Solid State Electronics : Electromechanical and Solid State Relays

 

1. Introduction to Relays:

   • Relays provide electrical connections between two or more points in response to a control signal.
   • Commonly used in HVAC systems to turn on and off like a switch.

2. Electromechanical Relays (EMRs):

   • Consist of input terminals for control signals and operating contact terminals.
   • Available in various shapes, sizes, and power ratings.
   • Types of operation: SPST (Single-Pole Single Throw), SPDT (Single-Pole Double Throw), DPDT (Double-Pole Double Throw).
   • Contacts can be normally open (NO) or normally closed (NC).
   • Used in heating systems as sequencers to time and stage electrical heat backup coils or time blowers.

3. Solid-State Relays (SSRs):

   • Provide electrical isolation between input and output contacts.
   • Act like conventional electrical switches but use semiconductors (SCR, TRIAC, or switching transistors) instead of mechanical contacts.
   • Designed to switch both AC and DC currents.
   • Main disadvantages of EMRs (mechanical parts, slower switch speeds, electrical noise) are addressed by SSRs.

4. Comparison of EMRs and SSRs:

   • EMRs: Mechanical devices with moving parts, limited contact life cycle, slower switch speeds, and electrical noise issues (contact bounce).
   • SSRs: No moving parts, high switching speed, high reliability, long life, reduced electromagnetic interference, zero voltage turn-on, and zero current turn-off.

5. Advantages of SSRs:

   • No moving parts to wear out.
   • Faster switching than EMRs.
   • Eliminates electrical noise and transients.
   • High degree of reliability and long lifespan.
   • Reduced electromagnetic interference.