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Solid state relays

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Solid state relays

All relays available on the market can be divided into electromagnetic and semiconductor ones. Both of them perform the same function in the circuits, i.e. they close and open the electric circuit, influencing the operation of other devices in a given control system. However, they perform this task differently, due to differences in construction. Their specificity is also different, which predisposes the relays to suit their specific applications.

Semiconductor relays in electrical installations

Solid state relays, or SSR (Solid State Ralay), unlike their electromagnetic counterparts, are entirely electronic devices. They do not contain moving parts, and they use thyristors, triacs (or alternistors) and transistors for switching. The most common solution is the use of triacs and a pair of thyristors.

Solid State Relays - Noise Protection, Speed, Durability, and More

Semiconductor relays are very often used in widely understood electrical engineering. In relation to electromagnetic devices, they have many advantages that affect their popularity in industrial automation, residential buildings and public utility facilities. The characteristics of solid state relays are:

  • durability,

  • shock and vibration resistance,

  • resistance to chemicals and dust,

  • mechanical resistance through permanent bond with the housing,

  • speed of action,

  • possibility of switching on high inrush currents,

  • protection against interferences electromagnetic, electromechanical and radio frequencies,

  • no electric arc,

  • no moving parts (quiet operation),

  • low power needed to drive.

The disadvantages of semiconductor relays, in turn, include large voltage drops, leakage current and lack of overvoltage resistance. These parameters are much better coped by electromagnetic relays, which are characterized by a negligible voltage drop, no leakage current and resistance to overvoltage.

Semiconductor relays generate large amounts of heat under heavy loads. This is the main reason for reducing the long service life of the device. However, it can be eliminated by using a properly selected heat sink, and also by using thermal paste during assembly and always ensuring adequate air circulation. It is worth installing the relays for rail mounting with a heat sink so that there is plenty of free space around the system.

How do solid state relays work?

SSRs are devices designed to control the current load with the use of a semiconductor controlled by a galvanically isolated electronic circuit. This separation is possible thanks to the use of an optoelectronic system (optocoupler), which consists of a transmitter (an infrared emitting diode) and a receiver (e.g. a photo transistor) and converts the electrical signal into an optical one at the input and vice versa on the output side.

In the idle state of a semiconductor relay, the current does not flow through the optocoupler - it remains switched off and has a very high equivalent resistance. After applying voltage to the input, the optocoupler diode is activated, the element begins to conduct light, and consequently switches on the load circuit.

Solid state relays - lighting and electric drive control, temperature control and other functions

Semiconductor relays are widely used in industry, medicine, agriculture, as well as in households and the broadly understood service sector. Areas where solid state relays are used include:

  • temperature control,

  • turning on transformers,

  • lighting control (especially industrial and public),

  • electric drive control.

SSRs are elements of various devices and systems: dryers, electric and induction furnaces, injection molding machines, X-ray machines, incubators, photocopiers and many others.