 Welcome to this session, myself Mr. Giridhar Jain, Assistant Professor in Electronics and Telecommunication Engineering, Valchan Institute of Technology, Sholapur. Now, today I am going to explain you on-off temperature controllers. Learning outcomes are, at the end of this session, students will be able to draw circuits and explain on-off temperature controllers. And second outcome is, students will be able to design on-off temperature controllers. Contents of the session are on-off temperature controller without dead band, on-off temperature controller with dead band, and design of on-off temperature controllers. Now, first is on-off temperature controller without dead band. So, figure shows the circuit A and this figure B shows the transfer curve. Now, let us understand the basic circuit. Now, if you look at the circuit, the input stage is a transducer bridge. Now, in this transducer bridge consists of resistance R A, R B, R C and a sensor P T 100, which is used as a temperature sensor. So, these four elements are connected to form a bridge. Now, voltage between this point and this point is the output of the bridge. Now, when bridge is balanced at 0 degree centigrade, the output from the bridge is 0 volt. Now, output of the transducer bridge is proportional to the temperature because P T 100 is a thermistor and the resistance of P T 100 increases with increase in temperature. Now, output of transducer bridge which is proportional to the temperature is given as a input to the instrumentation amplifier. So, instrumentation amplifier is designed using the three operational amplifiers and the few resistances and one potentiometer as shown in the figure. Function of instrumentation amplifier is to amplify the output of transducer bridge which is very small voltage to a required voltage level at the output voltage V o as shown in figure. Now, output of instrumentation amplifier is connected as a input to the error detector. Now, one input to the error detector is a set point that is proportional to the set temperature and the another input is the output of instrumentation amplifier a voltage which is proportional to the actual temperature. So, error amplifier compares the actual process variable that is a voltage proportional to the actual temperature to the set point voltage. A set point voltage is a voltage proportional to the set temperature and comparing this two a error is generated. So, this error detector is nothing but a subtractor circuit using op-amp. So, output of the error amplifier is again connected as an input to the comparator and the reference voltage for comparator is 0. This means when the error voltage is positive then output of the control output is on means controller is on and when the error is negative controller is off. So, this figure B shows the transfer curve for a positive error voltage controller is on and for negative error voltage controller is off as shown in figure. What will be limitation of this controller? Think on this. Now, basic limitation of this controller is that this controller does not have a dead band means whenever the temperature exceeds the set point controller becomes off and whenever temperature comes below a set point controller becomes on. So, there is on and off of the controller when the temperature deviates about a set point by a small amount. Therefore, stability of such controller is less and unnecessary the controller will become on and off frequently. So, this is limitation or drawback of the controller. So, this type of controller is very sensitive to the noise. Now, how to solve this problem? Now, to solve this problem a on-off temperature controller with a dead band is used. Now, in this controller a dead band is introduced. Now, this dead band is symmetrically placed around the set point. For example, let the set temperature is say 50 degree centigrade and let the dead band is 10 degree. Now, this dead band is 10 degree which is divided into two parts that is 10 by 2 is 5 degree. So, 5 degree is added to the 50. So, 50 plus 5 is 55 and 50 minus 5 is 45. So, initially when the temperature is less than the lower border that is 45 degree centigrade then the controller becomes on. Once the controller becomes on it will remain on till a temperature exceeds the target value and it reaches to 55 degree centigrade then controller will become off. Once it becomes off it will remain off till a temperature falls below 45 degree centigrade. So, such controller is having a dead band and due to this a effect of noise is reduced. Now, circuit diagram for such controller. Now, transducer bridge and error instrumentation amplifier that circuit which is used in earlier controller will remain same. Output of instrumentation amplifier is a VPV that is a process variable or a voltage VPV which is proportional to the actual temperature and this is a set point. So, this is error detector. So, circuit from the error detector is shown earlier circuit is same. After error detector instead of connecting the output of error detector directly to the comparator as it was done earlier a one more circuit is introduced in between and this circuit is a smith trigger. So, for this smith trigger it is possible to introduce a dead band. So, dead band is introduced by this smith trigger and this dead band depends on the value of resistance R1 and R2 and this is the characteristics of the on-off temperature controller with a dead band. Now, when the error voltage is positive when the error voltage exceeds this point this is called as UTP the controller becomes on. Once it becomes on it will remain in on state till the error voltage reduces below this point B that is lower triggering point. Now, UTP and LTP of the smith trigger are calculated by the formula VUT is R2 upon R1 plus R2 into plus Vsat and this is VLT R2 upon R1 plus R2 into minus Vsat. So, let this is equation number 1 and this is equation number 2. Now, width of hysteresis is given by subtraction of VUT and VLT. So, width of hysteresis is VUT minus VLT that is equal to 2 into R2 upon R1 plus R2 into Vsat. So, width of hysteresis can be designed as per requirement by designing the proper values of R1, R2 and Vsat. Vsat depends on the supply voltage. Now, let us proceed further for solving the design problems on the on-off temperature control. Now, this is problem 1 which states that design on-off temperature controller to control a temperature in the range of 0 to 100 degree centigrade. Set point is 40 degree centigrade and require full scale output is 10 volt. Now, solution for this on-off controller we use a on-off controller without dead band circuit is used because a dead band is not given in the design. Step number 1, design of transducer bridge. So, use PT-100 as a temperature sensor. Its resistance is 100 ohm at 0 degree and 138.5 ohm at 100 degree centigrade. For design of transducer bridge, so this is a transducer bridge RA, RB, RC this is 1.4K and PT-100 and DC voltage selected is 5 volt. Select RA is equal to RB is equal to RC is equal to 1.5 kilo ohm and the series resistance in series with PT-100 is selected as 1.4K as shown in figure. At 100 degree centigrade potential of point A is determined by FA is equal to 1.5 upon in the bracket 1.5 plus 1.538 bracket close into 5 volt. Therefore, FA is coming as a 2.469 volt and VB is coming as a 2.5. Therefore, VB minus FA comes to be 0.031 volt. Now, this voltage 0.031 volt is to be amplified by instrumentation amplifier. So, output of instrumentation amplifier is given by FA is equal to RF by R1 in the bracket 1 plus 2 R4 by R5 bracket close in the bracket VB minus FA. Substituting the values FA is 10 volt that is full scale output RF by R1 we have selected select R1 is equal to RF is equal to 1K. Solving this we get R4 equal to 159.39 R5 select R4 is equal to 10K therefore R5 comes to be 62.73 ohm. So, design of error detector and comparator. So, this is the circuit. Now, in the circuit select op-amp as 741 select VCC as a plus 12 volt and minus 12 volt R1 is equal to R2 is equal to R3 is equal to RF is equal to 1K and select the potentiometer as a 10K. These are references. Thank you for watching the video.