Router UPS

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. 

Router UPS Circuit Diagram

The UPS consists of four parts: a backup  detector schema that monitors the supply  voltage from the AC mains adapter, a battery  schema 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.