Generating a 20hz 90v signal phone ringer

Firstly, do you absolutely need it to be 20Hz? I ask this because if you are trying to ring a newer phone which uses an electronic ringer you don't need 20Hz since they will work anywhere from 16 - 60Hz. 20Hz is only required to operate older style electromagnetic ringers with metal gongs, so the telco's still have to provide it as standard because some people still have these phones.

Typical telco electronic exchange ringers these days use fairly complex DC/AC converters using switch-mode techniques (much like you will find in a modern sine wave DC/AC converter found with solar power mains generating systems). For a hobbyist it is easier to use an audio power amplifier with a 70V public address type output transformer. The amplifier will need to have a 20Hz sine wave signal applied to its input. You will also need to work out how to produce your required cadence sequence and ring trip functions.

Simple way is an oscillator generating an approx sine wave feeding a pair of 500mW transistors in push pull off a 12 volt supply. These drive a low volt mains transformer in reverse. In the UK, I used a 240 - 12 volt one. Not very efficient in electrical terms but works fine. The oscillator can be triggered by a logic circuit to give the ring cycle as required.

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    Dave Plowman        dave@davenoise.co.uk           London SW
                  To e-mail, change noise into sound.

Very easy. A small 120v transformer with, say, a 12v secondary. Drive the secondary with a lower voltage AC waveform than 12v to get approx 90v out. You could use a transistor driven by a 555 timer IC running in astable mode at 20Hz to drive the transformer. The positive rail if the 'inverter' circuit could be supplied by a simple adjustable regulator to get the 90v out from the transformer.

Obviously the transformer will not be very efficient at 20Hz, but it will work well enough for this application.

Dave

I had an old Post Office local system ring generator that did the job very simply. It had a transformer with a split winding on one side, and a single winding on the other. The split windings were linked in series by a 2uF paper capacitor, and a single diode ( IN4007 style ) was in series with one of the outside terminals. The diode half wave rectified the mains going in, providing the transformer with half mains frequency pulses ( 25Hz in the UK ). The cap between the windings provided rough tuning, which took the waveform back to something approximating a sine wave. Out of the other side of the transformer came a reasonable sine wave at about 80v p-p.

I guess that the same thing could be reproduced now using a split primary power transformer, with a secondary of say 40 - 0 - 40. The value of the cap could be played with a bit for best waveshaping to suit the tranny. I doubt that you would notice the frequency being 30Hz US or 25Hz UK

Arfa

Dave-

This sounds like it will probably work OK. My concern is that a 60 Hz power transformer may have much too low of an impedance at 20 Hz.

The 555 circuit is probably stable enough for a resonant bell. There is also the Rube Goldberg approach: One could divide the 60 Hz power line frequency by 3. For battery-only, there is a chip that divides a 3.579545 crystal (tuned for 3.579540) to produce 60 Hz.

Fred

Check out the web page at

to get a pretty good idea of telephone ringing at the layman's level. Check out the ring tone generators at for a couple of circuit examples that may actually work (I haven't built any of them, but there's nothing blatantly wrong with the circuits as drawn). Here's another one:

Cheers!!!

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Dave M
MasonDG44 at comcast dot net  (Just substitute the appropriate characters in 
the address)

Never take a laxative and a sleeping pill at the same time!!