Basically for any electronic device it needs a DC supply as
a source power the different components inside it.
everybody knew that the telephone in our houe will not
contain any supply line (speaking about basic model) other
than the one that comes from the exchange. but how is the
Electronic board inside the telephone is working?
this line carries a DC voltage of 48V (42V to $8V approx).
this voltage supplies power to the components inside the
telephone as well as carries the voice signals from
exchange to our phone and vice versa.
- 48 VDC is used in telecom to prevent corrosion or
electrolysis. If you use +48VDC there are free electrons
raging to go somewhere, as each electron is lost, you lose
what is part of a molecule of copper, thus the copper
corrodes away, lines corrode away until they break! -48VDC
wants electrons so it attracts molecule = no loss of copper