JPS59229956A - Input protection system of call and answer detecting circuit - Google Patents

Abstract

PURPOSE:To attain an electronic constitution for the call and answer detection part in a switchboard by providing a circuit constitution which varies to low impedance when a voltage level exceeds the voltage level set only on the basis of the ground potential. CONSTITUTION:When call and answer detection operation is performed, a call signal current flowing through a resistance is converted by a current converting circuit 12 into a low level current proportional to an input, and the low-level current is converted by resistances 20-23 of an overvoltage limiting circuit 23 into a voltage, which is applied to a DC detecting circuit 18. The voltage which is applied to the DC detecting circuit 18 and proportional to the call signal current is filtered by a low-pass filter 16, and a comparator 17 outputs a call answer signal when the output of the low-pass filter 16 contains a DC component and its voltage level is higher than the reference voltage set by a DC voltage source 9. The overvoltage limiting circuit 23 has low impedance when the output current of the current converting circuit 12 increases, so that an overvoltage input to the DC detecting circuit 18 is suppressed by setting the resistance to a proper value.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a paging response detection circuit for a telephone terminal in a telephone exchange, and particularly to an input protection system for a paging response detection circuit suitable for semiconductor integrated circuit implementation.

[Prior art]

Traditionally, telephone exchanges were mostly made up of electromagnetic parts.
With advances in semiconductor technology, electronic switching is gradually progressing, and switching equipment is becoming smaller and more sophisticated. However, the part of the telephone exchange that sends out ringing signals to subscriber terminals and detects responses from terminals handles extremely high signal levels, so it mainly handles signals of several volts. It had good compatibility with semiconductor integrated circuit technology (which had been lagging behind in computerization).

FIG. 1 shows an example of the configuration of a telephone call and response detection circuit of an exchange. In FIG. 1, telephone 1, which is a subscriber terminal,
The paging and response detection operations are performed as follows. First, connect the call retracting switch 6 indicated by the contact 4.5 to the resistor 7.
, 8, a DC voltage source 9 (usually -48V), and a calling signal source 1° (usually 75Vrms).

As a result, a ringing signal is sent from the two-line subscriber lines L1 and L2 to the telephone set 1 via the line resistance 43 and rings the bell. When the subscriber picks up the handset, the telephone 1 switches from an AC circuit to a DC circuit, and a DC current flows superimposed on the AC ringing signal.

On the other hand, the current conversion circuit 12 converts the voltage drop across the resistor 8 into a low-level current proportional to the input, and the low-pass filter 16.
The signal is transmitted to a DC detection circuit 18 composed of a comparator 17.

Note that the input voltage of the low-pass filter 16 is determined by the product of the resistor 26 and the output current of the current conversion circuit 12. Current conversion circuit 1
If a DC component is superimposed on the output of the low-pass filter 16, a DC voltage is generated at the output of the low-pass filter 16, and the comparator 17 outputs a calling response signal. Here, in order to prevent the comparator 17 from malfunctioning due to noise voltage, the voltage of the DC voltage source 19 is set as a reference voltage.

As described above, when the DC detection circuit 18 detects a call response, the call lead-in switch 6 is switched and the 2-line subscriber line L1
．． L2'lf: Connected to the call current supply circuit 11, and continues to send direct current while the subscriber is talking.

Now, in the call response detection circuit shown in FIG.
It is practical to divide the latter into a high-voltage IC using an NPN switch element or the like, and a low-voltage IC using a CNO technology or the like. Here, the output voltage of the current conversion circuit 12 is determined by the line resistance 2. Changes in Party B,
DC voltage source9. If fluctuations in the paging signal source 10 and the resistors 7 and 8 are considered, the range is four to five times larger. Therefore, if the detection sensitivity is set under the minimum voltage condition, when the voltage is maximum, the low withstand voltage IC
The power supply voltage of the DC detection circuit 18 is exceeded. Therefore, a hexagonal protection circuit is required for low voltage ICs, and in Figure 1, diodes D1, D2, DC voltage source 13,
A voltage clamp circuit 15 composed of 14 takes care of this. However, this voltage clamp circuit 15 has the following problems. It is practical to share the DC voltage sources 13 and 14 with the DC voltage source 19 (usually ±5 V) of the DC detection circuit 18, but as a result, when the voltage is clamped, the input voltage of the DC detection circuit 18 is lower than the power supply voltage. More diode DI,
The voltage must be exceeded by the forward voltage of D2. To prevent this, the DC voltage sources 13 and 14 must be made independent.
It is unfavorable in terms of economy and practicality.

Furthermore, when considering single conductor integration, the number of power supply terminals increases, which also impairs economic efficiency.

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to provide a call response detection circuit suitable for semiconductor integrated circuit implementation, which uses an input protection method that can protect against overvoltage input without increasing the number of power supply terminals.

[Summary of the invention]

To this end, the invention is characterized in that it mines overvoltage inputs with a circuit configuration that is referenced only to ground potential and changes to a low impedance when a set voltage level is exceeded.

[Embodiments of the invention]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 2 is a circuit configuration diagram showing a first embodiment of the present invention, in which the subscriber section and current supply shown in FIG.

A portion of the feeding section and the calling signal sending circuit are omitted. In FIG. 2, 8 is a resistor, 10 is a calling signal source, 12 is a current conversion circuit, and 25 is a resistor 20 to 22 and a diode I.
)3. 18 is a DC detection circuit comprised of a low-pass filter 16 and a comparator 17; and 19 is a DC voltage source.

In the configuration shown in FIG. 2, the ringing response detection operation involves converting the ringing signal current flowing through the resistor 8 into a low-level current proportional to the input using the current conversion circuit 12, and converting it into a voltage using the resistors 20 to 22 of the overvoltage limiting circuit 26. This is accomplished by converting the voltage and transmitting it to the DC detection circuit 18. here,
The following explanation will be given assuming that the input impedance of the DC detection circuit 18 is sufficiently large and can be ignored. The voltage proportional to the ringing signal current applied to the DC detection circuit 18 is converted into a p-wave by the low-pass filter 16, and a DC component exists in the output of this low-pass filter, and the voltage level is set by the DC voltage source 19. If it is wider than the reference voltage, comparator 1
A call response signal is output from 7. Here, the impedance of the overvoltage limiting circuit 23 changes with respect to the output current i of the current conversion circuit 12, and the peak value Vp of the input voltage of the DC detection circuit 1B becomes as shown below.

(1) Togi where i is small.

Vp - (几1 +R2+R11) 1 (2) i is thick.

a) Positive side voltage Vp(+) Vp(+) = R3・i (+) 10VFb) Negative side voltage Vp(-) Vp(-) = (R20 R3) -1(-)
+VF Here, R1 to R5 are resistances of 20 to 2, respectively.
2 resistance value, VF is the diode D3. It shows the forward voltage of D4, and i shows the peak value of the current. In this way, the overvoltage limiting circuit 23 exhibits low impedance when the current i becomes large, so by setting R1 to R6 to appropriate values, overvoltage input to the DC detection circuit 18 can be suppressed.

Furthermore, in the configuration according to this embodiment, since direct current is superimposed on the positive side of the calling signal current, the positive side has a lower impedance than the negative side.

The overvoltage limiting circuit 23 shown in FIG. 2 does not require an extra power supply terminal and is suitable for semiconductor integration.

FIG. 3 is a circuit diagram showing a second embodiment of the present invention, showing only the overvoltage limiting circuit portion of the call response detection circuit. In FIG. 6, the overvoltage limiting circuit 24 includes a resistor 25 and a constant voltage diode D5. Consists of D6.

In this circuit configuration, constant voltage diodes D5, D6
By setting the Zener voltage to an appropriate value, overvoltage can be suppressed.

Further, since there is no increase in the number of redundant power supply terminals and the number of elements is small, it is more suitable for semiconductor integration.

[Effects of the Invention] As described above, according to the present invention, an overvoltage limiting circuit can be obtained in a call response detection circuit that is suitable for semiconductor integration without increasing the number of power supply terminals. It is now possible to digitize the response detection part.
This contributes to making switchboards more compact, highly functional, and economical.

It should be noted that the present invention can be applied as an input protection system when a high-voltage signal is detected by a low-voltage detection circuit other than the call response detection circuit.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing a call and response detection section of a telephone according to the prior art, and FIG. 2 and FIG.
．． FIG. 3 is a circuit configuration diagram showing a second example. 8.20~22.25...Resistance, 19...
DC voltage source, 12...Current conversion circuit, 18...DC detection circuit, 25, 24-...Overvoltage limiting circuit, D1-D4...Diode, D5. D6... Constant voltage diode.

Claims (1)

[Claims] t. A calling response detection circuit in which the current output of the current conversion circuit is detected by a DC detection circuit, wherein an external voltage source is used as a reference between the current conversion circuit and the DC detection circuit. An input protection method for a call response detection circuit that is characterized by the provision of an overvoltage limiting circuit that limits the voltage relative to ground potential. 2 The above-mentioned overvoltage limiting circuit has an 11th 2nd. The third resistance and the first resistance. The second diode is composed of the first diode and the second diode. Second
．． A third resistor is connected in series between the main terminal and ground, the anode of the first diode and the cathode of the second diode are connected to the main terminal, and the cathode of the first diode is connected to the second diode. ．． At the connection point of the third resistor,
The anode of the second diode is the anode of the first diode. 2. An input protection system for a call response detection circuit according to claim 1, wherein the input protection system is connected to the connection point of the second resistor. 3 The above-mentioned overvoltage limiting circuit includes a resistor and a first . The resistor is connected between the main terminal and ground, and the first. 2. The input protection system for a calling response detection circuit according to claim 1, wherein the second constant voltage diode is connected in series on the same electrode side, and the series circuit is connected in parallel to the resistor.