Telephone Ring Generator Using Switching Supply

The telephone ring generator shown below generates the needed high voltage from a simple switching mode power supply (SMPS) which employs a CMOS Schmitt Trigger square wave oscillator, 10 mH inductor, high voltage switching transistor (TIP47 or other high voltage, 1 amp transistor) and a driver transistor (2N3053). The inductor should have a low DC resistance of 1.5 ohms or less. The switching supply must have a load connected to prevent the voltage from rising too high, so a 22K resistor is used across the output which limits the voltage to about 120 DC with the phone ringer disconnected and about 90 volts DC connected. The output voltage can be adjusted by changing the value of the 150K resistor between pins 10 and 11 which will alter the oscillator frequency (frequency is around 800 Hz as shown). The supply is gated on and off by a second Schmitt Trigger oscillator (pins 12/13) so that the phone rings for about 2 seconds and then the circuit idles for about a minute between rings. These times can be adjusted with the 10K and 300K resistors connected to pin 12.
Telephone Ring Generator Using Switching Supply - schematic

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faiz   Mar 20, 2008


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