Why do we need a galvanic isolation?

Very often in electrical devices, there is a need to eliminate the electrical connection between high power voltage and low voltage control circuits. In other words, it is necessary to protect low-voltage devices from the voltage of the power circuits in the hundreds, or even thousands of volts. Technically, this means that in this system or electrical device it is necessary to exclude the flow of current through common circuits. The absence of current means that there is a large ohmic resistance between the common wires of the two devices, which is equivalent to breaking the circuit. This task is solved by galvanic isolation - a device that excludes galvanic connection between electrical devices.

Imagine a conventional industrial electric motor. Under production conditions, most engines have an operating supply voltage above 200V, which is dangerous for the personnel. Therefore, supply of a supply voltage to the windings, i.e. The engine is switched on, with the help of additional devices, switching power circuits. On the other hand, switches also need to be controlled, for example, by a button, and thus galvanic isolation protects the operator from hitting with dangerous voltage.

By themselves, switching devices, such as contactors and starters, are devices in which the design excludes electrical contact between the input (coil contacts of the electromagnet) and the output (power contact group of the starter). Communication between them is carried out only through the mechanical interaction of the magnetic field with the construction elements of the starter, so that the high voltage of the motor supply does not fall on the control panel.

There are other options for a technical solution of the galvanic isolation. First of all, these are transformers. With their help, the galvanic isolation of power is easily solved. This method was especially widely used in electro-radio engineering for domestic use. The fact that the voltage supply of household appliances is dangerous for humans. For example, in the absence of a galvanic isolation between the household electric grid and the television signal processing board, a life-threatening potential will be on all metal elements of the TV's design, and access to the "TV internals" is quite accessible to home "self-made". The problem of protection against electric voltage for such devices is solved simply: at the entrance of the household appliance a transformer is placed between it and the power grid. Its primary winding is connected to the network, and the secondary winding supplies the current induced in it to power the TV. This is where one of the useful features of the transformer manifests itself - with its help galvanic isolation of the analog signal is realized , which is widely used in various devices.

With the development of power semiconductor devices, switching devices - optiothyristors - with an optocoupler (light) control channel were widely used. The input (control) circuit of the optocoupler contains a light bulb or LED that is turned on when a control signal is applied. The light flux hits the photosensitive thyristor control electrode, which includes an anode-cathode power circuit. This ensures 100% absence of galvanic input-output connection. Another variant of optocouplers are opto-transistors, which easily solve the galvanic isolation of an analog signal, for example, in gauges of measuring instruments.

The use of galvanic interchanges in engineering has a much larger range of solved problems than is covered in this article. Modern technologies constantly add to the list of such devices for engineering applications.