1. About Process Signals

1.1. Term: Transmitters

1.2. Term: Converter

1.3. Term: Isolator

1.4. Term: Transducer

2. Types Of Transmitters

2.1. Auxiliary Powered Transmitters (4-wire)

2.2. Loop Powered or 2-Wire Transmitter/Isolator

2.3. 3-Wire Transmitter/Isolator

2.4 Signal Powered Transmitter/Isolator

Over the last 60 years a variety of electrical control signals have replaced pneumatic or capillary signalling in the process control industry. With the development of electronic sensing principles and the introduction of electronic evaluation, recording and control equipment, a number of analogue communication signals have evolved.

1. About Process Signals

The most popular process control signal still used today is a load independent current signal, in the format of 4-20mA. This current, produced by the sensor/transmitter, remains constant up to a specified maximum load resistance, so that type and length of cabling used is of little consequence.
The receiving instrument has typically a low input resistance (10-250 ohm) which will render the current signal highly immune to electrical interference. The low drive voltage required, usually 12-24Vdc provides a very high degree of safety. Electrical control signals were introduced at an early stage in Europe - normally 0-1mA or 0-20mA. The advantage of a zero based signal is that the receiving instrument does not require any input offset and conversion from a current to voltage signal is simply accomplished by feeding the current through a suitably sized resistor. Scaling is made easy by direct ratio calculations. In the US, pneumatic signalling of 3-15psi emerged as an early standard. Consequently the transition from pneumatic to electronic control produced the live zero 4-20mA and 10-50mA signals first used in the US.

The 4-20mA signal has a number of features different from zero based signals. Commencement of the scale at 4mA, known as live zero, clearly distinguishes between zero and failure conditions of the signal. Further the signal segment below 4mA can be utilised to provide power to a loop or signal powered type of transmitter, converter or isolator. The signal information is contained in the 16mA swing. This requires the deduction of 4mA while scaling calculations are carried out. An almost indefinite number of signal types used in process control around the world have produced a variety of interfacing and signal conditioning equipment, and various definitions.

1.1. Term: Transmitters

Transmitters convert a low level electric signal from any sensing device into a standardised process signal. These transmitters sometimes incorporate electromechanical devices such as strain gauges for measurement of physical values - pressure, flow, level, etc.

1.2. Term: Converter

Converter converts one specific type of signal into another type of signal, i.e. 0-10Vdc to 4-20mA.

1.3. Term: Isolator

Isolator is normally used to provide galvanic isolation from one signal (input) to another (output) and usually combines the converter function.

1.4. Term: Transducer

2. Types Of Transmitters

2.1. Auxiliary Powered Transmitters (4-wire)

Auxiliary powered transmitters also known as 4-wire transmitters are, in general, transmitters, isolators or converters requiring a power supply of some kind to operate. The input signal is converted or isolated and re-powered to give a high-drive output signal. The transmitter becomes a source for the process signal. Any combination of input/output/supply can be obtained using this principle. Even applications requiring conditioning for signals with polarity change e.g. ±10V can be accommodated. The power supply is usually ac-mains or dc power from larger dc power supplies. This enables the 4-wire transmitters to employ opto-coupling for signal isolation and output drive voltage of 22V typically. As a number of sensing devices require excitation voltage to operate, the transmitter can provide the correct voltage to suit the sensor. Transducers with auxiliary supply are also referred to as active transducers.

2.2. Loop Powered or 2-wire Transmitter/Isolator

Loop powered or 2-wire transmitters are connected in series with all instruments driven by the same signal or current loop. Loop powered transmitters must have a live zero output signal, typically 4-20mA as the power to operate the transmitter is taken from the output signal segment below 4mA (quiescent current). This is achieved by tolerating a voltage drop of up to 12V across the output of the transmitter. Maximum power available as such is 12V x 0.004A = 48mW. Modern components and special low power circuits result in power consumption below half of the above value. The external loop power supply sinks a signal current through the transmitter. A loop powered transmitter will act like a variable resistor except that signal current produced remains load independent up to supply limitations. If the transmitter requires a 12V power supply the maximum load resistance (that is all instruments connected in series within the loop) is calculated by deducting the transmitter voltage and dividing the result by 20mA. The obvious advantage of loop powered transmitters lies in the reduction of cabling and installation costs. Only one DC power supply is required for a group of transmitters further reducing the hardware costs. Loop powered transmitters combining isolation and signal conversion utilise transformer coupling for power and signal coupling.

Block Diagram Loop Powered Isolator

2.3. 3-Wire Transmitter/Isolator

Note: *RL is input load of PLC or other process instrument.

3-wire transmitters have one power supply terminal in common with the output signal. These transmitters require a separate power supply but can support zero based output signals. As the power is supplied separately the transmitter can also have higher output drive and produce auxiliary power supplies to support input sensors such as loadcells.

2.4 Signal Powered Transmitter/Isolator

The input signal (signal being measured) is used to power the transmitter. Signal powered transmitters must have a live zero input and are typically used for input loop currents isolation and ac voltage measurement.

Application Drawing

Note: *12.- Voltage drop across a signal powered isolator is typically as high as 12V.

Care must be taken when designing systems using signal powered isolators. The maximum loop resistance relies entirely on the loop drive voltage. A signal powered device would represent a 600 ohm load in the loop. Again, the low installation and hardware cost, especially when a requirement for isolation becomes apparent during the commissioning phase, makes this concept very attractive.

An ac-current transducer powered by 5A from a current transformer does not function properly below 10% of its input signal. The output is also restricted to a zero based signal. Voltage transducers, when signal powered, will usually be calibrated for a signal swing around the normal operating point i.e. 200...260V, 50Hz for 240V, 50Hz mains monitoring.