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What is a passive transmitter repeater?

A passive transmitter repeater comprises an input for a 4 … 20 mA transmitter signal from a measuring transducer with a 2-conductor system. The measuring transducer obtains its entire supply via this 2-wire circuit.

The output is connected to the process control or a PLC at 4 … 20 mA. The passive transmitter repeater requires no additional voltage supply because the auxiliary energy is obtained from the feeder circuit without the results becoming falsified.

Can the isolation amplifier supply a transmitter?

Yes, a transmitter repeater or transmitter repetition amplifier can supply the 2 or 3-wire transmitter with energy by galvanically isolated means.

A 2-wire measuring transducer takes its voltage supply from the measuring signal, the current for which is provided by the transmitter repeater. In the case of 3-wire measuring transducers an additional input terminal with feeding voltage is available to the transmitter.

Is it possible to electrically isolate HART transmitters?

Yes, when using the HART DC 52500, a 2-wire measuring transducer can be fed and operated by means of HART communication. The 4 - 20 mA measured value with the modulated HART signal is conveyed smoothly over the isolating distance; HART communication is bi-directional, i.e. it is transported in both directions across the electrical isolation.

What is meant by HART Communication?

Hart Communication is a standardized method of additionally transferring bi-directional communication on an analog measuring value. The modulation and the data log are defined exactly in the specifications of the HART Communication Foundation (

This is an FSK signal which is modulated in the measuring transducer to the measuring value, 4 - 20 mA output current of the transmitter. A HART modem is installed in the process controller; it transports the transmitted digital data to a process computer.

The great advantage of this concept is that the analog measured value can be evaluated conventionally. Furthermore, but without any additional cabling work, all kinds of information such as measured values, configuration data or set parameters can be made available to the user in digital form.

Can I connect up my 3-wire transmitter?

Yes, some of our transmitter repeaters have a terminal for the feeder voltage. So you can supply your transmitter from the feeder voltage and deliver the measuring signal to the transmitter repeater on separate terminals (the output current).

Transmitter supplied from the isolation amplifier

Our transmitter repeaters supply the feeder voltage for your transmitter.

2-wire transmitters regulate their own power consumption proportionate to the measured value; here the 4 - 20 mA connection provides the auxiliary energy for the transmitter and the level of the current is at the same time the output measured value.

3-wire transmitters usually have an active current output for the measured value and additionally terminals for the supply voltage (auxiliary energy).

HART Communication across the isolation distance

In addition to the analog 4…. 20 mA current signal, many 2-wire transmitters permit digital communication, which is modulated on the output current. A popular protocol is the Hart Communication of the HART Communication Foundation (

It is the function of an appropriate transmitter-repeater amplifier to also transmit the modulated frequency in unadulterated form across the isolation distance. In view of the fact that HART Communication must function in both directions, the HART transmitter repeater has a return channel which transmits the digital queries from the master to the slave (field device, transmitter) against the normal analog data flow.

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