The Difference Between Electrical Relays

Electromechanical Relays vs Solid-State Relays

By: Megan Tung

A relay is a power switching solution used to allocate power without manually opening and closing a switch. A relay is utilized when it is necessary to switch a small amount of power to a larger amount of power. There is a very high, almost infinite resistance when nonconducting (open) and a very low resistance when conducting (closed). They can be used in various applications, such as heating, lighting, and motion control.

Electromechanical Relays (EMR)

Electrical Relay The basic parts and functions of electromechanical relays include the frame, coil, armature, and contacts. A heavy-duty frame contains and supports the parts of the relay. A wire is wound around a metal core, and the coil of wire causes an electromagnetic field. The armature is the moving part of the relay, which opens and closes the contacts. An attached spring returns the armature to its original position. The contacts are the conducting parts of the switch that makes (closes) or breaks (opens) a circuit. When the contacts make a connection there is typically a “click” sound.

In the image above the plastic outer case of the relay is removed. On the left are the two spring contacts. On the right is the electromagnet coil. When the current flows through the coil it turns into an electromagnet, which pushes a switch to the left, forcing the spring contacts together, and completing the circuit they are attached to.

Solid-State Relays (SSR)

Solid-State Relay Solid-State relays consist of an input circuit, a control circuit, and an output circuit. The input circuit is the portion of a relay’s frame to which the control component is connected. It performs the same function as the coil of the EMRs. The circuit is activated when a voltage higher than the relay’s specified pickup voltage is applied to the relay’s input. The control circuit is the part that determines when the output component is energized or de-energized. It functions as the coupling (providing electrical isolation) between the input and output circuits. The output circuit is the portion of the relay that switches on the load, performing the same function as the mechanical contacts of EMRs. Instead of a physical switch, an optoisolator or optocoupler is used. The input will turn on an internal LED, which will shine a beam of light onto a photodiode. The diode will turn on an SCR, thyristor, or MOSFET, allowing flow to the current to flow to the output pins, causing it to “switch” on.

To learn how to wire a solid state relay with ease, check out Tech Tip: Wiring a Solid-State Relay.

Differences

Unlike EMRs, SSRs have no moving parts that will wear out, and therefore no contact bounce issues. Due to an optoisolator rather than moving parts, the life span of the SSR is often longer than the EMRs. The SSR is able to switch “ON” and “OFF” much faster than a mechanical relay’s armature can move. When SSRs are in the on-state, substantial resistance is present, which can lead to significant heat generation when current is flowing. Therefore, SSRs should be mounted on heatsinks. Solid-state relays normally have only one output contact, while electromechanical relays can multiple output contacts.

Relay Applications


Solid-state relays can be used for:
  • Applications requiring high speed and frequent switching operations
  • Applications in high vibration environments
  • Applications where the relay has to be located near sensitive automation components, such as PLC’s, HMIs, and temperature controllers

Electromechanical relays can be used for:
  • Applications requiring a wide output range from the relay
  • Applications using motors and transformers requiring high starting amps
  • Applications where the initial installation budget is very limited
Megan Tung is an intern at Jameco Electronics. Megan is a rising junior at the University of California, Santa Barbara. She is interested in photography, music, business, and engineering.