It can be handy to have your phone and Internet router continue working for a while after a power failure for example, if they provide access to a security system. This requires a backup power supply for the router. The version described here consists of a 12-V lead-acid battery and a voltage converter capable of supplying an output voltage in the range of 15 to 30 V. It has built-in protection to prevent excessive battery discharge. This DIY uninterruptible power supply (UPS) operates in standby mode as long as the mains voltage is present.
The UPS consists of four parts: a backup detector circuit that monitors the supply voltage from the AC mains adapter, a battery circuit that monitors the battery voltage to prevent it from dropping below 11.8 V, a FET switch between the battery and the voltage converter, and a voltage doubler (inside the dashed outline). To understand how it works, first consider the situation with a router supply voltage above 20 V, for which the voltage doubler is not required. In this case the outputs of com-parators IC1a and IC1b (pins 1 and 7) are connected directly to the gate of the FET (G1 is connected to G2).
Under normal conditions the router, which is connected to K3, is powered from the voltage on connector K1. In this situation the voltage on pin 2 of comparator IC1a is higher than 5.6 V. The output on pin 1 is therefore low, and the FET is switched off. If the external volt-age on K1 drops out, the voltage on pin 2 of IC1 drops and the output on pin 1 goes high, switching on the FET. In this state the battery and the voltage converter supply power to the router. The battery will gradually discharge, and to prevent the battery voltage from dropping below 11.8 V the output of the second comparator (on pin 7) goes low when the voltage reaches this threshold level, switching off the FET. The battery voltage may rise quickly after the FET is switched off, so capacitor C3 is included to ensure that this does not cause the FET be switched on again.
Switch S1 allows the UPS to start up without an external supply voltage on K1, and capacitor C4 enables the comparators to continue operating in the event of a brief dropout of the two supply voltages on K1 and K2. The emergency stop switch S2 and fuse F1 are included for safety reasons. The voltage converter has a high inrush current, so F1 must be generously dimensioned.
If the router supply voltage is below 19 V, the comparator output level in the high state is too low to achieve a gate–source voltage of 4.5 to 5 V, since the source voltage is always the same as the battery voltage under continuous charging, which is 13.8 V. This means that the gate voltage must be at least 18.3 to 18.8 V, which is difficult or impossible with a router supply voltage under 19 V. This can be remedied by including the voltage doubler, which is built around the well-known 555 timer IC (CMOS version). The frequency of the oscillator (IC2) is approximately 40 kHz. Components C6, D5 and D6 add the AC voltage to the switched supply voltage delivered by T2, which is driven by the comparators in parallel with the timer reset. An 18-V Zener diode (C7) protects the FET gate–source junction against overvoltage.
Be careful to select a 555 with a maximum rated supply voltage sufficient for this application; they are available in 16-V and 18-V versions. The voltage converter of this UPS is a note-book power converter designed for in-car use, with an input voltage of 12 V, selectable output voltage, and a minimum current capacity of 0.5 A. Most voltage converters can handle this easily. The battery must be connected to a good charger capable of maintaining a lead-acid battery in good condition under prolonged no-load operation. Various designs for this have been described in Elektor in the past.
Adjust P1 for a voltage of approximately 7 V. With a lab power supply connected in place of the battery, adjust P2 for a threshold volt-age of 11.8 V.
