Patent Description:
As the above-mentioned volumetric flowmeter, there has been known, for example, a volumetric flowmeter proposed by the applicant of the present application as described below as Patent Literature <NUM>. Now, a configuration and structure of a related-art volumetric flowmeter are described with reference to the drawings.

In <FIG>, a volumetric flowmeter <NUM> includes a main body <NUM> and a calculation unit <NUM> mounted on the main body <NUM>. As illustrated in <FIG>, the volumetric flowmeter <NUM> is a flowmeter (instrumentation device), which is mounted in a midway part of a pipe <NUM>, and can meter a fluid to be measured flowing through an inside of the pipe <NUM> and perform calculation regarding a flow rate to display the calculation result in the calculation unit <NUM>. The above-mentioned display is performed by a display unit <NUM> described later.

In <FIG>, a strainer <NUM> is arranged between the pipe <NUM> on an upstream side and an upstream-side flange <NUM> of the main body <NUM>. There is illustrated a bypass pipe <NUM> in <FIG>. Further, there are illustrated valves <NUM>. The bypass pipe <NUM> is arranged as a portion to be used for an overhaul of the volumetric flowmeter <NUM>.

In <FIG> and <FIG>, the main body <NUM> includes: a front lid portion <NUM> on which a mounting base <NUM> of the calculation unit <NUM> is mounted; a metering chamber forming portion <NUM> including a metering chamber <NUM>; two rotors <NUM> and <NUM>, which are accommodated in the metering chamber <NUM>, and are rotated by rotor shafts <NUM>; and the upstream-side flange <NUM> and a downstream-side flange <NUM> contiguous to the metering chamber forming portion <NUM>. The front lid portion <NUM> is formed into a member for covering the metering chamber <NUM> under a state in which the rotors <NUM> and <NUM> are accommodated in a rotatable manner in the metering chamber <NUM>. The front lid portion <NUM> is fixed to the metering chamber forming portion <NUM> with screws <NUM>.

The rotor <NUM> includes two magnets <NUM>, that is, an S-pole magnet and an N-pole magnet. The two magnets <NUM> are arranged at a distance as illustrated in the figure. The two magnets <NUM> are detected by a detection unit accommodated in a recessed portion <NUM> of the front lid portion <NUM>, that is, a flow rate detection unit (not shown) of the calculation unit <NUM>.

There is illustrated a pulse transmitter <NUM>. The pulse transmitter <NUM> is connected to, for example, an F/I converter (not shown) and an integrator (not shown) through an output cable <NUM>. The F/I converter and the integrator are provided in, for example, a remote management chamber.

A display main body unit <NUM> includes a unit case <NUM>, a substrate having the display unit <NUM> mounted thereon, and a battery (not shown) for supplying electric power to the substrate. The substrate is accommodated in the unit case <NUM>, and is fixed thereto with screws. The battery is also accommodated in the unit case <NUM>. A cover <NUM> includes a transparent viewing portion <NUM> facing the display unit <NUM>, and switches <NUM> and <NUM> for operation.

[PTL <NUM>] <CIT>
<CIT> discloses a metering system for operating a combustion engine having a fuel and an air flowmeters both providing pulsatory electrical signals. <CIT> discloses a circuit protection device for protecting a switching element.

The above-mentioned related-art flowmeter (instrumentation device) has a problem in that, when wrong wire connection occurs at a time of connection of a power supply from outside, and as a result, for example, a large current flows, a switching element (transistor) (not shown) arranged in the pulse transmitter <NUM> is broken. In view of the foregoing, provision of a fuse has been considered. However, in this case, it is required to replace the fuse, with the result that there is a problem of an unsatisfactory maintenance property.

The present invention has been made in view of the above-mentioned circumstances, and it is an object of the present invention to provide an instrumentation device having a pulse output function, which is capable of preventing breakdown of a switching element from occurring even when wrong wire connection occurs.

In order to solve the above-mentioned problem, according to one embodiment of the present invention, as described in claim <NUM>, there is provided an instrumentation device having a pulse output function, the instrumentation device including: a positive power supply terminal; a negative power supply terminal; a signal terminal; a control circuit connected to the positive power supply terminal through a power supply circuit; an NPN-type transistor having a collector terminal connected to the positive power supply terminal, an emitter terminal connected to the negative power supply terminal, and a base terminal connected to the control circuit; a feedback circuit connected between the negative power supply terminal and the emitter terminal and connected to the control circuit; a PTC thermistor connected in series to the signal terminal; and an N-channel type MOSFET having a drain terminal connected to the signal terminal through the PTC thermistor, a source terminal connected to the negative power supply terminal on a downstream side of the feedback circuit, and a gate terminal connected to the control circuit.

According to the present invention as described in claim <NUM>, for example, in a case of wrong wire connection in which an external power supply terminal (+) is connected to the signal terminal, and an external power supply terminal (-) is connected to the negative power supply terminal, and further even in a case in which a large current flows, there is an effect that a switching element can be protected by a resettable protection element, that is, a PTC thermistor. Further, according to the present invention, for example, even in a case of wrong wire connection (reverse wire connection) in which the external power supply terminal (+) is connected to the negative power supply terminal, and the external power supply terminal (-) is connected to the signal terminal, there is an effect that the switching element can be protected in the same manner as in the foregoing, because the N-channel type MOSFET is used as the switching element, and the resettable protection element (PTC thermistor) is provided. In addition, according to the present invention, there is also an effect that an error of a signal can be suppressed, which is described later in Example section. Thus, according to the present invention, there is an effect that breakage of the switching element can be prevented even in a case of wrong wire connection, and hence a more satisfactory instrumentation device can be provided.

A calculation unit of a flowmeter includes a positive power supply terminal, a negative power supply terminal, and a signal terminal, a control circuit, an NPN-type transistor, a feedback circuit, a PTC thermistor, and an N-channel type MOSFET. The control circuit is connected to the positive power supply terminal through a power supply circuit. The NPN-type transistor has a collector terminal connected to the positive power supply terminal, an emitter terminal connected to the negative power supply terminal, and a base terminal connected to the control circuit. The feedback circuit is arranged between the negative power supply terminal and the emitter terminal, and is connected to the control circuit. The PTC thermistor is a resettable protection element, and is connected in series to the signal terminal. The N-channel type MOSFET has a drain terminal connected to the signal terminal through the PTC thermistor, a source terminal connected to the negative power supply terminal on a downstream side of the feedback circuit, and a gate terminal connected to the control circuit.

Now, description is given of Example with reference to the drawings. <FIG> is a block diagram for illustrating a circuit configuration of a calculation unit of a flowmeter serving as an instrumentation device having a pulse output function according to one embodiment of the present invention. Further, <FIG> and <FIG> are each a block diagram for illustrating a state in which wrong wire connection occurs in <FIG>.

In <FIG>, a calculation unit <NUM> included in, for example, a volumetric flowmeter includes a positive power supply terminal <NUM>, a negative power supply terminal <NUM>, and a signal terminal <NUM>. The number of signal terminal <NUM> in Example is one, but the number is not particularly limited thereto. Further, a basic configuration of the above-mentioned volumetric flowmeter is the same as that of the related-art volumetric flowmeter <NUM> described with reference to <FIG>, and hence description thereof is omitted here.

A power supply circuit <NUM> is connected to the positive power supply terminal <NUM>. A control circuit <NUM> is connected to the power supply circuit <NUM>. The control circuit <NUM> is provided so as to perform various controls in the calculation unit <NUM> (as examples of the controls, there are given on/off control of an NPN-type transistor <NUM> and an N-channel type MOSFET <NUM> described later, and control of a feedback circuit <NUM>. Further, the NPN-type transistor <NUM> is also connected to the positive power supply terminal <NUM>.

The NPN-type transistor <NUM> is a known NPN-type transistor. A collector terminal (reference symbol is not shown) of the NPN-type transistor <NUM> is connected to the positive power supply terminal <NUM>. Further, an emitter terminal (reference symbol is not shown) thereof is connected to the negative power supply terminal <NUM> through the feedback circuit <NUM> for current control. Further, a base terminal (reference symbol is not shown) thereof is connected to the control circuit <NUM>.

A PTC thermistor <NUM> is connected in series to the signal terminal <NUM>. The PTC thermistor <NUM> is a polymer-based device, and is also a device having a resistance value that changes abruptly when a temperature inside an element becomes higher than a certain temperature. More specifically, the PTC thermistor <NUM> is a device in which, when heat is generated due to an excess current and the element temperature rises to increase the resistance value, a circuit current can be limited minutely. The resistance value of the PTC thermistor <NUM> returns to an original value when the element temperature lowers, and hence the PTC thermistor <NUM> has an advantage in that the PTC thermistor <NUM> is not required to be replaced unlike a fuse.

The PTC thermistor <NUM> exhibits an important function of the present invention when being used in combination with the N-channel type MOSFET <NUM> described later.

The N-channel type MOSFET <NUM> is an N-channel type Metal-Oxide-Semiconductor-Field Efect (MOFET). A drain terminal (reference symbol is not shown) of the N-channel type MOSFET <NUM> is connected to the signal terminal <NUM> through the PTC thermistor <NUM>. Further, a source terminal (reference symbol is not shown) thereof is connected to the negative power supply terminal <NUM> on a downstream side of the feedback circuit <NUM>. Further, a gate terminal (reference symbol is not shown) thereof is connected to the control circuit <NUM>. The N-channel type MOSFET <NUM> is an element capable of applying a voltage between the gate terminal and the source terminal to control a current from the drain terminal with the magnitude of the voltage. Further, the N-channel type MOSFET <NUM> is an element in which an input resistance to the gate terminal is significantly high, and a current hardly flows.

As is understood from <FIG>, in the calculation unit <NUM>, two circuits, that is, an open collector output circuit and an open drain output circuit are formed. The calculation unit <NUM> is configured to control the level of an instant signal value, which is currently being measured, with a current amount drawn in from an external power supply <NUM>. In Example, the measured instant signal is transmitted outside by changing the draw-in amount of a consumption current within a range of from <NUM> mA to <NUM> mA, and the signal is used in, for example, an F/I converter (not shown) and an integrator (not shown).

Consideration is given of the case of wrong wire connection in which an external power supply terminal (+) <NUM> is connected to the signal terminal <NUM>, and an external power supply terminal (-) <NUM> is connected to the negative power supply terminal <NUM>, for example, as illustrated in <FIG>. When the N-channel type MOSFET <NUM> is in an ON state, the N-channel type MOSFET <NUM> is protected from an excess current due to the presence of the PTC thermistor <NUM>. Meanwhile, when the N-channel type MOSFET is in an OFF state, a large current does not flow, with the result that the N-channel type MOSFET <NUM> is not broken. Thus, even when the above-mentioned wrong wire connection occurs, there is an effect that the N-channel type MOSFET <NUM> can be protected.

In <FIG>, there is illustrated the case of wrong wire connection (wire connection reverse to the example of <FIG>) in which the external power supply terminal (+) <NUM> is connected to the negative power supply terminal <NUM>, and the external power supply terminal (-) <NUM> is connected to the signal terminal <NUM>. In this case, when the N-channel type MOSFET <NUM> is in an ON state, a current flows, but the N-channel type MOSFET <NUM> is protected from an excess current due to the presence of the PTC thermistor <NUM>. Meanwhile, when the N-channel type MOSFET <NUM> is in an OFF state, a large current flows to the PTC thermistor <NUM> through a body diode (not shown, a function originally owned by the N-channel type MOSFET <NUM>) of the N-channel type MOSFET <NUM>, with the result that the N-channel type MOSFET <NUM> is not broken. Thus, even when the above-mentioned wrong wire connection occurs, there is an effect that the N-channel type MOSFET <NUM> can be protected.

Now, description is given of Comparative Example with reference to <FIG>. <FIG> is a block diagram for illustrating a circuit configuration of a calculation unit of a flowmeter according to Comparative Example. Further, <FIG> and <FIG> are each a block diagram for illustrating a state in which wrong wire connection occurs in <FIG>. Comparative Example adopts an NPN-type transistor <NUM> in place of the N-channel type MOSFET <NUM> of <FIG>.

In <FIG>, according to Comparative Example, a collector terminal (reference symbol is not shown) of the transistor <NUM> is connected to the signal terminal <NUM> through the PTC thermistor <NUM>. Further, an emitter terminal (reference symbol is not shown) thereof is connected to the negative power supply terminal <NUM> on the downstream side of the feedback circuit <NUM>. Further, a base terminal (reference symbol is not shown) thereof is connected to the control circuit <NUM>. In Comparative Example, as illustrated in a portion of the transistor <NUM>, there is a risk in that a base current Ib may leak outside of the feedback circuit <NUM> for current control. When leakage of the base current Ib occurs, an error of a current signal may be caused (this error does not occur in the present invention illustrated in <FIG>).

Consideration is given of the case of wrong wire connection in which the external power supply terminal (+) <NUM> is connected to the signal terminal <NUM>, and the external power supply terminal (-) <NUM> is connected to the negative power supply terminal <NUM>, for example, as illustrated in <FIG>. When the transistor <NUM> is in an ON state, the transistor <NUM> is protected from an excess current due to the presence of the PTC thermistor <NUM>. Meanwhile, when the transistor <NUM> is in an OFF state, a large current does not flow, with the result that the transistor <NUM> is not broken. Thus, even when the above-mentioned wrong wire connection occurs, there is an effect that the transistor <NUM> can be protected.

However, in the case of wrong wire connection (wire connection reverse to the example of <FIG>) in which the external power supply terminal (+) <NUM> is connected to the negative power supply terminal <NUM>, and the external power supply terminal (-) <NUM> is connected to the signal terminal <NUM> as illustrated in <FIG>, when the transistor <NUM> is in an ON state, a large current flows backward from the emitter terminal to the collector terminal, and hence there is a risk in that the transistor <NUM> may be significantly degraded. Meanwhile, when the transistor <NUM> is in an OFF state, a breakdown voltage between the emitter terminal and the collector terminal is small, and hence a large current flows even in the absence of a base current, with the result that the transistor <NUM> is broken. It is conceivable to add a diode in order to give the function of the N-channel type MOSFET <NUM> (see <FIG>). However, it is not effective to add the diode because a leakage current from the diode leads to an error of a current signal.

As described above with reference to <FIG>, when two circuits, that is, the open collector output circuit and the open drain output circuit are not electrically insulated from each other (see <FIG>), it is effective to use the PTC thermistor <NUM> and the N-channel type MOSFET <NUM> in combination. Further, as is understood from the comparison with Comparative Example (see <FIG>), as a matter of course, the present invention (see <FIG>) is more effective.

The configuration and effects of the present invention are summarized as follows. The calculation unit <NUM> in the volumetric flowmeter includes the positive power supply terminal <NUM>, the negative power supply terminal <NUM>, the signal terminal <NUM>, the control circuit <NUM>, the NPN-type transistor <NUM>, the feedback circuit <NUM>, the PTC thermistor <NUM>, and the N-channel type MOSFET <NUM>. The control circuit <NUM> is connected to the positive power supply terminal <NUM> through the power supply circuit <NUM>. The NPN-type transistor <NUM> has the collector terminal connected to the positive power supply terminal <NUM>, the emitter terminal connected to the negative power supply terminal <NUM>, and the base terminal connected to the control circuit <NUM>. The feedback circuit <NUM> is arranged between the negative power supply terminal <NUM> and the emitter terminal, and is connected to the control circuit <NUM>. The PTC thermistor <NUM> serves as the resettable protection element, and is connected in series to the signal terminal <NUM>. The N-channel type MOSFET <NUM> has the drain terminal connected to the signal terminal <NUM> through the PTC thermistor <NUM>, the source terminal connected to the negative power supply terminal <NUM> on the downstream side of the feedback circuit <NUM>, and the gate terminal connected to the control circuit <NUM>.

Thus, according to the present invention, there is an effect that breakage of the switching element can be prevented from occurring even in a case of wrong wire connection, and hence the more satisfactory calculation unit <NUM> can be provided.

In Example, the volumetric flowmeter (flowmeter) is given as one embodiment of the instrumentation device having a pulse output function.

Claim 1:
An instrumentation device (<NUM>) having a pulse output function, the instrumentation device (<NUM>) comprising:
a positive power supply terminal (<NUM>);
a negative power supply terminal (<NUM>);
a signal terminal (<NUM>);
characterized in that the instrumentation device (<NUM>) further comprises:
a control circuit (<NUM>) connected to the positive power supply terminal (<NUM>) through a power supply circuit (<NUM>);
an NPN-type transistor (<NUM>) having a collector terminal connected to the positive power supply terminal (<NUM>), an emitter terminal connected to the negative power supply terminal (<NUM>) through a feedback circuit (<NUM>), and a base terminal connected to the control circuit (<NUM>);
the feedback circuit (<NUM>) being connected to the control circuit (<NUM>);
a PTC thermistor (<NUM>) connected in series to the signal terminal (<NUM>); and
an N-channel type MOSFET (<NUM>) having a drain terminal connected to the signal terminal (<NUM>) through the PTC thermistor (<NUM>), a source terminal connected to the negative power supply terminal (<NUM>) on a downstream side of the feedback circuit (<NUM>), and a gate terminal connected to the control circuit (<NUM>).