It's 12 volts dc modem, the battery is 12 volts and the charger also. I want to make this switching circuit. Anyone plz help...

 

What switching circuit? Can you explain more clearly?

 

Last time I used it it was working. Now... nothing. Don't what happened.

 

Here. Try this.

 

Here. Try this.

Thanks, but I'd rather repair the one I have... I think there is something wrong with the switching circuit.

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He Naveed, just kidding. We need more details, as many as you have.

 

Sentimental value? I understand. I would never give up my magic wand, no matter how expensive to repair it.

 

Last time I used it it was working. Now... nothing. Don't what happened.

From here it looks like you have some cross threaded photons

 

Ahah! The light just came on.

What the OP wants is a circuit that switches the power source from the AC adapter to the 12V battery when AC mains power fails (when light is off).

Use two diodes. One diode from the adapter to the modem. The other diode from the battery to the modem.

The other thing you can try is unplug the adapter and throw it away.

 

I tried by just eliminating adapter, but this way, battery was not being charged properly. perhaps all the current was being drawn by modem. I tried using larger wattage charger as well but failed.

Now how to use diods? I think diods will simply pass the current, how to switch between adapter or battery?

 

Must always be careful using diodes to choose between batteries and adaptors. The device will always prefer the higher voltage of the battery rather than the lower voltage of the adaptor thus permanently using the battery.

Alternative is to use a lower voltage battery of course.

 

A simple but nasty solution would be to use a normally closed relay for the battery. Power the relay to hold open from the adaptor, when the adaptor power fails the battery will be switched in as fast as the relay can switch.

 

about transister I know that it switches more fast than a relay and more reliable. But I don't know how to use transister for this circuit. Can you please help me?

 

If the battery voltage was less than 2 volts above that of the adaptor then you could use a p channel mosfet in a quick and nasty manner.

The suggestion to remove the adaptor was probably the best though, just put a fixed 13.8V power supply on the battery and power the modem off the battery permanently.