Scope
Cordless phones are great! Without going into all their
benefits I really like their grid power independence, but there is one
problem. Although the cordless phones work on batteries, the base station (and
all connecting phones) becomes useless in case of a power failure since the
base station needs power to work! To overcome this limitation, I designed and built
a simple circuit which powers the base station with the benefit of battery
backup.
Although I could have
easily tapped a 24v or 12v supply to power the base station from the house
batteries, I had a problem with this setup because basically the base station
stopped working. In my scenario, I’m also powering the telephone provider modem
using my in-house batteries instead of the supplied power adaptor. It seems
that the power used for the modem has to be isolated from the power used for
the base station. Don’t know why this is but that’s how it has to be!The requirement was to build a small power supply of 6.5v at a maximum current of 350mA. These are the ratings of the power supply which came with the phone. Here are more details of the phone system and how I'm charging the phones using solar energy.
How it works
AC power is supplied from a 16v ac power adapter, rectified by diodes bridge B1 and smoothed by capacitor C1. LED D8 indicates that the ac power is available. This voltage is then applied to voltage regulator U1, the 1 amp regulator 7812 through diode D1. Diodes D2, D3& D4 will boost U1 output voltage by 0.7v increments and therefore the output from U1 is 13.8v. This is fed to an external battery through fuse F2. The 13.8 volts is enough to keep the battery fully charged or better trickled charged. If this voltage is too high, it can be decreased by removing diodes D2, D3, D4, each diode decreasing the voltage by 0.7 volts.
Diode D5 will direct combined power from the power supply
and battery to another voltage regulator U2, the 1 amp regulator 7808. Diode D5 reduces the voltage and linear
regulator U2 will stabilise the voltage to 8v. This is smoothed by capacitor C2
and further reduced by diodes D6 & D7 to 6.8v which is the base station
operating voltage. LED D10 indicates that power is being fed to the base
station.
