Signal Conditioning Circuits for RTD

Hello friends, today we will see Signal Conditioning Circuits for RTD. We know that sensitivity of RTD depends on the temperature resistance coefficient of the metal used for RTD. The value of this coefficient is very small and thus RTD requires the amplifying circuit which is the first signal conditioning circuit for RTD which we are going to see in next post.

Linearization of RTD:

Another signal conditioning circuit required for RTD is linearization circuit. We know that temperature vs resistance curve of RTD is non linear and therefore for wide range of measurements we need to use Linearization circuit for RTD.

Signal Conditioning Circuits for RTD
Linearization of RTD

See also: How to Linearize RTD Output.

Lead resistance elimination:

We know that RTD is a low resistance device that means it has a very small range of resistance and therefore there should not be any lead resistance effect on the output of RTD i.e. Lead wire resistances should not be added with the RTD resistance. Therefore another signal condition circuit for RTD includes lead resistance elimination.

Sensor fault detection is also one of the important signal conditioning circuits for RTD. Sometime due to corrosion of connecting leads RTD may get opened. In such cases signal conditioners may indicate some finite voltages. So to avoid these wrong readings we have to design a signal conditioning circuit for RTD.

In short There are four main signal conditioning circuits for RTD as follows:

  • Bridge amplifier
  • Linearization circuit for RTD output
  • Lead resistance elimination
  • Sensor fault detection circuit

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Signal Conditioning Circuits

We know that signal conditioning is a process in which signals from different sensors are transferred into a form necessary to interface with other modules of system.

For example, we know that thermocouple produces very low output voltage and this voltage is not sufficient to operate the other controlling modules. Therefore there is need to amplify such signals. For this purpose we use different signal conditioning circuits. In case of thermocouple, we have to use amplifier, linearization circuits, etc. the purpose of using linearization circuits is that, thermocouple has non linear characteristics but in most of the cases we need linear controlling action.

Signal conditioning circuit
Measurement System Block Diagram

 

Signal Conditioning Circuits:

There are different types of signal conditioning operations such as amplification, filtering, isolation, linearization, excitation, etc. we will discuss all these operation one by one.

Amplification

We know that most of the sensors produce output in the form of change in resistance, voltage or current. All these parameters are having very low strength i.e. very small voltage in case of thermocouple, small change in resistance in case of RTD, etc. Therefore we have use current or voltage amplifiers in case of sensors which produces output in the form of current or voltage.

If the sensor produces output in the form of change in resistance (such as resistance thermometer) we have to use bridge amplifiers. We can make use of operational amplifiers to amplify the signal.

Filtering

Another important signal conditioning circuit is filter. As mentioned earlier most of the sensor produces very low output and therefore electromagnetic noise may get added in the original output. To remove the electromagnetic noise from sensor output we have to use different filter circuits. Filter circuits eliminates noise i.e. undesired frequency components from original signal without affecting it.

Active filters, passive filters, bypass filters are the common types of filter circuits.

Isolation

Isolation circuits are required to differentiate signals from unwanted common mode voltages. Another advantage of isolation circuit is that, it protects measuring devices (sensors) if high voltage is applied to other circuit. It also breaks ground loops.

Linearization

There are many sensors which produces non linear output such as thermocouple, thermistor, etc. linearization circuits are used to convert non linear signal into linear one. It can be achieved by varying the gain of an amplifier as a function of input signal.

Excitation

Another signal conditioning operation is current or voltage excitation. Signal conditioning circuits provide the required voltage or current excitation to some passive sensors such as strain gauge, RTD, etc.

 

In the upcoming posts we will see signal conditioning of RTD, thermistor and thermocouple.

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Temperature Controlled DC Fan using Thermistor (Mini Project)

Hello friends, in this post we are going to make one simple mini project which is temperature controlled DC fan using thermistor. In this mini project we are going to control the speed of the DC fan automatically as the surrounding temperature changes. Thus when temperature of surrounding increases speed of fan also increases and when temperature decrease speed of fan also decreases. This is achieved by using the principle of thermistor.

Components required:

  • R1 = 4.7K
  • R2 = 47 Ohm
  • NTC Thermistor = 4.7K
    bead type thermistor
    bead type thermistor
  • Potentiometer (Vr) = 500K
  • OP AMP IC 741
  • Transistor T1 = BD140 (or other PNP transistor may work)
  • 12V DC fan which is also used in computer (CPU)
  • Diode 1N4007
  • 12 V DC power supply

Circuit diagram:

Following figure shows circuit diagram of temperature controlled DC fan using thermistor.

temperature controlled dc fan using thermistor
Temperature Controlled DC fan using Thermistor

Working:

The basic working principle of temperature controlled DC fan is based on the working principle of thermistor. Thermistor is component which changes its resistance as its temperature changes. There are two types of thermistor available which are NTC i.e. negative temperature coefficient and other is PTC which is positive temperature coefficient.

In temperature controlled DC fan we have used a NTC type thermistor. It is called NTC because its resistance increases when its temperature decreases and vice verse. Similarly in PTC its resistance increases when temperature increases and vice verse.

Op amp IC741 is used as a voltage comparator which compares the voltage between its two inputs i.e. inverting and non inverting terminals. Pin number 2 is inverting terminal which is connected to the potentiometer and pin number 3 is non inverting terminal which is connected in between thermistor and R1 which makes a voltage divider circuit. Thus the output of op amp is responsible for the speed of fan.

When the temperature of surrounding increases, temperature of thermistor also increases which causes its resistance to decrease, therefore voltage divider circuit causes more voltage across pin number 3. Thus the output voltage increases causing speed of fan to increase.

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Experiment to determine characteristics of Thermistor

Experiment: Characteristics of Thermistor 

This section provides you a detailed steps to determine the characteristics of thermistor.

Aim: To determine the characteristics of thermistor (resistance temperature characteristics).

Apparatus required: Bead type thermistor, multimeter, thermometer, water bath, heater connecting wires, etc.

bead type thermistor
bead type thermistor

Introduction:

Thermistors are the temperature sensitive resistors that exhibit a negative temperature coefficient of resistance. In other words electrical resistance of a thermometer will be reduced when it is placed in an environment of higher temperature likewise its temperature decreases. thus the characteristics of thermistor provides an information about how its resistance changes with the changes in temperature.

It is very essential in temperature measurement, thermistors are manufactured and formed into rods, discs, and washes, beads for special applications, they can be directly or indirectly heated. Temperature determines the resistance of those that are directly heated in environment. The resistance of indirectly heated thermistor is determined by temperature of self-contained heater.

Diagram:

types of thermistor
types of thermistor

Procedure:

Following are the steps to determine the characteristics of thermistor.

1)    Take water in container and place a heater to heat water.

2)    Immerse thermistor and thermometer in water bath.

3)    Switch on the power supply.

4)    Measure the temperature on the thermometer from room temperature (30 C) to 98 C and corresponding resistance of thermistor at that temperature.

5)    Switch off the power supply, and then take reading in decreasing order of temperature in an interval of 10 C.

6)    Plot a graph of temperature on X-axis and Resistance on Y-axis. This graph shows the characteristics of thermistor.

Observation Table:

By taking following readings we can plot the characteristics of thermistor on a graph paper.

Sr. No.: Temp in degree Celsius Resistance(Uploading) Resistance(Downloading)
       
       
       
       

 When we plot characteristics of thermistor it will look like as follows:

Characteristics of thermistor
Characteristics of thermistor

Calculations:

We have To= 30 C, Ro=980Ω, β=4000.

R(t) = Ro*exp [  β* (1/T  –  1/To) ]

Conclusion:

1)    Input and output relationship is non-linear for thermistor (i.e. characteristics of thermistor are non-linear).

2)    In comparison with RTD change in resistance for a given change in temperature is very large.

Analysis:

1)    What is negative temperature coefficient of resistance ?

Answer: The property of a material in which resistance of a material decreases with increase in temperature that material is said to have negative temperature coefficient of resistance.

2)    What are common shapes of most commercially available thermistors.

Answer: The common shapes are bead type, disc type, rod type ans IC chips.

3)    What the difference between directly heated and indirectly heated thermistor?

Answer: In directly heated thermistor, there is direct contact between source and thermistor, but in indirectly heated thermistor there is no direct between source and thermistor.

4)    What is the relationship between thermistor resistance and temperature?

Answer: In thermistor, resistance is inversely proportional to the temperature.

5)    What is the range of temperature for thermistor?

Answer: The temperature range of thermistor is -50 C to 15C

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Selection criteria for transducers

Selection criteria for transducers:

Introduction:

Transducers are the instruments which converts non-electric signals into an electric signal. So while selecting any type of transducers for any special purpose, we should think about its specifications or characteristics. Any transducer is based on a simple concept that physical property of a sensor must be altered by an external stimulus to cause that property either to produce an electric signal or to modulate an external electric signal. Selection criteria of a transducer is based on different factors, such as availability, cost, power consumption, environmental conditions, etc. After considering all these factors we can select a best one for our use.

Selection of the transducer among the many available mainly depends upon:

  1. Input characteristics
  2. Transfer characteristics
  3. Output characteristics

The following points must be considered, while selecting a transducer for any application or a particular application

Input characteristics

  1. This is one of the most important characteristic, while selecting a transducer. By considering input characteristics we can determine, what type of input is needed for that transducer? What is the operating range for that transducer? What is the loading effect on that transducer?
    1. Type of input
    2. Operating range
    3. Loading effect
  1. Transfer characteristics:

  2. Transfer characteristics also plays very important role in selection of transducer. Transfer characteristics means, the effects on the signal when it is being processed. Errors and hysteresis also occurs when the signal is being processed. Following are some major transfer characteristics  which we should keep in mind while selecting a transducer for any special purpose:
    1. Transfer function ( input output relation)
    2. Error and hysteresis
    3. Accuracy and precision
    4. Response of transducer to the environment influences
    5. Calibration
  1. Output characteristics:  

  1. As we all know, while we are doing some work, we always set some goal or aimed for output. Similarly for our use we should first think about what type of output we required? So here output characteristics plays a vital role while selecting a special type of transducer. Some of the output characteristics are summarized below:
    1. Type of output
    2. Output impedance
    3. Useful range
    1. Life span:  It determines how long the selected transducer will work.
    2. Availability: While selecting a transducer we should think about its availability.
    3. Cost
    4. Stability and reliability
    5. Purpose: indication, recording or control

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Introduction to sensors and transducers

Introduction to sensors and transducers:

1. Sensor:

A device detecting the presence of parameter or measured variable and converting it into suitable form is called a sensor. It is a device which converts one physical parameter into other.

2. Transducer:

It is information receiving device, in a form of quantity. It converts the information in the same or other quantity, for indication, recording or control. The device used for conversion of a non electrical quantity into electrical quantity is called transducer.

A transducer is a device which converts non-electrical or physical quantity into electrical quantity. In most of the electrical system the output is not in the form of electrical form, but in non-electrical form. So if we want to measure that output using electrical methods we are required to use transducers.

Transducer = sensor + transduction

Transduction means conversion of input information, into some suitable form.

E.g. Glass. Thermometer is a sensor as it converts temperature into displacement of mercury column (non-electrical) and thermistor is a transducer as it converts temperature into resistance change (electrical).

Block diagram of transducer is shown in figure below:

block diagram of transducer
block diagram of transducer

1) Sensing element:

Sensing element senses the physical or non-electrical quantity like temperature, pressure, etc. or its rate of change.

2) Transduction element:

The output of the sensing element goes to the transduction element. This transduction element is responsible for conversion of non-electrical or physical quantity into its equivalent electrical quantity.

In some cases this transduction element performs both the action of sensing as well as transduction. For example if we consider thermocouple, it generates emf (electrical quantity) corresponding to the heat generated at the ends of two dissimilar metals.

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