Dear All,

i need help, i want to control my DSL Modem with a relay, pls bit a sight on my requirment
my modem have 12v Dc Adapter
i have 12v dc battery 7amp.
also i have battery charger approximately 18 V
when i have ac electric failure my modem turn off and i need to connect manually to my modem with battery, and when battery empty i need to connect manualy it with charger.
i want that battery charger, modem and battery always connect with each other and my modem should operate with relay. meanz i want when Ac supply working my modem should get power supply from main supply and same time battery should charge from charger, and whenever main supply fail it should automatic convert on battery and when main electric supply resume again it should cut from battery and convert on main supply and battery charging.
pls help me
thanks to all

 

This is easier than you had imagined. The way to do it is to use the battery charger to keep the battery full at all times and run the modem from the battery at all times. When power fails, battery just keeps on powering modem until power comes back on. No relay required. No human required.

Is this OK with you?

 

but i want to control battery, modem and charger through relay. if anyone have experience pls help me

 

Use a DPDT relay with a 12VDC coil. Connect the coil of the relay with the 12VDC adaptor. The 12VDC should also connect to the N/O contacts and the battery/charger can connect to the N/C contacts of the relay. The modem then connects to the common contacts of the relay. When the mains is good the 12VDC adaptor will power the relay on closing the N/O contacts and power goes through to the common to the modem. If power fails the contacts open and power is derived through the N/C contacts from the battery/charger.

One question. What kind of battery charger do you have, meaning is it solar or mains powered?

 

Here's a drawing.

 

Here's a drawing.

I guess I was a little lazy on that aspect. Thanks for picking up my slack and drawing it out.

 

Doing a drawing in Paint is a slow PIB, but someone has to do it.

ps, which voltage you use to fire the relay coil is negotiable. As BSomer said, you can use 12 VDC or the power line, or you can complicate it with something like a 5 volt relay and a 5 volt regulator.