Relays

Question 1

What is a switch, and explain its different modes of operation and their meaning.

Answer 1

• Any device that can make or break an electrical circuit.
• make means close switch
• break means open swtich

Question 2

What are switches rated by?

Answer 2

The amount of current they can interrupt and by voltage.

Question 3

Explain some different aspects of switch contacts

Answer 3

• switches hace mechanical contacts, usually metal
• Choice of metal is based on conductivity, resistance to coorosion, hardness, mechanical stregth, low toxicity and low cost.
• mechanical contacts can exibit contact bounce

Question 4

explain some machanical opertations of switches

and some problem that can occur

Answer 4

• High power swtiches have spring mechanisms to make and break circuits as quickly as possible to minimize arching.
• Breaking a circuit is harder on contacts that making beaucse arcing degrades conductivity
• some contacts designed to rub as they close to remove arc deposits

Question 5

Name some different types of switch contacts

Answer 5

Normally open = form A

normally closed = form b

Form c = SPDT- has a comman terminal that moves between a NO and NC contact

Question 6

What can cause contacts life to be shortened

Answer 6

1. Switching dc loads - The supply does not pass through 0v twice per cycle to interrput an arc when the contacts are closed
2. Switching inductive loads- inductance tries to keep current flowing through the contacts after they open

Question 7

What is consitered an inductive load?

Answer 7

Relay coils, motor windings and solenoids

Question 8

Explain the different aspects of toggle switches

Answer 8

• Simplest are two positition, on and off
• single pole = two terminals
• 0 ohms between terminals when contacts closed
• infinite ohms when open
• contacts can be maintained or momentary (spring retrun to open or closed

Question 9

What is an acuator

Answer 9

Any device that provides the force to open the switch contacts, such as toggles, rockers, pushbuttons, cams, keys or mechanical linkage

Question 10

why is the number of poles significant

Answer 10

Defines how many electrically seperate circuits can be controlled by the sam acuator

Question 11

Explain some aspects of selector switches

Answer 11

Allow selection of one or more posistions

SPDT can be used as a single pole selector

some haave spring return to a center posistion

Question 12

Name some different contacts types of selector switches

Answer 12

Single break- breaks in one place

Double break- breaks circuit in two places

bifurcated - split contacts for low power applications

Question 13

What are some different push button components

Answer 13

Industial PB’s consist of operators and contact blocks which bolt on

Contact blocks can have combination of NO and NC contacts

Question 14

What is an electromechanical relay

Answer 14

consists of one or more sets of contacts and an electomagnet coil

contacts state depends on/off state of coil

Question 15

what are some differnt uses for eletromechanical relays

Answer 15

Used to ‘relay’ on / off logic between circuits

small amount of coil power can control larger amounts of power in contact circuits

one relay can control mutiple circuits depending on number of poles

Question 16

What do electormechanical relays allow you to do

Answer 16

To control a high current or high voltage load with a low current, low voltage control circuit

Question 17

what are contactors

Answer 17

Relays with contacts designed to switch larger amounts of power

Amount of power that can be switched detwrmined by NEMA or CEMA ‘ size’

Question 18

What is a solid state relay?

Answer 18

low power input circuit controls higer power output circuit with no contacts or moving parts

Question 19

What is used for swithcing dc loads and ac

Answer 19

Transistor output solid state relay

triac output for AC

Question 20

WHat are the advantages and disadvantages of Solid state relays

Answer 20

advantages : No moving parts, acrless swtiching, quiet, shock and vibreation ressitance, no contact bounce, high life expectancy, easily interfaced to electronics

Disadvantage: no multiple contact configurations, cannot withstand surdge currents, moretemperature sensative.

Question 21

Draw a SPST toggle switch

draw a DPST

draw a SPDT

draw a DPDT

Answer 21

SINGLE POLE SINGLE THROW

Double pole single throw

Question 22

What type of button is this

Answer 22

Industial Pushbuttons

Question 23

what are these?

Answer 23

Contact Blocks

Question 24

What is this

Answer 24

Double acting contacts also called Double throw

Question 25

what does a dashed line though double throw mean

Answer 25

It means a mechanical connection

Question 26

What are the different relay contact arrangements

Answer 26

Question 27

What is this

Answer 27

General purpose relay with SPDT contacts

Question 28

What is this

Answer 28

industiral control relay with 2 sets of NO and 2 sets NC

Question 29

whats this

Answer 29

SS relay and symbol