Production of backup power supply for cordless phone

Production of backup power supply for cordless phone

　Cordless phones are becoming more and more popular because of their convenience. However, since common cordless phones require an external power supply, they cannot be used when there is a power outage, which limits their practicality. The cordless phone backup power supply device (see Figure 1) introduced here can solve this problem.

　　Working principle: The whole circuit is mainly composed of the cordless phone power adapter and the backup power supply. When the power supply is normal, the 12V power output by the power adapter supplies power to the cordless phone through diodes D1 and D2. LED and R1 constitute the power indication circuit. Diode D3 is in reverse bias cut-off state at this time. At the same time, the constant current source circuit composed of the three-terminal stabilizer IC1 (LM317) and R2 charges the backup power nickel-cadmium battery pack. The charging current is Icharge = 1.25/R2+50uA. When there is a power outage, D1 and D2 are cut off, D3 is turned on, and the nickel-cadmium battery pack supplies power to the cordless phone through D3. Thereby realizing the automatic switching of the backup power supply and the cordless phone is powered uninterruptedly.

　　The circuit is simple to make and can work without debugging. Please refer to Figure 1 for the selection of components. Nickel-cadmium battery packs should be selected as genuine products with reliable quality to ensure the use effect. For different battery packs, different charging currents can be obtained by adjusting the resistance value of R2, but the impact of long-term charging should be considered and it should not be too large to avoid damaging the battery pack. If space permits, the entire circuit (in the dotted box) can be directly installed in the power adapter box.