Submitted below a very low-cost UPS for the modem. Modems / WiFi routers are usually powered by an adaptor and while the utility power fails then internet connectivity is disrupted even if the power is interrupted for less than a second. Providing a commercial 500VA UPS only for the purpose is an expensive proposition. Small duration power interruption is well taken care of with zero change over time as it uses no relays. The majority of apartments start the backup generator automatically within minutes while the modem power is interrupted. Even with an inverter or mains ups feeding the modem fail to avoid a reset of the modem as those UPSs use relay change over (for off kine UPS) resulting in power interruption for few milliseconds that the modem is not able to tolerate.
Video here https://youtu.be/kcugHjLLIPU
Materials required :-
- 3 nos 4 volt mini lead acid sealed maintenance-free batteries any capacity from 300 mAH to 1.2 mAH =Rs 60 ( for 3 nos 300 mAH being Rs 20 each in electronics local market)
- One DC to DC booster converter ,input 5 to 32 v, 3A, output 1.5 to 35volt=Rs150
- 5 nos 1N4007=Rs 2
- One LED and a series resistor (optional)Rs=2
- One 10 ohm 2 watt resistor Rs1/-
- One DC socket= Rs 5
- One DC pin as per modem socket=Rs 2
- One junction box from electrical shop= Rs20
- Miscellaneous= Rs 8
- Total Rs 250/-
Final Circuit operation
Dc to Dc module has one common ground, one input, and one output. It has adjustments to increase the output voltage (or even decrease) whatever may be the input from 5 volts to 32 volts. Thus while 12 volts are fed from the adaptor it is adjusted by a multi-turn type inbuilt pot to 13.2 volts once for all. Other components like 3 batteries in series, diodes, resistors, and LEDs are as per the circuit diagram. While power from the adaptor is available then 13.2 volts is fed through D3 and R2 to charge the batteries and the modem load is just shown as R4 ( not required) is fed through D2 & D4. This time D5 & D6 remain reverse biased being at a lower potential than the D2 & D4, hence no current flows from the battery. The higher potential at the top end of R4 is 13.2, minus 0.6, minus 0.6= 12 volts where 0.6v is the forward voltage of silicon diodes, and the path D3, R2, D5, D6 delivers 13.2 minuses 3x 0.6= 11.4 ignoring a very small drop at R2. While the mains fail the Dc to Dc module delivers no power and the load is fed directly from the battery through D5 & D6 with zero change over time. The battery is maintained at 13.2 minus 0.6 minus a small drop across R2 at say = 12v. Thus the load voltage now is 12v minus 0.6 minus 0.6 = 10.8 volts. The modem can work well within these voltages of 12 volts and 10.8 volts on mains available or not available respectively. Circuits using relay often noticed in internet schematics always have a small change over time that resets the modem while it changes from mains power to backup power of the battery.