Relays are either electromechanical relays or solid-state relays. In electromechanical relays (EMR), contacts are opened or closed by a magnetic force. With solid-state relays (SSR), there are no contacts and switching is totally electronic. The decision to use electromechanical or solid state relays depends on an application’s electrical requirements, cost constraints and life expectancy.
Although solid-state relays have become very popular, electromechanical relays remain common. Many of the functions performed by heavy-duty equipment need the switching capabilities of electromechanical relays. Solid State Relays switche the current using non-moving electronic devices such as silicon controlled rectifiers.
These differences in the two types of relays result in advantages and disadvantages with each system. Because solid state relays do not have to either energize a coil or open contacts, less voltage is required to “turn” Solid State Relays on or off. Similarly, Solid State Relays turn on and turn off faster because there are no physical parts to move.
Although the absence of contacts and moving parts means that Solid State Relays are not subject to arcing and do not wear out, contacts on Electromechanical Relays can be replaced, whereas entire Solid State Relays must be replaced when any part becomes defective. Because of the construction of Solid State Relays, there is residual electrical resistance and/or current leakage whether switches are open and closed. The small voltage drops that are created are not usually a problem; however, Electromechanical Relays provide a cleaner ON or OFF condition because of the relatively large distance between contacts, which acts as a form of insulation.
Although Solid State Relays accomplish the same results as Electromechanical Relays, the physical structure and functionality of Solid State Relays is different from that of Electromechanical Relays.