JPS642509Y2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a safety barrier, and more particularly to an improvement in a safety barrier using a current limiting circuit having a transistor.

In order to make the instrumentation of a chemical plant, etc., an intrinsically safe explosion-proof system, a safety barrier is designed to combine an intrinsically safe circuit located on the hazardous area side and a non-intrinsically safe circuit located in the control room on the safe area side. It is inserted into the connecting transmission line and limits the energy transmitted from the safe area to the dangerous area to a safe value, even in the event of an accident. As a safety barrier using a current limiting circuit using a transistor as a current limiting element, the first
A configuration as shown in the figure has been proposed. In Figure 1, the emitter of the transistor Q that constitutes the current limiting circuit is connected to the transistor protection resistor Ro.
It is connected to the non-intrinsically safe terminal A on the safe side via the terminal A, and is connected to the intrinsically safe terminal B on the hazardous area side via the current detection resistor _R1 , and the base is connected to the intrinsically safe terminal B.
The collectors are each connected to a ground terminal G. In this safety barrier, when the current Io supplied to the intrinsically safe circuit SC on the hazardous area side is within the normal range (for example, 4 to 20 mA), the voltage drop IoR ₁ generated across the resistor R ₁ is The value of R ₁ is chosen to be less than the emitter voltage V _BE and Q is off. By the way, when a short-circuit accident occurs on the intrinsically safe circuit side and the output current Io increases, the voltage drop IoR ₁ of R ₁ exceeds V _BE and Q turns on, causing the output current Io to become the safe value determined by V _BE /R ₁ . Limit to value. Moreover, since the emitter and collector of Q are connected between the non-intrinsically safe terminal A and the ground G, a short circuit accident between the emitter and collector of Q will reduce the output current Io, which has the advantage of working on the safe side. however
If a short circuit occurs between the base and emitter of _Q1 , the only thing that limits the output current Io is the resistor Ro, and to reduce the output current it is necessary to increase the value of Ro. For example, to set the limiting current to 50 mA when the voltage V _IN between non-intrinsically safe terminal A and ground is 30 V,
Ro must be 600Ω. However, Ro
Increasing the value increases the internal resistance of the safety barrier,
The voltage drop during normal operation becomes large. This voltage drop is limited by the minimum operating voltage of the intrinsically safe circuit. For this reason, Ro cannot be made very large, and the first
In the circuit shown, the base of the current control transistor
If a short circuit occurs between the emitters, the output current of the safety barrier may not be limited to a safe value.
There was a practical problem.

An object of the present invention is to realize a safety keeper that can limit the output current to a safe value even in the event of a short circuit between the base and emitter of a current limiting transistor.

FIG. 2 is a connection diagram showing one embodiment of the safety barrier of the present invention. The difference in Fig. 2 from the conventional example shown in Fig. 1 is that two current limiting circuits CL ₁ and CL ₂ are used, and a series circuit of CL ₁ and resistor R ₃₁ and a series circuit of CL ₂ and resistor R ₃₂ are used.
The series circuit is connected in parallel and inserted between the non-intrinsically safe terminal A and the intrinsically safe terminal B. That is, CL ₁ ,
The emitters of transistors Q ₁₁ and Q ₁₂ of CL ₂ are commonly connected to the non-intrinsically safe terminal A via a resistor Ro, and the collectors of Q ₁₁ and Q ₁₂ are also commonly connected to the ground terminal G, respectively.
The base of Q ₁₁ is connected to intrinsically safe terminal B through resistor R ₃₁ ,
The bases of Q ₁₂ are each connected to the intrinsically safe terminal B via a resistor R ₃₂ . Note that there are current detection resistors R 21 and R ₂₁ between the emitter and base of Q ₁₁ and Q ₁₂ , respectively.
R ₂₂ is connected.

In the present invention configured in this way, the output current
Io is shunted into a series circuit of resistors R ₂₁ and R ₃₁ and a series circuit of resistors R ₂₂ and R ₃₂ . Now R ₂₁ = R ₂₂ = R ₂ , R ₃₁ =
When R ₃₂ = R ₃ , a current of 1/2Io flows through each series circuit. Therefore, the voltage drop 1/2IoR ₂ occurring across the resistors R ₂₁ and R ₂₂ is compared with the base-emitter voltage V _BE of Q ₁₁ and Q ₁₂ , respectively. When Io is in a normal range (for example, 4 to 20 mA), 1/2 IoR ₂ is smaller than V _BE and Q ₁₁ and Q ₁₂ are off. By the way, when a short-circuit accident occurs on the intrinsically safe circuit side and the output current Io increases, the voltage drop of R ₂₁ and R ₂₂ exceeds V _BE and Q ₁₁ and Q ₁₂
is turned on. When Q ₁₁ and Q ₁₂ turn on, R ₃₁ ,
The current through R ₃₂ is limited to V _BE /R ₂ , respectively.

As a result, the output current Io is limited to a safe value determined by _2VBE / _R2 , and the limit value can be adjusted by the value of _R2 . With the output current Io limited in this way,
Even if _Q11 becomes abnormal and its base-emitter is shorted, _the current _I2 flowing through resistor _R32 is
Since it is limited to the value of _VBE / _R2 , the current _I1 flowing through resistor _R31 is also limited to a value of _VBE / _R2 (1 + _R2 / _R3 ), and the output current Io (= _I1 + _I2 ) is a value determined by _2VBE / _R2 + _VBE / _R3 . This limited current increases by _VBE / _R3 compared to when _Q11 is normal, but there is no problem if the limit value is set to a safe value taking this increase into account.
The internal resistance of the safety barrier is Ro + 1/2 (R ₂ + R ₃ ).
The resistance increases by the amount of _R3 connected. However, if _R3 is set to about 50Ω, the increase due to _R3 is 25Ω.
Therefore, it can be made sufficiently small.

Note that when transistors Q ₁₁ and Q ₁₂ are turned on and collector current flows, the collector current
If an increase in V _BE becomes a problem, as shown in Figure 3, current limiting circuits CL ₁ and CL ₂ include transistors Q ₂₁ and Q ₂₂ for collector current amplification of Q ₁₁ and Q ₁₂ , and transistors for current shunting. transistors Q ₃₁ and Q ₃₂ are provided,
Q ₁₁ and Q ₁₂ may be configured to be used only for voltage or current detection. In this case Q ₃₁ ,
Since the Q ₃₂ collector is grounded, it has the advantage of making heat dissipation measures easier. Furthermore, if it is desired to limit the output voltage, Zener diodes DZ ₁₁ and DZ ₁₂ may be connected between the intrinsically safe terminal B and the ground as shown in FIG. Further, in the above description, the case where two series circuits of a current limiting circuit and a resistor are connected in parallel is illustrated, but the number can be increased as necessary.

As explained above, the present invention provides a safety keeper that can limit the output current to a safe value even in the event of a short circuit between the base and emitter of a current limiting transistor.

[Brief explanation of drawings]

Figure 1 is a connection diagram showing an example of a conventional safety barrier, Figure 2 is a connection diagram showing one embodiment of the safety barrier of the present invention, and Figure 3 is another embodiment of the safety barrier of the present invention. It is a connection diagram. CL, CL ₁ , CL ₂ ... Current limiting circuit, Q, Q ₁₁ ,
Q ₁₂ ...transistor, Ro, R ₁ , R ₂₁ , R ₂₂ ,
R ₃₁ , R ₃₂ ...Resistance, A...Non-intrinsically safe terminal, B...
Intrinsically safe terminal, G...ground terminal.

Claims (1)

[Scope of utility model registration request] First and second current limiting transistors each having a current detection resistor connected between their base and emitter, and the emitters of these first and second current limiting transistors are commonly connected to a non-intrinsically safe terminal. a protective resistor, a resistor that connects the base of the first current limiting transistor to the intrinsically safe terminal, a resistor that connects the base of the second current limiting transistor to the intrinsically safe terminal; and means for connecting the collector of the second current limiting transistor to a ground terminal.