Limitations of Electromechanical Relays

Limitations of Electromechanical Relays

Although electromechanical relays are versatile, they do suffer from a number of limitations. Compared to modern semiconductor devices, they can be expensive to manufacture, have limited contact cycle life, take up a lot of space, and switch slowly.

These limitations are especially true for high power contactor relays.

solid state relay

To address these limitations, many relay manufacturers offer "solid state" relays that use SCR, TRIAC, or transistor outputs instead of mechanical contacts to switch controlled power supplies. The output device (SCR, TRIAC or transistor) is optically coupled to the LED light source inside the relay.

The relay is turned on by energizing this LED, usually using a low voltage DC power supply. This optical isolation from input to output rivals the best performance an electromechanical relay can offer.

Solid State Relay Limitations

Advantages of Solid State Devices

As solid state devices, there are no moving parts to wear out, and they are able to open and close faster than any mechanical relay armature can move. No sparks between contacts, no contact corrosion problems.

A significant advantage of solid state SCR or TRIAC relays over electromechanical devices is their natural tendency to open the AC circuit only at the point of zero load current. Because SCRs and TRIACs are thyristors, their inherent hysteresis maintains circuit continuity after the LED is powered down until the AC current drops below the threshold (holding current).

In practice, this means that the circuit never breaks in the middle of a sine wave peak. Such untimely interruptions in circuits containing a large number of inductances often result in large voltage spikes due to sudden magnetic field collapses around the inductance.

This does not happen in circuits broken by SCRs or TRIACs. This feature is called zero-cross switching.

Disadvantages of Solid State Relays

One disadvantage of solid state relays is that they tend to fail "short" on their output, whereas electromechanical relay contacts tend to fail "open". In either case, the relay could fail in the other mode, but these are the most common failures.

Because the "fail open" state is generally considered safer than the "fail closed" state, electromechanical relays are still preferred over solid state relays in many applications

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