JPH05122836A - Safety holder for two-line transmitter - Google Patents

Abstract

PURPOSE:To realize operation with no terminal voltage drop by providing a second transistor having collector and emitter connected between transmission lines through resistor elements and base to be applied with collector current from a first transistor, and a voltage limiting element connected between the transmission lines. CONSTITUTION:A transistor Q1 is cut off within a normal measuring range and a current signal IL does not flow therein. When the current signal IL exceeds the normal current range, the transistor Q1 is turned ON to cause turn ON of a transistor Q2 and thereby the current signal IL does not flow to the side of a transmitter (not shown) but it is shunt thus realizing current limitation. On the other hand, the voltage is limited by a Zener diode DZ3 functioning as a voltage limiting element and thereby voltage rise at the output terminals T3, T4, of the transmitter can be avoided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to two transmission lines from the receiver side, which are arranged between a transmitter arranged in an intrinsically safe area and a receiver side arranged in a non-intrinsically safe area. Limit the excessive current and voltage from entering the transmitter via
The present invention relates to a safety guard for a wire type transmitter, and more particularly, to a safety guard for a two wire type transmitter improved so as to enable current limiting without lowering a terminal voltage supplied to the transmitter.

[0002]

2. Description of the Related Art FIG. 5 is a block diagram showing the overall configuration of a system including a conventional safety protector for a two-wire transmitter. The 2-wire type transmitter 10 with a built-in sensor is an intrinsically safe explosion-proof area A1.
The transmitter 10 measures physical quantities such as differential pressure and pressure with the built-in sensor and outputs them as current signals corresponding to the physical quantities to the output terminals T1 and T2.

The output terminals T1 and T2 are connected to the input terminals T3 and T4 of the safety barrier 11, respectively, and their output terminals T1 and T4 are connected.
5, T6 is a receiver 12 via two transmission lines L1 and L2
It is connected to the side terminals T7 and T8. This terminal T7,
A DC power supply 13 and a receiving resistor 14 are connected in series to T8. The safety holder 11 and the receiver 12 are arranged on the non-intrinsically safe explosion-proof area A2 side.

At least about 4 mA as a current signal IL is transmitted from the DC power source 13 to the transmitter 10 through the safety guard 11 by the two transmission lines L1 and L2. The current of about 4 mA is transmitted. A minimum of power to drive the appliance 10 is provided. Then, the transmitter 10 transmits this electric power as base electric power up to 20 mA as a current signal IL corresponding to the physical quantity detected by the sensor.

In this case, the safety retainer 11 prevents the excessive current and voltage from being mixed from the non-intrinsically safe explosion-proof area A2 side to the transmitter 10 arranged in the intrinsically safe explosion-proof area A1.
It has the function of ensuring intrinsic safety. This function will be described below.

Between the input terminal T3 and the output terminal T4,
Resistors R1, R2, and R3 that limit the current are connected in series, and the input terminal T4 and the output terminal T5 are connected by a connection line CN.

A voltage limiting Zener diode Dz1 is provided between the connection point of the resistors R1 and R2 and the connection line CN.
A voltage limiting Zener diode Dz2 is connected between the connection point of the resistors R2 and R3 and the connection line CN.

[0008]

However, the safety barrier as described above has, for example, resistance (R1 + R2 + R3).
As a total of about 300Ω, when the current signal IL flows up to 20mA, the voltage drop across the safety device 11 is about 6V, and the receiving resistor 14 is selected to about 250Ω, so 250Ω × 20mA = 5V. As a result, the voltage drops by 11V as a whole.

The DC voltage 13 cannot be set higher than the voltage of the Zener diodes Dz1 and Dz2, and the voltage of these Zener diodes Dz1 and Dz2 is limited to a predetermined value for intrinsic safety. There is.

For this reason, the terminal voltages generated at the output terminals T1 and T2 of the transmitter 10 must be lowered,
There is the trouble that a transmitter that can operate at low voltage must be developed separately for intrinsically safe explosion-proof purposes.

[0011]

In order to solve the above problems, the present invention provides a physical quantity to be measured by receiving a current signal supplied from a receiver side by a DC power supply through two transmission lines. Correspondingly, in the safety holder for 2-wire transmitter that is inserted between the transmitter and the receiver that changes the current signal and transmits it to the receiver, the previous transmission A diode connected in series to the line through which the current signal flows in the forward direction, and a diode voltage generated in the previous diode having a smaller saturation current than the saturation current of this diode. Is input between the base and the emitter
A voltage limit connected between the transistor and the previous transmission line and the second transistor to which the collector and the emitter are connected through a resistance element between the first transmission line and the collector current of the first transistor is applied to the base. And an element.

[0012]

[Operation] A diode is connected in series to the transmission line connecting the transmitter and the receiver, and a current signal is sent in the forward direction from the DC power supply. The first transistor, which has a smaller saturation current than the saturation current of this diode, is in the off state with a small voltage drop of this diode within the range of the normal current signal, but when an excessive current flows, The diode voltage generated in the diode increases and the diode becomes conductive.

When the first transistor is turned on, the collector current flows into the base of the second transistor to turn it on, and the current signal does not flow to the transmitter 10 side, but returns from the safety holder to the receiver 12 side. .. On the other hand, the excessive voltage is limited to a predetermined value by the voltage limiting element and is not transmitted to the transmitter 10. Therefore, the voltage drop in the safety device caused by the excessive current is a slight value between the base and the emitter of the first transistor.

[0014]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a circuit diagram showing the configuration of one embodiment of the present invention. The parts having the same functions as those of the conventional safety guard for a two-wire transmitter shown in FIG. 5 are designated by the same reference numerals, and the description thereof will be appropriately omitted.

The Zener diode Dz3 is connected to the input terminals T3 and T4 of the safety protector 15, and the diode D1 is connected between the input terminal T3 and the output terminal T5. Is connected to the output terminal T5, and the cathode is connected to the input terminal T3. The base of the transistor Q1 is the cathode of the diode D1 and the emitter is the resistor R4.
Are connected to the node of the diode D1 via the respective terminals.

The collector of the transistor Q2 is a resistor R5.
Is connected to the cathode of the diode D1, the emitter is connected to the connecting line CN, and the base is connected to the collector of the transistor Q1 via the resistor R6.

In this case, the diode D1 and the transistor Q1 may be selected such that the saturation current ISD of the diode D1 has an extremely larger value than the saturation current ISQ of the transistor Q1. For this purpose, for example, diode D
It may be selected so that the emitter area of 1 is extremely larger than the emitter area of the transistor Q1. Also,
Resistor R4 is the saturation current of diode D1 and transistor Q
0 if necessary when increasing the difference from the saturation current of 1
It can be any value from ~ ∞.

Next, the operation of the embodiment shown in FIG. 1 configured as above will be described. Since the resistor R1 is selected to be large in the normal measurement range, the transistor Q1 is cut off, and the current signal IL does not flow to the transistor Q1 but only to the diode D1. The generated drop voltage VD1 is VT
Is a thermal voltage, it increases according to the equation: VD1 = VT.Ln (IL / ISD).

However, when the current signal IL exceeds the normal current range and becomes large, the transistor Q1 is turned on, and accordingly, the transistor Q2 is turned on.
The current signal IL does not flow to the transmitter 10 side and is shunted to limit the current.

On the other hand, the voltage is limited by the Zener diode Dz3 which functions as a voltage limiting element, and the voltage rise at the output terminals T1 and T2 of the transmitter 10 can be avoided. In this case, the voltage drop in the safety holder 15 is only a slight voltage drop of about 0.6 V between the base and the emitter of the transistor Q1.

FIG. 2 is a circuit diagram showing the configuration of another embodiment of the present invention. In this case, the case where the current is detected on the return side of the transmission line is shown. The Zener diode Dz3 is connected to the input terminals T3 and T4 of the safety guard 16, and the diode D2 is connected between the input terminal T4 and the output terminal T6, and the node thereof is the output terminal. A cathode is connected to the input terminal T4 at T6.

The base of the transistor Q3 is the anode of the diode D2, and the emitter is the cathode of the diode D1.
Connected to each of the terminals. The collector of the transistor Q4 is connected to the anode of the diode D2 via the resistor R7, the emitter is connected to the input terminal T3, and the base is connected to the resistor R8.
Are respectively connected to the collector of the transistor Q3 via. In this case, diode D2 and transistor Q
As for 3 as well, the relationship of saturation current is selected as in the case of Fig. 1.

FIG. 3 shows a case in which a resistor R9 is connected in series to the diode D1 in the embodiment shown in FIG. 1, and FIG. 4 shows a diode in which a resistor R4 is connected in series to the embodiment shown in FIG. De D3
The figure shows the case in which is connected.

If desired, a plurality of safety holders 15 may be connected in a cascade between the receiver 12 and the transmitter 10. For example, when 3 units are used in a cascade, the total voltage drop of 6 × 0.6V = 1.8V is 0.6V per unit, but FIG.
In the case of the safety holder 11 shown in, the voltage drop of 6V
A voltage drop of 4.2V can be prevented.

[0025]

As described above in detail with reference to the embodiments, according to the present invention, the voltage drop in the safety device can be made smaller than in the conventional case, so that the 2-wire type transmitter is essentially explosion-proof. Even when used in the area, it can be operated without lowering the terminal voltage generated at the output terminal,
A transmitter that can operate at a low voltage does not need to be specially developed for intrinsically safe use, and can be shared with the same transmitter as a normal one.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a configuration of an embodiment of the present invention.

FIG. 2 is a circuit diagram showing the configuration of another embodiment of the present invention.

FIG. 3 is a partial configuration diagram in which a part of the embodiment shown in FIG. 1 is modified.

FIG. 4 is another partial configuration diagram in which a part of the embodiment shown in FIG. 1 is modified.

FIG. 5 is a configuration diagram of an entire system including a conventional safety guard for a two-wire transmitter.

[Explanation of symbols]

 10 transmitter 11, 15, 16 safety guard 12 receiver 13 DC power supply 14 reception resistance Dz1, Dz2, Dz3 Zener diode D1 diode IL current signal

Claims (1)

[Claims] 1. A transmitter for receiving a current signal from a receiver side through a two-transmission line from a DC power source, changing the current signal in accordance with a physical quantity to be measured, and transmitting the current signal to the receiver side. In a safety holder for a two-wire type transmitter, which is inserted between the receiver and the receiver side to limit voltage / current, a diode connected in series to the transmission line and allowing the current signal to flow in a forward direction. A first transistor having a saturation current smaller than that of the diode and having a diode voltage generated in the diode inputted between a base and an emitter; and the transmission line. A second transistor having a collector and an emitter connected through a resistance element between which a collector current of the first transistor is applied to the base; and a voltage limiting element connected between the transmission lines. Characteristic 2-wire transmission Use safety cage.