Monday, February 08, 2016

Make Mini UPS For DSL Modem PART-I

This is the first post in a series of posts on making your own mini UPS for DSL modem. Be sure to read all parts as each part contains useful information.

Power Crisis In Pakistan

The concept of electric power load shedding is not new to Pakistanis who are facing power black outs on daily basis. These blackouts occur for a total of around eight hours a day with some areas especially the rural ones facing 14 hours of power outages.

Today nearly all households in Pakistan make use of an arrangement commonly called UPS to keep their non-critical loads running. This UPS typically consists of a combination of a battery charger plus inverter in one package and an automotive battery.

Need Of A Mini UPS

Reading on the internet I found many people asking for a small UPS to run just a DSL modem and a wireless router and considered the above mentioned UPS to be an overkill or expensive. If you are under similar situation then you should definitely read on.

In this and the following posts I will describe how I was able to make a mini UPS and how you can make one too. To keep things simple I’ve not used the microcontroller. Using microcontroller would have made the process much complicated for those with no prior knowledge of electronics and computer programming.

I have tried to write in such a way that both the readers with or without electronics background would not feel any difficulty to understand.

Block Diagrams

Let us first see the block diagram of this mini UPS for DSL modem (Figure 1).

Block Diagram of Mini UPS
Figure 1: Block Diagram of Mini UPS

If we define a UPS with zero transfer time as an online UPS then this mini UPS is an online UPS. Other UPS types are offline or standby UPS and line interactive UPS.

I can’t explain these types of UPSes as this is not the topic of this post. This mini UPS is an online UPS because the battery is always connected with the load. In electrical terminology, the load is anything that consumes power. In our case the load is our DSL modem. There is no transfer switch involved thus the transfer time is zero. The transfer time is the time taken by a UPS to switch from one operating mode to another e.g., from its normal operating mode to battery mode.

How this mini UPS functions when ac mains power is present? To understand this, see the block diagram in Figure 2.

Block Diagram of Mini UPS - AC Mains Present
Figure 2: Block Diagram of Mini UPS - AC Mains Present

And when the ac mains power is absent (see Figure 3).

Block Diagram of Mini UPS - AC Mains Absent
Figure 3: Block Diagram of Mini UPS - AC Mains Absent

As you can see when ac mains power is present the battery charger is not only charging the battery but is also powering the DSL modem. And when ac mains power goes out, the rectifier and battery charger simply drop out and our DSL modem continues to operate without even restarting.

The block diagram has two blocks which need further explanation. These are rectifier and UVLO. To prevent this post from getting too long I’ve explained these terms in sufficient detail in separate posts. For now simply be informed that a rectifier is responsible for converting AC (Alternating Current) to DC (Direct Current). The electricity from the electrical outlet in your home is ac while the DSL modem needs dc.

Under Voltage Lock Out

The UVLO stands for Under Voltage Lock Out. The function of a UVLO is to protect the connected equipment from damage when the voltage is beyond predefined thresholds. In our case the UVLO protects the battery from getting too deeply discharged.

This post and the following posts, all were written taking into mind the DSL modem. However, you can substitute the DSL modem with the WiFi router or any other device as long as the device runs on 12 V DC.

While this mini UPS was designed to provide backup power to a dsl modem, you can also use this mini UPS (after some modification) as a big power bank for charging your phones or tablets.

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