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Table of Contents

1. Abstract……………………………………………………………………………………..2

2. Introduction……………………………………………………………………………….2

3. Components………………………………………………………………………………..3

a. Transformer 4 b. Bridge Rectifier 5 c. Transistor as a switch 6 d. Battery 6 e. Load 6

4. Working………………………………………………………………………………………7

5. Applications………………………………………………………………………………..12

6. Conclusions………………………………………………………………………………..13

7. References…………………………………………………………………………………14

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220 to 12 V D.C POWER SUPPLY & STORAGE

1. Abstract:

This project is to create a 220 A.C to 12V DC power supply that can also store the power for a long time. The circuit used is an efficient one and has brought many improvements into current D.C power supplies such as voltage regulation and removal of ripples in output.

The 220 A.C voltage is first converted into 12 V A.C by a step down transformer, then a full wave rectifier bridge (based on wheat stone bridge) is used to convert the A.C into D.C. Then this output is filtered twice by two mechanisms.

i. In order to remove the ripples in the waveform of the bridge circuit.ii. Create a regulated and efficient power supply.

A NPN transistor with a base connected to the Zener diode is also used as a switch in the circuit. Then 12 V is received as output voltage. The circuit s and waveforms are created using PSpice. Due to the voltage regulation and removal of ripples in the output this power supply can also be used as a “battery eliminator” [6] which provides constant and efficient output to the load without the need for a battery.

2. Introduction:

The need of D.C Power supply is always there in the field of electrical engineering. The main advantages these D.C power supplies bring are portability and cost effectiveness as compared to A.C power supplies but sometimes the cheapness of these D.C power supplies results in the lack in efficiency of the output. That is the output of most of the D.C power supplies present in the market has ripples in it and

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is not pure D.C. Moreover the output voltage is not exact due to losses in the circuit.

To remove these draw backs in D.C power supplies we have made an efficient circuit that not only removes the ripples in the output voltage to make a pure D.C signal but also regulates the voltage to a constant and desired value. This is achieved by using a filter circuit and transistor which is used as a switch. We have used a bridge rectifier instead of two diode rectifier (which also produces full wave rectification) because the bridge rectifier does not require a high “peak inverse voltage” as it utilizes most of the windings of the transformer. We have also used a simple transformer instead of center tap because it is cheaper and produces compact and cheaper power transmission. The use of a RL circuit as a filter has improved the efficiency of the circuit by removing the ripples in the D.C that is rectified by the bridge. The use of transistor as a switch has brought the other improvement in the circuit I.e. it has regulated the voltage to a constant value that has saved our load from voltage fluctuation damages. Rechargeable Nickel Metal Hydride batteries are used, which are being used extensively in consumer electronics these days. These also have lower charging time and are very long lasting. Due to the efficient combination of the values of the circuit elements a variety of loads can be attached to the output circuit i.e., any circuit element having voltage 12 V and resistance greater than 10 Ωs.

(The Circuit Diagram is placed on the next page)

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3. Components:

a. Transformer [2] : This is a step down transformer that converts 220 V A.C into 12 V A.C. A maximum current of 1200mA can flow through it and it works on a frequency of 50 Hz and it is easily available in market in low cost. It is not a center tap transformer and it is easy to connect it into the circuits to get actual and desired results, as required.

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S t e p D o wn Tra n s f o rm e r

2 2 0 -1 2 V

The relation between number of turns and voltages of input and output coils is:

Vs/Vp =Ns/Np

We have Vs= 12 Vrms and Vp= 220 Vrms

Then

Ns/Np=.054

The principle of the transformer is mutual induction. [2] The output of the transformer is pure 12 V A.C in this project.

b. Bridge Rectifier: The rectification is conversion of A.C into D.C with the help of Diodes. There are two types of rectificationsa. Half wave rectificationb. Full wave rectification

We use Full wave rectification by using bridge rectifier. This rectifier consists of four diodes (D1, D2, D3, and D4) connecting each other as shown in figure:

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- +

D 51

2

3

4

The advantage of bridge rectifier over two diode rectifier is that:

1. Peak inverse voltage of bridge rectifier is very low as compared to the two diode rectifier.

2. Power dissipation of bridge rectifier is lower.3. The output of bridge rectifier is very efficient than two diode rectifier.

Due to these advantages we have used bridge rectifier in our circuit.

Principle of bridge rectifier is based on wheat stone bridge. The output of bridge rectifier is pure D.C (+12 v). That can be shown by connecting a resistance at its output terminals. This D.C output has ripples which are not a

pure D.C (Figure.3). These ripples can be removed by connecting a capacitor

which acts as a filter.

c. Transistor as a switch [3] : The transistor, having a Zener diode at its base is used in this circuit as a switch. The transistor works in an active region that is its base-emitter junction is in forward bias and collector-base junction in reverse bias. The

transistor used in our circuit is shown in the figure:

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2 N 2 2 2 2 A

1 3 V

Z e n e r

d. Battery: We have used Nickel Metal Hydride (Ni-MH) rechargeable cells in this project instead of a 12 V battery as it is very cheap. Each cell has 1.2 Volts so we have added 10 cells in series to make 12 V storage.

V= V1+V2+V3+V4+V5+V6+V7+V8+V9+V10

V= 1.2+1.2+1.2+1.2+1.2+1.2+1.2+1.2+1.2+1.2V=12 volts

e. Load: The loads connected in the circuit can have resistance greater than 10 Ω. As our transformer has maximum current rating of 1200mA and our Voltage is 12 V so by Ωs law:

R=V/IR=12/1200mR=10 Ω

4. Working:The process of the getting a 12 V D.C from 165 V to 285 V A.CIs divided into following steps:1. A step down Transformer is used to convert 220 V A.C into 12 V A.C.

This is done by mutual induction phenomena in the transformer.

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The 220 v A.C input waveform is shown below:

(Figure.1)

The 12 V A.C stepped down output waveform of transformer is shown:

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(Figure.2)

2. In this step the full wave rectification is done. That is conversion of 12 V A.C into D.C. Bridge rectifier is used for this purpose. When the positive half cycle of A.C wave form reaches the upper terminal of the bridge rectifier this terminal becomes +12 V and lower terminal becomes -12 V with respect to upper terminal. Then diodes D3 and D2 become forward biased and conduct while D1 and D4 become reverse biased. The current flows from this path ( D2 to D3). This + half cycle is observed at the output of bridge rectifier. Now when Negative half cycle reaches the upper terminal of bridge rectifier the lower terminal becomes positive with respect to upper terminal and diode D1 and D4 conduct and current

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flows through this path (D1 and D4) while diode D3 and D2 are switched off. This Negative half cycle is converted to positive half cycle at the output as shown below (by red wave):

(Figure.3)

3. In the previous step the D.C waveform is not pure because it has ripples which are not suitable for our circuit. So these ripples can be removed by connecting a filter circuit at the output of bridge rectifier. This filter contains a capacitor in parallel with a resistor. This capacitor has a suitable value that can be calculated by some mathematics which is very important in calculating the time constant of the capacitor (T).

T=RCT=2.13k X 4.7uT=10ms

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Capacitor is charged after 5 time constants. So the ripples are removed by using this kind of capacitor. The output of the filter circuit is shown in red waveform where green is input of bridge:

(Figure.4)

4. Up till now we have 12 V D.C waveform. Now the enhancement we have brought in this circuit is the use of transistor having a Zener Diode connected to its base. It is used as a switch as well as 12 volts regulator. The biasing of the transistor is the major issue which could be solved by connecting D.C batteries but we solved this problem by using a resistor as a potential divider to avoid the use of batteries (an additional component of the circuit). Now the collector has voltage 12 volts and base has the voltage of 2.59 volts this makes the collector-base junction

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reverse biased. Initially the bypass capacitor connected at emitter is uncharged so the potential at emitter terminal of transistor is 0 volts. While the Zener Diode connected at base regulates 12 volts across itself so base-emitter junction becomes forward biased and capacitor begins to charge. This operation is called “switching of transistor”. After 5 time constants that can be calculated by (T=RC) capacitor becomes fully charged. When the capacitor is fully charged base-emitter junction becomes reverse biased and transistor goes to cut off state. And all the voltage is observed at this capacitor. The output wave form is shown below:

(Figure.5)

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5. This capacitor is now connected to our rechargeable battery. This battery is charged in approximately 120 minutes. The power stored in the battery is calculated as follows:

6.P=VIP=12 X 1200mP=14.4Watt

The maximum load is connected to this battery is calculated (as in section 2.5). So the maximum load which can be connected to this circuit is greater than 10 Ω.

5. Applications:This circuit has many applications:

a. The circuit can be used as a “battery eliminator” [6] i.e. as it provides constant regulated voltage and ripple free output. It can be used to give output directly to a load instead of a battery first. This reduces the cost of battery.

b. It can be used as Battery charger. This can be detached from the circuit and then used to supply power to a variety of electronic devices.

c. It can be used as a D.C voltage regulator which is capable of providing ripple free voltage.

d. To recharge an electric vehicle (EV) battery pack [5].

e. To recharge a fuel vehicle's starter battery, where a modular charger is used [5].

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6. Conclusions:We have made a storing power supply. Which takes a 220 volts A.C input and steps down and rectifies it into 12 volts D.C output and stores a power of 14.4 watt in the rechargeable battery which can be stored for as many days as you want? It has some major advantages over common power supplies e.g., it uses a step down transformer instead of center tape, bridge rectifier instead of two diode rectifier is used and most important is transistor followed by a Zener diode at its base acting as switch which prevents damages due to false input as well as any fault in output voltages. The power can be stored in battery for many days and Load should be greater than 10Ω.

7. References:

1. Microelectronic Circuit by Sedra-Smith2. Power Electronics by McKenzie3. Awais Haider (eleshah86@gmail.com)4. www.pspice.com 5. www.wikipedia.org 6. http://en.wikipedia.org/wiki/Battery_eliminator