I want to use the blog to start showing some projects in progress. For my current PBX Telephone Exchange project, I needed to develop a circuit that could generate the proper waveform to ring an old mechanical ringer telephone (i.e. Model 500). The problem is that these bells require 50-100Vrms at 20Hz. That's annoying because the voltages in that range are high and because I have no transformers for those voltages. Plus, even if I did, the frequency would be 60Hz. The 20Hz AC frequency is absolutely critical, or the phone won't ring at all.

The correct way to do this is to build a DC to AC inverter from scratch. In my first attempt, I took 30VDC from a homemade power supply and sent it through an H-bridge that chopped the DC up into a 20Hz, 30 to -30V square wave. Then, I sent this large waveform through a 20Hz RC filter to break down the square wave into its sinusoidal components (Fourier series in reverse). This generated a 20Hz shark-fin wave that was good enough to be sent through an output transformer that boosted the voltage up to 70Vrms. Unfortunately, the RC filter was such a waste of power (throwing away enough power to bend a square wave into a sinusoidal wave creates a lot of heat) that I scrapped that plan. The bipolar H-bridge was also difficult to control. It did, however function and it could ring three telephones at once.

The new circuit, which I completed today does away with RC filters and square waves. Now, a +/-30VDC power supply powers an LM1875 high-voltage, high-power amplifier chip. This chip takes in a low-level 20Hz sine wave and boosts it up to +/-24 VAC 20Hz. A transformer ramps up the voltage to 53Vrms. The new circuit is much simpler and more professional. It still, however, creates a lot of heat. I still have to design a sine wave generator and figure out a way to split across the x-axis it so that its center-line is at 0V. I also need to play with the gain of the LM1875, as well.

Schematic of the amplifier circuit I used. Not shown is the power supply or sine wave generator.

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