Telephone converter circuit that can be used to power circuit added to the basic phone is shown in figure 1. This may be more economical/convening than adding a separate power supply from the AC power line or using battery power. It takes the -48V provided by the telco and efficiently converts it to a well regulated 5V.
Circuit Description of Telephone Converter Circuit
Telephone converter circuit does not require the bulk and expensive transformer which is usually used in these design. It takes the -48V (it can actually function well with input of -35V to 75V) and provides up to 150mA at 5V. The switching regulator (IC1 operates) in the boost configuration powered by a zener regulated 6.2V with reference to the -48V. If therefore acts like it was converting 48V to 53V. A feedback signal is shifted in level from 5V to the IC’s feedback input (pin 10) by the current source transistor T1. Transistor T3 is included to compensate for the Vbe temperature drift in transistor T2. Transistor T3 can be omitted if about -2mA/0C voltage variation are acceptable.
The switching FET (T1) exhibits a typical RDS(on) of 1Ω when operation with the 6-V gate driver provided by the circuit. The efficiency of this circuit is about 75% and the 1Ω sense resistor (R2) limits peak current to about 200mA.
PARTS LIST OF TELEPHONE CONVERTER CIRCUIT
|Resistor (all ¼-watt, ± 5% Carbon)|
|R1 = 47 KΩR2 = 1 ΩR3, R4 = 22 KΩ|
|C1 = 4.7 µF/100VC2 = 0.33 µFC3 = 0.1 µF
C4 = 330 µF/10V
|IC1 = MAX721
T1 = 2N7004
T2, T3 = 2N5401
ZD1 = 1N753A(6.2V)
D1 = 1N5804
|L1 = CD54-151M(sumida)|