Switching Supply In Telephone Ring Generator

  
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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 D
Switching Supply In Telephone Ring Generator - schematic

C 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. The push button shown is used to manually ring the phone. The 25Hz ringing frequency is generated by another Schmitt Trigger oscillator (pins 1/2) which controls the H bridge transistor output circuit. The 6 transistors in the output stage (4 NPN, 2 PNP) should be high voltage types rated at 200 volts collector to emitter or more. The ringer will only draw around 10 mA, so the output transistors can have a low current rating but must have a high voltage rating. I used TIP47s and small signal PNPs of unknown numbers that I had on hand, but other types such as NTE287 (NPN) and NTE288 (PNP) should work. Both have a 300 volt C-E rating and cost about $0. 95 from mail order houses. The two 470 ohm resistors...



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