RTD PT100 Mains Powered Transmitter

This is a RTD Pt-100 Transmitter, It can Convert Thermocouple mV or RTD mV to 4-20 mA Current. A Mains powered Transmitter not loop powered. The part numbers on Circuit and silk screen may not match. But the PCB may be usable. The output is a current source and not a current sink. It goes to a 4-20mA Mux with a common ground.

Supervisory Control and Data Acquisition

This low-cost transmitter was made to send RTD information in a 4-20mA over long distance. The 4-20mA was the input to a Analog-Mux which interfaced to a PLC system and a 486 Computer.

The Analog Multiplexer Built around CD4051 was was controlled by PLC to get data from numerous channels. This reduced the cost of the System. This is ok when Real-Time Acquisition is not required. The Analog 4-20 to 4-20 Mux-Buffer circuit can also be seen at Industrial Process Control Circuits.

This was done as customer budget could not afford dedicated analog input modules for every channel. There were more than 50 Inputs. The thermal inertia of system did not need a real time or fast correction.

PNG-Schematic,     PDF-Circuit,    PCB - DT  L1  SS  SM

RTD PT100 Mains Powered Transmitter - del30006


The PNGs of PCB is 600dpi 16 color. It is to be scaled appropriately.

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.

Analog Mux for Data Acquisition

Now with new Technologies like Zigbee and Modbus, 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 by many, like Monitoring the Weather Attributes.

Significance of Current Loop 4 to 20 mA Standard

You have a three wire RTD, there is also a 4 wire RTD type, This circuit can be used for both.

In the 3W RTD you have two wires coming from the same point (zero ohms between them) the third wire will be 100 ohms from these two. From the two wires that are at same point take one wire and give 1mA from Q3 + OUT. The other wire of the pair connect to input +..... input - can be connected to the 100E end.

• 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.

RTD 3-W Mains Powered Temperature 4-20mA Transmitter.

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