Barclay McKenna, Marketing Manager at Omron, Electronic Components Business Europe, explores the trade off of Electromechanical versus solid state relays. Electromechanical Relays (EMRs) have been around since the 1980s but were destined to be rivalled by newer technologies such as that of solid state switching devices.

With current environmental concerns and energy conservation being major drivers, both electromechanical and Solid State relays (SSRs) have a key part to play in effective energy management, to ensure that systems or sub-systems are fully powered down and do not consume energy when they are not in use. The power efficiency of a system can be notably influenced by the way in which power is switched and applied. At the same time, the relays themselves consume energy.

A negative aspect of an SSR is that a semiconductor switch is never completely on or off. In the on-state, the closed resistance of the switching circuit will be higher; leading to more heat being generated when current is flowing. SSRs therefore need to be mounted on heatsinks, often several times the relays own weight. They are sensitive to ambient heat and must be derated if used in hot environments.

EMRs typically have an On resistance in the range of 100milliohms, whereas SSRs have an On resistance in the range of 10ohms. The low On resistance of the EMR allows for greater load current capability and less signal attenuation.

In applications where a circuit must be completely on or off with minimal on-state voltage drop, or no danger of injury or damage from leakage current, the electromechanical relay is the preferred choice. EMRs are also the best choice if heavy surge currents or spike voltages are likely to occur. EMRs cannot be falsely switched due to voltage transients, which can beset SSRs due to their faster switching characteristics.

Switching characteristics

Switching characteristics are a key consideration when making a choice. SSRs offer faster switching times than EMRs as there are no moving parts. Typical switching times of an SSR are in the order of micro to milliseconds where as an EMR will have switching times in the order of 5 to 10mS.

The highly versatile electromechanical relay can switch any ac or dc load up to their maximum rating. Contact resistance reduces as the load increases, eliminating any need for heatsinks. Although EMRs require substantial coil power, they can operate at full load over a wide temperature range. Most EMRs come with multiple poles and can control multiple voltages and circuits simultaneously.

EMRs typically have an output capacitance of less than 1 picoFarad, whereas SSRs typically have a capacitance of greater than 20 picoFarads. Capacitance becomes an issue in high frequency signals, which means that EMRs are a better option for high frequency applications.

Electrical isolation and the contact gap are important characteristics where EMRs triumph over SSRs. An EMR provides a complete physical break in the electrical switching circuit, this can be important for safety purposes.

The lifetime of a relay can be partly influenced by the number of switching activations. EMRs have moving parts (coil, core, arm, contact lever arms, spring mechanism) which over time can become physically degraded or worn out. Relay lifetimes are continually improving, though, and normally other components in a system will fail before the relay’s service life is exceeded. However, SSRs do have an advantage as they do not possess any moving parts so maintain integrity throughout their lifetime. SSRs are often preferred where the primary requirement is the ability to perform for tens of millions of operations, or more. The output resistance of an SSR will remain constant regardless of the amount of use. A typical EMR can provide an electrical lifetime of between 500K to 1M switching operations.

Solid state relays are becoming the choice in many applications, especially throughout the telecommunication and microprocessor control industries. The high reliability and long life mean less field failures and improved product performance. Low input signal levels are ideal for TTL or CMOS applications, and less power consumption translates to longer battery life in portable devices.

Solid-state relays are however still too expensive to build in very high current ratings, and so electromechanical contactors continue to dominate that application in industry today.

Other benefits of SSRs include totally silent operation and decreased electrical noise. SSRs will function with a lower minimum latching current over a wide range of input voltages and consume little power even at high-voltage limits. With a clean, bounce-less switching operation, SSRs don’t produce a switching arc, making them suitable for use in hazardous environments. Additionally, SSRs will be less sensitive to environmental storage and operating conditions such as mechanical shock, vibration and humidity.

As manufacturers respond to opportunities presented by new applications from hybrid vehicles to set top boxes, to industrial control networks and wireless base stations, EMR and SSR designs are each continually progressing to keep pace.