Ringing waveform

Simple question, is the POTS ringing waveform a simple sinewave or something else?

[ukcol]

I don't understand the "simple question".

POTS ?

[TonyDuell]

POTS = 'Plain Old Telephone Service' or something similar, meaning a normal analogue telephone line.

And yes, AFAIK the ringing voltage is a simple sine wave.

Thanks TonyDuel, and ukcol, I must never assume (I assumed a posting with POTS in it on the telephony section would have been understood), it annoys me too, FX (effects as in radio etc.) slaps self on wrist.

Now to build a ring generator, either a high voltage (+/- 100V or so) amplifier or a low voltage one and a transformer, that could be tricky (and expensive) at 20Hz. Or ignore the losses at 20Hz for a normal 'mains' transformer and feed it with one of those quite remarkable digital amplifiers, we use one which will give 1.5W in stereo from a thing about 1/4" square with no heatsinking.

[russell_w_b]

It's a sine-wave, however...

Our old 'INDeX' telephone switch at work used to build it up digitally, like a step-stair waveform. And the 'Herald' system before that had the ringing waveform sat on the 50V d.c!

Here's one I made earlier with an old wall-wart transformer. Not my idea, BTW...

[Dave Moll]

Originally Posted by TonyDuell

POTS = 'Plain Old Telephone Service' or something similar, meaning a normal analogue telephone line.

I seem to remember being told that it originally stood for "Post Office Telephone Service" - presumably ceasing to be so after the telephone network was no longer run by the Post Office.

[TonyDuell]

I've seen 'POTS' (in this sense) in a lot of American publications on telephony, so I doubt it has much to do with 'Post Office'.

As for the ringing generator, there are many ways it's been done. I doubt you want to make a motor-generator set (although they do turn up second-hand). The GPO had a little rining supply ('Converter Ringing 7' IIRC) that fed half-wave rectified mains into a transfrormer appropriately resonated by a capacitor. Of course a special transformer.

The Telephone 704 (Linesman's telephone) and the 'Converter Ringing 9A' (used on Planphones with external extensions) were simple transistor oscillators again with a special transformer. The latter is not impossible to obtain.

My commerical line simulator uses an 8049 microcontroller feeding a DAC to generate the sine wave. This is filtered and fed to an audio amplifier IC (I forget which one, but it is a common one) then to a step-up transformer. This approach allows you to vary the frequency and amplitude to generate just about any country's ringing voltage (important in that applications).

Finally, the book 'Telephone Projects for the Evil Genius' has a design for a ringing generator that consists of a flyback converter to generate the DC supply and a full-H circuit to turn that into AC (possibly more 'square' than 'sine' but it should work). I hesitate to mention this book as it appears every circuit has at least one error (OK I exagerate, but only slightly) but this one looks not too hard to get working.

[G6Tanuki]

The PABXes I used to deal with provided 48V 25Hz 'ring current' from a little rotary motor-alternator that spun all the time and was really rather rude as regards its waveform.

With a single REN load it remained kinda sinusoidal; if there were a dozen lines ringing at any one time the waveform got lumpy. And the frequency wouldn't be 25Hz either!

Don't get paranoid over this: the telephones you connect to your ring-current source will have been trained to handle all sorts of truly depraved and corrupt ring-current waveforms!

[julie_m]

Originally Posted by russell_w_b

the 'Herald' system before that had the ringing waveform sat on the 50V d.c!

I thought that was how it was supposed to be anyway? Don't you need the DC there, to detect when the phone is answered?

[Biggles]

I made a ringer unit for testing telephones at work which is a simple cross coupled 6-0-6 mains transformer in reverse similar to the one described earlier in this thread. I use the 110V primary windings to keep the output voltage to a sensible level. It just sends the ring voltage for as long as you have your finger on the button, but it's still realistic enough to fool the other techs in the workshop that the "real" phone is ringing if you get the timing right. I suspect the output waveform is more of a square wave but most phones seem tolerant of this. What I have noticed is that some phones seem to load the oscillator more than others and in extreme cases the phone will only make one "peep" instead of ringing properly. The same unit produces the 50V line voltage via a DC-DC converter to run the test phone to mimic a normal line as well as having DTMF decoding etc. The whole lot is portable and runs on batteries. Please bear in mind that the line and ringing voltages produced by these units can be dangerous.
Alan.

[russell_w_b]

I suppose it must do... I must've looked at it on the pair incoming to the bell capacitor.

Don't get paranoid over this: the telephones you connect to your ring-current source will have been trained to handle all sorts of truly depraved and corrupt ring-current waveforms!

Ah, the missus is very sensitive to the wrong ring on her 'new' 746. I have a small PBX (got from a fellow member) that works well, but not up to SHMBOs standards. So it is build it yourself time, a fun project.

The GPO had a little rining supply ('Converter Ringing 7' IIRC) that fed half-wave rectified mains into a transfrormer appropriately resonated by a capacitor. Of course a special transformer.

I can't see how that would work, 50Hz would be the lowest harmonic, I would love to see the circuit and learn something.

[TonyDuell]

Take a look at diagram N654, available from Sam Hallas's site,

http://www.samhallas.co.uk/repositor...&file=N654.pdf

Unfortunately while it gives a schematic, it doesn't give any details of the transformer. I do have such a unit somewhere but have never stuck a 'scope on the output. It does get telephone bells ringing, though.

[russell_w_b]

Originally Posted by merlinmaxwell

'I would love to see the circuit and learn something.'

What you want is Atkinson, Volume 1 (September 1947 but reprinted and available).

[OscarFoxtrot]

N651 and N654 diagrams
http://www.samhallas.co.uk/repositor.../0000/N651.pdf
http://www.samhallas.co.uk/repositor.../0000/N654.pdf

[ukcol]

(Re POTS)

Originally Posted by Dave Moll

I seem to remember being told that it originally stood for "Post Office Telephone Service".

I should have know this because when I left school in the 1960s I desparately wanted to be an apprentice Radio & TV service engineer but I could not find a firm that would take me on, so I joined Post Office Telephones as a YIT.

YIT was the Post Office's acronym for a Youth In Training.

[Dave Moll]

Here is the circuit for Power Unit no. 100A, which produces 25Hz ringing in the manner described.

[russell_w_b]

Originally Posted by ukcol

YIT was the Post Office's acronym for a Youth In Training.

As was the BBC's (where the work ethos and environment was very similar to that of the GPO), but a long, long time ago!

Thanks for the diagrams, I will have to have a play in Splice.