JPS6253981B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a surge protection circuit for a telephone that uses an electronic circuit for a telephone call circuit, among telephones that send dial pulse signals by turning on and off switch elements.

Conventionally, telephones that send out dial pulse signals have used mechanical rotary dials as the sending means, and the switch section (hereinafter referred to as DP switch) that connects and disconnects the DC loop between the exchange and the telephone also has a telephone circuit. Mechanical contacts were also used in the switch section for disconnecting or shorting (hereinafter referred to as MT switch). Additionally, a hybrid transformer was used in the communication circuit. Therefore, even if a lightning surge or the like is induced into the telephone, the inflow of surge current is limited by the inductance of the hybrid transformer, and since the DP and MT switches are mechanical contacts, their current capacity is relatively large. Surge protection using only a protector attached to an indoor line or the like outside the telephone circuit is sufficient, and such surge protection circuits have been put into practical use.

However, in recent years dial pulse signal generation
In so-called electronic telephones constructed using ICs (hereinafter referred to as dial pulse ICs) and ICs for communication circuits, electronic components such as ICs generally have low withstand voltage and small current capacity, so a surge protection circuit is installed inside the telephone. It has become necessary to establish one.

FIGS. 1 and 2 are block diagrams showing the circuit configuration of a general telephone set using the dial pulse IC, respectively. However, the switch and diode bridge are not shown.
In both figures, 1a and 1b are telephone terminals connected to subscriber lines (not shown). 2 is a telephone communication circuit constructed using electronic components such as an IC, and 3 is a dial signal generating section using a dial pulse IC. 4 is an MT switch, and 5 is a DP switch, each of which is composed of an electronic switch.

The operation of the circuit shown in FIG. 1 will be explained. First, during a call, a control signal from the dial signal generator 3
The MT switch 4 is controlled to be on and the DP switch 5 is off.
The DC loop current supplied via a and 1b is supplied to the communication circuit 2. During dialing, the MT is also controlled by the control signal from the dialing signal generator 3.
After the switch 4 is turned off and the loop current is cut off from flowing into the communication circuit 2, the DP switch 5 is turned on and off to send out dial pulses from the telephone terminals 1a and 1b.

In the circuit operation shown in Figure 2, when talking, MT
The switch 4 is in the off state and the DP switch 5 is in the on state. When dialing, the MT switch 4 is in the on state and the DP switch 5 is turned on/off to send dial pulses to the telephone terminal.
Send from a and 1b.

Next, a conventional surge protection circuit for a telephone configured using an electronic circuit as shown in FIGS. 1 and 2 will be explained with reference to FIG.

Normally, the withstand voltage of ICs, etc. is 20 to 30V, so if you connect a surge protection element with voltage clamping characteristics of about 20V between the terminals of the telephone, it will prevent the DC loop from breaking during dial pulse transmission (so-called dial pulse break). However, a loop current flows through the surge protection device, making it impossible to send out a normal dial signal.

For this reason, as shown in FIG. 3, a circuit using two types of surge protection elements is known. In the same figure, 1a, 1b, 2-5 indicate telephone terminals or circuit blocks similar to those in FIGS. 1 and 2, respectively. In addition, 6 is a surge protection element that protects switches 4, 5, etc. and has voltage clamping characteristics (80 to 100V) that does not affect dial pulses. 7 is a surge protection element having voltage clamping characteristics (approximately 20V) for protecting the communication circuit 2. It goes without saying that electronic switches having a withstand voltage of 80 to 100 V or higher are used for the switch sections 4 and 5 constituting the DP and MT switches.

In the above configuration, due to the voltage clamping characteristics of the surge protection element 6 during dialing (when the MT switch 4 is in the OFF state) and when the dial pulse breaks, no loop current flows through the element 6, and surges such as those caused by lightning are prevented. When applied, the telephone circuit is protected by the surge protection element 6. However, when a surge is applied during a call (MT switch 4 is on), the surge current passes through the MT switch 4 and flows into the surge protection element 7.
Although the communication circuit 2 is protected, the MT switch 4 will be destroyed by the surge current flowing through the switch 4 itself. This is because when the MT switch 4 is on, the voltage between the telephone terminals 1a and 1b is determined by the characteristics of the surge protection element 7, which has lower voltage clamping characteristics than the surge protection element 6, so the surge protection element 6 operates. The voltage between the terminals 1a and 1b will not rise to the maximum voltage, and therefore a surge current will flow through the MT switch 4, causing it to be destroyed. To prevent this destruction, it is necessary to use a large capacity electronic switch as the MT switch 4.

Although Fig. 3 shows the case where two types of surge protection elements are added to the circuit of Fig. 1, it is also possible to add two types of surge protection elements to the circuit of Fig. 2 as well. The same can be said. That is,
In that case, the DP switch 5 (on state during a call) in FIG. 2 will be destroyed.

The present invention has been made to eliminate the drawbacks of the prior art as described above, and an object of the present invention is to provide a telephone that uses two types of surge protection elements even when a surge is applied during a call. To provide a surge protection circuit for a telephone in which a switch connected in series to a telephone circuit when seen from a telephone terminal is effectively protected from being destroyed even if the switch is not a large-capacity switch.

The main point of the configuration of the present invention is that either one of a switch element that connects and disconnects the DC loop and another switch element that disconnects or shorts the communication circuit (this is referred to as the first switch element) is used to switch the communication circuit when viewed from the telephone terminal. A dial pulse signal generated by turning on and off a switch element connected in series with the circuit and the other (this is referred to as a second switch element) in parallel with the telephone circuit, and which connects and disconnects the DC loop, is transmitted through the telephone terminal. In a telephone set for transmitting data to a telephone line, a first surge absorbing element is connected between the telephone terminals on the telephone terminal side of the first switch element, and a first surge absorbing element is connected between the telephone terminals on the telephone terminal side of the first switch element. A second surge absorbing element is connected in parallel with the communication circuit, and a means for detecting the current flowing into the second surge absorbing element is provided, and when the detected current is equal to or higher than a predetermined value, the first switch is activated. The present invention further includes on/off control means for rendering the element non-conductive.

Next, an embodiment of the present invention will be described with reference to the drawings. FIG. 4 is a block diagram showing one embodiment of the present invention. In the figure, 1a, 1b, and 2 to 7 refer to the same items as described above, so their explanation will be omitted. In addition, 8 is a control unit that has a current detection function and controls the on/off of the MT switch 4.

Explain circuit operation. In FIG. 4, when a surge such as lightning is applied during a call and the surge current flows into the surge protection element 7, the current is detected by the control unit 8 with current detection function, and the detected current reaches a certain current value I. When the temperature exceeds _SM , the MT switch 4 is turned off by a control signal from the control section 8. Therefore, the surge current flows into the surge protection element 6. Therefore, by setting the current value I _SM to an appropriate value, an electronic switch with a small current capacity or power capacity can be used as the MT switch 4.

Here, when the surge current I _SM flows in, the surge protection element 7 and the control unit 8 with current detection function are set so that the voltage between the communication circuit terminals 2a and 2b shown in the figure does not exceed the withstand voltage of the communication circuit 2. It goes without saying that constant settings must be made.

FIG. 5 is a circuit diagram showing a specific embodiment of the present invention. In the figure, 1a, 1b, 2-8 are the same as or corresponding to those in FIG. 4. In addition, ZD ₁ and ZD ₂ indicate Zener diodes used as surge protection elements 7 and 6, respectively; as protection elements,
Other elements such as varistors may also be used. CD shows an example in which a constant current circuit is used to supply current to the dial pulse generator 3, and a circuit element having a breakdown voltage of 80 to 100 V or more is used.

Next, the circuit operation will be explained. In Fig. 5, when a surge is applied through telephone terminals 1a and 1b during a call, the current flows through the Zener diode ZD ₁
and flows into resistor _R1 . Assuming that this surge current is I _S , when the voltage drop due to I _S (I _S ×R ₁ ) exceeds the on-voltage V _{BE (ON)} (approximately 0.6 V) between the base and emitter of transistor T _r1 , transistor T _r1 until it turns on, the transistor T _r3
The current flowing between the base and emitter of transistor T r1 is absorbed by the transistor T _r1 side, and no current flows between the base and emitter of transistor T _r3 , so transistor T _r3 is turned off, and therefore transistor T _r2 is also turned off, causing a surge. Current is Zener diode
Now flowing into ZD ₂ .

Therefore, the transistor T _r2 has V _BE(ON) /
A surge current greater than the current value determined by R ₁ will not flow. Here, the main constant values can be set as follows.

V _C ; Maximum voltage between call circuit terminals (2a and 2b) during a call V _CT ; Withstand voltage of call circuit 2 V _ZD1 , V _ZD2 ; Zener voltage of ZD ₁ and ZD ₂ V _DP ; Telephone terminal when sending dial pulses (1
Maximum voltage between a and 1b) V _DT ; Telephone terminals such as DP and MT switches (1a and 1b)
1b) The withstand voltage of the circuit elements connected between

V _C <V _ZD1 V _CT > (V _ZD1 +V _BE(ON) ) V _DP <V _ZD2 <V _DT As explained above, according to the present invention, even if a surge due to lightning or the like is applied during a call, the electronic Since it is possible to prevent current exceeding a certain value from flowing into the switch, there is no need to use electronic components with large current capacity or large power in the electronic switch.
A surge protection circuit for a telephone can be realized economically and in a small size. It also constitutes a telephone circuit.
Electronic components such as ICs and transistors have low breakdown voltage and can be made from common components, making it economical.

[Brief explanation of the drawing]

Figures 1 and 2 are for diamond pulse generation, respectively.
A block diagram showing the circuit configuration of a general telephone configured using an IC, FIG. 3 is a block diagram showing an example of a conventional surge protection circuit for a telephone, and FIG. 4 shows an embodiment of the present invention. The block diagram in FIG. 5 is a circuit diagram showing a specific embodiment of the present invention. Code explanation, 1a, 1b...telephone terminal, 2...
Call circuit, 3...Dial signal generator, 4...
MT (miyuto) switch section, 5... DP (dial pulse) switch section, 6, 7... surge protection element, 8... control section with current detection function.

Claims (1)

[Claims] 1. Either one of a switch element that connects or disconnects the DC loop and another switch element that disconnects or shorts the communication circuit (this is referred to as the first switch element) is connected in series with the communication circuit when viewed from the telephone terminal; A second switch element (hereinafter referred to as a second switch element) is connected in parallel with the telephone circuit, so that a dial pulse signal generated by turning on and off the switch element which interrupts the DC loop is sent to the telephone line via the telephone terminal. In the telephone set in which the first
A first surge absorbing element is connected between the telephone terminals on the telephone terminal side of the switch element, and a second surge absorbing element is connected in parallel with the telephone circuit on the telephone circuit side of the first switch element, A means for detecting a current flowing into the second surge absorbing element is provided, and further comprising an on/off control means for rendering the first switch element non-conductive when the detected current is equal to or higher than a predetermined value. A surge protection circuit for telephones characterized by: