Basic Components of Control Boards

7/25/2019 Basic Components of Control Boards

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Chapter 1

Basic components of control

boards

A simple control board is illustrated in Figure 1.1. The board contains two

push-buttons. One is used to operate the load (a flashing buzzer) and the

other is used to stop it. This board, similar to most other boards consists

of two circuits: A power circuit and a control circuit . The power circuit

is used to be a high current one connecting the electric power to the load

while the control circuit is a low current circuit implements the methodin which the drivers of the load are excited. It may contain switches, push

buttons, relays, timers, and counters. The drivers of the load are likely to be

contactors, relays, or solid state relays. This chapter is devoted to provide a

through description of the basic components encountered in control boards.

1.1 Electrical switches

An electrical circuit switch is any device used to interrupt the flow of elec-

trons in the circuit. It is usually an electromechanical part used to control

continuity between two points. However, there are other types of switches

which are more complex, containing electronic circuits able to turn on or

off depending on some physical stimulus (such as light or magnetic field)

sensed. In any case, the final output of any switch will be a pair of wire-

connection terminals that will either be connected together by the switch’s

1



2 CHAPTER 1. BASIC COMPONENTS OF CONTROL BOARDS

Figure 1.1: A simple control board.

internal contact mechanism (closed ), or not connected together (open).

Any switch designed to be operated by a person is generally called a hand

switch, and they are manufactured in several varieties. The most famous

types are toggle switches and push-button switches.

Toggle switches which are illustrated in Figure 1.2 are actuated by a lever

angled in one of two or more positions. The common light switch used in

household wiring is an example of a toggle switch.

Push-button switches are two-position devices actuated with a button that

is pressed and released. Push-button switches have an internal spring

mechanism returning the button to its “out,” or “unpressed,” position, for

momentary operation. The contacts of the push-button switch can be de-

signed so that the contacts “close” (establish continuity) when actuated, or

“open” (interrupt continuity) when actuated. The direction that the spring

of the push-button returns it to with no applied force is called the normal

position. Therefore, contacts that are open in this position are called nor-

mally open (N.O.) and contacts that are closed in this position are called

normally closed (N.C.). Push-button switches which has N.C. contacts areused to have red color while push-button switches which have N.O. con-

tacts are used to have green color. However, there are some push-button

switches which have both N.O. and N.C. contacts. Some types of push-

button switches are illustrated in Figure 1.3.

Some switches are specifically designed to be operated by the motion of

a machine rather than by the hand of a human operator. These motion-



1.1. ELECTRICAL SWITCHES 3

Figure 1.2: Toggle switches.

Figure 1.3: Push-button switches.




Figure 1.4: Lever actuator limit switches.

operated switches are commonly called limit switches, because they are

often used to limit the motion of a machine by turning off the actuating

power to a component if it moves too far. There are three basic types

of limit switches: lever actuator limit switches, magnetic proximity limit

switches, and electronic proximity limit switches.

Lever actuator limit switch is illustrated in Figure 1.4. It is closely resem-

ble rugged toggle fitted with a lever pushed by the machine part. Often,

the levers are tipped with a small roller bearing, preventing the lever from

being worn off by repeated contact with the machine part.

Magnetic proximity switches are simple non-electronic devices. They sense

the approach of a metallic machine part by means of a permanent magnet.

This magnet is used to actuate a sealed switch mechanism whenever the

machine part gets close (typically 2 cm or less). A magnetic proximity

switch widely used in security alarm systems is illustrated in Figure 1.5.Note that its electric symbol is identical to the lever actuator limit switch.

Electronic proximity switches are illustrated in Figure 1.6. They encom-

pass inductive, capacitive, and optical sensors.

In many industrial processes, it is necessary to monitor various physical

quantities with switches. Such switches can be used to sound alarms, in-



1.1. ELECTRICAL SWITCHES 5

Figure 1.5: Magnetic proximity limit switches.

Optical Capacitive Inductive

Figure 1.6: Electronic proximity limit switches.




speedpressure temperatureflow level

Figure 1.7: Symbols of common types of process switches.

normally open switch

normally close switch

Figure 1.8: Generic symbology for switch contacts.

dicating that a process variable has exceeded normal parameters, or they

can be used to shut down processes or equipment if those variables have

reached dangerous or destructive levels. This type of switches is called

process switches. Speed, pressure, temperature, level, and flow switches

are examples of this category. They are illustrated in Figure 1.7.

As with push-button switches, limit switches and process switches may

have N.O., N.C., or double contacts. The normal state of a switch is that

where it is unactuated. This is the condition it’s in when sitting on a shelf,

uninstalled. There is a generic symbology for any switch contact, using

a pair of vertical lines to represent the contact points in a switch as illus-

trated in Figure 1.8. Normally-open contacts are designated by the lines

not touching, while normally-closed contacts are designated with a diag-

onal line bridging between the two lines. If switches are designated with

these generic symbols, the type of switch usually will be noted in text im-mediately beside the symbol.

When movable contact(s) can be brought into one of several positions

with stationary contacts, those positions are sometimes called throws. The

number of movable contacts is sometimes called poles. Selector switches

shown in Figure 1.9 are with one moving contact and seven stationary con-

tacts would be designated as single-pole, seven-throw (SP7T) switches. If



1.2. RELAYS 7

Common

1

2

34

5

6

7

Figure 1.9: A selector switch.

two identical single-pole, five-throw switches were mechanically ganged

together so that they were actuated by the same mechanism, the whole

assembly would be called a double-pole, five- throw (DP5T) switch. Fig-

ure 1.10 illustrates a few common switch configurations and their abbrevi-

ated designations.

1.2 Relays

A relay is a simple device that uses a magnetic field to control a switch.

When a voltage is applied to the input coil, the resulting current creates

a magnetic field. The magnetic field pulls a metal switch towards it and

the contacts touch, closing the switch. SPDT, DPDT, and 3PDT switch

types are commonly used in relays as illustrated in Figure 1.11. Their

current ratings may vary from few to tens of Amperes and may withstand

few hundreds of volts. The relay coil on the other hand, is usually rated at

much less values. Commercial relays usually have 3PDT contacts rated at

5 A and 220 V and their windings are rated at 24 V DC and 0.03 A.




single-pole, single-throw

single-pole, double-throw

double-pole, single-throw

three-pole, four-throw

SPST

SPDT

DPST

3P4T

Figure 1.10: Common switch configurations.

Figure 1.11: Common relays.



1.2. RELAYS 9

Load

GND1

+24V

R

S

220V24V

Relay (R)

Sensor (S)

GND2

220V AC

Load

R

Beginners sketch

Professionals sketch

Figure 1.12: A circuit which uses a relay as a driver.

There are two basic functions for relays. First, they may be used to let one

power source close a switch for another high current source, while keeping

them isolated. In other words they are extremely useful when we have

a need to control a large amount of current and/or voltage with a smallelectrical signal. The relay coil which produces the magnetic field may

only consume fractions of a watt of power, while the contacts closed or

opened by that magnetic field may be able to conduct hundreds of times

that amount of power to a load. In effect, a relay acts as a digital amplifier.

Figure 1.12 illustrates an example of this case. A relay contacts are used

in the power circuit where they are used to switch a lamp on when the

relay coil is excited. The relay coil is in the control circuit which has a low

voltage and low current photocell.

The second function of relays is to implement logic functions. Assume

that we want to design a Set/Reset control circuit for a lamp. In the control

circuit, we will have two switches denoted by S and R for setting and

resetting the relay coil denoted by C. In the power circuit we will have one

of the relay contacts, which is also denoted by C, connected in series with

the lamp and its power supply. To deduce the connection of the control

circuit we may utilize the standard techniques used in logic design to find

the boolean function of the coil C. Starting by the truth table and passing by




S R C Ct-1 t

0 0 00

0

0

0

0

0

0

1

1

1

1

1

1

1

11

1

1

1 0

0

01

0

0

1

1

x

x

S {

{

R

Ct-1

111 x x

Ct=S+C R’

t-1

S {

{

R

Ct-1

x x

0 0 0 C’t=R+C’ S’t-1

Ct=R’(C +S)

t-1

Figure 1.13: Driving the boolean equation of the relay.

GND

+24V

C

CS

R

Ct=S+C R’

t-1

GND

+24V

C

CS

Ct=R

’(C +S)t-1

R

Figure 1.14: Circuits which use relays to implement certain logic.

the Karnough map as illustrated in Figure 1.13 one may find the equation

of C as sum of products

or product of sums

These equations are implemented as illustrated in Figure 1.14. Try to figure

out which one is superior to the other.

Sophisticated modern control circuits use relays for driving loads and rarely



1.3. CONTACTORS 11

Figure 1.15: A typical contactor.

use them for logic. The logic functions of the relays are implemented in

software using programable logic controllers.

1.3 Contactors

When a relay is used to switch a large amount of electrical power through

its contacts, it is designated by a special name: contactor . Contactors typ-

ically have three power contacts, and those contacts are usually normally-open, so that power to the load is shut off when the coil is de- energized.

Another low power N.C. or N.O. contact is used to be available also in

contactors. The power contacts are used in the power circuit while the

low power one may be used in the control circuit. A typical commercial

contactor is illustrated in Figure 1.15.

When there is a need for extra contacts for the control circuit, auxiliary




Figure 1.16: Auxiliary contacts.

contacts may be attached to the contactor. These auxiliary contacts are

available in different capacities and different configurations (N.C and N.O.)

as illustrated in Figure 1.16.

Perhaps the most common industrial use for contactors is the control of

electric motors. Figure 1.17 illustrates a common control circuit for a three

phase ( ) induction motor. The contactor power contacts switch the re-

spective phases of the incoming 3-phase AC power, typically 480 Volts

line-to-line for motors greater than one horsepower. The three devices in

series with each phase going to the motor are called overload heaters. Each“heater” element is a low-resistance strip of metal intended to heat up as

the motor draws current. If the temperature of any of these heater elements

reaches a critical point (equivalent to a moderate overloading of the motor),

a N.C. switch contact (F) will spring open. This normally-closed contact

is connected in series with the contactor coil, so that when it opens, the

contactor will automatically de-energize, thereby shutting off power to the

motor.

Overload devices usually have adjustable current ratings and may have N.C

contacts along with the N.O. contacts. The overload devices may be avail-

able as stand alone devices or as integrated devices which are tighteneddirectly to contactors as illustrated in Figures 1.18 and 1.19

Overload heaters are intended to provide over-current protection for large

electric motors, unlike circuit breakers and fuses which serve the primary

purpose of providing over-current protection for power conductors.

Overload heater function is often misunderstood. They are not fuses; that



1.3. CONTACTORS 13

R S T

M

3

C

GND

220V

C

CON

F

OFF

F

Figure 1.17: Control board circuits of a 3-phase induction motor.

Figure 1.18: Overload devices.




Figure 1.19: A contactor attached to an overload device and an auxiliary

contacts.

is, it is not their function to burn open and directly break the circuit as a

fuse is designed to do. Rather, overload heaters are designed to thermallymimic the heating characteristic of the particular electric motor to be pro-

tected. All motors have thermal characteristics, including the amount of

heat energy generated by resistive dissipation (

), the thermal trans-

fer characteristics of heat “conducted” to the cooling medium through the

metal frame of the motor, the physical mass and specific heat of the ma-

terials constituting the motor, etc. These characteristics are mimicked by

the overload heater on a miniature scale: when the motor heats up toward

its critical temperature, so will the heater toward its critical temperature,

ideally at the same rate and approach curve. Thus, the overload contact,

in sensing heater temperature with a thermo-mechanical mechanism, will

sense an analogue of the real motor. If the overload contact trips due to ex-

cessive heater temperature, it will be an indication that the real motor has

reached its critical temperature (or, would have done so in a short while).

After tripping, the heaters are supposed to cool down at the same rate and

approach curve as the real motor, so that they indicate an accurate pro-

portion of the motor’s thermal condition, and will not allow power to be

re-applied until the motor is truly ready for start-up again.



1.4. SOLID STATE RELAYS 15

Figure 1.20: A solid state relay.

1.4 Solid state relays

A solid state relay (SSR) uses an SCR, TRIAC, or transistor output instead

of mechanical contacts to switch the controlled power. The output device

(SCR, TRIAC, or transistor) is optically-coupled to a Light Emitting Diode

(LED) inside the relay. The relay is turned on by energizing this LED,

usually with low-voltage DC power. This optical isolation between inputto output rivals the best that electromechanical relays can offer. Figure 1.20

illustrates a SSR and its simplified schematic.

Being solid-state devices, there are no moving parts to wear out, and they

are able to switch on and off much faster than any mechanical relay arma-

ture can move. There is no sparking between contacts, and no problems

with contact corrosion. However, solid-state relays are still too expensive

to build in very high current ratings, and so electromechanical contactors

continue to dominate that application in industry today. One significant

advantage of a solid-state SCR or TRIAC relay over an electromechanical

device is its natural tendency to open the AC circuit only at a point of zero

load current. Because SCR’s and TRIAC’s are thyristors, their inherent

hysteresis maintains circuit continuity after the LED is de-energized un-

til the AC current falls below a threshold value (the holding current). In

practical terms what this means is the circuit will never be interrupted in

the middle of a sine wave peak. Such untimely interruptions in a circuit

containing substantial inductance would normally produce large voltage




spikes due to the sudden magnetic field collapse around the inductance.

This will not happen in a circuit broken by an SCR or TRIAC. This featureis called zero-crossover switching. One disadvantage of solid state relays

is their tendency to fail “shorted” on their outputs, while electromechanical

relay contacts tend to fail “open”. In either case, it is possible for a relay

to fail in the other mode, but these are the most common failures. Because

a “fail-open” state is generally considered safer than a “fail-closed” state,

electromechanical relays are still favored over their solid-state counterparts

in many applications.

1.5 Problems

1.1 What are the main differences between relays and contactors?

1.2 What are the differences between an overload device and the circuit

breakers used in traditional home electrical wiring system?

1.3 Apart of the devices described in this chapter, write down 3 electrical

components used in control boards and provide brief description and

some illustrations for each one.

1.4 Assume that , , , and are switches. Sketch the electricalcircuits which implement the following outputs:

a)

b)

c)

d)

1.5 Implement a master-slave toggle flip-flop using relays. Hint: You

need 4 relays.

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Basic Components of Control Boards