Signal conditioning of thermistor
Signal conditioning means manipulating an analogue signal in such a way that it meets the requirements of the next stage for further processing. Operational amplifiers(op-amps) are commonly employed to carry out the amplification of the signal in the signal conditioning stage. The signal conditioning equipment may be required to do linear processes like amplification, attenuation, integration, differentiation, addition and subtraction. They are also required to do non-linear processes like modulation, demodulation, sampling, filtering, clipping and clamping squaring, linealizing or multiplification by another function etc. the signal conditioning or data acquisition equipment in many a situation be an excitation and amplification system for passive transducer. It may be an amplification system for active transducer. In both the applications, the transducer output is brought up to a sufficient level to make it useful for conversion, processing, indicating and recording. Excitation is needed for passive transducers because these transducers do not generate their own voltage or current. Therefore passive transducers like strain gauges, potentiometers, resistance thermometers, inductive and capacitive transducers required excitation from external sources. The active transducers like techno generators, thermocouples, inductive pick ups and piezo-electric crystals. The thermistor constitute one arm or more than one arm of a wheatstone bridge which is excited by an isolated DC source. The bridge can be balanced by a potentiometer and can also be calibrated for unbalanced conditions. Thermistor is a concentration of the term "Thermal Resistor". It is essentially a semiconductor, which behaves as a resistor with a high negative temperature coefficient of resistance. That is, as the temperature of the thermistor increases, its resistance decreases. The temperature co-efficient is expressed in ohms per unit change in degree Celsius (° C). thermistors with high temperature co-efficient of resistance are more sensitive to temperature change and are therefore well suited to temperature measurement and control. CONTENTS
1. Wheatstone bridge:
Whetstone bridge is the most accurate method available for measuring resistances and is popular for laboratory use. The circuit diagram of typical Wheatstone bridge is given in figure
Rx is the unknown resistance to be measured; R\, R2 and R^ are resistors of known resistance and the resistance of R2 is adjustable. If the ratio of the two resistances in the known leg (R2 / R\) is equal to the ratio of the two in the unknown leg (Rx / R3), then the voltage between the two midpoints (B and D) will be zero and no current will flow through the galvanometer Vg. R2 is varied until this condition is reached. The direction of the current indicates whether R2 is too high or too low. Detecting zero current can be done to extremely high accuracy (see galvanometer). Therefore, if R\, R2 and R3 are known to high precision, then Rx can be measured to high precision. Very small changes in Rx disrupt the balance and are readily detected. At the point of balance, the ratio of R2 / R\ = Rx / R3
Alternatively, if R\, R2, and R3 are known, but R2 is not adjustable, the voltage difference across or current flow through the meter can be used to calculate the value of Rx, using Kirchoff s Circuit laws (also known as Kirchhoff s rules). This setup is frequently used in strain gauge and resistance thermometer measurements, as it is usually faster to read a voltage level off a meter than to adjust a resistance to zero the voltage. In practical Wheatstone bridge, at least one of the resistance is made adjustable, to permit balancing. When the bridge is balanced, the unknown resistance (normally connected at Rx) may...