JPH021685A - Loop closing switch circuit - Google Patents

Abstract

PURPOSE:To prevent fuming or iginition at parts in a loop by constituting a voltage holding means as the serial connected body of a thermistor and a Zener diode. CONSTITUTION:The thermistor 7 and the Zener diode 6 are inserted in series into a loop closing switch circuit, so that minimum line voltage at the time of a closed loop is maintained. When the output of a terminal call detection circuit 5 is OFF, since an FET1 goes also to OFF, no breakdown occurs in an element in a loop even though overvoltage is impressed. On the other hand, when the output of the circuit 5 is ON, the FET1 goes to ON, as well. At that time, when the overvoltage is impressed, since the thermistor 7 operates, its resistance value is raised so as to limit a current. Accordingly, the fuming or the ignition in the parts in the loop can be prevented.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a loop closing switch circuit for closing a loop based on the detection of a calling signal from a terminal, and particularly relates to a loop closing switch circuit for closing a loop based on the detection of a calling signal from a terminal. This relates to a loop-closing switch circuit designed to prevent smoke or fire from parts inserted therein. [Prior Art] A loop-closing switch that closes a loop based on detection of a call signal from a terminal. As shown in Figure 2, the circuit has so far included a loop closing FET 1, a reverse current prevention diode 3 using a FET parasitic diode 2, and a minimum line voltage holding resistor to ensure peripheral circuit operation. It is made up of 4. When a call signal from the terminal is detected by the terminal call detection/circuit 5, the detection is outputted and the FET 1 is closed.

Regarding detection of overvoltage and overcurrent and protection from them, for example, see pages 181 to 184 of Practical Electronic Circuit Book (5) J (November 1, 1980, published by CQ Publishing). It is described throughout.

[Problem to be solved by the invention]

However, in the past, if an overvoltage was applied when the loop was closed, all the voltage would be concentrated on the minimum line voltage holding resistance, which would cause the power consumption there to exceed the rated power. This causes problems such as the line voltage holding resistor emitting smoke or catching fire.

An object of the present invention is to prevent smoke and ignition from occurring in the parts that constitute the loop closing switch circuit even if an overvoltage is applied to the loop when the loop is closed. [Means for solving the problem] The above problem is to maintain the minimum line voltage when the loop is closed using a thermistor and a Zener diode. is achieved.

[Effect]

The problems observed so far can be solved by changing the line voltage holding resistor to a thermistor, but with this method, the resistance value of the thermistor varies widely and if the resistance value falls below the specified value, , it may not be possible to maintain the minimum line voltage. Therefore, set the resistance value to a large value in advance, and even if the resistance value varies, set the resistance value to a specified value or higher ( It is possible that -
Generally, there is an upper limit to the loop resistance when the loop is closed, and this method cannot be used. Therefore, it is necessary to solve the above problems (by lowering the resistance value of the thermistor to reduce the effect of variation in the resistance value, and by adding a Zener diode to maintain the line voltage. When an overvoltage is applied when the loop is closed, the thermistor increases its resistance and takes on the voltage that cannot be borne by the Zener diode, which can cause smoke and other problems. Ignition is to be prevented.

〔Example〕

The present invention will be explained below with reference to FIG.

FIG. 1 shows the configuration of a loop closing switch circuit according to the present invention together with its surroundings. As shown in the figure, a loop closing FET 1, a diode 5 for preventing reverse current by a FET parasitic diode 2, and a thermistor are shown. 7 and a Zener diode 6. Note that the reference numeral 8 in the figure indicates a circuit that operates with line voltage.

Now, to explain its operation, when the output of the terminal call detection circuit 5 is OFF, the FET 1 is also OFF, so even if an overvoltage is applied, there is no damage to the elements in the loop.

On the other hand, if the terminal call detection circuit 5 is ON, the FET
If t is also turned on and a steady current I flows through the loop in this state, a voltage Vx of (RI+Vz) (R: thermistor resistance value, v2: Zener voltage) will be generated between the lines. However, assuming that R varies within a range of ±α (0<α × 1), the line voltage Vx will exist within the following voltage range.

(1-Q)RI +V2<V, r<(1+α)R
I+V, -(1) Therefore, considering the case where a voltage vl is always required for the circuit 8 connected between the lines to operate stably, v2 must satisfy the following equation (2). be.

V2≧VA −(i −C1) RI −
” (2) That is, as Zener diode 6, equation (2)
If all the thermistors having a Zener voltage V2 that satisfies the above are used, the minimum required VZ can be maintained even if the resistance value R of the thermistor 7 varies. If overvoltage is applied when the loop is closed, thermistor 7
As the loop operates, its resistance value R increases and current is limited, thereby preventing smoke or fire from occurring in the components within the loop.

〔Effect of the invention〕

As explained above, according to the present invention, the thermistor and Zener diode have the effect of preventing smoke and ignition when overvoltage is applied.

[Brief Description of the Drawings] Fig. 1 is a diagram showing the configuration of an example of the loop closing switch circuit according to the present invention together with peripheral circuits, and Fig. 2 is a diagram showing the configuration of the loop closing switch circuit according to the prior art. be. 1... Loop closing FET, 6... Diode, 6
... Zener diode, 7... Thermistor. Representative Patent Attorney Katsuri Ogawa J No.

Claims (1)

[Claims] 1. Voltage holding means for holding the minimum operation guaranteed voltage of the loop closure, especially the peripheral circuit, and connected in series with the means,
A loop closing FET that is closed when a terminal makes a call, and the FET
A loop closing switch circuit comprising a reverse current prevention diode for preventing reverse current due to a parasitic diode, the loop comprising a voltage holding means as a series connection of a thermistor and a Schenner diode. Closing switch circuit.