JPS5944159A - Surge protecting circuit of telephone set - Google Patents

Abstract

PURPOSE:To protect a telephone set from a surge at the time of talking, by disconnecting the 1st surge absorbing element connected between the terminals of the telephone set when current flowing into the 2nd surge absorbing element exceeds a prescribed value. CONSTITUTION:When a surge is impressed through telephone terminals 1a, 1b at the time of talking, the current is passed through a Zener diode ZD1 and made flow into a resistor R1. If voltage drop due to the surge current IS exceeds the on voltage VBE applied between the base and emitter of a TR1, the TR1 is turned on and a TR3 is turned off. Consequently, a TR2 is also turned off and the surge current is made flow into a Zener diode ZD2. Therefore, the TR2 is prevented from the inflow of surge current more than the current value found by VBE/R1, and even if the surge is impressed at the time of talking, the telephone circut is prevented from the inflow of current more than a prescribed value.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a surge protection circuit for a telephone that uses an electronic circuit as a communication circuit, among telephones that transmit 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 communication circuit. Mechanical contacts were also used in switch sections (hereinafter referred to as MT switches) for disconnecting, disconnecting, or shorting. Also, a no-brid transformer was used in the communication circuit. Therefore, even if a lightning surge or the like is induced into the phone, the inflow of surge current is limited by the inductance of the hybrid transformer, and since the DP and MT are inch mechanical contacts, their current capacity is relatively large. Therefore, surge protection using only a protector attached to an indoor line outside the telephone circuit is sufficient, and such surge protection circuits have been put into practical use.

However, in recent years, so-called electronic telephones configured using ICs that generate dial pulse signals (hereinafter referred to as dial pulse ICs) and ICs for communication circuits,
Since electronic components such as
It has become necessary to provide surge protection circuits inside telephones as well.

FIGS. 1 and 2 are block diagrams showing the circuit configuration of a general telephone set using the dial pulse IC, respectively. However, the hook 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 communication circuit constructed using electronic components such as IC, 3 is a dial signal generator using a dial pulse IC, and 04 is an MT, a switch,
5 is a DP switch, each of which is composed of an electronic switch 0. The operation of the circuit shown in FIG. 1 will be explained. First, during a call, the MT switch 4 is activated by a control signal from the dial signal generator 3.
is controlled so that the DP switch 5t is in the on state and the DP switch 5t is in the off state.

The DC loop current supplied through 1b is connected to the communication circuit 2.
is supplied to During dialing, the JMT switch 4 is also turned off by the control signal from the dialing signal generator 3, and after cutting off the flow of loop current into the communication circuit 2, the D
By turning the P switch 5 on and off, dial pulses are sent out from the telephone terminals 1a and 1b.

In the circuit operation in FIG. 2, during a call, the MT switch 4 is in the off state and the DP switch 5 is in the on state, and when dialing, the MT switch 4 is in the on state and the DP switch 4 is in the on state.
By turning on/off the P switch 5, a dial pulse is sent to the telephone terminal 1a.

Send from 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. 3.

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). In this case, a loop current flows through the surge prevention 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, ib
, 2 to 5 are the 1st. 2 shows a telephone terminal or circuit block similar to that in FIG. 2; In addition, 6 are
It is a surge protection element that protects the switches 4, 5, etc. and has voltage clamping characteristics (80 to 1 ooV) that does not affect dial pulses. Reference numeral 7 denotes a surge protection element having voltage clamping characteristics (approximately 20 V) for attaching the communication circuit 2. Here, 1) P, M '1'
80 to 1oo to the switch parts 4 and 5 that constitute the switch.
Needless to say, an electronic switch having a breakdown voltage of V or more is used.

In the above configuration, when dialing (MT switch 4 is in the OFF state) and when the dial pulse breaks, the ↑t1 pressure cut clamp characteristic of the surge protection element 6 prevents a loop current from flowing through the entire element 6, and lightning, etc. When a surge of I/'i is applied, υ11λ
The speaker circuit is pleuralized. However, when a VC 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, so the call 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 Ib is determined by the characteristics of the surge protection element 7, which has a lower pressure clamping characteristic than the surge protection element 6. The voltage between the terminals ia and Ib does not rise to the operating voltage, and therefore a surge current flows 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.

Note that although Fig. 3 shows the case where two types of surge protection elements are added to the circuit of Fig. 1, two types of surge protection elements are added to the circuit of Fig. 2 in the same way.
Exactly the same is true when adding other types of surge pleural elements. That is, in that case, the I)P switch 5 (on state during a call) in FIG. 2 will be destroyed.

The present invention has been made in order to eliminate the drawbacks of the prior art as described above, and therefore, the purpose of the present invention is to solve the problem of a telephone using two types of surge protection elements, in which surges are not fully applied during a call. Also, even if the switch connected to line 1a of the communication circuit when viewed from the telephone terminal is not a large-capacity switch, it will remain effective and protected without being destroyed. The purpose of this invention is to provide a surge protection circuit for J-machines.

The main point of the configuration of the present invention is that either the switching element that connects or disconnects the DC loop or the other switching element that disconnects or shorts the communication circuit (this is referred to as the first switching element) is connected to the One switch element is connected in series with the telephone circuit, and the other (which is referred to as a second switching element) is connected in parallel with the telephone circuit, and a dial pulse signal generated by turning on and off the switching element that connects and disconnects the DC loop is transmitted through the telephone terminal. In the telephone set, 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 means for detecting the current flowing into the second surge absorbing element is provided, and when the detected current is more than a predetermined value, the first The present invention further includes on/off control means for rendering the switch 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 same figure, la, 1b, and 2 to 7 refer to the same items as described above, so their explanation will be omitted. In addition, 8 has a current detection function, and thereby the iMT switch 4
This is a control unit that controls on/off of the

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 is greater than or equal to the current value ISM. To become and,
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 ISM to an appropriate value, an electronic switch with a small current capacity or 11 t force capacity can be used as the MT switch 4.

Here, when the surge current ISM flows in, the surge protection element 7 and the current detection function (=t ff1U It goes without saying that the constants for the control section 8 must be set in advance.

FIG. 5 is a circuit diagram showing a specific embodiment of the present invention. In the figure, 1a, 1b, 2 to 8 are the same as or corresponding to those in FIG. 4. In addition, Zener diodes are shown which are used as surge protection elements 7 and 6, respectively, and other elements such as varistors may also be used as the protection elements.

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 the telephone terminals 1a and Ib during a call, the current passes through the Zener diode ZD1 and is applied to the resistor H.
flows into. If this surge current is Is, the voltage drop T (Is
When the above happens, the transistor Tri turns on and ib, the current that was flowing between the base and emitter of the transistor Tr3 is absorbed into the transistor Tri side, and no current flows between the base and emitter of the transistor Tr3, so the transistor Tr3 is off, so the transistor Tr
2 is also off, surge current is caused by Zener diode ZD
2.

Therefore, the transistor Tr2 has VBE (oN)
A surge current exceeding the current value determined by /Rt often flows. Here, the main constant values can be set as follows.

Maximum voltage VOT between the communication circuit terminals (2a-2b) during a VOp call; pressure of the communication circuit 2 VzDhvzD2 ; ZD11Z1] 2 Zener voltage ■DP;
b) Maximum voltage VDT between telephone terminals (1a-l) such as DP9MT switch
b) Assuming the withstand voltage of the circuit elements connected between them, constant values are set so as to satisfy the following relationships.

■○＜VZDI VOT＞ (VZDI +VBE(ON))VDP
< VZD2 < VDT As explained above, according to the present invention, even if a surge due to lightning or the like is applied during a call, it is possible to prevent current exceeding a certain value from flowing into the electronic switch, thereby preventing frost. Therefore, it is not necessary to use electronic components with large current capacity or large power in the slave switch, and it is possible to realize a surge protection circuit for a telephone in an economical and compact manner. Also, an IC that constitutes a communication circuit.

Electronic components such as transistors have low breakdown voltages, and common components can be used, making it economical.

[Brief explanation of the drawing]

Figures 1 and 2 are block diagrams showing the circuit configuration of a typical telephone configured using a dial pulse generation IC, and Figure 3 is a block diagram showing an example of a conventional surge smoke prevention circuit for a telephone. 4 is a block diagram showing an embodiment of the present invention, and FIG. 5 is a circuit diagram showing a specific embodiment of the present invention. Description of symbols 1a, ib...Telephone terminal, 2...Telephone circuit, 3...Dial signal generator, 4...
・-・MT (mute) switch section, 5...D
P (dial pulse) switch section, 6, 7...
Surge protection element, 8...Control unit with current detection function Representative Patent attorney Akio Namiki Patent attorney Kiyoshi Matsuzaki @ I Figure ■ 2 Figure 3 Figure 4

Claims (1)

[Claims] 1) Either one of the switching element that connects and disconnects the DC loop and another switching element that disconnects or shorts the communication circuit (this is referred to as the first switching element) is connected in series with the communication circuit when viewed from the telephone terminal, The other (this is referred to as a second switching element) is connected in parallel with the telephone circuit, and a dial pulse signal generated by turning on and off the switching element that interrupts the DC loop is sent to the telephone line via the telephone terminal. In the telephone terminal of the first switch element (1111), a first surge absorbing element is connected between the telephone terminals, and a second surge absorption element is connected to the communication circuit side of the first switch element. An absorbing element is connected in parallel with the communication circuit, and 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 switching element is turned off. A surge protection circuit for a telephone, further comprising on/off control means for conducting.