Galvanic separation

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Under galvanic separation (also galvanic isolation ) is meant the avoidance of the electrical line between two circuits between which power or signals are to be exchanged. The electrical line is separated by electrically non-conductive coupling elements. With galvanic separation, the electrical potentials are separated from each other at this point and the circuits are then potential-free to each other .

This means that direct current or a current with a low frequency can no longer flow from one circuit to the other. The terms "galvanic separation" and galvanic coupling (direct current coupling ) were coined in connection with the research of the Italian doctor Luigi Galvani .

Galvanic isolation is necessary if circuits are to act on one another, but their reference potentials are to remain separated.

This can be necessary to avoid interference (common-mode interference or equalizing currents via earth loops), for example when transferring measurement signals, high-frequency or audio signals or with digital signals. For safety reasons, electrical isolation is used to separate touchable parts of electrical circuits from the dangerous mains voltage.

Another reason can be to avoid corrosion. Different metals laid in the ground form a galvanic element and would corrode if they are connected.

Technical implementation options

The galvanic separation always includes a non-electrical coupling. Various components such as transformers , capacitors , optocouplers , fiber optic cables or relays can be used for power or signal transmission . According to the current state of technical development, the coupling is realized almost exclusively by induction , influence , radiation or by means of a potential-free contact .

Inductive separation

Galvanic isolation in a transformer: The lines of the primary circuit (above, mains winding) do not come into contact with those of the secondary circuit (below, extra-low voltage winding).

Inductive galvanically isolated power and signal coupling requires isolating transformers, transmitters or coils that are close together. With them, an alternating voltage required for the respective application is transmitted and at the same time a galvanic separation is created between the primary and secondary circuit . With mains transformers (with the exception of autotransformers), the secondary side can be operated potential-free or at any other potential. For safety and to prevent capacitive leakage currents, the secondary side should only be earthed if the supply is from a medium-voltage or high-voltage network .

A protective insulation for the safe electrical separation is given in those power transformers, which have an enhanced or double insulation and with signs of protection class are characterized II or III.

For the inductive galvanic separation of analog measurement or audio signals, transformers, measuring transducers , transformer sheath current filters or inductively coupled electronic isolation amplifiers are used . In systems with long cables or several earthing points, this prevents hum loops .

In older televisions there were transformers between the device and the headphone connection, as these devices had one of the mains connection poles as reference potential (chassis).

Inductive transmission is also used for contactless charging (electric toothbrushes) and is used for special contact safety (safe electrical separation) in a damp environment.

Capacitive separation

Capacitive separation with two capacitors

Capacitors can transmit alternating quantities by shifting their charge , but not constant quantities . With suitable dimensioning, low-frequency alternating voltages (e.g. the mains voltage ) can also be blocked and only higher-frequency voltages are allowed through. The prerequisite is a sufficiently large frequency spacing of the transmitted signal, which is often achieved by modulating the signal on a carrier . In this way, small voltage-resistant capacitors can be used for potential separation.

The capacitive galvanic separation is structurally often simpler than the inductive one. It requires a capacitor for each conductor. The above-mentioned sheath current filters and isolating amplifiers can be implemented with capacitive coupling, see also section Electronic assemblies .

Historic tube-equipped television sets had capacitively coupled antenna inputs (2 times 500  picofarads ), since their reference potential (chassis) had mains potential.

Optoelectronic separation

Separation optocouplers, the two photodiodes comprises: (when At the same transmission characteristics in the two branches ) are represented by the
feedback from the non-linearity and drift of the transmission compensated .

The galvanic separation by means of optocouplers is particularly important with digital signals. With simple optical coupling, only signals with two discrete values ​​can be transmitted due to tolerances and non-linearities . Circuit components that adapt to logic levels are often housed in the coupler itself.

Analog signals can also be transmitted in a more complex circuit as an isolating amplifier (picture on the right).

Power transmission is possible with optocoupling if the radiation source is strong enough and the receiver has a series connection of photodiodes with a sufficient total area. Laser diodes are also used as the radiation source. The method is used for the bidirectional coupling of sensors without an electrical supply line.

Separation by relay

Relays work with magnetic fields and are in principle potential separating. Relays that guarantee safe electrical isolation can switch mains circuits and operate with safe extra-low voltage ( SELV ).

Further procedures

An electrical separation is still u. a. possible by means of pneumatics, piezo elements (e.g. piezo buttons ) and electromagnetic waves.

Electronic assemblies

Isolation amplifier

Electronic circuits for potential separation are known as isolation amplifiers. They are designed as an integrated circuit or as a module for industrial use - with inductive, capacitive or optoelectronic coupling and with a loadable output; The input circuit, output circuit and supply circuit can each be potential-free and voltage-proof up to the kilovolt range.

For the transmission of direct quantities or low-frequency alternating quantities, these are modulated onto a carrier with inductive or capacitive coupling ; this is transmitted potential-free; then the signal is again separated from the carrier . There are simplified circuit diagrams of integrated circuits of this type. With the isolating amplifier using an optocoupler, a constant value can be transmitted without modulation in the circuit shown above.

Interface isolating transformer

Serial interfaces usually require an accompanying ground reference or are at least only able to bridge potential differences to a limited extent. While ethernet and comparable data networks basically have transformer separation points, this is a. not the case with USB , RS232 or RS485 . Isolating assemblies are therefore used in a disturbed environment or for bridging potential. With RS232 and USB these modules are fed from the line, even if, as with RS232, there are no dedicated supply lines; the rectified logic level is used here for supply.

The signal transmission of such isolators takes place optically or inductively, the supply voltage that may be transmitted is transmitted inductively.

Areas of application

Galvanic isolation is required in the following cases:

  • for safety (e.g. medical devices, electrical toys, all mains-operated devices with protective low voltage , such as plug-in power supplies, audio devices, chargers, etc.); see also protective separation .
  • For metrological reasons: Potential separation of the voltage supply of measuring devices from the circuit of the voltage to be measured or potential separation in the measurement signal path (e.g. current clamps , current transformers )
  • to prevent hum loops and electromagnetic interference (e.g. audio devices , analog and digital signals in industry, transmitters in data networks , e.g. Ethernet )
  • to bridge potential differences, e.g. B. Sensors in high voltage systems.

If several electrical quantities are measured simultaneously and z. B. to be recorded in a computer whose reference potentials differ, then the transducers must be galvanically isolated from each other. This can be achieved, for example, with current transformers, isolation amplifiers or analog optocouplers.

A consequent galvanic separation is an effective protection against electromagnetic interference (see also electromagnetic compatibility ). This also applies if there is no galvanic connection to other circuits - long cables can also absorb interference without a conductive connection and pass them on to signal inputs. This so-called common - mode interference can be kept away from the signal inputs by means of electrical isolation. Industrial plants and equipment must e.g. B. so-called. Burst tests exist in which steep high-voltage pulses are capacitively coupled to the cable. The protection against radiated electromagnetic waves is also improved by potential-separating signal inputs.

Galvanic isolation is still required for remote data transmission on high-voltage lines and for protection against EMP .

Potential equalization

With complete galvanic separation of two circuits, one circuit can become statically charged compared to the other or it can assume a potential that deviates from earth due to leakage currents . It can lead to malfunctions or even to such high voltages that a subsequent electrostatic discharge , characterized by short high current pulses, destroys electronic components. As a precautionary measure, the two circuits can be connected with a very high resistance (generally ground to ground). Although this removes the potential separation, the high resistance means that only a very low equalizing current can flow. However, this loose connection is sufficient to ensure a defined potential and to allow charges to flow away continuously.

Protection class II audio devices have a metal housing, connected to the potential-free signal ground. Apart from safety reasons, this is good for avoiding earth loops, on the other hand the undefined reference potential can lead to the capture of mains hum. For this purpose, the manufacturers offer a so-called functional earth connection, an earth connection on the housing that can be used to earth the device if necessary. If the device has another earth connection (for example to a PC of protection class I), the connection remains free. In order to prevent earth loops and still create a defined potential, the functional earth and the housing ground are sometimes connected to one another or to the protective earth with a parallel circuit of a capacitor and a resistor .

Web links

Individual evidence

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