Relays

What are the relays used for?

The primary function of the relays is usually to separate the control circuits from the power circuits for isolation purposes, e.g. to drive a high voltage or current from a lower control, and in some applications also to ensure operator safety. Relays were mainly used in very large quantities in electromechanical PSTN telephone switching systems; they are still used, but to a lesser extent as they have been replaced by electronics and computers. Relays can also be used to create suitable logic functions, as was the case with the first computers or in pinball machines. This is still the case in many signal boxes, designed before the 1990s. Relays were used in very large quantities in electromechanical PSTN telephone switching systems; they are still used, but to a lesser extent because they have been replaced by electronics and computers in today's switches. The service life of low-end electromagnetic relays is relatively short due to wear and tear on the contacts during repeated switching. However, there are solutions to extend the life of these relays. Some relays are designed and tested to withstand a minimum of 10 million operations.

Evolution of the electromechanical relays

The fully electronic, solid-state relays advantageously replace the electromechanical relays in many situations. Why is that so? Better switching speed, quiet operation, insensitivity to vibrations, are some of its advantages over electromechanics. On the other hand, it can poorly withstand high short-circuit currents, even though it is generally equipped with internal protection. It cannot be used either as a safety cut-off, because of the leakage current and the permanent short circuit that can be generated when the component flashes (unlike the electromechanical relay which sometimes "sticks" but, most often, opens when the current returns to zero). Several so-called "safety" relays are designed to offer a guarantee of opening in the event of a failure. For fixed railway applications (signaling) these relays must usually comply with certain standard which depend on the country. Most railway networks use relays of a very similar design. The search for alternative solutions using solid state relays has not been successful so far because the probabilities of the different failure modes are too close.

Which type of relay to choose?

The electromechanical relay, by its switching principle, is similar to a mechanical switch whose operation would not be manually, but from the power supply of a coil called excitation coil or control coil. The coil excitation field creates a magnetic field even when it is traversed by a weak current. It then causes the opening or closing of a switch connected in series in a circuit containing a motor or a lamp, consuming a high current. This is why it is called power adaptation because the relay can control a power greater than the excitation power. Most of the time, it will be associated with a transistor. On the other hand, a solid state relay has no moving parts. The transmission of information is carried out by infrared using photo-triac, phototransistor or photodiode associated with a control circuit. This absence of moving parts gives it a longer life and an absence of noise during use. If you have questions or concerns, please contact our technical department. The topic of relays may seem complicated, but we will be happy to share our experience with you and advise you on the best solution for your situation.