Thursday, July 25, 2019

Electromechanical Relays (EMRs) vs. Solid State Relays Comparison (SSRs)

Electromechanical Relays (EMR) Pros (Advantages)
  • Lower initial cost compared to solid state relays.
  • Provides complete electrical isolation.
  • Tolerates high current & voltage transients.
  • Insensitive to electromagnetic interference (EMI) / radio frequency interference (RFI).
  • Higher open resistance (air gap).
  • Lower closed resistance.
  • Available with many poles/circuits (up to 8 or more).
  • Many different circuit configurations available.
  • Multiple packaging & feature options.
  • Most typical failure mode is open.

Electromechanical Relays (EMR) Cons (Disadvantages)
  • Higher control (coil) power consumption.
  • Contact arcing can cause pitting & eventual open/short failure.
  • Contacts can be affected by corrosion, oxidation or contamination.
  • Contact bounce possible due to shock & vibration.
  • Generates electromagnetic interference (EMI) / radio frequency interference (RFI).
  • Can be orientation sensitive.
  • Can be affected by external magnetic fields.
  • Subject to mechanical degradation over time (residual magnetism, armature flexing, spring stretching).
  • Potential higher overall cost over equipment life.
  • Can be noisy.

Solid State Relays (SSR) Pros (Advantages)
  • Substantially longer life compared to mechanical relays.
  • Low control power consumption.
  • Faster on/off cycling.
  • Allows very fine proportional output control.
  • No arcing (safer in hazardous environments).
  • No contact bounce.
  • Not orientation sensitive.
  • Switching not affected by shock & vibration.
  • Not affected by external magnetic fields.
  • Less electromechanical interference.
  • Silent operation.
  • Allows many functions in a single package.
  • Potentially lower overall cost over equipment life.

Solid State Relays (SSR) Cons (Disadvantages)
  • Higher initial cost compared to mechanical relays.
  • Generate more heat compared to mechanical relays.
  • Current rating may require derating based on ambient temperature.
  • Voltage or current transients can damage or affect operation.
  • Susceptible to electromagnetic interference (EMI) / radio frequency interference (RFI).
  • Lower off state resistance.
  • Higher on state resistance.
  • Most are single pole/circuit (some available with up to 4).
  • Normally closed/ON function available on a limited basis.
  • Changeover form/circuit not supported.
  • Most typical failure mode is shorted/closed.

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