# help me design 1.5 second backup circuit

#### Sabih mahmood

Joined Jun 29, 2010
5
Hi my wireless router operates at 9V DC 1amp. My dc to ac inverter takes 1.5 seconds to turn on due to which my wireless router turns off and turns on. I want to design a circuit which provides a backup of 1.5 sec to my router.

I think that capacitor would solve this but i don't know the value ...

#### mcgyvr

Joined Oct 15, 2009
5,394
What is the DC supply the inverter? What signal is available to trigger this timer?

#### Sabih mahmood

Joined Jun 29, 2010
5
There is a step down transformer which converts 220v ac to 9v dc 1amp and supplies to the router ... I have inverter installed at my home in case of power failure it takes on ... it operated on a 12v dc batteray (150amp) and then convert it into 220v ac ... but it takes 1.5 sec to turn on ...

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#### mcgyvr

Joined Oct 15, 2009
5,394
You don't want a timer really..you need a secondary power supply with a faster transfer time. Why not just install a regular UPS for the router only? Or can you adjust the inverter transfer time?

#### kubeek

Joined Sep 20, 2005
5,795
Is the router the only device powered from the ups? If yes, then you should avoid the double conversion to 220 and back and feed the router straight from the battery. A simple low drop regulator should do the job, and since the ups will keep recharging the battery when online, you can get rid of the 9v adapter.

#### mcgyvr

Joined Oct 15, 2009
5,394
yeah I wasn't sure if his battery was always being charged (when needed). A regulator direct from the battery getting rid of the 9V adapter is the easiest.

#### ErnieM

Joined Apr 24, 2011
8,377
How big a cap? The cap equation is always correct:

I = C dV/dT

I and dT are 1A and 1.5 seconds in the post. What can we do with dV? Well, if we guess the router runs on 5V so it's power regulator needs 2 V then dV = 9V - 5V - 2V = 2V.

Solving for C:

C = I dT/dV

= 1A * 1.5 sec / 2V = 1.5 A-sec / 2V

= .75 F.

#### Sabih mahmood

Joined Jun 29, 2010
5
You don't want a timer really..you need a secondary power supply with a faster transfer time. Why not just install a regular UPS for the router only? Or can you adjust the inverter transfer time?
Regular ups is little bit expensive ... I want to do something cheap ... ...

#### Sabih mahmood

Joined Jun 29, 2010
5
Is the router the only device powered from the ups? If yes, then you should avoid the double conversion to 220 and back and feed the router straight from the battery. A simple low drop regulator should do the job, and since the ups will keep recharging the battery when online, you can get rid of the 9v adapter.
No, Router is not the only device which is powered from inverter, my whole home runs on inverter.

#### Sabih mahmood

Joined Jun 29, 2010
5
How big a cap? The cap equation is always correct:

I = C dV/dT

I and dT are 1A and 1.5 seconds in the post. What can we do with dV? Well, if we guess the router runs on 5V so it's power regulator needs 2 V then dV = 9V - 5V - 2V = 2V.

Solving for C:

C = I dT/dV

= 1A * 1.5 sec / 2V = 1.5 A-sec / 2V

= .75 F.

How did you calculate that power regulator needs 2 volts ...