[3] Hardwired Relay Logic Technology

Relay logic

a control system using electromechanical relays and wired connections

Relay logic

used before PLCs

1. Relay

2. Input Devices

3. Output Devices

Components of Relay Logic Systems

Relay

an electromechanical switch made up of an electromagnet and a set of contacts

Relay

allows a low-power electrical circuit to control a higher-power circuit

1. Coil

2. Armature

3. Contacts

4. Spring

5. Frame/Base

Relay Parts

Coil

relay part pertaining to a wire wound around a core that generates a magnetic field when energized; this magnetic field is responsible for moving the relay’s internal mechanical parts

Armature

relay part pertaining to a movable metal lever that is attracted by the magnetic field when the coil is energized

Armature

relay part that physically moves the contacts to either open or close the circuit

Contacts

relay parts pertaining to metal connections that either complete (closed) or interrupt (open) the electrical circuit; normally open, normally closed, common terminal

Spring

relay part that returns the armature to its original position when the coil is deenergized

Frame/Base

relay part pertaining to the physical structure that holds all the parts together

1. Electromagnetic Relay

2. Solid State Relay

3. Timer Relay

Types of relay

Electromagnetic relay

common and basic industrial relay

Solid state relay

a type of relay that uses semiconductors instead of mechanical parts

Timer relay

a type of relay that introduces a delay before switching

1. SPST (Single Pole Single Throw)

2. SPDT (Single Pole Double Throw)

3. DPST (Double Pole Single Throw)

4. DPDT (Double Pole Double Throw)

Types of relay based on the number of contacts

SPST (Single Pole Single Throw)

a type of relay containing 1 common terminal and 1 NO or NC terminal

SPDT (Single Pole Double Throw)

a type of relay containing 1 common terminal, 1 NO terminal, and 1 NC terminal

DPST (Double Pole Single Throw)

a type of relay containing 2 common terminals and 2 NO or NC terminals

DPDT (Double Pole Double Throw)

a type of relay containing 2 common terminals, 2 NO terminals, and 2 NC terminals

RELAY SPECIFICATIONS"

1. Coil Voltage

2. Coil Power

3. Contact Configuration

4. Contact Voltage Rating

5. Contact Current Rating

6. Switching Time

Relay specifications

Coil Voltage

relay specification pertaining to the required voltage to energize the relay coil; common values: 5V, 12V, 24V, 48V, 110V, 220V (DC or AC)

Coil Power

relay specification pertaining to the power required to operate the relay

Contact Configuration

relay specification pertaining to the number of pole and throw (SPST, SPDT, DPDT, 4PDT, etc.)

Contact Voltage Rating

relay specification pertaining to the maximum voltage that the contacts can switch safely; expressed in volts (DC or AC)

Contact Current Rating

relay specification pertaining to the maximum current that the relay contacts can handle; expressed in amperes

Switching Time

relay specification indicating the operate time and the release time

Operate Time

under switching time, this is the time required to activate the contacts

Release Time

under switching time, this is the time required to deactivate the contacts

Input Devices

these are devices that are connected to the relays in the circuit to act as inputs; include switches, push buttons, and sensors

Output Devices

devices that are connected to the relays in the circuit to act as outputs; include indicator lights, solenoids, motors, contactors, buzzers, and horns

Ladder Diagram

a symbolic representation of an electrical circuit using horizontal rungs and vertical rails

Rung

each represents a control action, such as starting a motor, turning on a light, or activating a relay

Ladder Diagram

used in relay-based systems and Programmable Logic Controllers (PLCs)

Relay Coil

What does this ladder diagram symbol represent?

Normally open relay contact

What does this ladder diagram symbol represent?

Normally closed relay contact

What does this ladder diagram symbol represent?

Normally open push button

What does this ladder diagram symbol represent?

Normally closed push button

What does this ladder diagram symbol represent?

Solenoid valve

What does this ladder diagram symbol represent?

Pilot light

What does this ladder diagram symbol represent?

Motor

What does this ladder diagram symbol represent?

Buzzer

What does this ladder diagram symbol represent?

Momentary

means that the contacts are actuated when pressed but will return to the initial state when released

Indicator lights

mall visual signaling devices used in relay logic circuits to indicate the status of a machine, process, or electrical system

Red

pilot light meaning fault, stop, emergency

Green

pilot light meaning normal operation, running

Yellow/Amber

pilot light meaning warning, caution, standby

Blue

pilot light meaning special condition, operator attention needed

White

pilot light meaning general system active, power on

1. Voltage Rating

2. Current Consumption

3. Mounting Type

4. Color Coding

5. Durability & Protection (IP Rating)

Pilot Lamp Specifications

Voltage rating

a pilot lamp specification that must match the system voltage (24V DC, 110V AC, 220V AC)

Current consumption

a pilot lamp specification where LED lamps are pertained as more energy-efficient

Mounting type

a pilot lamp specification that may be a panel mount, screw-type, or push-fit

Color coding

a pilot lamp specification wherein proper color standards for system status must be ensured

Durability & Protection (IP Rating)

a pilot lamp specification wherein industrial environments require IP65/IP67-rated lamps for dust/water resistance

On/Off-delay Circuit

a type of relay circuit that has an on or off delay coil wherein the set of contacts changes status depending on the set delay in the timer

On-delay

set of contacts change status after the set delay

Off-delay

set of contacts change status instantly but return to its initial state after the set delay