20 Hz Inverter from 9V Battery to Ring Old Phone - Page 1
Forum section: electronics / beginners

author: 4DoorToyota
author_rank: Newbie
timestamp: November 20, 2024, 02:12:23 am
content: So, I've got an old phone that I want to ring and I don't want to just go through the route of buying one since why not just design my own.I decided to try and use the following topology:9V Battery -> Single Phase Inverter -> Transformer -> Roughly 60V RMS ACI selected out a PIC Microcontroller that has a specific peripheral for driving an H bridge, selected my transformer, and gate drivers. I've attached a schematic for the H bridge portion of my design to this post. I’m struggling with if what I have conceived will work with my topology. Resources that I can find online generally all seem to use a high voltage DC bus which makes sense for a standard inverter, but I would like the simpler approach of using a transformer here.[ Specified attachment is not available ]I specifically have a couple of questions about the output of the H Bridge:- I know that at some point I am going to want to filter to remove the switching frequency from the 20Hz AC signal. Can this be performed after the transformer or should it be performed before? Could I filter just at one of the output legs or do I need something symmetrical?- The transformer is meant for mains frequency (50Hz), this shouldn't in my mind present an issue at my final frequency of 20 but will for sure not like the PWM frequency. Can I use this fact to my advantage and basically use it as a form of low pass filter?

author: Analog Kid
author_rank: Super Contributor
timestamp: November 20, 2024, 02:20:35 am
content: Instead of going to all the trouble of programming a PIC, why not just use a simple astable multivibrator as an oscillator? Dead simple to implement with just 2 transistors; pick RC values to get the 20Hz you want.

author: Andy Chee
author_rank: Super Contributor
timestamp: November 20, 2024, 02:35:47 am
content: I would use a self-oscillating half bridge driver IR2153, driving a centre-tapped 12-0-12V transformer in push-pull mode with low-side MOSFETs.If you wish to persist with your existing design;- a filter is not necessary, the telephone ringer is quite tolerant of dirty ring signals- high frequency PWM generation of a low frequency sinewave is not necessary. Just create a raw 20Hz square wave directly.Another alternative is to use a premade module from eBay:https://www.ebay.com.au/itm/204402618630?mkcid=16&mkevt=1&mkrid=705-154756-20017-0&ssspo=a26ASYzTSl2&sssrc=2047675&ssuid=&widget_ver=artemis&media=COPY

author: CaptDon
author_rank: Super Contributor
timestamp: November 20, 2024, 03:40:27 am
content: You are more than one full octave below the design frequency of the transformer. It will possibly have an unacceptably low inductive reactance at 20Hz and be very inefficient. I wonder what the power consumption is of an old mechanical ringer? B.T.W., a friend of mine lived along the Susquehanna River where the old P.R.R. ran the GG1 electric locomotives on 25Hz. His phone with the mechanical ringer would lightly chatter and ring when the trains passed by. Those GG1's were a common sight at least into the early 1980's. I almost got mowed down by a pair running 'light power' as they were so quiet and fast!

author: thermistor-guy
author_rank: Frequent Contributor
timestamp: November 20, 2024, 04:53:14 am
content: I don't know about Canadian standards. This Australian standard has some useful info, see page 35:https://www.commsalliance.com.au/__data/assets/pdf_file/0006/1122/S002_2010.pdfQuoteThe (Australian) standard ringing signal has a cadence, as per note 3 above. The exchanges I'm familiar with would generate this via relays.A "ring relay" for that customer line could connect/disconnect to a constant ring voltage source, typically -48 Vdc + 90 Vac (25 Hz). This relaywould switch on/off with the cadence required.From the spec above, you could use 50 Hz, to suit your transformer.

author: Terry Bites
author_rank: Super Contributor
timestamp: November 21, 2024, 07:45:15 am
content: Try this ringing generator its got a cadence oscillator too. Tweak it.https://www.qsl.net/yo5ofh/hobby%20circuits/telephone_circuits.htm

author: Zero999
author_rank: Super Contributor
timestamp: November 21, 2024, 09:37:52 am
content: Quote from: CaptDon on November 20, 2024, 03:40:27 amThat can be overcome by using a much lower voltage.The 9V peak from the battery will give 6.4V RMS out, so you need a ratio of 1:9.4 to step up to 60V, but you want to go a little higher to account for the voltage losses in the winding resistance and the fact the battery volage will drop over time. I would suggest a 240V to 18V mains transformer. The PWM duty cycle can be adjusted to account for the battery voltage dropping, as it discharges.