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Old 14th Dec 2004, 2:49 pm   #1
Station X
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Default Telephone Ringing Current.

Telephone Ringing Current, which is a completely different thing to Ring Tone, is AC. It is applied to the line and flows via a capacitor in the telephone to an AC Bell also in the telephone. The off-hook condition can then be detected by looking for a loop on the line. When line testing the continuity of the line is detected by looking for the capacitance of the capacitor at the far end.

When Ringing Current was generated by a vibrator it was nominally 17Hz. Later Ringing Machines replaced vibrators and the frequency was changed to 25Hz. A Ringing Machine is a 50V DC Motor driving an AC generator. This enables the machine to operate during power cuts by drawing current from the exchange batteries. The reason for the 2 rings, long pause, 2 rings used in the UK is that during the long pause other bells can be rung. This reduces the amount of ringing current that needs to be produced.

25Hz has the advantage that it can be derived from 50Hz mains by using a ringing convertor. However these were never used at public telephone exchanges due to the need to maintain service during power cuts. They would be used for small switchboards in customers premises.

By the way everything I have said here applies to Vintage Electro-Mechanical Exchanges, which I last worked on over 35 years ago. Digital Exchanges which I started working on in 1979 use different methods of generating ringing current.

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Old 14th Dec 2004, 11:25 pm   #2
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Default Re: Telephone Ringing Current.

The general ringing voltage is 16 2/3Hz, which as Graham states, is normally provided by a rotary machine, but can also be provided by a "Static ringing converter" from a 50Hz supply. The static converters used a saturable reactor to do the frequency division (I have the diagrams and theory of operation if anyone is interested).

In the last days of electro-mechanical PABXs (yes, I know there are still a few out there doing sterling service - we retired our last one two years ago), a number of elctronic units were marketed to replace the rotary machines. We fitted a dual electronic ringing machine on one shelf, and kept a rotary one as backup.

I have also seen transistor ringing generators in small PABXs, though the frequency tends to be a little erratic. The simplest one was a pair of OC25 transistors and a transformer, designed to replace the magneto of a LB (local battery) 700 series 'phone - a friend has a pair of these between kitchen and shed, the ringing generator runs off the microphone energising battery.

There is of course, the traditional magneto, with open circuit ouputs of up to 120V for long line versions.


Last edited by Station X; 27th Dec 2004 at 1:12 pm. Reason: Import
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Old 14th Dec 2004, 11:34 pm   #3
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Default Re: Telephone Ringing Current.

From what I can very vaguely remember from my days at BT. Ringing was 75VAC @ 25Hz. If you were on the end of it when someone was calling, you would picked up a fair packet from it
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Old 15th Dec 2004, 4:40 pm   #4
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Default Re: Telephone Ringing Current.

Not sure about the frequency, but the public network is still 75VAC. Most modern phones use a strangled budgie impersonator, which probably does not need such a voltage, but you can still use a proper bell for that classic ringing sound. Many business systems nowadays are digital & ringing is just a data-stream either on one of the transmission pairs or a dedicated data pair.

Last edited by Station X; 26th Dec 2004 at 6:44 pm. Reason: Import
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Old 15th Dec 2004, 11:04 pm   #5
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Default Re: Telephone Ringing Current.

Yes, "digital" has taken the poetry and romance out of almost everything. "Strangled budgie" is a generous description of the modern telephone ring ! A 1.8 uF capacitor, a 1K coil and pair of metal domes gave music. And couldn't be confused with the doorbell !!.

A thought for you all. The rotary dial (10p.p.s) and the 20 mA telephone loop were the first worldwide standard. A telephone from anywhere would work anywhere else.

I had a telephone "museum" (182 old desksets !) where everything worked on the public network. Then I got cable television+broadband internet+telephone, and the world changed. Pulse dialling no longer worked !

As some of you may have noticed, I now collect AVOs

Last edited by Station X; 26th Dec 2004 at 6:49 pm.
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Old 16th Dec 2004, 12:30 am   #6
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Default Re: Telephone Ringing Current.

Loop Disconnect (pulse dialling or LD) does work with cable, but the dial speed is critical. A "Strowger" type exchange is designed for 10pps, but will work between 9 and 11pps. The modern digital units are much less tolerant of inpulse speed, typically being 10pps +- 0.1, so LD phones will only work when set to close tolerance. I have got this to work, but only by very careful setting of the dial speed (anyone else got one of those electromechanical speed testers?)


Last edited by Dave Moll; 7th Jun 2007 at 8:48 pm. Reason: typo
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