Cordless telephone charger (Battery Backup)

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.