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The term Uninterruptible Power Supplies (UPS) has been applied both to an uninterruptible power system and to the specific battery-inverter equipment incorporated into the system. In this chapter, we will use the term UPS for the specific equipment.
The UPS was developed in parallel with digital computers and other IT equipment to provide a reliable uninterruptible electric power source to create a standard the electric-utility industry could not provide. The computer industry would not have developed without some type of UPS, which either used engine-generator sets or static inverters employing power electronic devices. If the concept of an independent UPS had not been developed, then all electronic equipment sensitive to disturbances in source voltage would have had to incorporate energy storage means—for example, batteries to counteract such disturbances in the source voltage.
1.1   History
The silicon-controlled rectifier (SCR) was introduced in 1957. The development of inverters using SCRs for UPS was expedited by the book, Principles of Inverter Circuits, by Bedford and Hoft, published in 1964.
An early publication by Fink, Johnston, and Krings in 1963 described three-phase static inverters for essential loads up to 800 kVA.
Kusko and Gilmore described in 1967 a four-module redundant UPS for 500 kVA for the FAA Air Route Air Traffic Control Centers. The overall MTBF requirement was 100,000 h and the MTTR was 1h. The individual modules were assumed to have an MTBF of 1100 h. Modern UPS modules are considerably more reliable.

Fig.1.1 block diagram
The major change in the design of inverters since their inception for UPSs is the replacement of the silicon controlled rectifiers (SCRs) as the switch devices by IGBTs. The SCRs require forced commutation to turnoff the current—that is, to open the switch. The IGBTs are controlled by the gate voltage. The forced commutation for SCRs required additional circuits and operations that increased the failure rates and reduced the reliability of the inverters. Furthermore, the IGBTs can be switched much faster than SCRs, which enabled pulse-width modulation (PWM)—in other words, the voltage and current waveform shaping to be used.An early concept of UPS that preceded the SCR is shown in Figure.
It was a rotary machine set consisting of a diesel engine, clutch, electric motor, flywheel, and generator. When utility power was available, the motor drove the generator. When utility power failed, the engine would be started while the flywheel and the generator slowed.
At a certain speed, the clutch was engaged so the engine could drive the generator to restore and maintain its synchronous speed. A Motor-Generator, which is a UPS, but employs batteries to provide energy when utility power fails.

1.2 Common Power Problems
There are various common power problems that UPS units are used to correct:
·         Power failure
·         Voltage spike
·         Over-voltage
·         Line noise
·         Harmonic distortion
1.3 Types of UPS
·         Offline / standby                      
·         Line-interactive                                           
·         Double-conversion / online       
·         Hybrid Topology / Double Conversion on Demand
·         Ferro-resonant                               
Fig: - 1

No. of Components
230v AC Primary to 12-0-12v, 1A Secondary Transformer
T1 BC548
T2 TIP127
IC 7809
IC 7805
Zener Diodes
ZD1 12.5v, 0.5w
ZD2 12v, 1w
R1 68Ω, 0.5w
R2=R3 1K
R4 47Ω, 1w
R5=R6 390Ω
470µF, 25v
VR1 10K
VR2 22K
12v 4.5 AH
1 Red, 2 White
On/Off Switch

1.6 Circuit Operation
This circuit provides an uninterrupted power supply (UPS) to operate 12V, 9V and 5V DC-powered instruments at up to 1A current. The backup battery takes up the load without spikes or delay when the mains power gets interrupted. It can also be used as a workbench power supply that provides 12V, 9V and 5V operating voltages. The circuit immediately disconnects the load when the battery voltage reduces to 10.5V to prevent deep discharge of the battery. LED1 indication is provided to show the full charge voltage level of the battery. Miniature white LEDs (LED2 and LED3) are used as emergency lamps during power failure at night.


1.7 Circuit Explanation

A standard step-down transformer provides 12V of AC, which is rectified by diodes D1 and D2. Capacitor C1 provides ripple-free DC to charge the battery and to the remaining circuit. When the mains power is on, diode D3 gets forward biased to charge the battery. Resistor R1 limits the charging current. Potentiometer VR1 (10k) with transistor T1 acts as the voltage comparator to indicate the voltage level. VR1 is so adjusted that LED1 is in the ‘off’ mode. When the battery is fully charged, LED1 glows indicating a full voltage level of 12V.
When the mains power fails, diode D3 gets reverse biased and D4 gets forward biased so that the battery can automatically take up the load without any delay. When the battery voltage or input voltage falls below 10.5V, a cut-off circuit is used to prevent deep discharging of the battery. Resistor R3, zener diode ZD1 (10.5V) and transistor T2 form the cut-off circuit. When the voltage level is above 10.5V, transistor T2 conducts and its base becomes negative (as set by R3, VR2 and ZD1). But when the voltage reduces below 10.5V, the zener diode stops conduction and the base voltage of transistor T2 becomes positive. It goes into the ‘cut-off’ mode and prevents the current in the output stage. Preset VR2 (22k) adjusts the voltage below 0.6V to make T2 work if the voltage is above 10.5V.
When power from the mains is available, all output voltages—12V, 9V and 5V—are ready to run the load. On the other hand, when the mains power is down, output voltages can run the load only when the battery is fully charged (as indicated by LED1). For the partially charged battery, only 9V and 5V are available. Also, no output is available when the voltage goes below 10.5V. If battery voltage varies between 10.5V and 13V output at terminal A may also vary between 0.5V and 12V, when the UPS system is in battery mode.
Outputs at points B and C provide 9V and 5V, respectively, through regulator ICs (IC1 and IC2), while output A provides 12V through the zener diode. The emergency lamp uses two ultra-bright white LEDs (LED2 and LED3) with current limiting resistors R5 and R6. The lamp can be manually switched ‘on’ and ‘off’ by S1.
The circuit is assembled on a general- purpose PCB. There is adequate space between the components to avoid overlapping. heat sinks for transistor T2 and regulator ICs (7809 and 7805) to dissipate heat are used.
The positive and negative rails should be strong enough to handle  high current. Before connecting the circuit to the battery and transformer, connect it  

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