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Definition ;Relays are devices that operates as an electrical switch, opening and closing under electromagnetic condition. It is a combination of electromagnet and solenoids with switch contact configuration.

Relay consist of four main parts:i. Coilii. Armature

iii. Springiv. Set of contact

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i. Coil – to generate the necessary magnetic field to actuate the armature and the contacts.

ii. Core – concentrate the magnetic fieldiii. Armature – the moving part of the

magnetic system which closes and opens the magnetic circuit and acts via an actuator to the moving relay contact.

iv. Spring – for quick return of the moving contact to the normal condition on removal of the coil power.

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v. Contact - Stationary contact - moving contact (contact being moved

by the magnetic system to switch the load circuit)

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Relay have two circuit: A control circuit ( shown in green) and a load circuit (shown in red). The control circuit has a small control coil while the load circuit has a switch. The coil controls the operation of the switch.

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Relay energized (On) - Current flowing through the coil (pin

1&3) create a small magnetic field which causes the switch to close, pin 2 & 4. The switch , which is part of the load circuit is used to control an electrical circuit that may connect to it.

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Relay De – energized (Off) - When current stop flowing through the

control circuit (pin 1&3), the relay become de-energized. Without the magnetic field , the switch opens and current is prevented from flowing through pin 2 and 4. The relay is Off.

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Relay Types

1. Horizontal Type Armature2. Bell Crank Type Armature3. Vertical Action Type Armature4. Clapper Type Armature

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Choosing a relayYou need to consider several features when choosing a relay:

Physical size and pin arrangement If you are choosing a relay for an existing PCB you will need to ensure that its dimensions and pin arrangement are suitable. Coil voltage The relay's coil voltage rating and resistance must suit the circuit powering the relay coil. Many relays have a coil rated for a 12V supply but 5V and 24V relays are also readily available. Some relays operate perfectly well with a supply voltage which is a little lower than their rated value.

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Switch ratings (voltage and current) The relay's switch contacts must be suitable for the circuit they are to control. Its need to check the voltage and current ratings. The voltage rating is usually higher for AC, for example: "5A at 24V DC or 125V AC".

Switch contact arrangement (SPDT, DPDT etc) Most relays are SPDT or DPDT which are often described as "single pole changeover" (SPCO) or "double pole changeover" (DPCO).

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Advantages of relays: Relays can switch AC and DC, transistors

can only switch DC. Relays can switch high voltages,

transistors cannot. Relays are a better choice for switching

large currents (> 5A). Relays can switch many contacts at

once.

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Disadvantages of relays: Relays are bulkier than transistors for

switching small currents. Relays cannot switch rapidly (except

reed relays), transistors can switch many times per second.

Relays use more power due to the current flowing through their coil.

Relays require more current than many chips can provide, so a low power transistor may be needed to switch the current for the relay's coil.

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Relay Symbols1. Coil Relay 5.Time Delayed After

energizing

2. NO Relay ContactNO

NC

3. NC Relay Contact 6. Time Delayed After Deenergizing

4. Time Delay Relay Coil NO NC

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CR

TR

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Relay Basic Circuits1. ON Circuit

L

+ -

PB CR

NO

Ladder Wiring

2. OFF Circuit

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L

+ PB -

CR

NC

3. ON-OFF circuit

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L1

L2

+ PB -

CR

NO

NC

4. AND Circuit

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CR1

L

CR2

+ -

PB1

PB2

NO1

NO2

5. OR Circuit

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CR1

L

CR2

+ PB1 -

PB2

NO 1

NO 2

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6. Self Holding Circuit

CR1

L

+ Start Stop -

NO 1

NO 2

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7. Interlock Circuit

CR1

L

CR2

L2

+ PB 1 -

PB 2 NCR2

NOR1

PB 3 PB 4 NCR1

NOR2

NOR1

NOR2

1. Litar Gerudi Automatik

22G

erudi

Silinder Pneum

atik

Meja Kerja

Suis Had1

Suis Had 2

Kendalian Litar Gerudi Automatik

Litar logik geganti adalah untuk mengawal lejang hadapan dan balikan silinder pneumatik.

Rajah di atas menunjukkan penggunaan silinder pneumatik untuk menarik dan menurunkan mata gerudi.

Kerja bermula dengan gerudi berada di bahagian atas. Semasa silinder dalam keadaan ‘retract’ motor gerudi akan OFF.

Bila ‘Start Buton’ ditekan, ‘Extension Solenoid’ akan dijana dan ‘retract solenoid’ tidak dijana. Ini menyebabkan silinder pneumatik menolak mata gerudi turun ke bawah. Ketika ini juga motor gerudi akan dihidupkan untuk memulakan kerja menggerudi.

Sehingga mata gerudi turun hingga lejang penuh, ia akan menyentuh suis had yang akan menyebabkan ‘retract solenoid’ dijana dan silinder akan retract kembali ke kedudukan asal.

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Relay Logic Circuit

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CR1

CR2

Start Stop

CR 1

CR 1 LS 1 LS 2

CR 2 LS 2

CR 3

CR 3 Extend Solenoid

CR 3 Retract Solenoid

CR3 Drill Motor Starter

2. Simple Compress Machine

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Silinder Pemampat

Suis Had 1

Suis Had 2

Suis Had 3 Meja Kerja

Silinder Penolak

Kendalian Sistem Operasi memampat dan menolak keluar tin yang telah

dimampatkan dilakukan oleh silinder pneumatik. Silinder pneumatik dikawal oleh injap solenoid jenis

balikan spring. Operasi bermula dengan Suis Start ditekan dan silinder

pemampat akan mula turun dan memampatkan tin. Bila silinder pemampat turun hingga menyentuh suis had

2, ia akan tertarik kembali kekedudukan asal dan silinder penolak akan menolak keluar tin.

Bila silinder penolak menyentuh suis had 3, ia akan kembali ke kedudukan asal. Proses tamat.

Jika operator ingin memulakan kitaran kembali suis start perlu ditekan.

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Litar Logik Sistem

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C1

C2

C3

Start Stop LS 3

C1

C1 C3

C2 Sol. 1

LS 2 LS 3

C3

C3 LS 1 Sol. 2