# Backup power for adsl modem

I want to create a circuitry for switching to backup power (small battery) in case of mains power failure for my ADSL modem (to avoid the internet going down in such scenario).

This is a basic circuitry however I have only very basic knowledge of electrical and electronics and hence need help.

The below diagram shows what I am trying to create. Request help on how to create this "instant switching circuitry".

Modem specs: Current=0.6A Voltage=9V

• Just to know, why don't you use UPS battery ? Any specific reason ? They're not expensive and are designed for that. – Emmanuel Istace Dec 29 '13 at 19:12
• @EmmanuelIstace (a) To get a kick out of creating something on my own (b) UPS batteries for supporting only the modem would indeed be costlier (use versus cost). – iCrus Dec 29 '13 at 19:23
• What are the current specs of your modem? – alexan_e Dec 29 '13 at 19:39
• @alexan_e Modem specs: Current=0.6A Voltage=9V – iCrus Dec 29 '13 at 19:45
• A small 9v battery with low capacity is not a very good plan for backup. I doubt you will get even minutes of power supply because the voltage will drop very fast on load. – alexan_e Dec 29 '13 at 20:03

The problem with this kind of circuit is that there is always a voltage drop on the diode so the voltage that reaches the load is Vsupply - Vf where Vf is the diode forward voltage drop.