م/ محمد أحمد زكي
28-10-2004, 06:36 PM
Telephone ringing circuits
Copyright Tomi Engdahl 1997,2000
Preface
Telephone circuit gain always interrest, because telephones are everywhere and quite often there are old telephone luying around somewhere. Those telephones can be used for many interresting experiments including small home intercom: connect telephones in series or parallel and feed suitable operating current (about 20 mA) to them through resistor from power supply.
The most problematic to home experimenter is how to get telephone ringing because the ringing voltage is over 50V and not at standard mains frequency (50/60Hz). Sometimes you want to get the information that telephone is ringing to your own circuits. This text tries to clear out those problems.
What is ring signal ?
The telephone company sends a ringing signal which is an AC waveform. Although the common frequency used in the United States is 20 HZ and in Europe is typically 25 Hz, it can be any frequency between 15 and 68 Hz. Most of the world uses frequencies between 20 and 40 Hz. The voltage at the subscribers end depends upon loop length and number of ringers attached to the line; it could be between 40 and 150 Volts. The ringing cadence - the timing of ringing to pause - varies from telephone company to company.
The usual arrangement is to feed the 75 V a.c. ringing current (backed by earth) down one wire of the phone line. On the other wire is placed a slugged relay (or equivalent) which is backed by -48V d.c. When you pick up the phone, the relay operates to the loop d.c. current and trips the ringing current. It also triggers a further device to put the transmission bridge in circuit to enable speech to take place, together with supervision of the calling and called loops. The ralay needs to be a slugged relay to prevent premature ring trip by the a.c. ringing current.
In USA minimum ring voltage supplied is 40Vrms (delivered into a 5 REN load). This is the must detect limit. There is also a minimum must ignore value of 10Vrms. Milage on individual PBX's will vary greatly. But most guarantee to deliver 40Vrms into a 3 to 5 REN load.
When the telephone ring signal is sen to the telephone, the ring voltage is not applied constanly to the line. Typically ring timing is 2 seconds on and 4 seconds off in the US. In the UK ring timing goes .4 sec on, .2 sec off, .4 sec on, 2 sec off then repeats. In toher countries the ring timign cna vary from country to country (even from operator to operator) and you should check the local regulations if you want to get to know the actual ring signal timing in use.
For more information, check Understanding Telephones article by Julian Macassey at http://www.egyed.com/phonework.html and appropriate BellCore documents.
لتكملة الموضوع انظر المرفقات
المصدر (http://www.epanorama.net/circuits/telephone_ringer.html)
Copyright Tomi Engdahl 1997,2000
Preface
Telephone circuit gain always interrest, because telephones are everywhere and quite often there are old telephone luying around somewhere. Those telephones can be used for many interresting experiments including small home intercom: connect telephones in series or parallel and feed suitable operating current (about 20 mA) to them through resistor from power supply.
The most problematic to home experimenter is how to get telephone ringing because the ringing voltage is over 50V and not at standard mains frequency (50/60Hz). Sometimes you want to get the information that telephone is ringing to your own circuits. This text tries to clear out those problems.
What is ring signal ?
The telephone company sends a ringing signal which is an AC waveform. Although the common frequency used in the United States is 20 HZ and in Europe is typically 25 Hz, it can be any frequency between 15 and 68 Hz. Most of the world uses frequencies between 20 and 40 Hz. The voltage at the subscribers end depends upon loop length and number of ringers attached to the line; it could be between 40 and 150 Volts. The ringing cadence - the timing of ringing to pause - varies from telephone company to company.
The usual arrangement is to feed the 75 V a.c. ringing current (backed by earth) down one wire of the phone line. On the other wire is placed a slugged relay (or equivalent) which is backed by -48V d.c. When you pick up the phone, the relay operates to the loop d.c. current and trips the ringing current. It also triggers a further device to put the transmission bridge in circuit to enable speech to take place, together with supervision of the calling and called loops. The ralay needs to be a slugged relay to prevent premature ring trip by the a.c. ringing current.
In USA minimum ring voltage supplied is 40Vrms (delivered into a 5 REN load). This is the must detect limit. There is also a minimum must ignore value of 10Vrms. Milage on individual PBX's will vary greatly. But most guarantee to deliver 40Vrms into a 3 to 5 REN load.
When the telephone ring signal is sen to the telephone, the ring voltage is not applied constanly to the line. Typically ring timing is 2 seconds on and 4 seconds off in the US. In the UK ring timing goes .4 sec on, .2 sec off, .4 sec on, 2 sec off then repeats. In toher countries the ring timign cna vary from country to country (even from operator to operator) and you should check the local regulations if you want to get to know the actual ring signal timing in use.
For more information, check Understanding Telephones article by Julian Macassey at http://www.egyed.com/phonework.html and appropriate BellCore documents.
لتكملة الموضوع انظر المرفقات
المصدر (http://www.epanorama.net/circuits/telephone_ringer.html)