Solid State vs. Electromechanical Relays

When it comes to selecting relays, two in particular really stand out. Here is some general information on what makes each of these relays stand apart from one another.

A relay is an electrical device that usually incorporates an electromagnet, that is activated by a current or signal in one circuit to open or close another circuit. Much like a message is “relayed” from one person to another, the electromagnet sends messages through the current or signal within the circuit and gives directions.

The switching of a load circuit is controlled by a low power, electrically isolated input signal. Because of price, function, and availability, electromechanical relays (EMRs) have usually been a more popular choice, but in recent years, solid state relays (SSRs) have been outperforming them. There are many factors to consider when choosing which relay to go with — “service life, reliability, isolation voltage, on resistance, capacitance, and package dimensions.” These specific characteristics should be explored when determining which relay to use. Obviously, it depends on which application the relay is needed for, but these features in particular are useful in researching which relays to ultimately go with.

Information on the electromechanical relay

The way in which the electromechanical and solid state relays work varies greatly. Because electromechanical relays can function in both AC and DC load circuits, they can be used for many various purposes. These relays do not allow any kind of leakage current to flow through them when their contact is open. The closed contact has low resistance. It creates only minimal loss of power in the load circuit. “Multiple contacts are also possible so that a control signal can simultaneously switch multiple contact and thus multiple load circuits.” When comparing electromechanical relays to solid state relays, the moveable contacts and reset springs that are found in electromechanical relays are much more prone to wear and tear than solid state relays. Most electromechanical relays are normally “rated for minimum input to output isolation voltages of 1500 to 2000 VAC.”

electromechanical relay

electromechanical relay

Information on the solid state relay

Solid state relays have many advantages, some of them bring “bounce-free operation, immunity to EMI…small package size, and multi-function integration.” These relays also have a long lifespan, and “superior reliability when exposed to shock and vibration.” Solid state relays tend to be small in size. They also require less control power, and their response times are very quick. There is no switching noise, and loads can be switched with no bounce. In comparison to electromechanical relays, however, solid state relays tend to experience more power loss on the load circuit. They are usually mounted on a heat sink for this reason.

solid state relay

solid state relay

Because of the fact that there are no moving parts, solid state relays have “established switching lives of more than 1010 cycles, and exhibit bounce-free operation.” The input LEDs need low signal levels in order to work. This makes solid state relays compatible for TTL and CMOS controlled circuits, or products that have low power consumption. Material properties of the molding compound and lensing material determines the input to output isolation of a solid state relay. This allows for a minimum isolation voltage of 2500 VAC, and sometimes up to 5000 VAC. The packing of solid relays has gotten much smaller over the years as components have shrunk in size, in turn “allowing the designer to conserve PCB space, and makes them valuable in PCMCIA application.”

As previously stated, there are many differences between the electromechanical and solid state relays. Each relay has it’s advantages, as well as some drawbacks. Every one of the above stated characteristics should be considered very carefully and thoroughly when deciding between the electromechanical and solid state relays. When it comes down to it, it really depends on which applications the relays are needed for.

*Additional information and quotations courtesy of http://www.ssousa.com/appnote040.asp

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