Shree Swami Atmanand Saraswati Institute Of Technology Kathiriya Krishna Kheni Vaidehi Submitted to Prof.Pratik Desai Prof.Hetal mem 6/6/20161 DEPT. OF ELECTRICAL ENGINEERING SSASIT, SURAT

Thermistor 26/6/2016

Basic characteristics of thermistors Thermistor are also temperature dependent resistors.They are made of semiconductor materials which have a negative temperature coefficient of resistivity. The variation of resistance with changes in temperature is nonlinear. Thermistors can be used to measure temperatures in the range of -100c to 300 c. The resistance decreases with increase in temperature, hence this is called as the negative temperature coefficient thermistor(NTC) Zero power resistance of thermistor β R=R 0 exp[ β (1/T-1/T 0 )] T, T 0 are ambient temperatures, R, R 0 are corresponding resistances b is Characteristics temperature Since thermistor is a resistor, power dissipation P=C(T 2 -T 1 ), where C is the thermal dissipation constant (mW/ºC). This causes self-heating. 36/6/2016

R-T characteristics of thermistor A common 10kOhm NTC thermistor It is nonlinear!! It is nonlinear!! Temperature goes up  more charges in semiconductor  resistance goes down! (NTC) Temperature goes up  more charges in semiconductor  resistance goes down! (NTC) 46/6/2016

Construction Thermistor are constructed using the material such as sintered mixtures of metalic oxides of Mn,Ni,Co,Fe,Cu etc. Thermistor are available in various configuration such as small beads,disks,rods,etc. The bead type thermistors have a diameter of mm to 1.25mm and are smallest in size. The beads may be sealed in the tips of solid glass glass roads to form probes that are easier to mount. Glass probes have diameter of approximately 2.5mm and their length varies from 2.5mm to25mm. The discs are made by pressing thermistor material under high pressure in to cylindrical flat shapes with diameter varing from 2.5mm to 25mm. 56/6/2016

bead thermistors bead thermistors 66/6/2016

Advantages Disvantanges They are suitable for temperature measurement, temperature control and temperature compensation. High Resolution Small size,compact,rugged and inexensive. T emperature versus resistance characteristic is non linear. Not suitable for wide range operation Low excitation current to avoid self heating. Needs external dc power supply for its operation. 76/6/2016

Potentiometer 8

Construction The resolution of the potentiometer influences the construction of their resistor elements. Normally the resistive element is a single wire of conducting material which gives a continuous step less variation of resistance as the wiper travels over it. Such potentiometer are available but their length restrict their use on account of space considerations. The resolution of potentiometer is dependent upon the construction of the resistive element and in order to get high value of resistance in a small wire wound potentiometer are used extensively. The resistance wire is wound on a mandrel or a cart for translational displacement. For the measurement of rotational motion these mandrels or cards are formed in to a circle or a helix. 96/6/2016

Principle of Working of Potentiometers The potentiometer is also called as pots and it one of the most commonly used devise for measuring the displacement of the body. The potentiometer is the electrical type of transducer or sensor and it is of resistive type because it works on the principle of change of resistance of the wire with its length. The resistance of the wire is directly proportional to the length of the wire, thus as the length of the wire changes the resistance of the wire also changes. 106/6/2016

The potentiometer are used for voltage division. They consist of a resistive element provided with a sliding contact. The slide contact is called as wiper. The contact motion may be linier (traditional)or rotational or combination of the two. The combinational potentiometer have their resistive element in helices form and are called helipots. The resistive body of the potentiometer may be wire wound.On the insulated former a thin film of 0.01 mm diameter of platinum or nickel is wound. 116/6/2016

A potentiometer with a resistive load, showing equivalent fixed resistors for clarity. The potentiometer can be used as a voltage divider to obtain a manually adjustable output voltage at the slider (wiper) from a fixed input voltage applied across the two ends of the potentiometer. This is their most common use.voltage divider The voltage across R L can be calculated by 126/6/2016

13 If R L is large compared to the other resistances (like the input to an operational amplifier), the output voltage can be approximated by the simpler equation: (dividing throughout by R L and cancelling terms with R L as denominator) 6/6/2016

Advantage Disadvantages They are easy to operate. They are cheap. Their electrical efficiency is high. They are useful for measurement of large amplitude of displacement. They require large force to move their sliding contacts. Environmental factors greatly affect the measurement. Use in laboratories but seldom used in industries. 146/6/2016

References Electrical and Electronic measurements and instrument by R.K.Rajput. 6/6/201615

166/6/2016