Relay Quick Reference Guide

A relay is an electrically operated switch that allows a low-power control signal to switch a high-power load.

It acts as a bridge between two circuits:

• Control side — low voltage signals from MCU, sensor, or microcontroller
• Load side — high voltage/high current devices like motors, heaters, or lights

• Complete Isolation or Galvanic Isolation: This protects the microcontroller from voltage spikes, electrical noise, and faults coming from the load side
• Control Power Loads: Enables microcontrollers to control motors, lamps, and industrial equipment, etc.
• Versatility: Handles both AC and DC loads interchangeably.
• Compact Control: Avoids bulky manual switches in control panels.

Electromechanical Relay (EMR)

1. A coil is energized by a control voltage.
2. The coil’s magnetic field pulls an armature, which moves the contacts.

Contacts open or close to control the load.

Solid-State Relay (SSR)

• No moving parts — uses optocouplers + semiconductor devices (Triac, MOSFET) for switching.
• Faster, silent, and longer life.

1) Based on the Operation Principle

Type	Image	Description
Electromechanical Relay (EMR)		Uses a magnetic coil to physically move an armature and close/open mechanical contacts. It provides physical "click" feedback and total electrical isolation, but is prone to contact wear and "bounce."
Solid-State Relay (SSR)		Uses semiconductor devices (like TRIACs or MOSFETs) to switch the load. Because there are no moving parts, they offer silent operation, extremely long lifespans, and no contact bounce.

2) Based on Load Type

Type	Image	Description
AC Relays		Optimized for switching Alternating Current (mains power). Designed to handle the rapid voltage reversals of AC, making them ideal for HVAC, lighting, and industrial motors.
DC Relays		Built for Direct Current loads like those in automotive systems or battery banks. They often use specialized contact materials because DC arcs are more persistent and harder to extinguish than AC arcs.

3) Based on Contacts

Type	Image	Description
SPST  (Single Pole Single Throw)		The simplest form. It has one moving contact (Pole) and one fixed contact (Throw).  It acts as a basic ON/OFF switch for a single circuit.
SPDT  (Single Pole Double Throw)		One pole that can switch between two different contacts.  This allows you to select between two different paths or toggle a state (e.g., switching between a "Green" and "Red" indicator light).
DPDT  (Double Pole Double Throw)		Essentially two SPDT switches operated by the same coil.  It can control two separate, isolated circuits simultaneously, such as reversing the polarity of a DC motor.
4PDT  (Four Pole Double Throw)		Controls four independent circuits at once.  These are often seen in industrial control panels where multiple signals or power lines need to be switched by a single control signal.

4) Based on Application

Type	Image	Description
Power Relay		High-capacity relay for switching heavy AC/DC loads in HVAC and large appliances.
Signal Relay		Low-profile, sensitive relay designed for switching low-level data or audio signals.
Automotive Relay		Ruggedized 12V/24V relay built to withstand the heat and vibration of vehicle engine bays.
Reed Relay		Fast-acting, magnetically actuated switch sealed in glass for low-power signal integrity.
Time-Delay Relay		Includes an internal timer to delay the switching action after a trigger signal is received.
Latching Relay		Bi-stable relay that maintains its last state without constant power to the coil.

• Rated Coil Voltage → Nominal voltage required to energize the relay
• Coil Resistance / Power → Determines current consumption and power loss
• Pull-in Voltage → Minimum voltage at which relay operates (turns ON)
• Drop-out Voltage → Voltage below which relay releases (turns OFF)
• Rated Contact Voltage → Maximum voltage the contacts can switch safely
• Rated Contact Current → Maximum current carrying capacity of contacts
• Switching Capacity (VA/W) → Maximum load power the relay can handle
• Contact Resistance → Resistance across closed contacts (affects efficiency & heating)
• Insulation Resistance → Isolation between coil and contacts
• Dielectric Strength → Maximum voltage the relay can withstand without breakdown

Example General purpose AWHSH112D00G Relay Specifications:

• Single SRD-05VDC-SL-C — low-cost PCB relay.
• Omron G5V-1 — miniature signal relay.
• Omron G3MB-202P — SSR for AC loads.
• Panasonic TQ2 — telecom-grade DPDT relay.

• MCU GPIO →Transistor/MOSFET driver → Relay coil.
• Always add a flyback diode for EMRs.
• For SSR, no diode is needed, but an input LED current-limiting resistor is required.

Interfacing an Electromechanical Relay with a MCU using Transistor Driver:

Interfacing a solid-state relay with an MCU:

• Keep high-voltage traces separate from low-voltage control.
• Use wide traces for high current loads.
• Maintain creepage and clearance distances as per safety standards.
• Place the flyback diode close to the relay coil pins.

• Flyback diode – absorbs the coil’s back EMF.
• Snubber network – RC combination across contacts to reduce arcing.
• MOV/TVS – for surge protection on AC loads.
• Contact material – choose gold for small signals, silver alloy for high power.

• Welded contacts – from overcurrent.
• Pitted contacts – from arcing.
• Coil burnout – from overvoltage.
• Stuck armature – mechanical wear.
• SSR leakage – inherent in semiconductor switches.

1. Measure coil resistance with a multimeter.
2. Apply rated coil voltage — listen for a click.
3. Measure continuity on NO/NC pins.
4. For SSR — test with load or diode-mode check.

Note: Some SSRs may show small leakage current in the OFF state, which is normal and not a fault.

• Home appliances (washing machine, AC compressor).
• Industrial automation (PLC control).
• Automotive (headlights, fuel pumps).
• IoT/smart home (mains control from Wi-Fi modules).

1. Load current & voltage rating.
2. Coil voltage (5V, 12V, 24V).
3. Drive method (MCU direct, transistor).
4. Contact configuration (SPDT, DPDT).
5. Isolation requirements.
6. Size & mounting style.
7. Protection components needed.