of the invention〕

According to the present invention, the Zener diode of the surge protection circuit is used to generate a constant voltage, which simplifies circuit construction and is advantageous in terms of implementation. *If used for m pressure etc.

This has the effect of simplifying the circuit and making the bottom thinner.

[Brief Description of the Drawings] Fig. 1 is a circuit diagram showing an embodiment of the air flow meter according to the present invention, Fig. 2 is an explanatory diagram of the surge voltage shown in Fig. 1, and Fig. 3 is a circuit diagram showing an embodiment of the air flow meter according to the present invention. FIG. 4 is a circuit diagram showing another example of a conventional air flow meter. l<l r R2r R6~ I<8 +
R23~Iku26...Resistor, H/W...Hot wire, C/W...Cold wire, 9...Transistor, 11.12...Operation amplifier, 27.28.
...Zena diode, Vout...output, ■8.
...Power supply voltage, Vs...constant voltage. Figure 1 υ mouth Figure 2 10 40 Line spacing (ms) Representative patent attorney Tadashi Akiki Diagram Figure

Claims (1)

[Claims] 1. At least one Zener diode of a surge protection circuit is connected between a power source and ground, a resistor is connected between the power source and one anode of the Zener diode, and the anode and the resistor are connected. A constant voltage circuit configured to output a constant voltage between a point and ground. 2. A constant voltage circuit consisting of a resistor and a Zener diode connected in series between the above connection point and ground. 3. An air flow meter having a configuration in which the output of the constant voltage circuit according to claim 1 or claim 2 is used as a reference voltage of an output amplifier of the air flow meter.