Abdul Kader (Babor)

The use of computers both in private and government sectors are spreading in Bangladesh. In the same time the use of high tech consumer electronic products such as TVs, Freezes, microwaves, AC coolers and other related products are even growing faster. There is a need for an extra power to meet such rapid growth.

All developing countries like Bangladesh, the power supply authorities are finding it difficult to meet such rapid growth. Due to this high growth the electricity supply gets difficult to manage and creates various problems, like,under voltage, over voltage, sudden spikes or even Power cut. An electronics device such as Computer requires a steady supply voltage all the time. In Bangladesh steady supply voltage varies on area to area. In some area supply voltage becomes as low as 140V or as high as 390V.

This type of power fluctuation is very common in almost all the third world countries. In Kenya, for instance, the legally allowed voltage fluctuation either side of the standard 240V is 5%. In reality 15% to 20% fluctuation is common, it is reportedly much the same in Zimbabwe and in Tanzania the situation is apparently even worse with fluctuation in the order of 30%.

Fluctuation in computer power supply can mean data corruption as well as blown power supplies if a spike comes down the line. These sudden high voltage surges of 300-400V generally caused by switching ON and OFF of heavy electronic product such as photo copier, last only a split second, but can cause everlasting damage to a computer.

Another problem in Bangladesh is that most, if not all, power lines are overhead, which can easily bedamaged and affected by accidents, violent weather and the like. Lack of maintenance has also resulted in lines not doing tightend properly in many places. Following shows the common power disturbances :


This is like the interference you get on a radio set (often caused by a ‘buzzing’ fluorescentlight). Unless filtered out, it can cause screen interference, corrupt data or cause read write errors.


These are dramatic unpredictable ‘jumps’ in power (such as a strike by lightening). As many as 6000 volts can ‘zap’ through your system enough to permanently damage your computer hardware.


These are slower and less powerful than spikes. They can be caused by switching on a nearby electrical appliance which creates an overvoltage (or un-dervoltage) thatcanscram-ble data and distort pro-grammes.


Circuit switching at your local powerstanon can create surges and brief interruptions that can fool your PC into thinking that it is experiencing a power failure. The result? Lost data any time!


A complete loss of mains power (no matter how brief) can completely  wipe out your data.

These fluctuations and disturbances can be overcome by using a device called voltage stabilizer. Its function is to supply a steady voltage, no matter what input condition is. Of course there are so many makes and models in market and not all provide the protection and function that it is supposed to do. It is advisable to shop around. A simple block diagram of a modern voltage stabilizer is shown below.

The filter is used to clean out spikes and noises which are very common in the mains supply. The power supply controller, controls the overall function of the stabilizer, i.e. it is an electronic intelligent circuit which decides how much voltage selection are required and then voltage selector selects the correct amount in order to give a constant supply voltage at the output. There are two major factors in choosing stabilizer namely (a) input voltage fluctuation range, (b) output power capacity.

(a)  Input Voltage Fluctuation Range: This   depends on the area you live in. But for Bangladesh, anything from 130V to 400V fluctuation range would be an ideal option.

(b)   Output Power Capacity : This depends on number of devices to be connected to it and also on the power consumption of each device. Freeze, for instance, may require anything from 300W to 600W whereas an average computer requires 200-250W. The simplest way to calculate is to add power consumption of each device to obtain the total power consumption then by a stabilizer which will take the total consumption. It is strongly recommended not to overload the stabilizer. However some good stabilizers have a overload protection which will either blow a fuse or trip out the circuit breaker.

In general this device can be used with any piece of electrical equipments such as Tvs, Freezes, Video recorders , or other electrical equipments where the equipment is not so critical when there is a sudden power cut. A device like a computer not only requires steady voltage all the time, it needs to be told before shutting it off so that it takes necessary action prior to switch off. Primarily because it has a device inside the computer called “Hard disk” on which the information or data is stored and it takes certain time to process such information or data. Therefore a sudden power cut could be catastrophic.

Secondly when the computer is in use the information or data is processed from one section of the computer to another at the very high speed. Which means when there is a sudden power cut the data in processed can be lost or damaged, that is why computers require a protection from this type of power cut. One way of protecting would be to have a back-up battery connected to the computer so that when there is a sudden power cut, the battery will take over to provide power until the computer finishes its tasks.Life would be very simple, if we could just use the battery to solve the problem. In a real world a device has been developed called UNINTERRUPTIBLE POWER SYSTEM (UPS). As the name suggests it does not interrupt the operation of the computer or any other device connected, i.e. it continues to provide power in the event of power cut. Here is a simple diagram of how UPS is used.


A properly designed UPS can protect you against random noise on the power lines high voltage, spikes and peaks of too high or too low voltage (surge and brown out). A computer and the data on it represent a big investment, so it is worth protecting.


A UPS consists of a battery, battery charging circuit, mains power detector and an inverter. The mains detectors, detects when there is a power cut occurs and as soon as power cut is detected it starts giving power from the battery. The inverter convert output of the battery DC (direct current) into mains 240V AC (alternating current)-50 Hz. The block diagram of a UPS shown below.

Mains power has what’s technically known as a sinusoidal waveforms. This means that it is just 50Hz, no nasty high frequency bits hidden away. The output of most UPS has near square wave, the computer power supply can cope with this for very short time but it is undesirable for long term use. However some UPS manufacturer offer UPS with pure sine wave output, it is always advisable to go for such option.

When a power failure is detected, it takes certain time to switch over to battery power this is known as transfer time, lower the transfer time the better the UPS and it is measured in milliseconds. Typical UPS has a transfer time from 3ms to 5ms. The problem is that you won’t know if it is fast enough, sensitive enough or even still working (in some cases) until the power fails.

Generally UPS must run the computer for at least 6 minutes to give it enough time for an orderly unpanicked shut down. Unfortunately, you can not just buy a 6 minute UPS, it is little more complicated than that. The amount of power stored in a UPS battery is actually measured in Ampere/hours, afigurethats rarely quoted. Instead UPS power supply ratings are given in Volt/ amps (VA).

First you need to determine the ampere hours required in order to supply that level of power for the time you need to shut down the system. Most UPS are about 70 percent efficient so you first have to divide the computer power requirements by 0.70. The computer power requirements are usually quoted in Watts, but one Watt is just 1VA. So 200W computer power supply will need a UPS rated 285VA. The majority of UPS manufacturers state the back up time in minutes, therefore if 285VA last for 5 minutes then 1140VA UPS with 200W computer will last 20 minutes.

There are three types of UPS’s namely:




For in depth details UPS specifications and different types available please read Part II in the next issue of Computer Jagat.

Abdul Kader (Babor)

49 Collindon Street

Luton, Beds,

England, (UK)

TEL : +44-582-487878

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