Tag Archive for Two Wire Transmitter

How Two Wire Transmitters Work


Two wire transmitters are one of the most used electronic instruments in the process industries.

Their flexibility and accuracy of measurements makes them the instrument of choice in many applications.

How They Work

A two wire transmitter is an electronic circuit which is designed to run on less than 4mA of current.

A Zero adjustment makes the circuit draw exactly 4.00mA when the measured signal is at its Zero Scale level.

The circuit will make the current rise to exactly 20.00mA when the measured signal rises to its Full Scale level.

The current drawn from the power supply is constant and is not dependent on the power supply voltage. This feature allows the desired signal to be read anywhere the power supply wire lead can be opened and a small resistor put in series with the lead. The voltage drop across the resistor is an accurate indication of the measured signal.

The constant current can drive total load resistors until the voltage dropped by the loads equals the saturation voltage of the Transmitter output circuit.  The compliance of the Transmitter is determined by the design of the circuits.

The universal standard of having the current go from 4.00mA to 20.00mA allows the two wire TX to be used anywhere in the world. A multitude of products exist which accept the 4.00mA to 20.00mA signal as an input signal.

One of the two major advantages of the 2 wire TX is the distance from the sensor to the signal conditioning circuit can be extremely short. This is a great advantage for thermocouple measurements. Often, the 2 wire TX is mounted on the TC sensor and no extra thermocouple wire is needed.

RTD sensors can often be 2 leads instead of 3 or 4 leads, because the 2 wire TX can be mounted on the sensor.

Where small signal levels are encountered, such as strain gauges, short wiring from the sensor to the 2 wire TX helps keep noise off the signal leads.

The other great advantage is the power supply leads for the 2 wire TX can be 1000’s of feet long and the lead can be opened anywhere so the signal can be read. Also since the signal is a constant current, the resistance of the power supply leads does not affect the output signal level.

Loop Powered Displays exist which can indicate the current in the wiring loop. The displays require very little current to operate. They usually have adjustments so the indication can be in the desired engineering units instead of just mA.

Modern technology has made 2 wire transmitters easier to design. Amplifiers are available which require only 10 microamps of current to fuction. These allow some complex circuits to be developed and function on less than 4mA. It expands the range of 2 wire TX’s which can be developed.

Wilkerson Instrument offers the following two wire instrumentation products

The SR Series Two Wire Transmitters offer inputs for DC , RTD, Thermocouple, Frequency, and Potentiometer Position

SR Series Two Wire Transmitter

SR Series Two Wire Transmitter

in a 50 mm Case size for DIN connection heads, flat panel or DIN rail mounting. Plug In range cards offer the ability to change ranges in the field. Some models offer galvanic input to output isolation.

TW303 Two-Wire Transmitter

TW303 – Specifications and I/O manual

The TW SeriesTwo Wire Transmitters offer an economic solution for applications requiring low cost transmitters for Potentiometer, RTD, Thermocouple, and Frequency inputs.

The SC Series Two Wire Transmitters feature universal temperature sensor inputs programmable via an onboard keypad or by remote RS232 interface. Linearization for thermocouple types J,K,T,R,S,E, and N is provided as well as inputs for 2, 3 and 4 wire Platinum RTDs.

SC 5010 Two Wire Transmitter

SC 5010 Two Wire Transmitter

The SC5010 Two Wire Transmitter also features a 4 1/2 digit digital display and galvanic input to output isolation.





Solutions provider for signal conditioning and process control instrumentation – Wilkerson Instrument Company Inc

Automatic Temperature Control In a Sugar Refinery



TW303 Two-Wire Transmitter

TW303 - Specifications and I/O manual


  • Provides 4/20 mAdc current output  proportional to an RTD input
  • Any 2-wire or 3-wire RTD  (100 ohms to 2000 ohms)
  • Linearized for platinum RTD
  • Any input range within RTD limit  (50°F minimum span)
  • Low-drift chopper-stabilized input
  • Operating temperature range -13°F to 176°F
  • Temperature stability of ±0.02% of span per °C
  • Operates on 12 V to 48 VDC
  • Conformal coated to resist corrosion,  moisture and fungus
  • Low cost
  • Small size – 50mm X 50mm
  • Our standard 5 year warranty

PROBLEM:                              Specification Sheet PDF      I/O Manual PDF

To replace manually read thermometers with 3-wire, 100 ohm, platinum RTDs allowing all temperature readings to be taken in a central control room. Also, to provide automatic temperature control for more efficient control of the refining process.


Historically, sugar refineries are reluctant to spend money on capital equipment. As a result, they tend to be slow in automating their process. Many still utilize manual thermometers that require some-one to “make the rounds” to read the temperatures and make manual adjustments to the steam valves.

This process is cumbersome and slow, particularly when trying to maintain constant temperature at multiple sites. Those refineries who have made the plunge have found that automation of the process has among other things, greatly improved the efficiency of the process.

In this particular application, shown in the flow chart, the customer must read and maintain temperatures at multiple locations including in/out temperatures of heat exchangers as well as the top/bottom temperatures in the pan. The pan temperatures must be exactly right in order to “seed” them to make crystal.


Steam Flow Chart - Temperature Control In a Sugar Refinery - TW303 - Wilkerson Instrument

Flow Chart

The customer chose to automate the plant with Moore 352E programmable loop controllers to control each temperature control loop, Fisher Controls steam valves and a central computer to poll each controller for CRT readout and data gathering in the control room.

Each Moore 352E can handle up to 5 analog inputs and 3 analog outputs. In addition, the 352E has a resident 24 V power supply capable of powering up to at least 3 current loops. However, in order to read an RTD directly, the 352E must be equipped with an optional RTD input at a cost of about $300 each. By using a low cost 2-wire transmitter, the customer can save ap-proximately $200 per RTD input.


The product chosen was the TW303 RTD input 2-wire transmitter. Its small size, low cost, reliability, and versatility in mounting (field or panel mount in a variety of industrial housings and enclosures) made it the ideal choice. The transmitter accepts any 2-wire or 3-wire RTD input and has a minimum span of 50°F. It can be calibrated to cover any range desired within the limits of the RTD selected. The module operates over a temperature range from -13°F to 176°F while providing a temperature stability of ±0.02% of span. It will also operate over a supply voltage range of 12 to 48 VDC.

The transmitter design provides accurate trouble-free operation. A single constant current source excites the RTD, but the low excitation current causes no appreciable self-heating in the RTD. An accurate and stable leadwire compensation circuit uses the third lead of the 3-wire RTD to compensate for the RTD lead resistance. Platinum RTD inputs are linearized and use of low-drift chopper-stabilized amplifiers and special filtering reduces susceptibility to transients and noisy operations. In addition, the electronic board and components are conformal coated to resist corrosion, moisture and fungus associated with harsh industrial environments.

These features, a generous 5 year warranty, customer service second to none in the industry, low cost and a recommendation from the Wilkerson representative, led the customer to choose the TW303.

The customer connected them as shown in the wiring diagram.

Wiring Diagram - Temperature Control In a Sugar Refinery - TW303 - Wilkerson Instrument

Wiring Diagram


Approximately $200 per RTD input.


Copyright Wilkerson Instrument Company Inc. – USA – 2012





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