Electromechanical vs. Solid State Relays: The Pros and the Cons

Electromechanical relays (EMR) and Solid State Relays (SSR) have fundamental differences that need to be considered in any design. In order to help simplify the process of deciding on the appropriate technology, we’ve identified the principal pros and cons for each type of device. While not an exhaustive list, we’ve covered the most important ones for you, whether you’re an engineering or purchasing professional.

Positive Points (Pro) of using EMRS

Electromechanical relays (EMR) are simple switching devices that have been around since their invention in 1835 for use in long distance telegraph circuits. They are rugged, uncomplicated, and elegant in their design. From a design engineering standpoint the PROS for using them in a product or system include the following:

  • Lower initial cost vs. solid state – their uncomplicated design makes them efficient to produce.
  • Complete electrical isolation – no current leakage across the switching junction.
  • Able to tolerate high current & voltage transients – lots of protection for your circuit.
  • Insensitive to EMI/RFI – not affected by noisy circuits or electrical fields.
  • Higher open resistance due to air gap – no circuit current flows.
  • Lower closed resistance – physical contact ensures closed circuit.
  • Available with numerous poles/circuits (up to 8 or more) – versatile use from one device.
  • Many configurations available – helps to reduce component count.
  • Multiple package & feature options – to match your device requirements.
  • Typical failure mode is open – easy to troubleshoot.

Negative Points (Con) of using EMRS

A technology and a product that has been around for over 180 years, however, is bound to have some engineering downsides. The CONS of using EMRs in your design include:

  • Higher control (coil) power consumption than SSRs – EMRs need more of a kick to actuates.
  • Contact arcing – over time can cause pitting and eventual open/short failure in your device.
  • Corrosion, oxidation or contaminates can degrade contacts – can lead to open/short failure.
  • Contact bounce possible – shock and vibration can affect circuit integrity.
  • Generate EMI/RFI – not good in noise intolerant systems or devices.
  • Can be orientation sensitive – installation orientation may affect performance on some devices.
  • Magnetic field sensitivity – performance of some devices can be degraded by external magnetic fields.
  • Subject to mechanical degradation – residual magnetism or internal component flexing and stretching can reduce performance over time.
  • Potential higher overall cost over equipment life – maintenance and repair must be considered.
  • Can be noisy – can generate physical noise (60 Hz “hum”, contact closure “click”, etc.).

Positive Points (Pro) of using SSRS

Solid State Relays (SSR) were invented in 1972 by Crydom engineers. For almost 50 years they have proven their worth in electrical and electronic circuits. The design PROS for using them include:

  • Substantially longer life vs mechanical – with no moving parts, little degradation over lifespan.
  • Low control power consumption – compatible with most IC logic families without the need of buffers or drivers.
  • Faster on/off cycling – more operations per given timeframe.
  • Very fine proportional output control – match power output precisely to your needs.
  • No contact arcing – safer for hazardous environments.
  • No contact bounce – less problem with digital circuits.
  • Not orientation sensitive – fewer errors at installation or replacement.
  • Not affected by shock and vibration – solid state means no moving parts to break or disengage.
  • Not affected by external magnetic fields - can be used in environments with magnetic noise.
  • Less electromechanical interference than EMRs – reduced effect on surrounding components.
  • Silent operation – no moving parts to produce noise, no 60Hz “hum”.
  • Many functions in a single package – can reduce component count on the board.
  • Potentially lower overall cost over lifetime – no moving parts means less repair or replacement.

Negative Points (Con) of using SSRS

Despite the fact that, by definition, SSRs have no moving parts to wear out or break, they do have their downsides when used in a circuit. The CONS for using SSRs are:

  • Higher initial cost than mechanical – more complicated to produce translates into higher cost at acquisition; may require extra heat dissipation components.
  • Generate more heat than mechanical – heat generated by semiconductor switching elements must be dissipated or parameters can be affected.
  • Current rating may be degraded based on ambient temperature – junction temperature must be kept within operating limits for proper operation.
  • Voltage or current transients can cause damage or affect operation – unplanned switching can be caused by transients.
  • Susceptible to EMI/RFI – noisy circuits can affect operation.
  • Lower off state resistance – lower ability to withstand momentary overload.
  • Higher on state resistance - higher threshold current needed to energize.
  • Most are single pole/circuit (some available with up to 4).
  • Normally Closed/ON function has limited availability.
  • Changeover form/circuit not supported.
  • Most typical failure mode is Shorted/Closed.