Background: I have a wireless router ASUS DSL_N12U. Input voltage and current is 12V , 1.5A respectively. I also have an Inverter (works on three modes: Solar, Battery, Mains) at my home.

Problem: What happens is whenever my Inverter switches its power source (From Battery to Mains or Main to Battery), electricity of my home turns off and on for a very short time (although this switching is visible as lights goes off and on). Earlier when the router was directly connected to the inverter, it used to hang up because of this switching and I had to restart in order to connect to it again. So in order to save my router from this voltage switching, I connected the router to a standby UPS (with transfer time approx: 4ms). But the problem is still there as router still hangs up when power switching happens and I have to restart it.

I searched this problem on internet and found that the wireless router's circuit may be very sensitive and connecting it to an Online UPS can work.

My requirement is to have a small online UPS that can power to the router. However I am not able to find such a small online UPS for this purpose.

So Is there any small online UPS available in the market for this purpose? or can I make one? or Is there any device which can be placed in between the router and current standby UPS in order to solve the problem? Any other suggestions/comments are welcome.

  • \$\begingroup\$ A big fat capacitor. That should do the trick. \$\endgroup\$ – Majenko Jul 6 '14 at 13:15
  • \$\begingroup\$ @Majenko Can you please explain a bit on how can I design such device/circuit, Consider me as a beginner \$\endgroup\$ – Deepak Jul 6 '14 at 13:20
  • \$\begingroup\$ Connect a large electrolytic (say 4700µF) capacitor (rated minimum 16V) across the 12V power input (capacitor + to power +, capacitor - to power -). It will act as a reserve providing power during the brief moment the UPS is switching over. \$\endgroup\$ – Majenko Jul 6 '14 at 13:31
  • \$\begingroup\$ @Majenko Thanks for this. Is there any ready made device or I need to make it? \$\endgroup\$ – Deepak Jul 6 '14 at 13:35
  • \$\begingroup\$ @Majenko is there any need to place resistor also. A circuit diagram may help me. \$\endgroup\$ – Deepak Jul 6 '14 at 13:41

A simple large capacitor placed in parallel with the DC supply input should do the trick. It will act as a power reserve for the brief period while the UPS is switching over. 4700µF should be adequate, and it wants to be rated at a minimum of 16V (12V PSU voltage plus some de-rating to cope with pulses and spikes from the switching):


simulate this circuit – Schematic created using CircuitLab

If you want to get really fancy you could build a little Pi filter, though it shouldn't be necessary:


simulate this circuit

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  • \$\begingroup\$ It worked!! .. However I used your suggested Pi filter circuit with 4.7mF 25v Capacitor is it ok? \$\endgroup\$ – Deepak Jul 7 '14 at 9:39
  • \$\begingroup\$ Yes, 25V is good. The voltage is that which if you exceed the voltage the capacitor may explode. Higher is better, but does mean a physically larger capacitor. The greater the voltage over your required voltage the longer the lifetime of the capacitor as it reduces the internal self-heating. \$\endgroup\$ – Majenko Jul 7 '14 at 9:50
  • \$\begingroup\$ I have one more query: incase I increase switching time of my UPS (current 4ms to say 10 or 15ms) so how much increase in capacitance will be required. \$\endgroup\$ – Deepak Jul 9 '14 at 12:31
  • \$\begingroup\$ Impossible to tell. It may work as it is, or you may need to add more capacitance - only experimentation will tell. It all depends on the current draw of the router at that specific moment in time. \$\endgroup\$ – Majenko Jul 9 '14 at 12:39
  • \$\begingroup\$ The Voltage and current drawn by the router is 12V, 1.5A.. No is it possible to tell? \$\endgroup\$ – Deepak Jul 9 '14 at 12:42

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