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7/27/2019 Auto Turn On

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EW 2__________________________

FINAL PROJECT

REPORT__________________________

Submitted by

1) V.S.Rao Veeravasarapu - 2007310102) K.Gowtham Raghunath- 200730015


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AIM: To build a circuit for Emergency Lamp.

ABSTRACT: Our circuit will have an emergency lampwhich uses white LEDs as lamp and rechargeable battery.In our

circuit, the connection is in such a way that when the mains is ON the

power is used for charging the rechargeable batteries and when the

mains is OFF the lamp turns on immediately by using the current

from the rechargeable battery..Also during charging of the battery,

when the battery gets completely charged since any further flow of

current through it may damage the battery we have kept a zener diode

in such a way that all the current passes through the zener diode when

the rechargeable battery is completely charged.

INTRODUCTION: Our whiteLED basedEmergency light has its own battery charger and rechargeable battery

in it.It uses the mains supply for charging battery and it automatically

stops charging when the battery is fully charged.This helps in

protecting the battery from being damaged.When the mains is on

battery gets charged and when the mains is off the battery discharges

for glowing the lights.We use a Relay for switching ON or OFF the

lamp.Relay is an electrical switch that opens and closes under the

control of another electrical circuit.

COMPONENTS USED:


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Serial No. Name No.of components

1. 12v Transformer 1

2. 1N4007 diode 3

3. 3904 NPN transistor 44. Zener diode 25. Capacitor(1000micro F) 16. Resistances(of required values) -------7. Relay switch 18. Rechargeable batteries(1.2v,1000mAhr) 49. Battery case 110. Potentiometer(5K) 1


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CIRCUIT EXPLANATION:

Our circuit mainly consists of two parts.They are charger

power supply and Led driver.LED driver actually consists of a Relayand some white LEDS where Relay is used as a swith for lamp.Inturn

Charger power supply consists of two sections.They are Voltage

stabilizer and Constant current source.Thus different sections of our

circuit are

1)Voltage stabilizer

2)Constant current source

3)LED Driver

VOLTAGE STABILIZER:

First of all we have to convert AC to DC.This is done by the

voltage stabilizer.The different stages involved in converting

AC to DC are shown in a block diagram.


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The block diagram is as follows

Constant

voltage

This gives us constant output voltage which is

required for constant current source which inturn is required for

charging the battery.The input of voltage stabilizer is AC mains.Since

its voltage is around 220v which is not safe for us.First we will reduce

that to 12v using a step-down transformer.The input and output

waveforms for transformer are shown below.

AC

mains

Transf-

ormer

Recti-

fier

Filter

Voltage

Regulator


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Now the transformer output is fully-rectified by using a full-wave

rectifier.A rectifier is an electrical device that converts Alternating

current(AC) to pulsating DC.Here the two diodes back-to-back(i.e.,

anode to anode) form a full wave rectifier.It converts the negative

cycle also into positive cycle so that the rms value is increased.The

input and output waveforms for rectifier are shown below.


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Now the rectified output is given to a filter whose output is almost a

dc but with some ripples.The output of the filter is shown below.

The voltage regulator uses a zener-diode of break-down voltage 5.3

v.In the voltage regulator circuit transistor(npn) Q2 is called pass

transistor because all the load current passes through it.The circuit

consists a voltage divider Rx and Ry(Actualy a potentiometer R3).The

voltage divider consists of two resistors and samples the output

voltage and delivers a negative feedback voltage to the base of

Q1.This negative feedback voltage VF controls the collector current oftransistor(npn) Q1.Let VBbe base voltage of Q1.From the figure it can

be easily seen that

VB=VZ+VBE

where

VZ=Break-down voltage of zener diode

VBE=Base-Emitter voltage of Q1 ~0.7v

The circuit diagram of voltage regulator is as follows.


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V1

220V

50Hz

0Deg

T110

D1

DIODE_VIRTUAL

D2

DIODE_VIRTUAL

C1

1000uF

R1

1500D

35.3V

Q1

BJT_NPN_VIRTUAL

R2

1k

R8

1000

U

1

D

C1e-009

6.092m

A

+-

XSC1

A

B

ExtTrig+

+

_

_

+

_

XMM1

R9

1k

R3

Key=A

1k

50%

Q2

BJT_NPN_VIRTUAL


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The voltage across Ry=VB=VZ+VBE.The output voltage

Vo across load resistance is equal to the voltage across (Rx+Ry).

Vo/(Rx+Ry)=(VZ+VBE)/Ry V0=(VZ+VBE)(1+Rx/Ry)

Since Rx and Ry is actually a potentiometer , the resistances are

adjusted in such a way that the output Vo equals approximately 15v.

i.e., Rx=300 ohm,Ry=200 ohm

The operation of the circuit is as follows.

1)suppose ,if the output voltage increases due to any reason the basevoltage of Q1 i.e., voltage across Ry increases and collector current

increases most of which is coming from the R2.So the base voltage of

Q2 decreases keeping the output voltage constant as the previous.

2)In the same way if the output voltage decreases due to any reason

the feedback voltage VF decreases.This reduces the current through R2

and Q1.This implies that base voltage of Q2 increases and thereby

increasing the output voltage to our constant value.

Constant Current Source:

Charging of a battery requires constant current.The constant currentsource requires a zener-diode and a constant voltage Vcc which we got

from voltage stabilizer .


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A switch is prepared using BJT which is used to switch on and off

depending on the potential difference across the battery. This switch

is used to change the current to zero through the constant currentsource when the battery is fully charged and the entire current flows

through the zener-diode.When the battery is charging, the transistor

will be in active-mode since the driving force is coming from

Vcc.When the battery is completely charged, since the maximum

voltage that the battery can be charged is kept almost equal to

breakdown voltage of zener diode the voltage across R1 equals

to 0 which inturn makes the transistor to turn from active

R1

100

R2

1.3k

Q2

BJT_NPN_VIRTUAL

D1

5 V

15V

VCC

Constant Current


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mode to satuaration mode.This switching of transistor from active

mode to satuaration mode acts as a switch and makes the current 0

through the battery when its charging is completed.

LED DRIVER:-

The third part of our circuit is LED driver. In this circuit LEDs

are connected to rechargeable battery through relay switch. if more

current is flow through a LED , then it will damage so We used more

than one LED in parallel connection because current divides to allLEDs that is reducing current through one LED. LEDs glows when

main power supply is not available then current is flows through

LEDs because of battery voltage.

In LED driver section, we use a total of three 10 mm white

LEDs. All the LEDs are connected in parallel with 100 ohms

resistances in series with each. We used a relay switch to connect the

common anode junction of all LEDs to battery. Relay switch is an

electrically operated switch. This relay switch has a total of terminals.

In those, two are input terminals and one is common terminal. current

flowing through the coil of the relay creates a magnetic field which

attracts a lever and changes the switch contacts. The coil current can

be on or off so relays have two positions and they are double throw or

change over switches. So we used a 12 volt relay switch, for this if we

apply a voltage greater than or equal to 12 volts between two input

terminals, then two of remaining three terminals will short and other

terminal doesnt connect to any terminal. If the voltage between two

input terminals is zero then the common terminal and second (other

terminal) terminal will shorted and the first terminal doesnt connect

to any terminal.

So we choose the two terminals of the relay switch such thatthese terminals can short when voltage between input terminals is


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zero and open when voltage between two input terminals is greater

than 12 volts. So we connected common terminal to anode of battery

and the other terminal to common anode junction of all LEDs. And

the cathodes of all LEDs are connected to the cathode terminal ofbattery. we applied the filtered output voltage in first part our circuit

between the input two terminals of the relay switch.

If the main power supply is not available then the two terminals

of relay switch, which we used, are closed i.e. LED driver circuit is

closed, therefore LEDs will glow using the battery voltage. And if the

mains is available then the two terminals of relay switch are open i.e.

LED driver circuit is open , therefore LEDS doesnt glow until mains

is off.

Then we connected a LDR (light dependent resistor) in parallel

to all LED. A LDR has zero resistance when light is falling on it

otherwise infinite resistance. Therefore, in the nights LDR has infinite

resistance and during day times it has zero resistance. So LEDs glows

when the mains is not present in the nights only, not in day time.

Application:

Our circuit can be used as an emergency lamp duringnights.

In household appliances when there is sudden power cutthis lamp automatically turns on itself.


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Alternative Approach tried:

Here in the LED driver circuit, since our aim is LEDs should glow

when there is no power and not to glow when there is mains we canuse a PNP transistor inplace of Relay switch.Once the base of a PNP

transistor is low the transistor turns ON and if base goes high the

transistor will be in OFF position.we connected the constant coltage

to base of the PNP and LEDs to its emitter and output of rechargeable

battery to collector.So when power is OFF, base is low =>current

flows from collector to emitter and LEDs glow.And when power is

ON no current flows through collector , LEDs will not glow.

Advantages:

No need of any battery charger since our circuit itself has ainbuilt battery charger.

During nights if the power is cut suddenly, there is no need ofsearching the lamp for switching it ON since the lamp will turn

on automatically when mains is OFF.

There will be no wastage of power in over charging the batteryonce the charging is completed since in our circuit once the

battery is completely charged the current through battery goes to0 and all the current passes through zener diode.


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What We Have Learnt:

1.Conversion of AC to DC.2.Working of a Relay(switch)3.Design of a constant voltage source.4.Design of a constant current source.5. Information about some batteries(Ni-Cd,....)6.Charging of a battery requires a constant current7.Working of LDR8.Using NPN transistor as a switch9.Usage of a zener diode to stop the charging of the battery

immediately after its complete charging.

10. Slow charging of batteries is more preferable than fastcharging because due to fast charging the durability(life span)

i.e., number of times the battery can be recharged decreases.

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