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From the Q&A

With TJ Byers


Antique Phone Ringer

Question:

At an antique telephone show, I ran across a collector with a tester clipped to his belt. I asked him what he had. He said it was a telephone ringer that was powered by (4) four AA cells. It produced a 20-Hz square wave of sufficient voltage to power a ringer. I asked him if he had a circuit, to which he said “in my head” and walked away. I am curious as to what he may have in that little black box.

Joe Patay


Answer:

The requirements for ringing a Ma Bell bell is an AC voltage of 40 to 150 volts at 20 Hz to 40 Hz. Lots of designs can do that, including a very simple DC-to-AC inverter with a step-up output transformer. The problem with most designs is that they are very inefficient. Want my take on it? I would first use a switching step-up converter to change six volts into 60 volts, then use a half bridge to create an alternating AC voltage — like in the figure. The switching regulator is a robust LM2577 with a 60-volt output at 100 mA. That part of the circuit is straightforward.

The unique part of this design is the IR2153. Normally found in fluorescent ballast, the IR2153 contains a 555-type oscillator in addition to a half-bridge with a dead band of 1.2 us so that the two MOSFET transistors are never on at the same time. Instead of driving a fluorescent tube, though, I turned the chip around to switch 60 volts to the telephone ringer. The frequency of the switcher is set by the RC network across the RT and CT pins and can be fine tuned — via the 100K pot — to the resonant frequency of the bell.


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