Creating a Backup Power Supply for Phone and Internet Router
Maintaining the functionality of your phone and internet router during power outages is crucial, especially when they are linked to a security system. To ensure uninterrupted service, a backup power supply for the router becomes essential. The system described here comprises a 12-V lead-acid battery and a voltage converter with an output voltage range of 15 to 30 V. It comes equipped with built-in protection mechanisms to prevent over-discharge of the battery. Operating as a DIY uninterruptible power supply (UPS), this system remains in standby mode as long as the mains voltage remains stable.
The Components of the UPS
The UPS comprises four essential components: a backup detector circuit that oversees the supply voltage from the AC mains adapter, a battery circuit that ensures the battery voltage remains above 11.8 V, a FET switch situated between the battery and the voltage converter, and a voltage doubler enclosed within the dashed outline.
Operation of the UPS in Normal Conditions
In a scenario where the router’s supply voltage remains above 20 V, the voltage doubler component is unnecessary. In this circumstance, the outputs of comparators IC1a and IC1b (pins 1 and 7) directly connect to the FET’s gate (G1 linked to G2).
Switching to Battery Power
Under standard operating conditions, the router, linked to K3, draws power from the voltage on connector K1. In this setup, the voltage on pin 2 of comparator IC1a exceeds 5.6 V. Consequently, the output on pin 1 is low, keeping the FET in the off position. If the external voltage on K1 declines, pin 2 of IC1 sees a voltage drop, causing the output on pin 1 to go high, turning on the FET. In this state, the battery and voltage converter jointly supply power to the router. To prevent the battery voltage from falling below 11.8 V, the output of the second comparator (pin 7) goes low when the voltage reaches this threshold, turning off the FET. To avoid inadvertent FET activation due to rapid battery voltage increases after shutdown, capacitor C3 is included.
Additional UPS Features
Switch S1 allows the UPS to initiate without an external supply voltage on K1, and capacitor C4 ensures the comparators can continue functioning during momentary losses of power on K1 and K2. For safety, an emergency stop switch (S2) and a fuse (F1) are incorporated. Since the voltage converter exhibits high inrush current, F1 must be appropriately sized. If the router’s supply voltage is less than 19 V, the comparator’s high state output level is insufficient to maintain a gate-source voltage of 4.5 to 5 V. This is because the source voltage remains the same as the battery voltage during continuous charging (13.8 V).
The gate voltage must exceed 18.3 to 18.8 V, which is problematic with a router supply voltage below 19 V. To address this, the voltage doubler is employed, employing a 555 timer IC (CMOS version). The oscillator’s frequency (IC2) is roughly 40 kHz. Components C6, D5, and D6 introduce AC voltage to the switched supply voltage from T2, controlled by comparators in parallel with the timer reset. An 18-V Zener diode (C7) safeguards the FET gate-source junction against overvoltage. Care should be taken to choose a 555 timer with a supply voltage rating suitable for this application, available in 16-V and 18-V versions.
Voltage Converter Details
The UPS’s voltage converter is a notebook power converter intended for in-car use, with a 12 V input voltage, selectable output voltage, and a minimum current capacity of 0.5 A. This is a requirement that most voltage converters can easily fulfill.
Battery Charging and Setup
The battery must be connected to a capable charger designed to maintain a lead-acid battery in optimal condition during extended no-load operation. Several designs for this purpose have been previously featured in Elektor.
Calibrating the System
To ensure proper operation, adjust P1 for an approximate voltage of 7 V. When using a lab power supply in place of the battery, adjust P2 to set a threshold voltage of 11.8 V.