The transmitter show has a ac or dc power supply that has power isolation from both the signal input and signal output.

The Auxiliary powered transmitter shown is also known as a 4-wire transmitter. The input signal is converted or isolated then re-powered to give a high-drive output signal. The auxiliary supply provides the necessary power for the transmitter to source (drive) the process signal output.

Only an isolator that provides true 3-way galvanic isolation will isolate all three paths (supply, input and output).

Any combination of input/output/supply can be obtained using this principle. Even applications requiring conditioning for signals with polarity change eg. ±10V can be accommodated.

(Back to top, Common Problems Due To Lack of Isolation

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 current isolation and ac voltage measurement.

(Back to top, Common Problems Due To Lack of Isolation

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.

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 transmitter lies in the reduction of cabling and installation costs.

(Back to top, Common Problems Due To Lack of Isolation

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.

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

Only an isolated 3 wire transmitter will separate the two paths of input and combined supply plus output.

(Back to top, Common Problems Due To Lack of Isolation

The transmitter show has a ac or dc power supply without power isolation from both the signal input and signal output.

The Auxiliary powered transmitter shown is also known as a 4-wire transmitter. The input signal is converted then re-powered to give a high-drive output signal. The auxiliary supply provides the necessary power for the transmitter to source (drive) the process signal output.

Due to the lack of an isolated power supply possible leakage paths must be considered that could potentially prevent the system from operating.

(Back to top, Common Problems Due To Lack of Isolation

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.

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

Due to the lack of an isolated power supply possible leakage paths must be considered that could potentially prevent the system from operating.