Tuesday, July 23, 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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