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Telephone ringing circuits

Telephone ringing circuits

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 to them through resistor from power supply.
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 and appropriate BellCore documents.

What is REN ?

REN stand for Ringer Equivalen Number. It is a measurment of how mugh ringing power certain telephone equipment takes. REN numbers are used in USA to determine how many telephoen equipments you can connect to same telephone line and still get them ringing properly (typical line can drive about 3-5 REN load).


The definition of 1 REN is the ringer power required by one ringer of an AT&T standard 500 series telephone set in single-party configuration (ringer placed ACROSS the line). One place to find the exact info: get a copy of 47CFR Part 68 – this is the FCC technical specs (and other info) regarding the PSTN (public switched telephone network). This info also may be available from the FCC’s web site.

What is ringing tone ?

Ringing tone is the ringing that can be heard while the receiver is on-hook and somebody tries to call you. The terms used for describing this telephone ringing are not always very clear whet they mean, because the same term has been used in differnet places to mean different things. ITU-T Q.9 indicates the preferred term is “ringing tone”, but that “ringback tone” is used in the USA. On the other hand, Bellcore (and the old Bell System), used “audible ringing tone” in many of their documents. In 5ESS switch documentation (according some news articles), RINGBACK is used only to describe various ways (other than a normal terminating call) by which a subscriber’s telephone may be rung. Usually people say “ringback” in place of “ringing tone”.

What is distinctive ringing

Distinctive ringing is a system where different ringing tone patterns can tel different thing about the telephone calls. Typical applications are PBXs where you can identify if the call is from inside buildign or from outside by hgearing different ring pattern. Aother applications are when multiple phone numbers are assigned to one physical line and the rign pattern tells which number of them has been called.

Distinctive Ringing and Call Waiting patterns and timing use in USA are covered in GR-506-CORE. Use of multiple patterns to identify the CALLED party (multiple DNs per line) is covered in the basic LSSGR (GR-505 and GR-506 in particular), in the ability to assign ringing patterns to numbers and to Centrex services. ANSI T1.401 identifies some other requirements for distinctive ringing involving inter-exchange carriers.

Normal telephone wiring

In normal telephoen wiring (used in Finland, USA and very amny other countries) the telephoen audio and sing signals share the same wire pair. Typical wiring for 6 pin modular connector.

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