Table of Content

S No. Content Page num1 Name of the Project 32 Statement about the project 33 List of parts used 34 Circuit Diagram to be referred 45 Brief description of the operation 56 Assembling and Testing 57 Description of main component(s) 68 Applications and Modifications 6

INTRODUCTION

The project idea put forth here can be used to turn on (or off) the load connected across the relay at a predetermined temperature. At the heart of this heat sensitive switch is IC LM35(IC 1), which is a linear

temperature sensor and a linear temperature to voltage converter circuit.The converter provides accurately linear and directly proportional output signal in millivolts over

temperature range of 0 deg. C to 155 deg. C. It develops an output voltage of 10mV per degree centigrade change in the ambient temperature. Therefore the output voltage varies from 0 mV at 0 deg. C to 1V at

100 deg. C at any voltage measurement circuit connected across the output pins can read the temperature directly.

CIRCUIT DIAGRAM AND

LIST OF PARTS USED

SEMICONDUCTORS

IC1 LM35 temperature sensor IC2 CA3130 comparator IC3 7805 DC voltage regulator T1 BC 549 NPN transistor T2 BD 139 NPN transistor D1-D5 1N4007 rectifier diode LED1, LED2 5mm light emitting diode

RESISTORS (all 1/4-watt,+ 5% carbon)

R1 1.2 kilo-ohmR2 1.0 kilo-ohmR3 12 kilo-ohmR4 680 kilo-ohmR5 15 kilo-ohmR6,R8,R9 1.5 kilo-ohmR7 1.0 kilo-ohm

CAPACITORSC1 47 micro farad(electrolytic)C2 1 micro farad(electrolytic)C3 0.17 micro farad(ceramic dis)C4 1000 micro farad, 35V(electrolytic)

MISCELLANEOUSX1 230V AC to 0-12V AC, 250mA secondary transformerRL1 12V, 200 ohm 1 C/o relay

BRIEF DESCRIPTION OF

OPERATION OF CIRCUIT

IC 1 temperature tracking output is applied to the non-inverting input (pin3) of the comparator IC 2. The inverting input (pin 2) of IC 2 is connected across the positive supply rails via a voltage divider network formed by potentiometer VR1.The voltage at pin 2 is used as reference level for comparator against the output supplied

by IC 1.

So, if pin 3 of IC 2 receives a voltage lower than the set level, its output goes low (approximately 650 mV). This low level is applied to the input of load-relay driver comprising NPN transistors T1 and T2 and they are in

cut-off. Hence, relay RL1 is in the de-energized state, keeping mains supply to load ‘off’ as long as the temperature at the sensor is low. Conversely, if pin 3 input receives a voltage higher than the set level, its output

goes high (approximately 2200 mV) and the load is turned ‘on’. This happens when IC 1 is at a higher temperature and its output voltage is also higher than the set level at pin 2 of IC 2.

Suppose, we want to switch on the load at 50 C. Heat the sensor with soldering iron until 50 mV is obtained at pin 2 of the sensor. Simultaneously, we have to vary VR1 such that pin 6 of CA3130 becomes high. This will enable to energize the relay and turn on the load. Keep the setting of VR1 at this position for future use, so that

whenever the temperature reaches 50 C, the circuit will automatically switch on the load.

DETAILED DESCRIPTION OF VARIOUS COMPONENTS

TEMPERATURE SENSOR- THE LM35

The LM35 is an integrated circuit sensor that can be used to measure temperature with an electrical output proportional to the temperature (in oC).

The LM35 series are precision integrated-circuit temperature sensors, whose output voltage is linearly proportional to the Celsius (Centigrade) temperature. The LM35 thus has an advantage over linear temperature sensors calibrated in ° Kelvin, as the user is not required to subtract a large constant voltage from its output to obtain convenient Centigrade scaling. The LM35 does not require any external calibration or trimming to provide typical accuracies of ±¼°C at room temperature and ±¾°C over a full -55 to +150°C temperature range. Low cost is assured by trimming and calibration at the wafer level. The LM35's low output impedance, linear output, and precise inherent calibration make interfacing to readout or control circuitry especially easy. It can be used with single power supplies, or with plus and minus supplies. As it draws only 60 µA from its supply, it has very low self-heating, less than 0.1°C in still air. The LM35 is rated to operate over a -55° to +150°C temperature range, while the LM35C is rated for a -40° to +110°C range (-10° with improved accuracy). The LM35 series is available packaged in hermetic TO-46 transistor packages, while the LM35C, LM35CA, and LM35D are also available in the plastic TO-92 transistor package. The LM35D is also available in an 8-lead surface mount small outline package and a plastic TO-220 package.

Features

• Calibrated directly in ° Celsius (Centigrade)

• Linear + 10.0 mV/°C scale factor• 0.5°C accuracy guaranteeable (at

+25°C)• Rated for full -55° to +150°C range• Suitable for remote applications• Low cost due to wafer-level

trimming• Operates from 4 to 30 volts• Less than 60 µA current drain• Low self-heating, 0.08°C in still air• Nonlinearity only ±¼°C typical• Low impedance output, 0.1 Ohm

for 1 mA load

Use of LM35s to Measure Temperature

o You can measure temperature more accurately than a using a thermistor. o The sensor circuitry is sealed and not subject to oxidation, etc. o The LM35 generates a higher output voltage than thermocouples and may not require that the

output voltage be amplified.

Typical application

Operation of LM35

o It has an output voltage that is proportional to the Celsius temperature. o The scale factor is .01V/oC o The LM35 does not require any external calibration or trimming and maintains an accuracy of 

+/-0.4 oC at room temperature and +/- 0.8 oC over a range of 0 oC to +100 oC. o Another important characteristic of the LM35DZ is that it draws only 60 micro amps from its

supply and possesses a low self-heating capability. The sensor self-heating causes less than 0.1 oC temperature rise in still air.

Typical Performance

The LM35 comes in many different packages, including the following. o TO-92 plastic transistor-like package, o T0-46 metal can transistor-like package o 8-lead surface mount SO-8 small outline package o TO-202 package. (Shown in the picture above)

Electrical Connections of LM35

o Here is a commonly used circuit.  For connections refer to the picture above. o In this circuit, parameter values commonly used are:

Vc = 4 to 30v 5v or 12 v are typical values used. Ra = Vc /10-6 Actually, it can range from 80 KW to 600 KW , but most just use 80 KW.

o Here is a photo of the LM 35 wired on a circuit board. The white wire in the photo goes to the power supply. Both the resistor and the black wire go to ground. The output voltage is measured from the middle pin to ground.

TRANSISTOR

A Bipolar Transistor essentially consists of a pair of PN Junction Diodes that are joined back-to-back. This forms a sort of a sandwich where one kind of semiconductor is placed in between two others. There are therefore two kinds of Bipolar sandwich, the NPN and PNP varieties. The three layers of the sandwich are conventionally called the Collector, Base, and Emitter. The reasons for these names will become clear later once we see how the transistor works.

Some of the basic properties exhibited by a Bipolar Transistor are immediately recognisable as being diode-like. However, when the 'filling' of the sandwich is fairly thin some interesting effects become possible that allow us to use the Transistor as an amplifier or a switch. To see how the Bipolar Transistor works we can concentrate on the NPN variety.

Figure 1 shows the energy levels in an NPN transistor when we aren't externally applying any voltages. We can see that the arrangement looks like a back-to-back pair of PN Diode junctions with a thin P-type filling between two N-type slices of 'bread'. In each of the N-type layers conduction can take place by the free movement of electrons in the conduction band. In the P-type (filling) layer conduction can take place by the movement of the free holes in the valence band. However, in the absence of any expernally applied electric field, we

find that depletion zones form at both PN-Junctions, so no charge wants to move from one layer to another.

Consider now what happens when we apply a moderate voltage between the Collector and Base parts of the transistor. The polarity of the applied voltage is chosen to increase the force pulling the N-type electrons and P-type holes apart. (i.e. we make the Collector positive with respect to the Base.) This widens the depletion zone between the Collector and base and so no current will flow. In effect we have reverse-biassed the Base-Collector diode junction. The precise value of the Base-Collector voltage we choose doesn't really matter to what happens provided we don't make it too big and blow

up the transistor! So for the sake of example we can imagine applying a 10 Volt Base-Collector voltage

Now consider what happens when we apply a relatively small Emitter-Base voltage whose polarity is designed to forward-bias the Emitter-Base junction. This 'pushes' electrons from the Emitter into the Base region and sets up a current flow across the Emitter-Base boundary. Once the electrons have managed to get into the Base region they can respond to the attractive force from the positively-biassed Collector region. As a result the electrons which get into the Base move swiftly towards the Collector and cross into the Collector region. Hence we see a Emitter-Collector current whose

magnitude is set by the chosen Emitter-Base voltage we have applied. To maintain the flow through the transistor we have to keep on putting 'fresh' electrons into the emitter and removing the new arrivals from the Collector. Hence we see an external current flowing in the circuit.

Some of the free electrons crossing the Base encounter a hole and 'drop into it'. As a result, the Base region loses one of its positive charges (holes) each time this happens. If we didn't do anything about this we'd find that the Base potential would become more negative (i.e. 'less positive' becuase of the removal of the holes) until it was negative enough to repel any more electrons from crossing the Emitter-Base junction. The current flow would then stop.

To prevent this happening we use the applied Emitter-Base voltage to remove the captured electrons from the Base and maintain the number of holes it contains. This have the overall effect that we see some of the electrons which enter the transistor via the Emitter emerging again from the Base rather than the Collector. For most practical Bipolar Transistors only about 1% of the free electrons which try to cross Base region get caught in this way. Hence we see a Base Current, IB, which is typically around one hundred times smaller than the Emitter Current, IE

Bipolar transistors, having 2 junctions, are 3 terminal semiconductor devices. The three terminals are emitter, collector, and base. A transistor can be either NPN or PNP.  See the schematic representations below:

     Note that the direction of the emitter arrow defines the type transistor. Biasing and power supply polarity are positive for NPN and negative for PNP transistors. The transistor is primarily used as an current amplifier. When a small current signal is applied to the base terminal, it is amplified in the collector circuit. This current amplification is referred to as HFE or beta and equals Ic/Ib.

Operational Characteristics of a Transistor

LIGHT EMITTING DIODE

LED's are special diodes that emit light when connected in a circuit. They are frequently used as "pilot" lights in electronic appliances to indicate whether the circuit is closed or not. A a clear (or often colored) epoxy case enclosed the heart of an LED, the semi-conductor chip.

The two wires extending below the LED epoxy enclosure, or the "bulb" indicate how the LED should be connected into a circuit. The negative side of an LED lead is indicated in two ways: 1) by the flat side of the bulb, and 2) by the shorter of the two wires extending from the LED. The negative lead should be connected to the negative terminal of a battery. LED's operate at relative low voltages between about 1 and 4 volts, and draw currents between about 10 and 40 milliamperes. Voltages and currents substantially above these values can melt a LED chip.

The most important part of a light emitting diode (LED) is the semi-conductor chip located in the center of the bulb as shown at the right. The chip has two regions separated by a junction. The p region is dominated by positive electric charges, and the n region is dominated by negative electric charges. The junction acts as a barrier to the flow of electrons between the p and the n regions. Only when sufficient voltage is applied to the semi-conductor chip, can the current flow, and the electrons cross the junction into the p region.

In the absence of a large enough electric potential difference (voltage) across the LED leads, the junction presents an electric potential barrier to the flow of electrons.

Cause and color of light emitted by an LED

When sufficient voltage is applied to the chip across the leads of the LED, electrons can move easily in only one direction across the junction between the p and n regions. In the p region there are many more positive than negative charges. In the n region the electrons are more numerous than the positive electric charges. When a voltage is applied and the current starts to flow, electrons in the n region have sufficient energy to move across the junction into the p region. Once in the p region the electrons are immediately attracted to the positive charges due to the mutual Coulomb forces of attraction between opposite electric charges. When an electron moves sufficiently close to a positive charge in the p region, the two charges "re-combine". Each time an electron recombines with a positive charge, electric potential energy is converted into electromagnetic energy. For each recombination of a negative and a positive charge, a quantum of electromagnetic energy is emitted in the form of a photon of light with a frequency characteristic of the semi-conductor material (usually a combination of the chemical elements gallium, arsenic and phosphorus). Only photons in a very narrow frequency range can be emitted by any material. LED's that emit different colors are made of different semi-conductor materials, and require different energies to light them.

Energy emitted by an LED

The electric energy is proportional to the voltage needed to cause electrons to flow across the p-n junction. The different colored LED's emit predominantly light of a single color. The energy (E) of the light emitted by an LED is related to the electric charge (q) of an electron and the voltage (V) required to light the LED by the expression: E = qV Joules. This expression simply says that the voltage is proportional to the electric energy, and is a general statement which applies to any circuit, as well as to LED's. The constant q is the electric charge of a single electron, -1.6 x 10-19 Coulomb.

Finding the Frequency from the Wavelength of Light

The frequency of light is related to the wavelength of light in a very simple way. The spectrometer can be used to examine the light from the LED, and to estimate the peak wavelength of the light emitted by the LED. But we prefer to have the frequency of the peak intensity of the light emitted by the LED. The

wavelength is related to the frequency of light by , where c is the speed of light (3 x 108 m/s) and

is the wavelength of light read from the spectrometer (in units of nanometers or 10-9 meters). Suppose you observed the red LED through the spectrometer, and found that the LED emits a range in colors

with maximum intensity corresponding to a wavelength as read from the spectrometer of = 660 nm or 660 x 10-9 m. The corresponding frequency at which the red LED emits most of its light is

or 4.55 x 1014 Hertz. The unit for one cycle of a wave each second (cycle per second) is a Hertz.

Finding the Energy from the Voltage

Suppose you measured the voltage across the leads of an LED, and you wished to find the corresponding energy required to light the LED. Let us say that you have a red LED, and the voltage measured between the leads of is 1.71 Volts. So the Energy required to light the LED is E = qV or E = -1.6 x 10-19 (1.71) Joule, since a Coulomb-Volt is a Joule. Multiplication of these numbers then gives E = 2.74 x 10-19 Joule.

CAPACITORS

Capacitors store electric charge. They are used with resistors in timing circuits because it takes time for a capacitor to fill with charge. They are used to smooth varying DC supplies by acting as a reservoir of charge. They are also used in filter circuits because capacitors easily pass AC (changing) signals but they block DC (constant) signals.

Capacitance

This is a measure of a capacitor's ability to store charge. A large capacitance means that more charge can be stored. Capacitance is measured in farads, symbol F. However 1F is very large, so prefixes are used to show the smaller values.

Three prefixes (multipliers) are used, µ (micro), n (nano) and p (pico):

µ means 10-6 (millionth), so 1000000µF = 1F n means 10-9 (thousand-millionth), so 1000nF = 1µF p means 10-12 (million-millionth), so 1000pF = 1nF

There are many types of capacitor but they can be split into two groups, polarised and unpolarised. Each group has its own circuit symbol.

Polarised capacitors (large values, 1µF +)

Examples:        Circuit symbol:   

Electrolytic Capacitors

Electrolytic capacitors are polarised and they must be connected the correct way round, at least one of their leads will be marked + or -. They are not damaged by heat when soldering.

There are two designs of electrolytic capacitors; axial where the leads are attached to each end (220µF in picture) and radial where both leads are at the same end (10µF in picture). Radial capacitors tend to be a little smaller and they stand upright on the circuit board.

.

Unpolarised capacitors (small values, up to 1µF)

Examples:        Circuit symbol:   

Small value capacitors are unpolarised and may be connected either way round. They are not damaged by heat when soldering, except for one unusual type (polystyrene). They have high voltage ratings of at least 50V, usually 250V or so. It can be difficult to find the values of these small capacitors because there are many types of them and several different labelling systems!

Many small value capacitors have their value printed but without a multiplier, so you need to use experience to work out what the multiplier should be!

For example 0.1 means 0.1µF = 100nF.

Sometimes the multiplier is used in place of the decimal point: For example:   4n7 means 4.7nF.

Unpolarised capacitors (small values, up to 1µF)

Examples:        Circuit symbol:   

Small value capacitors are unpolarised and may be connected either way round. They are not damaged by heat when soldering, except for one unusual type (polystyrene). They have high voltage ratings of at least 50V, usually 250V or so. It can be difficult to find the values of these small capacitors because there are many types of them and several different labelling systems!

Capacitor Number Code

A number code is often used on small capacitors where printing is difficult: the 1st number is the 1st digit, the 2nd number is the 2nd digit, the 3rd number is the number of zeros to give the capacitance

in pF. Ignore any letters - they just indicate tolerance and voltage rating.

For example:   102   means 1000pF = 1nF   (not 102pF!)

For example:   472J means 4700pF = 4.7nF (J means 5% tolerance).

Capacitor Colour Code

A colour code was used on polyester capacitors for many years. It is now obsolete, but of course there are many still around. The colours should be read like the resistor code, the top three colour bands giving the value in pF. Ignore the 4th band (tolerance) and 5th band (voltage rating).

For example:

    brown, black, orange   means 10000pF = 10nF = 0.01µF.

For example:

    wide red, yellow   means 220nF = 0.22µF

Note that there are no gaps between the colour bands, so 2 identical bands actually

appear as a wide band.

Colour Code

Colour Number

Black 0

Brown 1

Red 2

Orange 3

Yellow 4

Green 5

Blue 6

Violet 7

Grey 8

White 9

TRANSFORMER

A transformer is an electrical device that transfers energy from one circuit to another by magnetic coupling with no moving parts. A transformer comprises two or more coupled windings, or a single tapped winding and, in most cases, a magnetic core to concentrate magnetic flux. A changing current in one winding creates a time-varying magnetic flux in the core, which induces a voltage in the other windings. Michael Faraday built the first transformer, although he used it only to demonstrate the principle of electromagnetic induction and did not foresee the use to which it would eventually be put.

Basic principles

Coupling by mutual induction

A simple transformer consists of two electrical conductors called the primary winding and the secondary winding. Energy is coupled between the windings by the time-varying magnetic flux that passes through (links) both primary and secondary windings. When the current in a coil is switched on or off or changed, a voltage is induced in a neighboring coil. The effect, called mutual inductance, is an example of electromagnetic induction.

Simplified analysis

Practical transformer showing magnetising flux in the core

If a time-varying voltage is applied to the primary winding of turns, a current will flow in it producing a magnetomotive force (MMF). Just as an electromotive force (EMF) drives current around an electric circuit, so MMF tries to drive magnetic flux through a magnetic circuit. The primary MMF produces a varying magnetic flux in the core, and, with an open circuit secondary winding, induces a back electromotive force (EMF) in opposition to . In accordance with Faraday's law of induction, the voltage induced across the primary winding is

proportional to the rate of change of flux:

     and     

where

vP and vS are the voltages across the primary winding and secondary winding, NP and NS are the numbers of turns in the primary winding and secondary winding, dΦP / dt and dΦS / dt are the derivatives of the flux with respect to time of the primary and secondary

windings.

Saying that the primary and secondary windings are perfectly coupled is equivalent to saying that . Substituting and solving for the voltages shows that:

    

where

vp and vs are voltages across primary and secondary,

Np and Ns are the numbers of turns in the primary and secondary , respectively.

Hence in an ideal transformer, the ratio of the primary and secondary voltages is equal to the ratio of the number of turns in their windings, or alternatively, the voltage per turn is the same for both windings. The ratio of the currents in the primary and secondary circuits is inversely proportional to the turns ratio. This leads to the most common use of the transformer: to convert electrical energy at one voltage to energy at a different voltage by means of windings with different numbers of turns. In a practical transformer, the higher-voltage winding will have more turns, of smaller conductor cross-section, than the lower-voltage windings.

Where

vP and vS are the voltages across the primary winding and secondary winding, NP and NS are the numbers of turns in the primary winding and secondary winding, dΦP / dt and dΦS / dt are the derivatives of the flux with respect to time of the primary and secondary

windings.

Saying that the primary and secondary windings are perfectly coupled is equivalent to saying that . Substituting and solving for the voltages shows that:

    

where

vp and vs are voltages across primary and secondary,

Np and Ns are the numbers of turns in the primary and secondary , respectively.

Hence in an ideal transformer, the ratio of the primary and secondary voltages is equal to the ratio of the number of turns in their windings, or alternatively, the voltage per turn is the same for both windings. The ratio of the currents in the primary and secondary circuits is inversely proportional to the turns ratio. This leads to the most common use of the transformer: to convert electrical energy at one voltage to energy at a different voltage by means of windings with different numbers of turns. In a practical transformer, the higher-voltage winding will have more turns, of smaller conductor cross-section, than the lower-voltage windings.

The universal EMF equation

If the flux in the core is sinusoidal, the relationship for either winding between its number of turns, voltage, magnetic flux density and core cross-sectional area is given by the universal emf equation (from Faraday's law):

where

E is the sinusoidal rms or root mean square voltage of the winding, f is the frequency in hertz, N is the number of turns of wire on the winding, a is the cross-sectional area of the core in square metres B is the peak magnetic flux density in teslas P is the power in volt amperes or watts,

Classifications

Transformers are adapted to numerous engineering applications and may be classified in many ways:

By power level (from fraction of a volt-ampere(VA) to over a thousand MVA), By application (power supply, impedance matching, circuit isolation), By frequency range (power, audio, radio frequency(RF)) By voltage class (a few volts to about 750 kilovolts) By cooling type (air cooled, oil filled, fan cooled, water cooled, etc.) By purpose (distribution, rectifier, arc furnace, amplifier output, etc.). By ratio of the number of turns in the coils

Step-up

The secondary has more turns than the primary. Step-down

The secondary has fewer turns than the primary. Isolating

Intended to transform from one voltage to the same voltage. The two coils have approximately equal numbers of turns, although often there is a slight difference in the number of turns, in order to compensate for losses (otherwise the output voltage would be a little less than, rather than the same as, the input voltage).

Variable

The primary and secondary have an adjustable number of turns which can be selected without reconnecting the transformer.

Circuit symbols

Standard symbols

Transformer with two windings and iron core.

Transformer with three windings.The dots show the relative winding configuration of the windings.

Step-down or step-up transformer.

The symbol shows which winding has more turns,

but does not usually show the exact ratio.

Transformer with electrostatic screen,which prevents capacitive coupling between the windings.

Losses

Transformer losses arise from:

Winding resistance Eddy currents Mechanical losses Hysteresis losses Stray losses Magnetostriction …etc.

Transformer types Autotransformers Polyphase transformers Resonant transformers Instrument transformers

Current transformers Voltage transformers

Pulse transformers

Uses of transformers

For supplying power from an alternating current power grid to equipment which uses a different voltage.

Electric power transmission over long distances. Large, specially constructed power transformers are used for electric arc furnaces used in steelmaking. Rotating transformers are designed so that one winding turns while the other remains stationary. A

common use was the video head system as used in VHS and Beta video tape players. These can pass power or radio signals from a stationary mounting to a rotating mechanism, or radar antenna.

Sliding transformers can pass power or signals from a stationary mounting to a moving part such as a machine tool head.

RELAYS

A relay is an electrical switch that opens and closes under control of another electrical circuit. In the original form, the switch is operated by an electromagnet to open or close one or many sets of contacts. It was invented by Joseph Henry in 1835. Because a relay is able to control an output circuit of higher power than the input

circuit, it can be considered, in a broad sense, to be a form of electrical amplifier.

Operation

When a current flows through the coil, the resulting magnetic field attracts an armature that is mechanically linked to a moving contact. The movement either makes or breaks a connection with a fixed contact. When the current to the coil is switched off, the armature is returned by a force that is half as strong as the magnetic force to its relaxed position. Usually this is a spring, but gravity is also used commonly in industrial motor starters. Relays are manufactured to operate quickly. In a low voltage application, this is to reduce noise. In a high voltage or high current application, this is to reduce arcing.

If the coil is energized with DC, a diode is frequently installed across the coil, to dissipate the energy from the collapsing magnetic field at deactivation, which would otherwise generate a spike of voltage and might cause damage to circuit components. If the coil is designed to be energized with AC, a small copper ring can be crimped to the end of the solenoid. This "shading ring" creates a small out-of-phase current, which increases the minimum pull on the armature during the AC cycle. [1]

The contacts can be either Normally Open (NO), Normally Closed (NC), or change-over contacts.

Normally-open contacts connect the circuit when the relay is activated; the circuit is disconnected when the relay is inactive. It is also called Form A contact or "make" contact. Form A contact is ideal for applications that require to switch a high-current power source from a remote device.

Normally-closed contacts disconnect the circuit when the relay is activated; the circuit is connected when the relay is inactive. It is also called Form B contact or "break" contact. Form B contact is ideal for applications that require the circuit to remain closed until the relay is activated.

Change-over contacts control two circuits: one normally-open contact and one normally-closed contact with a common terminal. It is also called Form C contact or "transfer" contact.

By analogy with the functions of the original electromagnetic device, a solid-state relay is made with a thyristor or other solid-state switching device. To achieve electrical isolation, a light-emitting diode (LED) is used with a photo transistor.

Types of relay

Small relay as used in electronics

A solid state relay, which has no moving parts latching relays are available that have two relaxed states (bistable). These are also called 'keep' relays.

When the current is switched off, the relay remains in its last state. This is achieved with a solenoid operating a ratchet and cam mechanism, or by having two opposing coils with an over-center spring or permanent magnet to hold the armature and contacts in position while the coil is relaxed, or with a remnant core. In the ratchet and cam example, the first pulse to the coil turns the relay on and the second pulse turns it off. In the two coil example, a pulse to one coil turns the relay on and a pulse to the opposite coil turns the relay off. This type of relay has the advantage that it consumes power only for an instant, while it is being switched, and it retains its last setting across a power outage.

A reed relay has a set of, usually normally open, contacts inside a vacuum or inert gas filled glass tube. This protects the contacts against atmospheric corrosion. The two contacts are closed by magnetism from a coil around the glass tube. See also reed switch.

A contactor is a very heavy-duty relay used for switching electric motors and lighting loads. With high current, the contacts are made with pure silver. The unavoidable arcing causes the contacts to oxidize and silver oxide is still a good conductor. Such devices are often used for motor starters. A motor starter is a contactor with an overload protection devices attached. The overload sensing devices are a form of heat operated relay where a coil heats a bi-metal strip, or where a solder pot melts, releasing a spring to operate auxiliary contacts. These auxiliary contacts are in series with the coil. If the overload senses exess current in the load, the coil is de-energized.

A Buchholz relay is a safety device sensing the accumulation of gas in large oil-filled transformers, which will alarm on slow accumulation of gas or shut down the transformer if gas is produced rapidly in the transformer oil.

A solid state relay (SSR) is a solid state electronic component that provides a similar function to an electromechanical relay but does not have any moving components, increasing long-term reliability.

With early SSR's, the tradeoff came from the fact that every transistor has a small voltage drop across it. This collective voltage drop limited the amount of current a given SSR could handle. As transistors improved, higher current SSR's, able to handle 100 to 1,200 amps, have become commercially available.

Circuit symbols of relays. "C" denotes the common terminal in SPDT and DPDT types.

Since relays are switches, the terminology applied to switches is also applied to relays. According to this classification, relays can be of the following types:

SPST - Single Pole Single Throw. These have two terminals which can be switched on/off. In total, four terminals when the coil is also included.

SPDT - Single Pole Double Throw. These have one row of three terminals. One terminal (common) switches between the other two poles. It is the same as a single change-over switch. In total, five terminals when the coil is also included.

DPST - Double Pole Single Throw. These have two pairs of terminals. Equivalent to two SPST switches or relays actuated by a single coil. In total, six terminals when the coil is also included. This configuration may also be referred to as DPNO.

DPDT - Double Pole Double Throw. These have two rows of change-over terminals. Equivalent to two SPDT switches or relays actuated by a single coil. In total, eight terminals when the coil is also included.

QPDT - Quadruple Pole Double Throw. Often referred to as Quad Pole Double Throw, or 4PDT. These have four rows of change-over terminals. Equivalent to four SPDT switches or relays actuated by a single coil or two DPDT relays. In total, fourteen terminals when the coil is also included.

Applications

Relays are used:

to control a high-voltage circuit with a low-voltage signal, as in some types of modems, to control a high-current circuit with a low-current signal, as in the starter solenoid of an automobile, to detect and isolate faults on transmission and distribution lines by opening and closing circuit breakers

(protection relays), to isolate the controlling circuit from the controlled circuit when the two are at different potentials, for

example when controlling a mains-powered device from a low-voltage switch. The latter is often applied to control office lighting as the low voltage wires are easily installed in partitions, which may be often moved as needs change. They may also be controlled by room occupancy detectors in an effort to conserve energy,

to perform logic functions. For example, the boolean AND function is realised by connecting NO relay contacts in series, the OR function by connecting NO contacts in parallel. The change-over or Form C contacts perform the XOR (exclusive or) function. Similar functions for NAND and NOR are accomplished using NC contacts. Due to the failure modes of a relay compared with a semiconductor, they are widely used in safety critical logic, such as the control panels of radioactive waste handling machinery.

to perform time delay functions. Relays can be modified to delay opening or delay closing a set of contacts. A very short (a fraction of a second) delay would use a copper disk between the armature and moving blade assembly. Current flowing in the disk maintains magnetic field for a short time, lengthening release time. For a slightly longer (up to a minute) delay, a dashpot is used. A dashpot is a piston filled with fluid that is allowed to escape slowly. The time period can be varied by increasing or decreasing the flow rate. For longer time periods, a mechanical clockwork timer is installed.

VOLTAGE REGULATORS

A voltage regulator is an electrical regulator designed to automatically maintain a constant voltage level.

It may use an electromechanical mechanism, or passive or active electronic components. Depending on the design, it may be used to regulate one or more AC or DC voltages.

With the exception of shunt regulators, all voltage regulators operate by comparing the actual output voltage to some internal fixed reference voltage. Any difference is amplified and used to control the regulation element. This forms a negative feedback servo control loop. If the output voltage is too low, the regulation element is commanded to produce a higher voltage. If the output voltage is too high, the regulation element is commanded to produce a lower voltage. In this way, the output voltage is held roughly constant. The control loop must be carefully designed to produce the desired tradeoff between stability and speed of response.

Electromechanical regulators

Early automobile generators and alternators had a mechanical voltage regulator using one, two, or three relays and various resistors to stabilize the generator's output at slightly more than 6 or 12 V, independent of the engine's rpm or the varying load on the vehicle's electrical system. Essenially, the relay(s) employed pulse width modulation to regulate the output of the generator, controlling the field current reaching the generator (or alternator) and in this way controlling the output voltage produced.

The regulators used for generators (but not alternators) also disconnected the generator when it was not producing electricity, thereby preventing the battery from discharging back through the stopped generator. The rectifier diodes in an alternator automatically performed this function so that a specific relay was not required; this appreciably simplified the regulator design.

More modern designs now use solid state technology (transistors) to perform the same function as the relays performed in electromechanical regulators.

Mains regulators

Electromechanical regulators have also been used to regulate the voltage on AC power distribution lines. These regulators generally operate by selecting the appropriate tap on a transformer with multiple taps. If the output voltage is too low, the tap changer switches connections to produce a higher voltage. If the output voltage is too high, the tap changer switches connections to produce a lower voltage. The controls provide a deadband wherein the controller will not act, preventing the controller from constantly hunting (constantly adjusting the voltage) to reach the desired target voltage.

AC voltage stabilizers

A voltage stabilizer is a type of household mains regulator which uses a continuously variable autotransformer to maintain an AC output that is as close to the standard or normal mains voltage as possible, under conditions of fluctuation. It uses a servomechanism (or negative feedback) to control the position of the tap (or wiper) of the autotransformer, usually with a motor. An increase in the mains voltage causes the output to increase, which in turn causes the tap (or wiper) to move in the direction that reduces the output towards the nominal voltage.

An alternative method is the use of a type of saturating transformer called a ferroresonant transformer. These

transformers use a tank circuit composed of a high-voltage resonant winding and a capacitor to produce a nearly constant average output with a varying input. The ferroresonant approach is attractive due to its lack of active components, relying on the square loop saturation characteristics of the tank circuit to absorb variations in average input voltage. Older designs of ferroresonant transformers had an output with high harmonic content, leading to a distorted output waveform. Modern devices are used to construct a perfect sinewave .The ferroresonant action is a flux limiter rather than a voltage regulator, but with a fixed supply frequency it can maintain an almost constant average output voltage even as the input voltage varies widely.

DC voltage stabilizers

Many simple DC power supplies regulate the voltage using a shunt regulator such as a zener diode, avalanche breakdown diode, or voltage regulator tube. Each of these devices begins conducting at a specified voltage and will conduct as much current as required to hold its terminal voltage to that specified voltage. The power supply is designed to only supply a maximum amount of current that is within the safe operarating capability of the shunt regulating device (commonly, by using a series resistor). In shunt regulators, the voltage reference is also the regulating device.

If the stabilizer must provide more power, the shunt regulator output is only used to provide the standard voltage reference for the electronic device, known as the voltage stabilizer. The voltage stabilizer is the electronic device, able to deliver much larger currents on demand.

Active regulators

Because they (essentially) dump the excess current not needed by the load, shunt regulators are inefficient and only used for low-power loads. When more power must be supplied, more sophisticated circuits are used. In general, these can be divided into several classes:

Linear regulators ,Switching regulators ,SCR regulators

Linear regulators

Linear regulators are based on devices that operate in their linear region (in contrast, a switching regulator is based on a device forced to act as an on/off switch). In the past, one or more vacuum tubes were commonly used as the variable resistance. Modern designs use one or more transistors instead. Linear designs have the advantage of very "clean" output with little noise introduced into their DC output.

Entire linear regulators are available as integrated circuits. These chips come in either fixed or variable voltage

types.

Switching regulators

Switching regulators rapidly switch a series device on and off. The duty cycle of the switch sets how much charge is transferred to the load. This is controlled by a similar feedback mechanism as in a linear regulator. Because the series element is either fully conducting, or switched off, it dissipates almost no power; this is what gives the switching design its efficiency. Switching regulators are also able to generate output voltages which are higher than the input, or of opposite polarity - something not possible with a linear design.

Like linear regulators, nearly-complete switching regulators are also available as integrated circuits. Unlike linear regulators, these usually require one external component: an inductor that acts as the energy storage element. (Unfortunately, the inductor must be external because large-valued inductors tend to be physically large relative to almost all other kinds of componentry; because of this, they are impossible to fabricate within integrated circuits.)

Comparing linear vs. switching regulators

Sometimes only one or the other will work:

Linear regulators are best when low output noise is required Linear regulators are best when a fast response to input and output disturbances is required. Switching regulators are best when power efficiency is critical (such as in portable computers). Switching regulators are required when the only power supply is a DC voltage, and a higher output

voltage is required.

SCR regulators

Regulators powered from AC power circuits can use silicon controlled rectifiers (SCRs) as the series device. Whenever the output voltage is below the desired value, the SCR is triggered, allowing electricity to flow into the load until the AC mains voltage passes through zero (ending the half cycle). SCR regulators have the advantages of being both very efficient and very simple, but because they can not terminate an on-going half cycle of conduction, they are not capable of very accurate voltage regulation in response to rapidly-changing loads.

Combination (hybrid) regulators

Many power supplies use more than one regulation method in series. For example, the output from a switching regulator can be further regulated by a linear regulator. The switching regulator accepts a wide range of input voltages and efficiently generates a (somewhat noisy) voltage slightly above the ultimately desired output. That is followed by a linear regulator that generates exactly the desired voltage and eliminates nearly all the noise generated by the switching regulator. Other designs may use an SCR regulator as the "pre-regulator", followed

by another type of regulator.

COMPARATORS

In electronics, a comparator is a device which compares two voltages or currents and switches its output to indicate which is larger. More generally, the term is also used to refer to a device that compares two items of data.

A standard op-amp without negative feedback can be used as a comparator, as indicated in the following diagram.

When the non-inverting input (V+) is at a higher voltage than the inverting input (V-), the high gain of the op-amp causes it to output the most positive voltage it can. When

the non-inverting input (V+) drops below the inverting input (V-), the op-amp outputs the most negative voltage it can. Since the output voltage is limited by the supply voltage, for an op-amp that uses a balanced, split supply, (powered by ± VS) this action can be written:

Vout = VS sgn(V+ − V−)

where sgn(x) is the signum function. Generally, the positive and negative supplies VS will not match absolute value:

Vout <= VS+ when (V+ > V-) else VS- when (V+ < V-).

Equality of input values is very difficult to achieve in practice. The speed at which the change in output results from a change in input (often called the slew rate in operational amplifiers) is typically in the order of 10ns to 100ns, but can be as slow as a few tens of μs.

A dedicated voltage comparator will generally be faster than a general-purpose op-amp pressed into service as a comparator. A dedicated voltage comparator may also contain additional features such as an accurate, internal voltage reference and adjustable hysteresis. It is incorrect to consider a comparator as a device with a differential (bipolar) input and a logic (0/Vcc) output as the inputs of real comparators are not isolated. This means that not only their difference affects the output but also their voltages must not exceed the power voltage range: VS- ≤ V+,V- ≤ VS+. In the case of TTL/CMOS logic output comparators negative inputs are not allowed: 0 ≤ V+,V- ≤ Vcc. When comparing a noisy signal to a threshold, the comparator may switch rapidly from state to state as the signal crosses the threshold. If this is unwanted, a Schmitt trigger can be used to provide hysteresis and a cleaner output signal

DETAILED DESCRIPTION OF

OPERATION OF CIRCUIT

At the heart of this heat-sensitive switch is IC LM35 (IC1), which is linear temperature sensor and linear temperature-to-voltage converter circuit.

The input and ground pins of this heat-to-voltage converter IC are connected across the regulated power supply rails and decoupled by R1 and C1. Its temperature-tracking output is applied to the non-inverting input (pin 3) of the comparator built around IC2. The inverting input (pin 2) of IC2 is connected across

the positive supply rails via a voltage divider network formed by potmeter VR1.

Since the wiper of potmeter VR1 is connected to the inverting input of IC2. The voltage presented to this pin is linearly variable. This voltage is used as a reference level for the comparator against the output

supplied by IC1.

So if the non-inverting input of IC2 receives a voltage lower than the set level, its output goes low(approximately 650mV). This low level is applied to the input of load-relay driver comprising npn

transistors T1 and T2. The low level presented at the base of the transistor T1 keeps it non-conductive. Since T2 receives forward bias voltage via the emitter of T1, it is also kept non-conductive. Hence, relay RL1 is de-energised state, keeping mains supply to the load ‘off’ as long as the temperature at sensor is

low.

Conversely, if the non-inverting input receives a voltage higher than the set level, its output goes high (approximately 220mV) and the load is turned ‘on’. This happens when IC1 is at a higher temperature

and its output voltage also higher than the set level at the inverting input of IC2. so the load is turned on as soon as the ambient temperature rises above the set level. Capacitor C3 at this pin helps iron out any

ripple that passes through the positive supply rail to avoid errors in the circuit operation.

By adjusting potmeter VR1 and thereby varying the reference voltage level at the inverting input pin pf IC1, the temperature threshold at which energisation of the relay is required can be set. As this setting is

linear, the knob of potmeter VR1 can be provided with linear dial calibrated in degrees centigrade. Therefore any temperature level can be selected and constantly monitored for external actions like

turning on a room-heater in winter or a room-cooler in summer. The circuit can also be used to activate emergency fire extinguishers, if positioned at the probable fire accident site.

The circuit can be modified to operate any electrical appliance. In that case, relay RL1 must be heavy-duty type with appropriately rate contacts to match the power demands of the load to be operated.