Backup power (UPS) for ADSL modem

Thread Starter


Joined Jan 21, 2014

I have only basic knowledge of electronics and still I am wanting to build a low capacity mini UPS for my ADSL modem so that it keeps on running in case of mains failure.

Modem specs: Current=0.6A Voltage=9V

Request help on going about it. I do have basic idea about transformers, diodes and resistors in general.


Joined Oct 15, 2009
designing a complete UPS is not a simple task.. Why don't you just buy one and then start learning electronics with an easier task.


Joined Feb 5, 2010
A simple string of rechargeable using your existing wall wart as the charger. A diode to prevent reverse flow when power dies would give you plenty of time for an orderly completion of work and a proper log off.
What is powering your computer? Why can your adapter not get power from that back up?


Joined Mar 3, 2012
I've done this a few times and I would suggest trying to work out what the true power supply requirements were. If you feel confident to do so open up the modem and trace out the circuitry around the power supply noting down the numbers of the PSU chips and the voltage ratings of any capacitors across the input.

I've found that my various network hardware that's labelled for different fixed input voltages between 6 and 15V in fact contains a switching supply with a much wider input range. Most of mine switch down to 3.3V, some following with a linear regulator to 1.8V and the input switcher can tolerate 20V in. That means I can run them off a float charging 12V lead acid and they're completely happy with the 11-14V range (flat battery to fully charged).

Obviously you can just make a 9V UPS, but if you discover your modem can work anywhere between 5 and 20V as I strongly suspect it can then it seriously widens your options.


Joined Oct 2, 2009
Start with a small 12V SLA and a proper float-charger. Use a IC regulator to reduce the 13.7V to 11.5V from the battery to 9.6V. Use a 1N4007 diode to keep the existing wall-wart from back feeding the regulator.