Power interface

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A current interface or current loop interface works with a current adaptation . It is used for information transmission in harsh environments, where high interference levels must be expected, so that the otherwise usual transmission by means of voltage levels ( voltage adjustment ) does not work with sufficient reliability.

Analog telephones in industrial applications and in electronic musical instruments ( MIDI ) preferably work by means of an impressed electrical current . Striking also was Telegraph .

In the simple case of the current interface, the transmitter or transducer is an adjustable current sink that is supplied with energy from a voltage source via the signal lines. The receiver consists of an ammeter or a low-resistance terminating resistor ( shunt ) located in the current loop , on which the measuring current is determined via the voltage drop.

Circuit example

Analog power interface

If analog voltage values ​​are transmitted from a transducer to a receiver via a long cable, they will be falsified. The cause is the necessary measuring current, which creates a voltage drop in the cable due to the undesired cable internal resistance. The measurement current and thus also the measurement error can be minimized by using inputs with the highest possible resistance, but at the expense of a high sensitivity to capacitive interference. Inductively coupled interference voltages cannot be eliminated in this way.

For this reason, measured values ​​are preferably transmitted as current (4… 20 mA) over greater distances. The internal resistance and thus the voltage drop of the cable play no role in power transmission as long as the voltage is sufficient. The insulation resistance now has an influence, but it can be kept very high with today's insulation materials. The current of the measured value is often sufficiently large compared to the interference currents caused by capacitive coupling (e.g. 50 Hz hum). In principle, inductive coupling no longer has any interference.

If a current interface that corresponds to the 4… 20 mA standard is used, further advantages can be achieved:

  • The transducer (sensor) can be supplied with a minimum current of 4 mA, and only a forward and return line is required.
  • Cable break (current is <4 mA) can be detected.

A distinction is made between active and passive sensors and the inputs for measured value processing.

  • Active input for passive sensors: The input measures the current and has a voltage source. The sensor is only a controlled current sink and can therefore be 2-pole (2-wire)
  • Passive input for active sensors: The input measures the current (e.g. with a shunt ). The sensor is a controlled power source and must therefore have a power supply unit
  • Passive input and passive sensor: an additionally required external voltage source is connected in series with them.

Digital power interface

In digital transmission, instead of different voltage levels at the current interface, the presence or absence of a current is evaluated as binary information.

The 20 mA current loop interface is standardized according to DIN 66258. It operates with a maximum open circuit voltage level of 24  V . Because this voltage leads to considerable crosstalk with steep signal edges , transmission with 20 mA interfaces in telecommunication cables is not permitted.

See also

literature

  • Friedrich Wittgruber: Digital interfaces and bus systems. 2nd edition, Springer-Verlag, Berlin / Heidelberg 2013, ISBN 978-3-663-01615-1 .
  • Karl Friedrich Früh (Hrsg.): Manual of process automation. Process control technology for process engineering systems, 4th revised edition, Oldenbourg-Industrie-Verlag, Munich 2009, ISBN 978-3-8356-3142-7 .
  • Harald Schumny (Ed.): Personal computer in laboratory, test and test field. Springer Verlag, Berlin / Heidelberg 1988, ISBN 978-3-540-18871-1 .
  • Martin Storm (Ed.): Workshop information systems. Springer Verlag, Berlin / Heidelberg 1993, ISBN 978-3-540-53439-6 .

Individual evidence

  1. ↑ https://www.beuth.de/en/standard/din-iec-60381-1/1254835 DIN IEC 60381-1: 1985-11 Analog signals for regulation and control systems; Analog DC signals

Web links