JPS6111610Y2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an excitation circuit for an electromagnetic flowmeter that controls the on/off of a semiconductor switch such as a transistor to intermittent or change the direction of an excitation current flowing through an excitation coil.

In the excitation circuit of this type of electromagnetic flowmeter, both ends of the excitation coil may be short-circuited for some reason during instrumentation. If the excitation coil is short-circuited, an overcurrent will flow through a semiconductor switch such as a transistor, destroying the semiconductor switch. For this reason, conventionally, semiconductor switches have been protected by using current limiters to limit the current that flows in the event of an accident. However, with this protection means, although limited, the current flowing through the semiconductor switch during an accident is larger than the current during normal operation, so power consumption by the semiconductor switch increases and heat generation increases. Therefore, the heat dissipation plate of the semiconductor switch needs to be designed with a considerable amount of leeway, resulting in a drawback that the overall configuration becomes large.

The purpose of the present invention is to realize an excitation circuit for an electromagnetic flowmeter with a simple configuration that can effectively prevent damage to a semiconductor switch when the excitation coil is short-circuited, reduce power consumption, and reduce heat generation.

FIG. 1 is a connection diagram showing one embodiment of the excitation circuit of the present invention. In the figure, MF is an electromagnetic flowmeter transmitter, which is equipped with an excitation coil L, a pipe P through which fluid flows, and electrodes G and G', and generates an electromotive force e proportional to the flow rate of the fluid. E is a DC power supply, SW is an excitation current switching circuit having bridge-connected transistor switches _Q1 to _Q4 , and CON is a control circuit for controlling on/off of the transistor switches _Q1 to _Q4 . Transistor switch Q ₁
~ Q ₄ is a combination of Q ₁ and Q ₄ , which is turned on and off in synchronization with the control pulse CP ₁ from the control circuit CON, and Q ₂ and Q ₃
is paired with a control pulse that is 180° out of phase with CP _1.
It is turned on and off in synchronization with CP ₂ , and the direction of the excitation current I supplied from the DC power supply E to the excitation coil L is switched between forward and reverse. In addition, diodes D ₁ to D ₄ connected in parallel to Q ₁ to _{Q 4} release the magnetic energy stored in the excitation coil L when Q ₁ to _{Q 4} are turned off during switching, and generate an excitation current I. It is meant to keep flowing. DZ is a Zener diode, which is connected to the DC power supply E via a resistor _R2 and a fuse F. The regulated voltage VZ generated across this DZ
is given as the power supply voltage of the control circuit CON. R ₁ is a resistance for fault detection, transistor Q ₃ ,
It is connected between the emitter of _Q4 and the negative pole of DC power supply E, and detects the current flowing through the transistor switch. _Q5 is a transistor for protecting the transistor switches _Q1 to _Q4 , and its collector is connected to the positive side of the DC power supply E through the fuse F, resistor _R3 and diode _D8 , and is also connected to the control circuit CON. The line that gives the control pulse CP ₁ to Q ₁ and Q ₄ and the line that gives the control pulse CP ₂ to Q ₂ and Q ₃ are connected via diodes D ₅ and D ₆ , respectively, and the emitter is connected to a DC power supply. The negative electrode side of E is connected. At the base of the protection transistor _Q5 , the voltage _Vr generated across the fault detection resistor _R1 is connected to a diode.
A holding capacitor C is connected between the base and emitter _{of Q5} . Note that the value of resistor R ₃ is chosen to be sufficiently smaller than the value of resistor R ₂ so that sufficient current flows to blow fuse F when Q ₅ is turned on. Resistors R ₁ and R ₃ , capacitor C, diodes D ₅ to D ₈ , transistor Q ₅ and fuse F constitute a protection circuit.

In the excitation circuit of the present invention configured in this way,
When the excitation coil L is connected normally, the base voltage when the protection transistor _Q5 turns on is
The value of the resistor _R1 is selected so as to satisfy the relationship _Vbe >Vr between the emitter voltage _Vbe and the voltage Vr across the fault detection resistor _R1 so that the protection circuit does not operate. When both ends of the excitation coil L are short-circuited, the collector currents of transistor switches Q ₁ and Q ₄ (or Q ₂ and Q ₃ ) that are on increase, and the voltage V across the fault detection resistor R ₁ increases. _r is V _r > V
When it becomes _be , the protection transistor Q ₅ is turned on, so the control pulses CP ₁ and CP ₂ are turned off, and the transistor switches Q ₁ to _{Q 4} are immediately turned off and protected. After that, fuse F blows, turning off the power to control circuit CON. In this case, a hold circuit for V _r consisting of a diode D ₇ and a capacitor C is provided in the base circuit of the transistor Q ₅ , so even after Q ₁ to _{Q 4} are turned off, Q remains unchanged until the fuse F blows. ₅ is kept on. In this way, power consumption is large during the time until the protection circuit operates, but after that, all transistor switches are turned off, significantly reducing power consumption and generating less heat. Therefore, the heat dissipation plate of the transistor switch that is required during normal operation is sufficient.

Note that although we have illustrated the case where CP _{1 ′} and CP ₂ are used as control pulses to turn on and off _the transistor switches Q ₁ to _{Q 4} , it is possible to turns on and off in cycles,
Control pulse whose duty ratio can be set
CP ₃ ′ and CP ₄ are generated in synchronization with CP ₁ ′ and CP ₂ , and Q ₁
are controlled by CP ₄ , Q ₂ by CP ₃ , Q ₃ by CP ₂ , and Q ₄ by CP ₁ , and the steady-state value of the excitation current I is set to a predetermined value according to the duty ratio of CP ₃ ' and CP ₄ . You can also set it to a value. In addition, as shown in Fig. 2, an exciting current detection resistor _R4 is provided, and the voltage V _C across _it is detected by the control circuit CON, and CP ₃ ',
Various configurations can be used as needed, such as one that controls the duty ratio of CP ₄ to keep the steady-state value of the excitation current I constant. In FIG. 2, transistors Q ₆ to _{Q 9} are for level conversion of control pulses CP ₁ to _{CP 4} . Moreover, although the case where the direction of the excitation current I is switched using the transistor switches _Q1 to _Q4 has been described above, the excitation current may be configured to be intermittent, for example, in synchronization with the control pulse _CP1 . Furthermore, in the above _description , a semiconductor switch consisting of transistors Q ₁ to _{Q 4} was exemplified, but as _shown in FIG _. FETs, SCRs, photocouplers, etc. are often used as needed.

As explained above, in the present invention, an abnormality in the current flowing through the semiconductor switch is detected and the semiconductor switch is directly turned off, and then the power of the control circuit that controls the on/off of the semiconductor switch is turned off. It is possible to realize an excitation circuit for an electromagnetic flowmeter with a simple configuration that effectively prevents damage to a semiconductor switch when a short circuit occurs, reduces power consumption, and generates little heat.

[Brief explanation of the drawing]

Fig. 1 is a connection diagram showing one embodiment of the excitation circuit of the present invention, and Fig. 2 is a connection diagram showing another embodiment of the excitation circuit of the present invention. MF: electromagnetic flowmeter detector, L: excitation coil, SW: excitation current switching circuit, _Q1 - _Q4 : transistor switch, _Q5 : protection transistor, F: fuse, _R1 : fault detection resistor,
CON...control circuit, C...hold capacitor, _D5 to _D8 ...diodes, DZ...Zener diode.

Claims (1)

[Scope of utility model registration request] In an excitation circuit for an electromagnetic flowmeter, the excitation circuit of an electromagnetic flowmeter is configured to control the on/off state of a semiconductor switch using a control pulse from a control circuit to intermittent or change the direction of an excitation current flowing through an excitation coil. a resistor, a means for connecting a Zener diode to a DC power supply via a first resistor and a fuse, and applying a voltage across the Zener diode as a power supply voltage of the control circuit; a protection transistor whose emitter is connected to the positive side of the DC power supply via a sufficiently small second resistor and the fuse, and whose emitter is connected to the negative side of the DC power supply, and the voltage generated across the detection resistor. means for supplying a control pulse from the control circuit to the base of the protection transistor via a diode; means for connecting a line for supplying a control pulse from the control circuit to the semiconductor switch via a diode to the collector of the protection transistor; The base of the transistor for
and a capacitor connected between the emitters and the capacitor for holding the voltage from the detection resistor, and the protection transistor is turned on when the voltage generated in the detection resistor exceeds the voltage between its base and emitter. An excitation circuit for an electromagnetic flowmeter, characterized in that the semiconductor switch is turned off and the fuse is blown to turn off the power supply voltage of the control circuit to protect the semiconductor switch.