A simple yet effective circuit to generate a POTS-compatible ringing voltage can be made from National Semiconductor's LM4871 audio-amplifier IC and a dozen passive components (see the figure). This circuit delivers 1 W of sine-wave output at about 70 V rms. This is sufficient to power most ordinary telephone ringers.
The op-amp section of the LM4871 (pins 3, 4, and 5) is connected as an inverting gain stage. The stage functions as a sine-wave oscillator thanks to the phase-shift network, which consists of resistors R1, R2, and R3; and capacitors C1, C2, and C3. The noninverting input is biased by the built-in rail splitter on pin 2, and R3 doubles as the input resistor.
Therefore, the ratio of feedback resistor R4 to R3 determines gain. The values shown represent a compromise between waveform purity and robust oscillation. Ceramic capacitor C4 counteracts the LM4871's tendency to break into spurious high-frequency oscillation at certain output voltages. Bypass capacitors C5 and C6 are inexpensive, solid-tantalum types.
T1 can be an ordinary, off-the-shelf, 60-Hz power transformer, such as the Triad FS12-500, designed to yield 6.3 V rms at 600 mA or more from a 220- to 240-V source. The derating factors appropriate for sub-design-frequency operation have been applied.
As shown, the circuit oscillates at approximately 20 Hz. It can be retuned to about 25 Hz by scaling resistors R1, R2, and R3 down to 24.3 kΩ; and R4 to 845 kΩ. Although a conventional mechanical ringer forms a resonant circuit with its blocking capacitor, frequency isn't extremely critical in most ringer applications.
To signal devices that accept both the 20- and 25-Hz standards, the oscillator may be designed for 22.5 Hz (R1, R2, and R3 at 27.4 kΩ and R4 at 953 kΩ), permitting adequate frequency stability using inexpensive, 10% tolerance, polyester capacitors for C1, C2, and C3.
The ringing-voltage burst timing is conveniently controlled by applying a 5-V CMOS-level pulse to the Enable input. If this input is driven high (or left open), the LM4871 shuts down so that the circuit's total current drain is under 1 µA. When the input is pulled below 0.5 V, the oscillator starts smoothly, reaching full amplitude in 300 ms. Usually the start time isn't much of an issue because ringing bursts typically last at least one second.