Question

Precisely control of temperature in many industries is critical to ensure the quality of the product. Discuss how you will use the instrumentation amplifier using transducer bridge to measure the temperature. Use block diagram to illustrate your concept.

Answer 1

In many industrial applications it is necessary to measure various physical quantities such as temperature, humidity, water flow etc. These are measured by transducers. The output of transducer is required to be amplified for which instrumentation amplifier is used.

The Fallowing figure shows a practical instrumentation amplifier with a transducer bridge.

Let the block diagram illustrate in details in fallowing figure

In this circuit a resistive transducer (whose resistance changes as a function of some physical energy) is connected to one arm of the bridge.

Let RT be the resistance of the transducer and ΔR the change in resistance of theresistive transducer. Hence the total resistance of the transducer is (RT ± ΔR).

The condition for bridge balance is Vb = Va, i.e. the bridge is balanced when Vb = Va, or when

The bridge is balanced at a desired reference condition, which depends on the specific value of the physical quantity to be measured. Under this condition, resistors RA, RB and RC are so selected that they are equal in value to the trans­ducer resistance RT. (The value of the physical quantity normally depends on the transducers characteristics, the type of physical quantity to be measured, and the desired applications.)

Temperature Indicators Using Thermistor
The Thermistor is a relative passive type of temperature resistance transducer. They are basically semiconductors.

In many respects, a thermistor resembles a conventional resistor. It is usually a two-terminal device. It has resistance as its fundamental property. It is gener­ally installed and operated in the manner of an ordinary resistor. But its great difference is that it has a negative temperature coefficient (NTC) or positive temperature coefficient (PTC) type. Most thermistors exhibit an NTC charac­teristic. An NTC type is one in which its resistance decreases with increase in temperature. The temperature coefficient is expressed in ohms/°C.

Since it is a THERMally sensitive resISTOR, it has a high temperature coef­ficient of resistance and is therefore well suited for temperature measurement and control.

If in the bridge circuit of Fig. 14.25 the transducer used is a thermistor, the circuit can thus be used as a temperature indicator. The output meter is then calibrated in °C or °F. The bridge is balanced initially at a desired reference condition. As the temperature varies, the resistance of the thermistor also changes, unbalancing the bridge, which in turn produces a meter deflection at the output. By selecting the appropriate gain for the Differential Instrumentation Amplifier Transducer Bridge, the meter can be calibrated to read a desired temperature. In this circuit, the meter movement (deflection) depends on the amount of unbalance in the bridge, which is caused by a change in the value of thermistor resistance ΔR. The change ΔR for the thermistor can be determined as follows.

ΔR= temperature coefficient of resistance

(i.e. final temperature - referance temperature)

If the meter in this circuit is replaced by a relay, and if the output of the Differential Instrumentation Amplifier Transducer Bridge drives the relay that controls the current in the heat-generating circuit, a temperature controller can be formed. A prop­erly designed circuit should energise a relay when the temperature of the ther­mistor drops below a desired value, causing the heater unit to turn on.