Archive for Two-Wire Transmitters

How Thermocouples Work Type J


The thermoelectric effect of heated conductors was discovered by a German physicist Thomas Johann Seebeck in 1821. His discovery of any conductor with a thermal gradient would generate a voltage lead to the development of accurate temperature measurement with two dissimilar metal wires now commonly called thermocouples.

Thermocouples are made by connecting two dissimilar metal wires together at one end. The open end of the wires will have a potential difference between them if the open ends and the connected ends are at different temperatures. If the open ends of the wires are held at a constant temperature, the voltage measured between them is proportional to the temperature difference between the open ends and the connected ends. With this arrangement, temperatures can be measured because the voltage created is consistently repeated versus temperature differences if the open ends are held at a reference temperature. The open ends are connected to terminals referred to as the “cold junction terminals”.

In earlier times, zero degrees centigrade was the reference, but modern technology made reference temperatures easier to create. Electronic temperature measurements with thermistors, diodes, and other small electronic devices allowed the temperature of the cold junction terminals to be measured. The voltage the thermocouple creates at this temperature is added to the voltage measured at the cold junction terminals. The results make the signal level the same it would be if the cold junction terminals were at zero degrees centigrade. Zero degrees centigrade is the cold junction reference for the common voltage charts for thermocouples.

The National Institute of Standards and Technology (NIST), a division of the U. S. Department of Commerce, has created voltage charts for common thermocouples. Copies of these charts are available free from most thermocouple manufacturers and can be downloaded as PDF files from the internet.


The J thermocouple is made of an iron wire (+ positive lead colored white) and a copper-nickel (Constantan) alloy wire (- negative lead colored red). It has a sensitivity of approximately 50 microvolts / deg C and an overall temperature range of -210C to 1200C, but is normally limited to narrower ranges. It has a limited range of -40C to 750C due to the Curie point of the iron at 770C. The iron undergoes a molecular change and permanently loses its standard voltage output versus temperature. It does not recover when the iron is cooled.

It should not be used at high temperatures in an oxidizing atmosphere. A reduction atmosphere is desired. Use at low temperatures is also not recommended

The J thermocouple is one of the lowest cost thermocouples.

The linearity of the J thermocouple varies by -70 degrees C over it full range from -210C to 1200C. It has a very straight section from 100C to 500C which deviates about -0.5C. The lower and higher ranges can be extended with a loss in linearity.


SC5010 Two wire Transmitter

SC5010 Two wire Transmitter With Thermocouple Lookup Tables

Modern microprocessor based signal conditioners remove linearity problems by measuring the input voltage and using this value to check a memory chip to see what temperature it represents. It then creates the correct voltage via a digital to analog converter to make the output linear. It also does the same process if it is sending serial digital data instead of an analog signal. With a digital signal conditioner, the use of the J Thermocouple becomes limited by the environment and temperature range required.

A typical digital 2 wire transmitter has a membrane keyboard for setup. An LCD display is required so the setup process can be observed.

This transmitter will process signals from J,K,R,S,T,E,N thermocouples as well as .00385 and .00392 alpha platinum RTD’s. Linearization is accomplished with 256 point lookup tables stored in chip memory.

Linearity is important only if the absolute temperature measurement is needed over a large temperature spread. If one temperature is required for process control, repeatability of a given temperature is important, but linearity is not required.

Dc Input Isolated 2 Wire Transmitters and How They Work


An isolated DC input two wire transmitter is a valuable tool for many difficult measurements.  The device can measure DC voltage or current inputs and provide a 4mA to 20mA output without a galvanic electrical connection between the input and output.

The isolation between input and output is typically rated with an insulation capability of 1000 volts or more.


The transmitter has a DC amplifier as an input circuit.  It can typically be calibrated for a current or voltage input.  The power for this circuit requires an isolated power supply which uses a transformer for the isolation.

The signal out of this amplifier must be changed to a form which can be sent through the signal isolation circuit.

A common isolation circuit is to convert the DC signal to a square wave AC signal and transformer couple it to the output circuit.  The output circuit must convert the square wave AC signal back to a DC voltage which can drive the current output circuit.

Another common circuit is to convert the DC signal to a pulse signal where the data is determined by the width of the pulse.  This pulse is passed through an optical isolator and then converted back to a DC signal which drives the current output circuit.


The digital transmitter has a DC amplifier as an input circuit. The power for this circuit requires an isolated power supply which uses a transformer for the isolation.

The level of the DC output is measured with an analog to digital converter.  The binary code, in the form of pulses, is passed on to a digital to analog converter through optical isolators.  The analog output of the D/A converter drives the current output circuit of the transmitter.


Common Mode Definition

A DC input transmitter has 2 connections for the input signal.  These are the signal input and the common connection.  The common connection is the circuit common of the input amplifier.

If the input terminal and the common terminal are connected together, and a signal is connected to this connection, and the other signal lead is connected to its normal place, the transmitter has a common mode input.  The signal input and common terminal, being tied together, both have exactly the same signal attached.

The transmitter will not respond to this common mode signal.  It only responds to a difference in the level between the signal and common lead.

Application For Common Mode Capability

Measure current from a 138V battery pack by putting a .01 ohm resistor in series with the positive lead.  Connect the 2W transmitter input across the resistor.

The resistor drops 10mV per amp, which is processed by the transmitter and converted to the constant current output.

The common mode capability of the transmitter allows the signal input leads to be 138VDC above ground, but it can accurately process the low voltage across the shunt resistor.

The current output is not effected by the 138VDC battery voltage.

Two Wire Common Mode Diagram

Two Wire Common Mode Diagram

Application For PLC Input

Some PLC’s provide 24VDC at the input terminals of their analog inputs.  A 2 wire transmitter can be connected to the 24VDC.  The PLC will monitor the current drawn from the power supply and thereby ascertain the signal level at the remote 2W transmitter.

An isolated DC voltage or DC current 2W transmitter connected to the PLC allows many types of signals to be monitored in the field.  The isolation prohibits ground loops and maintains noise free signals into the PLC.


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