Temperature 3 Wire RTD meansurement circuit / transmitter

Issues in 3 wire RTD temperature transmitter.

• RTD varies its resistance with temperature which is not directly a measurable quantity. Change in RTD resistance with one unit of change in temperature is very less. It is very difficult to measure this little change.
• Change in resistance with temperature is not linear therefore one single equation cannot predict correct temperature by sensing the change in resistance.
• The resistance due to wire leads and connections can lead to false readings.

Properties of 3 wire RTD resistance temperature transmitter circuit.

• Such RTD measurement circuit is needed which can convert this resistance into measurable quantity like voltage or current.
• Therefore such RTD transmitter circuit is required which can not only correctly sense little variation in resistance but also amplify it to such level so that it can be utilized easily.
• RTD transmitter should be able to neutralize the nonlinear relation between temperature resistances.
• RTD measurement circuit should employ 3 wire connection method of RTD thermocouple so that possibility of false reading is eliminated.
• RTD transmitter circuit should have the facility to adjust the amplification of signal for ease in sensing the signal.
• Temperature 3 wire RTD meansurement circuit should have the facility to be calibrated at any point.

Working of 3 wire RTD meansurement circuit

Components which are required for RTD transmitter circuit are as follows

• Input source (5V)
• Fixed resistors of various values
• Variable resistors ( values depend upon requirement. 100k and 20k used in this circuit
• Operational amplifiers 741 or any suitable for this making instrumentation amplifier. IC having built in instrumentation amplifier can also be used.
• 

For the sake of understanding, we divide the circuit into two main parts.
1) Bridge circuit

2) Instrumentation amplifier

1) Bridge circuit.

It consists of four resistors connected in Wheatstone bridge configuration.  Three resistors R1, R2 and R3 are fixed resistors while fourth resistor is RTD Pt 100 thermocouple. How bridge circuit works and the whole theory behind it is not the scope of this article. To understand general concept of bridge circuit please visit the following link Analysis of bridge circuit 
Bridge circuit allows both 2 wire and 3 wire connection RTD Pt 100 as shown in circuit diagram.

The values of other 3 resistors can be used to fix the reference value (temperature). By reference value we mean that voltage difference between point A and B i-e V_AB will be zero at reference temperature. For instance if 0 ᵒC is to be set as reference temperature , the corresponding resistance of PT-100 at 0 ᵒC is 100Ω. By selecting other 3 resistors of 100Ω, voltage V_AB = 0 at 0 ᵒC. It is because same current flows in both branches of bridge and same voltage drop occurs in R3 and RTD PT-100.

Now if the 100 ᵒC is to be selected as reference value than such values of resistors is to be selected so that V_AB = 0 at 100 ᵒC.  According to data chart of PT-100, its resistance value is 140Ω at 100 ᵒC. So if all the resistors values are selected as 140 Ω than V_AB will be 0 at 100 ᵒC.  In this way any temperature can be selected as reference value by selecting appropriate value of resistors.

One other advantage of bridge configuration is that a slight change in resistance values can produces considerable difference in voltage V_AB which can easily be sensed.

2) Instrumentation amplifier

The second part of 3 wire RTD meansurement circuit is instrumentation amplifier. It consists of voltage followers and gain amplifier. To understand the concept and theory of instrumentation amplifier please visit the following explanation of instrumentation amplifier 

The gain of instrumentation amplifier is given by the following equation

To adjust the gain RV5 and RV6 can be varied. Due to this reason RV5 and RV6 are kept as variable.  This gives user the facility to adjust gain according to exact requirement.

Another advantage of using this arrangement is that input impedance of voltage follower is infinite (very high) so by taking input, it does not disturb the bridge and its resistances.

Using RV5 and RV6 the gain of the amplifier is set at 10.

It means the voltage VAB will appear as 10 times of original at output of 3 wire RTD temperature transmitter circuit.

This value can be given to any controller or circuit to convert it into temperature and display on appropriate screen.

Summary

Following points summarizes the advantages of given 3 Wire RTD transmitter circuit.

• Bridge configuration allows 3 wire connection modes which eliminate the error due to lead resistance.
• Connecting RTD PT-100 in bridge configuration allows any small change to be sensed.
• Any value of temperature can be selected as reference value by choosing appropriate values of resistors.
• Voltage follower (instrumentation amplifier) has infinite input impedance which does not disturb actual reading of bridge.
• By adjusting gain of instrumentation amplifier, the amplification can be set at required value. Variable resistors allows user to set according to his requirement.