INTRODUCTION: This is a standard circuit which can be used to adjust the brightness of mains lights and the speed of AC motors. It uses a triac, diac and has a radio-frequency interference (RFI) noise suppression circuit built into it as well. The circuit controls the average power to a load through the triac by phase control. The AC supply is applied to the load for only a controlled fraction of each cycle. The triac is held in an OFF condition for a portion of its cycle then is triggered ON at a time determined by the circuit.

Each time the triac is turned on, the load current changes very quickly – a few micro seconds – from zero to a value determined by the lamp resistance and the value of the mains voltage at that instant in time. This transition generates Radio Frequency Interference. It is greatest when the triac is triggered at 90 degree and least when it is triggered at close to zero or 180 degree of the mains AC waveform. L-C suppression network is thus used to suppress these electrical noises. HISTORY : Light dimming is based on adjusting the voltage which gets to the lamp.

Light dimming has been possible for many decades by using adjustable power resistors and adjustable transformers. Those methods have been used in movie theatres, stages and other public places. The problem of those light controlling methods have been that they are big, expensive, have poor efficiency and they are hard to control from remote location. The power electronics have proceeded quickly since 1960. Between 1960-1970 thyristors and triacs came to market. Using those components it was quite easy to make small and inexpensive light dimmers which have good efficiency.

Electronics controlling also made possible to make them easily controllable from remote location. These types of electronic light dimmers became available after 1970 and are nowadays used in very many locations like homes, restaurants, conference rooms and in stage lighting WHAT ISADIMMER? Various definitions for a dimmer are as follows: 1. In lighting, the electrical device (technically known as a potentiometer) that regulates the current passing through the bulb filaments and, thereby, the amount of light emitted from the lighting instruments. 2.

Electronic controls that allow stage lighting to fade up or down slowly, as opposed to being on or off only. 3. A control that regulates light levels. 4. Device which provides adjustable voltage to a lighting fixture to control light output. It can also be a term to refer to a mechanical device, such as a shutter, that controls output. Dimmer Types : Older lighting controls used either resistors or large variable transformers called auto-transformers; these units had hand wheels mounted on top that needed to be turned in order to adjust the level of output.

Needless to say these devices were very inefficient, large and expensive. Modern Triac or Thyristor based dimmers are either ON or OFF and hence dissipate very little heat. A typical voltage drop from these units when the power is transferred to the load is only a few volts with most of this being lost through the electrical noise suppression part of the circuit. The power devices have 3 connections a) Power in b) Power out c) Gate. The units conduct when a low power control signal is provided to the gate HOW DOES LIGHT DIMMER WORK? A light dimmer works by essentially chopping parts out of the AC voltage.

This allows only parts of the waveform to pass to the lamp. The brightness of the lamp is determined by the power transferred to it, so the more the waveform is chopped, the more it dims. Mains power is comprised of an alternating current that flows in one direction and then in the other, along the cable, at the rate of 50 or 60 cycles per second (known as Hertz). The value 50 or60Hz is dependent on the countries power system. The current alternates back and forth changing direction at the zero point. If we were to look at this waveform it would appear as a stretched S shape on its side ~.

Draw a line through the middle and this is what is called the zero crossing point. At this instant in time no current is flowing in either direction. This is the point at which a dimmer is electronically synchronized to turn the power ON or OFF. By chopping the waveform at the zero-crossing point, smooth dimming can be achieved without the lamp flickering. This turning on and off of the power device occurs every time the mains crossing point is reached (half phase), 100 or 120 times per second. Typically light dimmers are manufactured using a Triac or Thyristor as the power control device.

These electronic parts are semiconductors not dissimilar to transistors. A Thyristor is a Uni. -directional device and hence, because AC power flows in both directions, two are needed. A triac is a bidirectional device and therefore only one is needed. An electronic circuit determines the point in time at which they turn ON (conduct). The ON state continues until the next zero-crossing point, at which point the device turns itself OFF. The electronic circuit then provides a delay, which equates to the dimness of the lamp, before turning the control device back on.

The slight capacitance of the load, filters the chopped waveform resulting in a smooth light output. Some controllers use a microprocessor control with the above timing function being handled by an analogue circuit. More sophisticated systems, called digital dimmers, operate the switching direct from microprocessor. This has the advantage of greater reliability, quieter operation, lower cost and smaller controls. Below is a typical picture of the mains sine wave, and a phase-controlled waveform [pic] COMPONENTS CIRCUIT DESCRIPTION TRIAC: Triac are widely used in AC power control applications.

They are able to switch high voltages and high levels of current, and over both parts of an AC waveform. This makes triac circuits ideal for use in a variety of applications where power switching is needed. One particular use of triac circuits is in light dimmers for domestic lighting, and they are also used in many other power control situations including motor control. The triac is a development of the thyristor. While the thyristor can only control current over one half of the cycle, the triac controls it over two halves of an AC waveform.

A triac may be considered as two SCR’s (Silicon Controlled Rectifiers) connected in opposite directions. A triac is a 3 terminal ac semiconductor switch which is triggered ON when a low energy signal is applied to its Gate. Switching is fast. The low energy of switching means that a wide range of low cost control circuits can be used, for example, optically coupled switches. There is a gate which acts as a trigger to turn the device on. In addition to this the other terminals are both called Anodes, and Main Terminals. Since the triac is bilateral the terms anode and cathode have no meaning.

So the terms Main Terminal 1 and 2 (MT1, MT2) are used. Both MT1 and MT2 have very similar properties. It is standard to use MT1 as a reference. Triac symbol Triac symbol for use in circuit diagrams How does a triac work? Equivalent circuit of a triac. When the voltage on the MT1 is positive with regard to MT2 and a positive gate voltage is applied, one of the thyristors conducts. When the voltage is reversed and a negative voltage is applied to the gate, the other thyristor conducts. This is provided that there is sufficient voltage across the device to enable a minimum holding current to flow 3 DIAC The DIAC, or ‘diode for alternating current’, is a trigger diode that conducts current only after its breakdown voltage has been exceeded momentarily. When this occurs, the resistance of the diode abruptly decreases, leading to a sharp decrease in the voltage drop across the diode and, usually, a sharp increase in current flow  through  the diode. The diode remains “in conduction” until the current flow through it drops below a value characteristic for the device, called the holding current. Below this value, the diode switches back to its high-resistance (non-conducting) state.

This behavior is bidirectional, meaning typically the same for both directions of current flow. It is found that because of their internal construction and the slight differences between the two halves, triacs do not fire symmetrically. This results in harmonics being generated: the less symmetrical the triac fires, the greater the level of harmonics that are produced. It is not normally desirable to have high levels of harmonics in a power system and as a result triacs are not favoured for high power systems. Instead for these systems two thyristors may be used as it is easier to control their firing.

To help in overcoming the problem non-symmetrical firing ad the resulting harmonics, a device known as a diac (diode AC switch)is often placed in series with the gate of the triac. The inclusion of this device helps make the switching more even for both halves of the cycle. This results from the fact that the diac switching characteristic is far more even than that of the triac. Since the diac prevents any gate current flowing until the trigger voltage has reached a certain voltage in either direction, this makes the firing point of the triac more even in both directions.

Working of Standard Light Dimmers using Triac : The following circuit is of a light dimmer circuit using triac. The circuit is a quite typical TRIAC based dimmer circuit with no fancy special features. The triggering circuit is a little bit improved compared to the 120V AC circuit. This circuit is only designed to operate with non-inductive loads like standard light bulbs. The circuit is designed to dim light bulbs in 50-1000W range. Component list: C1 0. 01µF C2 0. 47µF D1 diac R variable resistor 47KY R1 50Y +/- 0. 05% TH1 bt136 500E

The circuit shows a triac firing circuit employing a diac. In this circuit, resistor R is variable whereas resistor R1 limits the current in the diac and triac gate when diac turns on, the value of C and potentiometer R are so selected s to give a firing angle range of0to 180 degrees. In practice however, a firing angle of 0 to 170degrees is only possible. Variable resistor R controls the charging time of the capacitor C and therefore the firing angle of the triac. When R is small, the charging time constant, equal to RC is small.

Therefore, source voltage charges capacitor C to diac trigger voltage earlier and firing angle will be small. Likewise, when R is high, firing angle of triac is high. When capacitor C with upper plate positive charges to breakdown voltage Vdt of diac, diac turns on. As a consequence, capacitor discharges rapidly thereby applying capacitor voltage Vc in the form of pulse across the triac gate o turn it on. After triac turns on at firing angle o, source voltage Vs appears across the load during positive half cycle for (n-o) radians. When Vs becomes zero at wt=n triac turns off.

After wt=n Vs becomes negative,he capacitor C now charges with lower plate positive. When Vc reaches Vdt of diac, diac and triac turn on and Vs appears across the load during the negative half cycle for (n-o) radians. At wt=2n the process repeats itself. Thos circuit is commercially use for controlling the power in lamp dimmers, heat converters, speed control of fans etc. Radio Frequency Interference The modern thyristor (Triac or SCR) dimmer has one fairly severe drawback in its performance in that it dims by switching on the current to the load part-way through each mains cycle.

Cutting the leading smooth-part off a mains cycle produces a current with a very rapid turn-on time which generates both mains distortions and EMI. Chokes are included in dimmers to slow down the rapid switch-on (rise time) of the chopped current. The longer the rise time the less EMI and mains distortion produced. Turn on of the triac in the middle of the phase causes fast voltage and current changes. A typical thyristor/triac starts to fully conduct at around1 microsecond time after triggering, so the current change is very fast if it not limited in any way.

Those fast voltage and current changes cause high frequency interference going to mains wiring unless there are suitable radio frequency interference (RFI) filter built into the circuit. The corners in the waveform effectively consist of 50/60Hz plus varying amounts of other frequencies that are multiples of 50/60Hz. In some cases the interference goes up to 1-10 Mhz frequencies and even higher. The wiring in your house acts as an antenna and essentially broadcasts it into the air. Cheap bad quality light dimmers don’t have adequate filtering and they cause easily lots of radio interference.

Dimmer circuits typically use coils that limit limit the rate of rise of current to that value which would result in acceptable EMI. Typical filtering in light dimmers causes the current rise time(current rises from 10% to 90%) to be in range of 30-50microseconds. This gives acceptable results in typical dimmer applications in home (typically this limitation is made using 40-100uH coil). If the dimmers are used in places where dimmer is a serious problem for sensitive sound equipments (theatres, TV-studios, rock concerts etc. ) a slower current rise time would be preferred.

Typically the current rise time in light dimmer packs made for stage applications have a current rise speed of around 100-350 microseconds. If noise is a big problem (TV studios etc. ), even slower current rise times are sometimes asked. Those current rise times up to 1 millisecond can be achieved with special dimmers or suitable extra coil fitted in series with the dimmer. The coil itself does not typically solve the whole problem because of the self-capacitance of the inductor: they typically resonate below 200 kHz and look like capacitors to disturbances above the resonance frequency.

That’s why there must be also capacitors to suppress the interference at higher frequencies. If your dimmer circuit cause interference, you can try to filter out the interference by adding a small capacitor (typically 22nF to 47 nF) in parallel with the dimmer circuit as near as possible to the electronics inside the circuit as possible  BUZZING PROBLEM: A Lamp A consists of fine coils of wire held in place by a series of supports. When the current flow abruptly changes the magnetism change can be stronger than when supplied with a simple sine wave.

If the dimmer has inadequate suppression the filaments will vibrate against the support posts (this is called lamp singing). Hence, the filaments of the bulb will tend to vibrate more with a dimmer chopping up the wave form, and when the filaments vibrate against their support posts, you will get a buzz. A good quality dimmer will filter the abrupt current changing cause buzzing. This is known as EMI reduction. Changing the lamp brand to a higher quality one may also help to reduce lamp sing. In very high power dimming systems the wiring going to lighting can also cause buzzing.

The fast current makes the electrical wiring to vibrate a little bit and if the wire is installed so that the vibration can be transferred to some other material then the buzzing could be heard. Better filtered dimmers can reduce the problem because the filter makes the current changes slower so the wires make less noise. Power harmonics caused by dimmers All phase control dimmers are non-linear loads. A non-linear load is one where current is not in proportion to voltage. The non-linear load on dimming systems is caused by the fact that current is switched on for only part of the line cycle by a phase control dimming system.

This non-linear load creates harmonic distortion on the service feeder. Harmonics are currents that occur at multiples of the power line voltage frequency. When load currents are non-linear and have substantial harmonic content, they cause considerably more heating than the same undistorted current. In heavily dimmed system, you might not be able to utilize more than around 70 % of the rated transformer power rating because of harmonic induced heating have to worry much about the harmonics and transformer loads, because the light load of few hundred watts is clearly just a small fraction of the total transformer loads Advantage of Dimmers:

We have dealt with simple knob operated in wall dimmers but the more sophisticated types are programmable. These dimmers has following advantages : Convenience & Ambience Intelligent lighting forms part of home automation where the circuit levels are pre-programmed according to use and according to other factors such as time of day. Light fittings can be controlled individually or grouped together in circuits. Each circuit or fitting can be set to be at a different level of brightness. These levels are then stored as a “scene” which can best be though of as being a complete look of a room.

Some systems have 10 or more programmable scenes. Once set up scenes can be easily recalled manually from touch screen, switch panels, infra-red or wireless remote controls. Be recalled automatically by time clock, or according to occupancy. Once a new scene is selected the lighting will fade to the new set of levels at a pre-determined fade rate. Energy Saving :When dimming a lamp the energy saved is as high as 98% of the proportion of unused energy. Because the human eye perceive slight non-linearly, it is possible to reduce light levels by over 10%before the reduction in brightness is noticed.

This would lead to an ear 10% saving in energy consumption. A 50% reduction in dimming levels would save around 40% of the energy Intelligent dimmers ramp or fade a lamp to a preset level. By fading the lamp to the set level, also know as “soft start”, a lamps life is extended considerably. Security: Lighting can play an important part in security, deterring intruders whether the property is occupied or not. Low levels of illumination can be programmed to operate at night in certain rooms or hallways. When the house is vacated lighting levels can be selected that copy normal usage.

This can be by time clock or by selecting a vacation mode. Dimmed or selectively switched levels of illumination will save energy and is more effective than leaving lighting on or using simple plug in timers Application: 1. Lamp dimmers 2. Heat convertors 3. Speed control of fans 4. Dimming electric motors(inductive loads) 5. Dimming switching power supplies Safety issues on building the circuits: Because light dimmers are directly connected to mains you must make sure that no part of the circuit can be touched when it is operating. This can be best dealt by building the dimmer circuit to small plastic box.

Remember to use potentiometer with plastic shaft and install it so that no potentiometer metal parts are exposed to user. Remember to make circuit board so that the traces have enough current carrying capacity for the maximum load. Make sure that you have enough separation between PCB traces to withstand mains voltage. Make sure that all components can handle the voltages they face in the circuit. For 230V operation use at least 400V triac (600V better). The capacitor which is connected between the dimmer circuit mains wires should be a capacitor which is rated for this kind of applications (those are marked with letter X on the case).

Use capacitors with enough high voltage rating. Make sure that the TRIAC has enough ventilation so that it does not overheat at full load. Summary: The operation of the triac light dimmer circuit has been demonstrated. Using simple devices such as a diac and a triac, power flow is regulated to a light bulb by intermittently applying a 70AC source across the load during each half cycle. The intensity of the light is determined by the proportion of the sine wave that is applied across the load Prior to being triggered, the triac provides a barrier in the circuit, preventing current flow from a 70V AC source through the light bulb.

During this time voltage across a capacitor within in the circuit builds up until it exceeds the break over voltage of a diac. Once the break over voltage is exceeded, the diac ? fires? the triac into a conducting state and current flows through the light bulb. The amount of voltage seen over the light bulb is determined by the firing angle of the triac which is set by the RC time constant of the circuit. This process then repeats every half cycle.

The one main drawback of this circuit is the vast amount of harmonics that are present in the system, however for its simplicity, the design is quite impressive BIBLIOGRAPHY: • Reference book: POWER ELECTRONICS-Dr. P. S. Bhimbra • Power electronics- M. H. Rashid • http://howstuffworks. com • http://doityourself. com • http://sprags. com Thermistors are thermally sensitive resistors whose prime function is to exhibit a large, predictable and precise change in electrical resistance when subjected to a corresponding change in body temperature.

Negative Temperature Coefficient (NTC) thermistors exhibit a decrease in electrical resistance when subjected to an increase in body temperature and Positive Temperature Coefficient (PTC) thermistors exhibit an increase in electrical resistance when subjected to an increase in body temperature. U. S. Sensor produces thermistors capable of operating over the temperature range of -100 ° to over +600 ° Fahrenheit. Because of their very predictable characteristics and their excellent long term stability, thermistors are generally accepted to be the most advantageous sensor for many applications including temperature measurement and control.

Since the negative temperature coefficient of silver sulphide was first observed by Michael Faraday in 1833, there has been a continual improvement in thermistor technology. The most important characteristic of a thermistor is, without question, its extremely high temperature coefficient of resistance. Modern thermistor technology results in the production of devices with extremely precise resistance versus temperature characteristics, making them the most advantageous sensor for a wide variety of applications.

A thermistor’s change in electrical resistance due to a corresponding temperature change is evident whether the thermistor’s body temperature is changed as a result of conduction or radiation from the surrounding environment or due to “self heating” brought about by power dissipation within the device. When a thermistor is used in a circuit where the power dissipated within the device is not sufficient to cause “self heating”, the thermistor’s body temperature will follow that of the environment.

Thermistors are not “self heated” for use in applications such as temperature measurement, temperature control or temperature compensation. When a thermistor is used in a circuit where the power dissipated within the device is sufficient to cause “self heating”, the thermistor’s body temperature will be dependent upon the thermal conductivity of its environment as well as its temperature. Thermistors are “self heated” for use in application such as liquid level detection, air flow detection and thermal conductivity measurement.

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