I’ve got your back

I built a small UPS for my network setup at home about 4 years ago to keep my VoIP, phone and router alive during a power outage.  Since the equipment already run on DC, I would only need to deal with low voltage DC.  This approach eliminates 4 bulky wall warts and the inefficiencies of DC/AC and AC/DC conversion.  To deal with my equipment inconsistent voltage requirements, I added a few DC/DC converters on the PCB.  The design was subsequently replaced with one that uses a modular DC/DC converters approach as I didn’t want to rebuild the entire PCB to play catchup.  The modules as shown from top to bottom are: 5V for ATA, 26V for DSL modem and a 12V (non-backup) for scanner.

Network backup power supply V3 + modules

Individual plug-in DC/DC converter modules can be used with this UPS.  I used a pin out similar to the 78XX regulator so that I can also use them in my other projects.

The power outputs are breakout to the right hand side with Insulation Displacement Connectors.  A pair of Schottky Or’ing diodes automatically select the AC/battery source during normal/blackout conditions.  A BQ2002 NiCd/NiMH Fast-Charge Management IC is used for charging a NiCd backup battery connected to the front panel.  NiCd and Pb batteries are the few batteries types that are suitable for continuous trickle/float charge.

The following graph shows the NiCd battery voltage during a test blackout and the picture shows my old setup.  The battery voltage can be monitored by a (red) LED bar graph at the front panel.

Battery Discharge


A TL7702 Power Supply Supervisor is used as a under-voltage shutdown circuit to protect the battery from over-discharge.  The active high RESET pin disable Q1 when the input voltage UB+ drops below the preset threshold of 9.4V.  Once triggered, /RESET connects R36 in parallel to R24 in the voltage divider raising the recovery threshold to 12.5V.  As the voltage of a discharge battery rises slowly once its load has been removed, the 3V hysteresis ensures that the UPS would only recover after the main power is restored.

Undervoltage cutoff

A couple of years ago when I upgrade my network equipments, I added a SLA (Sealed Lead Acid) battery from a small discarded APC UPS to supplement the backup time to about 4 hours.  I built a dual level charger for the SLA using the BQ2906 Charger IC.

As most of my new equipments now standardize on 12V input on similar coax power connectors, I modified my ATA for 12V with an internal 5V DC/DC module to avoid accidents.  Unfortunately, there are still the odd ones that requires a module.  e.g. My cordless phone requires an isolated 6V supply (flyback module) as both my ATA and phone assume that the other piece of equipment has proper isolation between the power supply and the phone line.

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