Ringing control apparatus

A ringing control apparatus comprises a bidirectional switching circuit connected in series to a ringing source, which generates an alternating current of a large amplitude, for delivering to a called subscriber the AC signal as an interrupted ringing signal, a phase detector circuit in the form of a zero level detector for detecting the phase of the AC signal generated from the ringing source, and a control circuit for generating a signal for turning on and off the switching circuit in synchronism with the output signal of the phase detector circuit. The delivery of the ringing to the called subscriber is started at a point of time when the ringing voltage comes to a zero level and is stopped at a point of time when the ringing current comes to a zero level, thereby to prevent generation of a surge voltage.

LIST OF PRIOR ART REFERENCE (37 CFR 1.56 (a)) 
The following references are cited to show the state of the art: 
1. Japanese Patent Laid-Open No. 50-104508 
2. Japanese Patent Laid-Open No. 51-75309 
3. IBM Technical Disclosure Bulletin Vol. 12, No. 11, April 1970, pp. 
1736-1737, entitled "Ringing System for Telephone Lines", written by J. 
Garcia. 
BACKGROUND OF THE INVENTION 
1. FIELD OF THE INVENTION 
This invention relates to an apparatus for delivering a ringing signal to a 
called subscriber, or more in particular to a ringing control apparatus 
used with a telephone exchange for controlling the start and stoppage of 
the delivery of a ringing signal to a called subscriber. 
2. DESCRIPTION OF THE PRIOR ART 
An example of the conventional control system 10 for controlling a ringing 
signal to be sent to a called subscriber will be described below with 
reference to FIGS. 1 and 2. A circuit diagram associated with the delivery 
of a ringing signal to a called subscriber from a ringing trunk is shown 
in FIG. 1, and the waveform of a signal generated in that circuit is shown 
in FIG. 2. 
In FIG. 1, reference numeral 1 designates a telephone of a called 
subscriber, numeral 2 a channel network for transmitting a signal, numeral 
3 a ringing trip detector circuit, numeral 4 a ringing device, and numeral 
5 a continuous ringing source in the form of an AC oscillator of AC 75 
V.sub.RMS having a frequency of 16.6 Hz. Numeral 6 designates an interrupt 
relay for generating, through contacts 7 thereof, an interrupted ringing 
signal of one second on and two seconds off, from the continuous ringing 
signal. The ringing device 4 is provided in common on the exchange. The 
interrupted ringing signal of one second on and two seconds off which is 
produced by the ringing device 4 is delivered to a called subscriber. When 
a ringing signal is delivered to the called subscriber 1, a connection 
loop as shown in FIG. 1, starting from ground through continuous ringing 
source 5, contact 7, ringing trip detector circuit 3, channel network 2, 
called subscriber's telephone 1, channel network 2, and power supply, back 
to ground, is formed by the control apparatus. As long as the called 
subscriber is on hook, an interrupted ringing signal of one second on and 
two seconds off is delivered to the called subscriber 1 in asynchronism 
with the phase of the AC signal from the continuous ringing source 5 by 
means of the interrupted relay 6. When the called subscriber 1 responds to 
the ringing, the response is detected by the ringing trip detector circuit 
3 so that the delivery of the ringing signal is stopped in asynchronism 
with the phase of the ringing. The delivery of the ringing is also stopped 
in asynchronism with the phase of the ringing when the called subscriber 1 
gives up the call on the way. 
As explained above, the conventional control systems are such that when the 
delivery or stoppage of a ringing signal to a called subscriber is 
effected, the ringing signal of high voltage and large current from the 
ringing source 5 is interrupted in asynchronism with its phase. As shown 
in FIG. 2, therefore, a sudden voltage change of +120 V or -120 V may 
occur at the beginning of the delivery of the ringing signal. Also, when 
stopping the delivery of the ringing signal, the current in the bell coil 
of the telephone set of the called subscriber is suddenly cut off. As a 
result, the energy accumulated in the coil may present itself as a surge 
voltage with a peak value of high voltage. This surge voltage of large 
value is applied to the channel network 2, thereby causing an erroneous 
actuation of the channel network due to a high voltage exceeding the 
breakdown voltage. 
A channel network using integrated semiconductor switching elements has 
come to be used in recent years. The ability of the integrated 
semiconductor switching elements to withstand a surge voltage is lower as 
compared with that of the mechanical contacts of the conventional 
crossover switch, and such elements are apt to be erroneously actuated by 
a high-level surge voltage having a pulse width of several to several tens 
of .mu.s which is generated instantaneously. 
In order to produce an integrated semiconductor switching element having a 
high breakdown voltage of more than several hundred volts which can 
withstand such a surge voltage, it is necessary to widen the intervals 
between elements. This leads to lower integration, resulting in offsetting 
the advantages of economical efficiency, compactness and light weight. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a ringing control 
apparatus which can prevent a surge voltage of large value from being 
generated at the time of start or stoppage of the delivery of a ringing 
signal. 
Another object of the invention is to provide a ringing control apparatus 
which is economical and simple in construction and does not adversely 
affect the integrated semiconductor switching elements. 
According to the present invention, there is provided a ringing control 
apparatus comprising a bidirectional switching circuit connected in series 
with a ringing source for delivering to a called subscriber an AC signal 
in the form of an interrupted signal, a phase detector circuit providing 
zero level detection for detecting the phase of the AC signal generated 
from the ringing source, and a control circuit for generating a signal for 
operating the switching circuit in synchronism with the output signal of 
the phase detector circuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The circuit diagram of FIG. 3 shows an embodiment of the ringing control 
apparatus according to the present invention. In this drawings, reference 
numerals 1 to 5 designate like component elements as in FIG. 1, and 
numeral 8 a phase detector circuit for detecting the phase of the AC 
signal generated from the continuous ringing source 5. When the ringing 
voltage changes from a negative polarity to a positive polarity, a logic 
"1" signal is produced at the output terminal of the circuit 8. The phase 
detector circuit 8 is of the type commonly referred to as a "zero level 
detector" and is so well-known that it will not be described in detail 
here. Reference characters D.sub.1, D.sub.2 and D.sub.3 designate 
light-emitting diodes, and TH.sub.1, TH.sub.2 and TH.sub.3 light-receiving 
thyristors. The light-emitting diodes D.sub.1, D.sub.2 and D.sub.3 are 
respectively optically coupled with the thyristors TH.sub.1, TH.sub.2 and 
TH.sub.3. The thyristors TH.sub.1 and TH.sub.2 make up a bidirectional 
switching circuit for delivering an AC signal in the form of an 
interrupted signal to a called subscriber. Characters FF.sub.1 and 
FF.sub.2 designate flip-flops, character S a set terminal of the 
flip-flops and R a reset terminal thereof. Upon application of a logic "0" 
signal to the set terminal S, the flip-flops are set, with the result that 
the output 1 is converted into "1" and the output 0 into "0". Characters 
N.sub.1, N.sub.2, N.sub.3, N.sub.4 and N.sub.5 are NAND circuits. The 
flip-flops FF.sub.1 and FF.sub.2, NAND circuits N.sub.1, N.sub.2, N.sub.3, 
N.sub.4 and N.sub.5, and light-emitting diodes D.sub.1, D.sub.2 and 
D.sub.3 make up a control circuit for generating a signal for operating 
the thyristors TH.sub.1 and TH.sub.2 of the switching circuit in 
synchronism with the output signal of the phase detector circuit 8. The 
switching circuit and the control circuit are located in each ringing 
trunk. The reason for this is that the phase difference between the 
voltage and current of the ringing signal is determined by the capacity 
and inductance within the telephone set of the called subscriber and the 
line capacity, and therefore in order to accurately cut off the apparatus 
at zero level of ringing current according to the object of the invention, 
the functions of the present invention are required for each ringing trunk 
connected for each called subscriber. The operation of the apparatus 
according to the invention will be described below with reference to FIGS. 
3 and 4. 
An interrupting timing signal of one second on and two seconds off for 
producing an interrupted ringing signal from the continuous ringing signal 
and the output signal from the phase detector circuit 8 are distributed to 
the control circuit within each ringing trunk from the ringing device 4 or 
another device not shown in the drawings . 
First, explanation will be made of the case of normal operation where a 
ringing signal begins to be delivered. Assume that an order transmit a 
ringing signal to the called subscriber 1 is issued to the NAND circuits 
N.sub.1, N.sub.3 and N.sub.5 within the control circuit from the device 
generating the interrupting timing signal, with the interrupting timing 
shown in FIG. 4. At this point in time, the NAND conditions of the NAND 
circuit N.sub.1 are not yet satisfied since flip-flop FF.sub.1 is reset , 
so that no current flows in the light-emitting diodes D.sub.1 and D.sub.2, 
and the corresponding light-sensitive thyristors TH.sub.1 and TH.sub.2 are 
in the off state. When the ringing generated from the ringing source 5 
changes from a negative polarity to a positive polarity, the output 
terminal of the phase detector circuit 8 detects the zero crossing and 
produces a "1" pulse. Since one of the input terminals of the NAND circuit 
N.sub.5 is already supplied with an interrupting timing signal to be at 
the "1" level, the NAND conditions of the NAND circuit N.sub.5 are 
satisfied and the flip-flop FF.sub.1 is set, thereby making the output 1 
thereof "1" in state. The input to the NAND circuit N.sub.1 carrying the 
signal for giving up a call on the way is at the "1" level because of 
normal operation, and the input carrying the signal for ringing trip is 
also "1" in level since the called subscriber has not yet responded. 
Further, the input carrying the signal for interrupting timing is also "1" 
in level since the interrupting timing signal of one second on is being 
received. As a result, the NAND conditions of the NAND circuit N.sub.1 are 
satisfied the very instant the ringing voltage generated from the ringing 
source changes from negative polarity to a positive polarity. Current 
flows in the diodes D.sub.1 and D.sub.2, so that a gate current is 
supplied to the thyristors TH.sub.1 and TH.sub.2 thereby to turn on the 
light-sensitive thyristors TH.sub.1 and TH.sub.2. In view of the fact that 
the thyristors TH.sub.1 and TH.sub.2 are turned on only as the ringing 
voltage changes from negative polarity to a positive polarity, no high 
voltage is abruptly applied to the channel network 2 but instead an AC 
signal increasing slowly from zero level is provided. The ringing signal 
flows through the thyristors TH.sub.2 and TH.sub.1 respectively during the 
positive and negative half cycles of the ringing operation. 
The operation of transfer from one-second delivery of the ringing signal to 
two-second cut-off will be explained below. 
After one second, the interrupting timing signal changes to "0" level and 
the NAND conditions of the NAND circuit N.sub.1 fail to be met, with the 
result that no current flows in the diode D.sub.1 or D.sub.2, thereby 
terminating the gate drive of the corresponding thyristors TH.sub.1 and 
TH.sub.2. Due to the holding characteristics of the thyristors the 
thyristors are held on when the current is more than the holding current 
even if the gate drive is stopped, and they are turned off only after the 
current is reduced below the holding current, however, the ringing 
continues to be transmitted even after the interrupting timing signal 
becomes "0" level as shown in FIG. 4. At the point in time when the 
ringing current reaches zero, the thyristor TH.sub.2 is turned off. Since 
the thyristor TH.sub.2 is turned off at the time when the ringing current 
is reduced to zero, a large surge voltage which otherwise might be caused 
due to the inductance within the telephone set is prevented. Because of 
the phase difference between the voltage and current of the ringing 
signal, the turn off at zero current is liable to cause an abrupt voltage 
change. Actually, however, the current fails to lead the voltage as much 
as 90.degree. but leads only by about 60.degree., and therefore the actual 
voltage change is about 80 volts. In order to detect the response by the 
called subscriber at the time of signal turn-off, on the other hand, the 
thyristor TH.sub.3 is required to be turned on to form a loop after 
turning off the thyristors TH.sub.1 and TH.sub.2. The NAND circuits 
N.sub.3, N.sub.2 and N.sub.4 and flip-flop FF.sub.2 are logic elements for 
controlling the thyristor TH.sub.3. In other words, when the interrupting 
timing signal becomes "0" in level and the ringing voltage changes from a 
negative polarity to a positive polarity so that the output signal of the 
phase detector circuit 8 becomes "1" in logic level, the NAND conditions 
of the NAND circuit N.sub.2 are satisfied and the output of the NAND 
circuit N.sub.4 becomes zero in level. As a result, current flows into the 
light-emitting diode D.sub.3, thereby turning on the light-sensitive 
thyristor TH.sub.3. 
In the case of giving up a call in the interim or ringing trip, the input 
signals (for giving up a call in the interim) (or for ringing trip) of the 
NAND circuit N.sub.1 become zero, respectively. Thus, the NAND conditions 
of th NAND circuit N.sub.1 fail to be satisfied, so that a current stops 
flowing into the diodes D.sub.1 and D.sub.2. As explained above, the 
thyristors TH.sub.1 and TH.sub.2 are turned off when the current flowing 
therein becomes zero by the stoppage of the gate current thereof. It will 
be understood from the foregoing description that the subscriber ringing 
control apparatus according to the present invention presents a high surge 
voltage from being generated at the time of initial delivery or stoppage 
of the ringing signal, and therefore makes possible a superior electronic 
telephone exchange system without adversely affecting the advantages of 
the integrated semiconductor switching elements. 
The aforementioned embodiments refer to the case where light-sensitive 
thyristors are connected in reverse parallel to each other as a switching 
circuit for converting the AC signal generated by the ringing source 5 
into an interrupted ringing. As such a switching circuit, it is of course 
possible to employ alternatively a semiconductor switching element of PNPN 
four-layer structure with the same current-holding characteristics. Such a 
switching element may be controlled in transformer coupling with the 
control circuit. The described embodiments also refer to the case where 
the delivery of the ringing is stopped at zero level of the ringing by the 
current-holding characteristics of the thyristors or the like. If the zero 
ringing current from the ringing source is detected by the phase detector 
circuit to open the switching circuit, no switching element of 
current-holding type is obviously required in the switching circuit.