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INTELLIGENT MONITORING OF UPS SYSTEM B Tech Mini-Project Report

Submitted in partial fulfillment for the award of the Degree of  Bachelor of Technology in Electrical and Electronics Engineering

A V VINAY GOVIND G JITHIN RAJEEV TUSHAR MENON

B080166EE B080091EE B080100EE B080358EE

Under the guidance of  Dr. T.L JOSE

Department of Electrical Engineering  NATIONAL INSTITUTE OF TECHNOLOGY CALICUT  

 NIT Campus P.O., Calicut - 673601, India  

2011

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CERTIFICATE “

 

This is to certify that the thesis entitled INTELLIGENT MONITORING OF UPS SYSTEM is a bona fide record of the mini-project done by A.V Vinay ( Roll   Roll No. B080166EE), Govind G  (Roll No. B080091EE) , Jithin Rajeev (Roll No. B080100EE) and Tushar Menon ( Roll   Roll No. B080358EE) under my supervision and guidance, in partial fulfillment of the requirements for the award of Degree of Bachelor of Technology in Electrical & Electronic Engineering from National Institute of Technology Calicut for the year 2009. 



Dr. T.L Jose 

Dr. Sreeramkumar 

(Guide)

Professor & Head  

Professor 

 Dept. of Electrical Engineering  

 Dept. of Electrical Engineering

Place: Date: 2

 

ACKNOWLEDGEMENT  

It is with deep sense of gratitude that we thank our Project Guide Project Coordinator 

Dr. Ananthakrishnan

Dr. T.L Jose

and

whose valuable advice and suggestions

helped us a lot for the completion of the Project Work.

We also thank the Head of Electrical Department,

Dr.Sreeramkumar

for giving us

 permission to work in labs and also to the Lab assistants who helped us a lot in getting the required components and assembling the circuit.

Last but not the least we thank our friends who were with us at all times and also Almighty God who showered his blessings upon us.

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CONTENTS 1.  ABSTRACT 2.  PROBLEM STATEMENT 3.  LITERATURE 3.1  UPS SYSTEM

5 6 7 7

3.2 BATTERY AN OVER VIEW 3.3 PIC 18F452 4.  THEORETICAL BACKGROUND 4.1 ADC PROGRAMMING IN PIC 4.2 PROBLEM DEFINITION AND FUNCTIONAL REQUIREMENTS 5.  SALIENT FEATURES OF THE PROJECT 5.1 INTRODUCTION INTRODUCTION-- HOW WE PROCEEDED 5.2 COMPONENTS USED 5.3  INTERFACING THE MOSFET SWITCHES TO MICRO CONTROLLER 5.4 CONSTANT CURRENT BATTERY CHARGING 5.5 VOLTAGE ANALYZER CIRCUIT 5.6 VOLTAGE RECTIFICATION

10 13 1 14 20 22 22 23 26 28 30 31

5.7 PROGRAMMING 5.8 PROGRAM 6.  CONCLUSION 7.  REFERENCES

32 33 39 40

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CHAPTER-1

ABSTRACT Uninterrupted Power Supplies (UPS) are widely used to provide emergency power to critical loads in case of utility mains failure in areas like computer networks, networks, communication links, biomedical instrumentation etc. The rampant popularity of digital control and automation has not eluded this area too. Here we intend to propose the design of an off line UPS incorporating an intelligent switching of power from the supply side to the battery and back. The microcontroller forms the brain of the circuit and constantly monitors the battery as well as the supply side. At the same time the battery is constantly charged using the constant current charging scheme while the battery is feeding the load. This microcontroller based design incorporates several features like overvoltage protection of the battery, supply side undervoltage switching. Such a design can be used as part of the initial stage of the UPS. It also involves a voltage analyzing circuit which provides effective user interface by indicating the current voltage level of the battery. The use of microcontroller thus greatly increases the flexibility of the whole design as any change in the logic warrants a change in just the software structure while essentially keeping the rest of the associated circuitry to remain unchanged.

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CHAPTER-2

PROBLEM STATEMENT

   

Provide a backup power to critical load in the event event of a power outage outage and also to limit the duration of backup power power needed to a suitable length to permit a user to save unsaved data and properly shut down.

 

One of the project main goals is to provide information for user about the backup battery and status of the UPS during power outage. A voltage level analyzer circuit has to be there in order to indicate the state of charge.

 

A charging circuit also needs to be built for this project which is to ch charge arge a12V DC for the backup battery. The charging, discharging and switching processes are to be controlled by microcontroller.

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CHAPTER-3 

LITERATURE

3.1 UPS SYSTEMS An Uninterruptible power supply (UPS), also known as a battery back-up, provides emergency power and, depending on the topology, line regulation as well to connected equipment by supplying power from a separate source when utility power is not available. It differs from an auxiliary or emergency power system or standby generator, which does not provide instant protection from a momentary power interrupts. While not limited to safeguarding any particular type of equipment, a UPS is typically used to protect computers, data centers , telecommunication equipment or other electrical equipment where an unexpected power disruption could cause injuries, fatalities, serious business disruption or data loss. UPS units come in sizes ranging from units which will back up a single computer without monitor (around 200 VA) to units which will power entire data centers, buildings, or even an entire city. (Several megawatts). An UPS contains an internal rechargeable battery that gets charged from the power line then gets used to generate line power to the load when the power line fails. To accomplish that they also contain an inverter, an electronic device capable of generating 110/220v AC from batterybattery level DC voltage. The general categories of modern UPS systems are on-line, on- line, line-interactive, and standby. An online UPS uses a "double conversion" method of accepting AC input, rectifying to DC for passing through the battery (or battery strings), then inverting back to 120v AC for powering the protected equipment. A line-interactive UPS maintains the inverter in line and redirects the battery's DC current path from the normal charging mode to supplying current when power is lost. In a standby ("off-line") system the load is powered directly by the input power and the backup power circuitry is only invoked when the utility power fails. Most UPS below 1 kVA

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3.1.1 OFFLINE UPS  The Offline / Standby UPS (SPS) offers only the most basic features, providing surge protection and battery backup. Usually the Standby UPS offers no battery capacity monitoring or self-test capability, making it the least reliable type of UPS since it could fail at any moment without warning. These are also the least expensive. The SPS may be worse than using nothing at all, because it gives the user a false sense of security of being assured protection that may not work when needed the most. With this type of UPS, a user's equipment is normally connected directly to incoming utility power with the same voltage transient clamping devices used in a common surge protected plug strip connected across the power line. When the incoming utility voltage falls below a predetermined level the SPS turns on its internal DC-AC inverter circuitry, which is powered from an internal storage battery. The SPS then mechanically switches the connected equipment on to its DC-AC inverter output. The switchover time Is usually less than 4 milliseconds, but typically can be as long as 25 milliseconds depending on the amount of time it takes the Standby UPS to detect the lost utility voltage 3.1.2

LINE INTERACTIVE UPS 

The Line-Interactive UPS is similar in operation to a Standby UPS, but with the addition of a multi-tap variable-voltage autotransformer. This is a special type of electrical transformer that can add or subtract powered coils of wire, thereby increasing or decreasing the magnetic field and the output voltage of the transformer. This type of UPS is able to tolerate continuous under-voltage brownouts and over- voltage surges without consuming the limited reserve battery power. It instead compensates by auto-selecting different power taps on the autotransformer.

3.1.3 COMMON POWER PROBLEMS There are various common power problems that UPS units are used to correct: 1. Power failure 2.Voltage spike 3.Over-voltage 4.Line noise 5.Harmonic distortion

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3.1.4

POWER FAILURE:

A power outage (also known as a power cut, power failure ,power loss, or blackout) refers to the short- or long-term loss of the electric power to an area. Power outages are categorized into three different phenomena, relating to the duration and effect of the outage: •

A dropout is a momentary (milliseconds to seconds) loss of power typically caused by a

temporary fault on a power line. Power is quickly (and sometimes automatically) restored once the fault is cleared. •

A brownout is a drop in voltage in an electrical power supply, so named because it typically

causes lights to dim. Systems supplied with three-phase electric power also suffer brownouts if  one or more phases are absent, at reduced voltage, or incorrectly phased. Such malfunctions are particularly damaging to electric motors. •

A blackout refers to the total loss of power to an area and is the most severe form of power

outage that can occur. Blackouts which result from or result in power.

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3.2 BATTERY-AN OVERVIEW 3.2.1 Introduction A battery converts chemical energy into electric energy through an electro-chemical process. The basic unit is called a "cell" and can be manufactured in a wide variety of shapes and sizes. Batteries are made up of one or more cells in series or parallel combinations to create the desired voltage and output capacity. The electro-chemical cells consist of two terminals suspended in an electrolyte. The terminals are called the anode (-) and the cathode (+). An electrical current is essentially a flow f low of  electrons, and the battery can be regarded as an electron pump. The chemical reaction between the anode and the electrolyte forces electrons out of the electrolyte ele ctrolyte and into the anode metal, through the circuit, then back to the cathode. From the cathode metal, the electrons re-enter the electrolyte. This direction may seems strange, from negative to positive. We regard ‘current’ as flowing from positive down to negative, but in i n fact, this current is a flow of electrons in the opposite direction! The anode and cathode both get converted during this reaction, one is ‘eaten away’, and the other has a build -up of material on it. When a rechargeable battery is recharged, this chemical reaction is reversed, and the terminals are restored. 3.2.2 Primary and Secondary cells Batteries can be divided into two classes: primary, and secondary. Primary batteries are designed for a single discharge cycle only, i.e. i .e. they’re non-rechargeable. Secondary cells are

designed to be recharged, typically, from 200 to 1000 times. For use in robot wars, primary cells would be far too expensive. Among secondary cells, the options open to us include NiCd, NiMH, rechargeable Lithium, or lead-acid. 3.2.3 Ampere hour ratings.  ratings.  The energy stored in a battery is measured in the units Ampere-hours (Ah). The units of energy are actually Volts × Amps × Seconds, but since the voltage of the cell iiss constant, it is only the product of current and time which determines the amount of energy in the battery. Rechargeable cells generally have a lower energy density (that is the total amount of energy they can hold, Volts × current × time, divided by the physical size of the cell) than primary cells. A Ni-Cd cell provides 5Ah at 1.2V, a lead acid D cell provides 2.5Ah at 2V, and the com comparable parable alkaline cell provides 10Ah at 1.5V. 3.2.4 Lead acid acid batteries batteries In lead-acid batteries, the positive electrode is lead dioxide, while the negative electrode is metallic lead. The electrolyte is sulphuric acid. As the cell discharges, the acid electrolyte is consumed producing water and both electrodes change into lead sulphate. When the cell is 10

 

recharged, the chemical reaction reverses. As in most battery types, it is good to store them at cool temperatures and operate them at warm temperatures. Most lead-acid batteries have the highest energy density at about 30-40ºC. If the cell undergoes overcharging, hydrogen and oxygen gassing will occur with the loss of water.

The sealed type of lead(SLA battery) is similar to sealed type except a few the electrolyte. This takes the form of a gel or by the separator. Methods recombine oxygen formed overcharging are Although specifically for the reduction of excess pressure valves usually are

acid battery the nonchanges in commonly is absorbed to at employed. designed gassing, installed to

vent excess pressures to atmosphere. The sealed construction allows a maintenance-free life restricted to a horizontal orientation.

the

which is not Fig. 3.1. Lead Acid Battery 

3.2.5 BATTERY CHARGING

Charging Schemes The charger has three key functions   Getting the charge into the battery (Charging) 



   



Optimizing the charging rate (Stabilizing) Knowing when to stop (Terminating)

The charging scheme is a combination of the charging and termination methods. Charge Termination Once a battery is fully charged, the charging current has to be dissipated somehow. The result is the generation of heat and gasses both of which are bad for batteries. The essence of good charging is to be able to detect when the reconstitution of the active chemicals is complete and to stop the charging process before any damage is done while at all times maintaining the cell temperature within its safe limits. Detecting this cut off point and terminating the charge is critical in preserving battery life. In the simplest of chargers this is when a predetermined upper

11

 

voltage limit, often called the termination voltage has been reached. This is particularly important with fast chargers where the danger of overcharging is greater.

,

Safe Charging If for any reason there is a risk of overcharging the battery, either from errors in determining the cutoff point or from abuse this will normally normally be accompanied accompanied by a rise in temperature. temperature. Internal fault conditions within the battery or high ambient temperatures can also take a battery beyond its safe operating temperature limits. Elevated temperatures hasten the death of batteries and monitoring the cell temperature is a good way of detecting signs of trouble from a variety of causes. The temperature signal, or a resettable fuse, can be used to turn off or disconnect the charger when danger signs appear to avoid damaging the battery. This simple additional safety precaution is particularly important for high power batteries where the consequences of failure can be both serious and expensive. Charging Scheme There are different charging schemes like constant current, constant voltage charging etc. We use constant current charging for this project.

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3.3 PIC18F452 PIC18F452 has a RISC architecture that comes with some standard features such as on on-chip -chip program (code) ROM, data RAM, data EEPROM, timers, ADC, USART,and input /output ports. The 40 pin PIC18F452 has five ports. They are PORTA ,PORTB ,PORTC ,PORTD ,PORTE. Port A has 7 pins; Ports B,C and D each have 8 pins; and port E has only 3 pins. In addition to being used for simple input/output ports, each port has some other functions such as ADC, timers, interrupts and serial communication pins. Each port has three SFRs associated with it. They are designated as PORTx, TRISx, and LATx.

Fig. 3.2 Pin Diagram of PIC18F452 PIC18F452

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CHAPTER-4

THEORETICAL BACKGROUND 4.1 ADC PROGRAMMING IN PIC The signals in nature are usually analog in nature (here it is voltage ).But most of our computer (or Microcontrollers) are digital in nature. They can only differentiate between HIGH or LOW level on input pins. For example if input is more than 2.5v it will be read as 1 and if it is below 2.5 then it will be read as 0 (in case of 5v systems). So we cannot measure voltage directly from microcontrollers. To solve this problem most modern MCUs have an ADC unit. ADC stands for analog to digital converter. It will convert a voltage to a number so that it can be processed by a digital systems . Most important specification of ADCs is the resolution. This specifies how accurately the ADC measures the analog input signals. Common ADCs are 8 bit, 10 bit and 12 bit. For example if the reference voltage(explained latter) of ADC is 0 to 5v then a 8 bit ADC will break it in 256 divisions so it can measure it accurately up to 5/256 v= 19mV approx. While the 10 bit ADC will break the range in 5/1024 = 4.8mV approx. So you can see that the 8 bit ADC can't tell the difference between 1mV and 18mV. The ADC in PIC18 are 10 bit. So we have 10 bit ADC. Other specification include (but not limited to) the sampling rate, that means how fast the ADC can take readings. Microchip claims that pic18f4520's ADC can go as high as 100K samples per second. ADC Terminology Reference Voltage: The reference voltage specifies the minimum and maximum voltage range of analog input. In PIC 18 there are two reference r eference voltage, one is the Vref- and one is Vref+. The Vref- specifies the minimum input voltage of analog input while the Vref+ specifies the maximum. For example if the input signal Vref- is applied to analog input channel then the result of conversion will be 0 and if voltage equal to Vref+ is applied to the input channel the result will be 1023 (max value for 10bit ADC).

14

 

Fig.4.1 ADC Reference Voltage. Voltage. The Vref+ and Vref- pins are available in PIN5 and PIN4 of the PIC18F4520 chip. So you can connect the reference voltage here. For a simple design the Vref- is GND and Vref+ is Vcc. As this is such a common configuration that the ADC can be set up to use these reference internally. Therefore you do not need to connect these on the external Vref pins, so you can use them for other purpose. ADC Channels: The ADC module is connected to several channels via a multiplexer. The multiplexer can connect the input of the ADC to any of o f the available channels. This allows you to connect many analog signals to the microcontroller. In PIC18F452 there are 15 analog input channels, they are named AN0, AN1 etc. Acquisition Time: When an specific channel is selected the voltage from that input channel is stored in an internal holding capacitor. It takes some time for the capacitor to get fully charged and become equal to the applied voltage. This time is called acquisition time. The PIC18F452's ADC provides a programmable acquisition time, so you can setup the acquisition time. Once acquisition time is over the input channel is disconnected from the source and the conversion begin. The acquisition times depends on several factor like the source s ource impedance, Vdd of the system and temperature. A safe value is 2.45uS, so acquisition time must be set to any value more than this. ADC Clock: ADC Requires a clock source to do its conversion, this is called ADC Clock. The time ti me period of the ADC Clock is called TAD. It is also the time required to generate 1 bit of  conversion. The ADC requires 11 TAD to do a 10 bit conversion. It can be derived from the CPU clock (called TOSC) by dividing it by a suitable division factor. There are Seven possible option. optio n. * 2 x TOSC * 4 x TOSC 15

 

* 8 x TOSC * 16 x TOSC * 32 x TOSC * 64 x TOSC * Internal RC For Correct A/D Conversion, the A/D conversion clock (TAD) must be as short as possible but greater than the minimum TAD. It is 0.7uS for PIC18FXXXX device. We are running at 20MHz in our PIC Development board so we set prescaler of 32 TOSC. Our FOSC = 20MHz Therefore our FOSC = 1/20MHz = 50nS 32 TOSC = 32 x 50 nS = 1600nS = 1.6uS 1.6 uS is more than the minimum minimu m requirement.

You can calculate the value for division factor using the above example in case you are using crystal of other frequency. Also now we have the TAD we can calculate the division factor for acquisition time. Acquisition time can be specified in terms of TAD. It can be set to one of the following values. * 20 x TAD * 16 x TAD * 12 x TAD * 8 x TAD * 6 x TAD 16

 

* 4 x TAD * 2 x TAD * 0 x TAD As we saw in above paragraph that the safe acquisition time is 2.45uS, so we select 2 x TAD as acquisition time. TACQ=2 x TAD =2 x 1.6uS (Replacing TAD= 1.6uS) =3.2uS 3.2uS is more than required 2.45uS so its ok.] The converted binary output data is held by two special function registers called ADRESL(A/D result low ) and ADRESH(A/D result high). There are two ADCON registers available in PIC18F242, they are ADCON0 AND ADCON1 . The ADCON0 is used to set the conversion time and analog inp input ut channel. ADCO ADCON1 N1 is used to set the vrefvoltage . The detailed description of both the registers are given below.

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Fig. 4.2. ADCON0 REGISTER

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Fig 4.3. ADCON1 Register

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4.2. PROBLEM DEFENITION AND FUNCTIONAL REQUIREMENTS Figure illustrates the functional requirements in the development of the UPS system. The designer is confronted with two fundamental problems, namely providing continuous power to a critical load, such as a PC, and suppressing the effects of disturbances on the power lines between the supplies and the load. Continuous power flow implies the need for a secondary energy storage device to replace the utility mains in case the latter fails. f ails. Energy taken from the secondary source is replenished from the mains when the latter is restored. The UPS continues to operate on battery power for the duration of the backup time or, as the case may be, until the AC-input supply voltage returns to within the specified tolerances, at which point the UPS returns to its normal mode. Hence, a power interface is necessary between the AC supply and the back-up source. These elements have to take into account the types of disturbances that need to be overcome in the present system from the given specifications. Hence, one fundamenta fundamentall requirement for this function is the continuous monitoring of current and voltage on the power lines at points where protection is to be offered by microcontroller. The AC input line is continuously monitored for detecting black blackout out condition for the facility. The load is transferred to the battery bank via transfer transfer switch TI as in when the input from AC source is no longer available. When that happens,the transfer switch must operate to switch the load over to the battery backup power source . In case the battery is not sufficiently charged to supply the load during a mains failure, uncontrolled shutdown will occur to UPS.

Fig 4.4. Monitoring System System block diagram

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Fig. 4.5. Flow Chart of Operations When an outage had occurs, the select switch within the UPS will isolate the load from the utility power and switches it to UPS battery as a backup power sources. As the load been switched to UPS, LM324 indicating circuit will be needed in order to monitor the power of the UPS battery and also to display the status of the UPS during the operation .The backup power will provide a sufficient time for user to continue their work before safely power down the load. When UPS battery had reached the critical value, uncontrolled shutdown will occur to the load as the battery weakened.

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CHAPTER-5

SALIENT FEATURES OF THE PROJECT 5.1 Introduction- How we proceeded A 230V ac main supply is the input to the total setup. The 230V ac main supply is given to a 12 12-0-12 transformer. The output voltage of the transformer is 24V ac .This 24V is rectified using a bridge rectifier . The bridge rectifier was made by four P600 diodes d iodes . The output of this rectifier is filtered using 4700pf .The output at this stage is 24V dc . We have two connections from this point. One connection is o the load and the other is to the battery . The connection to the load is given through an ic LM7812 and a MOSFET MOSFET IRF640 switch. The input to the LM7812 can be from 14V to 35V , but the output will be a constant 12V . We use this IC to get this constant 12V output . This 12V output is given to the load through the IRF640. The switch is triggered from the microcontroller . It is triggered on when the input supply voltage is greater than 16V . So it will switch off when the supply is off . The connection from the supply to the battery is given through an LM317 . It maintains a 1.25 volt potential difference between the OUT pin and the ADJ pin . This 1.25 is made use of using a 1ohm 5 watt resistor which causes a constant charging current of 1.25 A . When we have to stop charging the switch is turned on which causes the ADJ pin to be shorted to the ground . This in turn reverse biases the diode thereby stops charging current . The logic to the switch is given from the microcontroller . When the battery voltage is more than 12.5V the battery is overcharged and it should stop charging and so we switch on that particular switch . There is a connection from the battery to the load which is done through a mosfet IRF640 switch . this switch is also controlled by the microcontroller. microcontroller. This switch is turned on when the suppy is off and the battery is not in the deep discharge stage . the deep discharge stage is when the battery goes below 9.8 voltage . The battery voltage and the supply voltage is constantly monitored by the microcontroller and the output logic is given to the switches . The microcontroller is powered up from the battery through LM7805. LM7805 gives a 5V output when the input is between 7V to 35V . A battery level indicating circuit is connected from the battery . This T his indicates the battery charge level . the level is indicated with the help of four LEDS . LM324 is the ic used in this circuits . the resistors connected to it are such that they show the correct battery stage (among the four stages).

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5.2

COMPONENTS USED

5.2.1 LM7805 It is part of the LM78XX series of positive voltage regulators. LM7805 is used to regulate the input voltage to 5V which is used as the Vcc for the PIC. The typical circuit employed for this positive regulation is as shown. 5.2.2 LM7812 It serves the same purpose as LM7805 except that the voltage is now regulated to 12V instead of 5V. The associated circuitry is as shown. One of the prime requirements of all positive regulators of this series is that the input voltage must be at least 2 -3 V more than the expected output voltage

Fig.5.1 Internal structure structure diagram of of LM 78XX 5.2.3 

IR 2110

IR2110 is a high voltage high speed MOS gated power device driver with independent high side and low side referenced out channels. The floating channel can be used to drive a N-channel power power MOSFET that operates between between high voltage rails.

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Fig 5.2 Internal structure structure of IR2110 IR2110

5.2.4 CD4049 CD4049 is an inverting type hex buffers and feature logic level l evel conversion using only one supply voltage. The input-signal high level can exceed the V cc supply when used as logic levels.

Fig. 5.3 Internal structure of CD 4049

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5.2.5 

LM317/117 LM317 is part of the series of 3 terminal adjustable positive voltage regulators. Here it is used to provide a constant charging current to the battery. This is achieved by utilizing the constant 1.25V potential difference between the ADJ pin and OUT to cause a constant 1.25 A charging current

Fig 5.4. Typical connection diagram for LM317/117 5.2.6  LM324 LM324 is a JFET based differential input quad opamp consisting of 4 opamps. The opamps here are used as comparators

Fig 5.5. Typical connection diagram for LM324 25

 

5.3 

INTERFACING THE MOSFET SWITCHES TO THE MICROCONTROLLERS

5.3.1

POWER MOSFET (IRF640) The power mosfet in use is the IRF640. It is highly conducive for high current applications and is used primarily in UPS applications. As a result we decided to stick with this MOSFET even though our load requirement is pretty small. Another plausible reason for including this power mosfet is that it showed pretty satisfactory satisfactory results during simulation. Upon comparing it with the mosfet 2SJ48, it was observed that the latter although it can be gated ON using a very low gate voltage, its behavior remains unchanged when the mosfet is desired to be in OFF state. In any switching circuit clearly defining the switching levels is of utmost importance. importance. As a result the IRF640 was found to be more suitable in our experimentation. e xperimentation.

5.3.2 

NEED FOR A BUFFER AMPLIFIER The control outputs of the PIC are normally no followed f ollowed by a buffering stage. This serves the dual purpose of isolating the output stage of the PIC from the switches and also safeguards the PIC from surge voltages that are bound to occur across the switches in lieu of the charging and discharging of the gate source and gate drain capacitanc capacitances. es. Here we use a CD4049 which is a inverting stage hex buffer. It effectively functions in the circuit as two cascaded NOT gates. An obvious downside of using such an intermediate stage would be introduction of an additional delay. However we have compensated for that delay in the control algorithm.

5.3.3  THE MOSFET DRIVER( IR2110) The introduction of a power MOSFET in our circuit obviously brings in the additional headache of designing an appropriate driver circuit. This is because the output logic levels of the PIC are 0-5V which are not enough to cause the MOSFET to switch from the cut-off to saturation region. Therefore we bring in a high speed high frequency MOSgated power device driver IR2110. High speed is desired since intermediate i ntermediate buffer stage is already accounting for a propagation delay and we don’t want any further increase in

it.

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Fig 5.6 Typical connection diagram of IR2110 Here we need use the high side outputs of the IR2110. A typical connection is shown above where the high side is that between 7 and 5. The application note was referred for the capacitor and diode values. The supply for the IC was obtained from the supply side.

Fig 5.7 High side connection of MOSFET to IR2110

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5.4 CONSTANT CURRENT BATTERY CHARGING A constant current supply is switched on and off as required by a micro-controller. The microcontroller senses the battery voltage and internally uses an analog to digital converter to read the battery voltage. The micro-controller requires its own 5V regulated supply.

Fig 5.8 Block diagram of constant charging charging of battery The microcontroller used here is the PIC18F452. The constant current supply supply to the battery while charging is realized using the LM317. This will maintain 1.25V between the OUT pin and the ADJ pin. We used a large 1ohm, 5W resistor here to ensure a constant 1.25A supplied. You can select this value as suits your application. The IN5404 diode ensures that the circuit charges the battery, and prevents the battery running the circuit, should the input power be turned off. If this does occur with a fully charged pack, the diode isolates the circuit and upon turn on the charger will find the battery peak again and return to a trickle charge after just a few minutes. The MOSFET switch is biased on by default, to ensure that the unit is "fail "fail-safe". -safe". The PIC can turn the transistor off by shorting the base to ground, and this thi s allows the LM317 to provide a regulated, constant current output. The drive current for the PIC is in the order of 1mA which is within the rated range of the PIC and MOSFET.

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Fig. 5.9 Complete Block Block diagram diagram

The full circuit is illustrated above. When the charging of the battery is to be stopped the MOSFET switch is turned ON. This shorts the ADJ to the ground which in turn reverse biases the diode cutting off the charging current.

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5.5 VOLTAGE ANALYSER CIRCUIT The battery voltage analyzer circuit is built around the popular quad op-amp LM324 that has four separate op-amps (A through D) with differential inputs. Op-amps have been used here as comparators. Switch is a push switch, which is pressed momentarily to check the battery voltage level before charging the battery. The non-inverting terminals of op-amps A through D are connected to the positive supply rail via a potential divider chain comprising resistors R1 through R5. Thus the voltage applied to any non-inverting input is the ratio of the resistance between that non-inverting terminal and ground to the total resistance (R1+R2+R3+R4+R5). (R1+R2+R3+R4+R5). The resistor chain provides a positive voltage of above 5V to the non-inverting inputs of all op-amps when battery voltage is 12.5V or more. A reference voltage of 5V is applied to the inverting inputs of op-amps via 5V zener diode. When the circuit is connected to the battery and push switch S2 is pressed, the battery voltage is sampled by the analyzer circuit. If the supply voltage sample applied to the non-inverting input of an op-amp exceeds the reference voltage applied to the inverting inputs, the output of the op- amp goes high and the led lights up.

Battery Voltage Status Indication Battery Voltage <9.8v

Red

Green

Yellow

Orange

Comments

Off

Off

Off

Off

>9.8v 11.5v

On On

Off On

Off Off

Off Off

Deep discharge Danger level Low level

12v 12.5v

On On

On On Off On On On Table 5.1 Battery status indication

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Normal level High level

 

Fig 5.10 Connection diagram for voltage analyzing circuit

5.6 VOLTAGE RECTIFICATION The input voltage which we receive is from the supply mains and it is an ac supply . so we have to make it dc so that it can be used to charge the the battery . in the first stage we have to convert the 230V ac to 24V ac . for this we use a 12-0-12 transformer . the current rating of the transformer is 4A . the output of the transformer is still stil l ac. So we use a rectifier circuit to convert this ac to dc. We use four f our P600 diodes and connect it in the bridge rectifier manner in order to give a rectified re ctified output. The output will now have all positive cycles but not a dc . so we keep a capacitor of suitable value to filter out the ripples and give a dc output. We use a 4700pf  to smooth out the ripple.

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Fig. 5.11 Rectifying circuit 5.7

PROGRAMMING

In our project the microcontroller controls all the three switches. One switch is from the supply to load , another from the supply to battery for the purpose of battery charging and the last one from battery to load to supply the load when the supply is off. we have kept three mosfet switches. So to drive them or switch it on we have to make that particular bit connected to the mosfet high or low .so we accept input (analog input from the battery and the supply ) in A0 and A1 pins. this analog signal is converted top digital and the values will be available in ADRESL and ADRESH.According ADRESH.According to the values availablein ADRESL and ADRESH we control the three switches. The input to the microcontroller has to be below 5 volt .so we divide the input voltage from the supply (24V) in 5V range by dividing divi ding it by 8 using resistors .This value is given to the A0 pin and it goes to the ADRESH and ADRESL as digital value . For this action to take place we give the ADCON0 value to be 10000001 and ADCON1 va value lue to be 11000100 . This means we have selected A0 pin , conversion clock source as FOSC/64 and right justified. The voltage from the battery battery comes in A1. For the conversion of this value we give 1000100 10001000 0 in ADCON0 and 11000100 for ADCON1 . This means that the input pin is A1 ,conversion clock source as FOSC/64 and right justified . 32

 

The switch from supply to load is set such that it opens when the supply voltage goes below a particular value (16V) .So it works according to the input in PORTA0. PORTB0 is made an output port and a logic 1 is given if it has to be on and logic 0 if it has to be off. The switch from supply to battery is set such that it opens when the battery is overc overcharged harged ie its voltage goes beyond 12.5 V .So it works according according to the input in PORTA0. PORTD0 is made as a output port and a logic 1 is given for the switch to be on and logic 0 for the switch to be off. The switch from the battery to load is switched switched on or off according to the input from both both the battery and the supply.So it works according to the input in PORTA0 and PORTA1. PORTC0 is made as an output port and a logic 0 is given for the switch to be off and logic 1 for it to be on . The program is written in both assembly and c language and are written below .

5.8 PROGRAM #include<P18F452.h> #pragma config OSC=HS, OSCS=OFF #pragma config WDT=OFF #pragma config DEBUG=OFF, LVP=OFF, STVR=OFF unsigned char mainline1,mainline2,battery1,battery2; mainline1,mainline2,battery1,battery2; int open1,i; void mainline(); void battery(); void DELAY() { for(i=0;i<5000;i++) {;} } void main() { TRISAbits.TRISA0=1; 33

 

TRISAbits.TRISA1=1; TRISB=0; TRISC=0; TRISD=0; PORTB=1; PORTC=0; PORTD=1;

while(1) { void mainline();

switch(mainline1) { case 0x3: PORTB=1; open1=1; break; case 0x2: PORTB=1; open1=1; break;

case 0x1: if(mainline2>0x9A) { 34

 

PORTB=1; open1=1; } else { PORTB=0; open1=0; } break; default: PORTB=0; open1=0; break; }

if(open1==0) { switch(battery1) { case 0x3: PORTC=1; break; case 0x2: PORTC=1; break; 35

 

case 0x1: if(battery2>0xF6) { PORTC=1; } else { PORTC=0; } break; default: PORTC=0; break;

} }

if(open1==1) { switch(battery1) { case 0x3: PORTD=0; break; case 0x2: if(battery2>=0x80) 36

 

PORTD=0; else PORTD=1; break; default: PORTD=1; break; } } void mainline() { int j=0,k=0; ADCON0=0x81; ADCON1=0xC4; DELAY(); ADCON0bits.GO=1; while(ADCON0bits.DONE==1); for(i=0;i<200;i++) for(j=0;j<10;j++); mainline1=ADRESH; mainline2=ADRESL; battery();

}

void battery() 37

 

{ int j=0,k=0; ADCON0=0x89; ADCON1=0xC4; DELAY(); ADCON0bits.GO=1; for(i=0;i<200;i++) for(j=0;j<10;j++); battery1=ADRESH; battery2=ADRESL; mainline(); }

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CHAPTER-6

CONCLUSIONS

 

We initially thought working with an NiCd but switched over to lead acid battery as they are cheaper and methods of charging are better developed in case of lead acid.

 

Our initial design had a DC-DC converter. This however was not available in the market forcing us to use the LM7812 instead. Since our load requirements are low this did not cause much of a problem.

 

We initially used transistors as switching devices. Even the simulation was completed using them. However acting on some expert advice which said that BJTs do attract a lot of problems, we replaced them by MOSFETs.

 

In the simulation stage itself the MOSFETs refused to turn ON using the 5V output from the PIC. We then tried to use the ULN2803 IC which is normally used to drive relays. Using this meant that we could do away with these MOSFET switches as a whole. This was not to be as the load cannot be isolated and we dropped that idea. Next we tried the IR2110 MOSFET driver. We used two of them to drive two of our MOSFETs. In both both cases the high side output was used.

 

The part by part analysis of the circuit was conducted in simulation as well as on a PCB. The simulation platform used is Proteus 7.7 .The results obtained were satisfactory and commensurate with the desired results. The code was burned into the microcontroller and its working tested. The whole circuit was divided into three parts and assembled on different boards. The charging of the battery and the working of the switches was observed to be satisfactory. The voltage analyzing circuit ant rectifying circuit worked perfectly fine. FUTURE SCOPE:  

Our proposed design can be used as the initial stages of a microcontroller based UPS system. Adding an inverter circuit and conditioning elements would complete the fabrication of the UPS systems

 

The visual interfacing can be improved by adding an LCD module to the proposed circuitry

 

Other charging schemes can be implemented i mplemented for charging the battery

 

Buzzer can be integrated which gives an indication about the over or under voltage

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CHAPTER-7

REFERENCES 1.  Nik Mohamad Anis Bin Nik Haron, university Malaysia Pahang ‘Design of a Micro controller based passive stand by uninterrupted power supply’  2.  ‘Power Electronics Converters applications and design’ by Ned Mohan. Second edition 1989, John wiley and sons. 3.  ‘Uninterrupted power supply common topologies’ technical article by K.S Suresh Kumar, NITC. 4.  www.batteryuniversity.com  www.batteryuniversity.com  5.  Application note-AN-936 “ the Dos and Don’ts of using HEXFETIII”  6.  Application note – AN-967 ”a new gate charge factor “   7.  PIC microcontroller and embedded systems by M.A Mazidi, Pearson Publications, 2008. 8.  www.nationalsemiconductors.org

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