Temperature-Measurement

This is the Photo of a RTD 3-W Mains Powered Temperature 4-20mA Transmitter. The Circuits and PCB are here RTD PT100 Transmitter and Multiplexer.The Circuit RTD-Pt-100-Transmitter.

We can classify Transmitters as shown below. The Measured Parameter Temperature, Flow or Events Has to reach an Intelligent Data Storage and Analysis System. It may just be an Human Operator who jots the data on a Notepad and Turns a Few Dials based on his Experience or an embedded controller. It could even be a Computer Network or a Web Application used,

RTD PT100 Transmitter and Multiplexer PCB

  • Analog Transmitters – Like 4-20mA Loop.
  • Electrically Isolated Analog Transmitters.
  • Transmitters that need to be Intrinsically Safe.
  • Digital Transmitters and Optical Interface.
  • Wireless Transmitters and TCP-IP.

The job of the transmitter is to take the weak analog measured parameter information from sensor, be close to it, amplify, clean, linearize the signal if required and send strong – error free data over a long distance to an operator or system.

Process Control Instruments and Custom Solutions has been my core Strength. The RTD Pt-100 Transmitter with 4-20 mA output shown here was used in numbers with a 4-20 mA Analog Multiplexer. This was in turn monitored/controlled by a PLC SCADA System to monitor many points in a large Industrial Plant. The external multiplexer makes unlimited channels possible with a Standard Analog Input and Digital Output Module.

This circuit changes the gain of opamp U1B in four steps or segments. It can be used to get a linear output from most transducers to 1% levels.U1A is a amplifying buffer use it to boost the signal to the required level.
Linearizing Circuit for Thermocouples

The resistor values i have put are for an imaginary transducer, you have to design them. The buffered input signal is compared to reference switching points by LM339.

Temperature Measurement and Control

LM339 changes the gain resistors of U1B thru the mux switch 4066. JP1 to JP4 can select either amplification or attenuation of signal. The resistor switched by 4066 can be across R1 or R2 based on JP1 to JP4.

You may have to input transducer values into a spreadsheet and draw a graph. Then divide the graph into 5 segments and deduce the switch points and gain.

These are the circuits and boards of a Mini Temperature Transmitter for a Platinum hundred ohms temperature Sensor. Has a 4-20mA sink output. Current Loop can be used for multiple instruments.

Mini RTD Pt-100 Three Wire Transmitter

The customer wanted a two wire system, this needs low power opamps which were ten times the price. If this transmitter works at 3mA it could have been two wire. Those parts were in short supply and the budget of customer was low. So i made it with regular opamps but three wire. The customer was satisfied as he got a cost effective solution. But now the situation of markets and products are different.

Mini RTD Pt-100 Three Wire Transmitter

If you need to transmit the temperature from a high voltage area or toxic environment. You will need to hermetically seal the transmitters, convert the Voltage to frequency, frequency to IR LED flashes or RF. This can be then remotely monitored. Then the transmitter has to be battery-solar operated, low power too. ICL7135 is a simple solution. It has a serial output that can be used to Drive IR Leds. Remotely sense these flashes in a Micro-controller and you have a reading. You may be able to use optic-fibers too. Where volatile liquids are present so that the risks of sparks can be eliminated.

RTD Mains Power Transmitter

This is the same Mini RTD Pt-100 Transmitter but in its case. Encapsulated in epoxy, hermetically sealed against harsh industrial environment. This will work well even near fumes of Ammonia with no corrosion. But not near vapors which can be ignited by sparking as terminals are still open. The side view shows zero and full scale ten turn bourns trimpots heads, for calibration. After cal it can be sealed with RTV compound.

Thermocouple is the most common sensor in Industrial Temperature Measurement. The Signal Conditioning involves Cold Junction Compensation and High Gain DC Amplification. The output of a Themocouple is in millivolts.

The OP07 is a low offset 75uV opamp, here it is used to amplify the output of a Thermocouple, the gain of this stage is high. The zeners are to protect any high voltage at input zapping the opamp.

Thermocouple Amplifier Standard

The Resistor R6 limits the current. The zeners should be low leakage or use clamping pull-up and pull-down diodes to +5 and -5 respectively.

The RC low-pass filter formed by R6 and C2 reduce the mains hum or 50 Hz pickup of long thermocouple cables laid close to high current heater wiring. R1 is a offset null use or add if required. R11 is gain control of OP07. The TL072 is a FET input opamp used here as a summing amp.

Blind Dial Proportional Temperature Controller

Adding one more inverting amp with some gain to the output of this circuit can give you a 1-5V suitable for ADC or PC analog I/O cards. C1 also serves to filter, it is an integrator here. It suppresses EMI and RFI from motors, contacters etc., R13 sets an output value for 0mV input.