module 3 digital outputs

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Module 3 Digital Outputs

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Digital Outputs

Contents: Part 1:

Directional Control Valve

Part 2:

Relays

Part 3:

Time-delay Relays (Timers)

Module 3




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PART 1

Directional Control Valves




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SOLENOIDS

Solenoids are the most common actuator components. The basic principle of operation is there

is a moving ferrous core (a piston) that will move inside wire coil as shown in the following

Figure. Normally the piston is held outside the coil by a spring. When a voltage is applied to the

coil and current flows, the coil builds up a magnetic field that attracts the piston and pulls itinto the center of the coil. The piston can be used to supply a linear force. Well known

applications of these include pneumatic values and car door openers.

As mentioned before, inductive devices can create voltage spikes and may need snubbers,

although most industrial applications will be powered by 24Vdc and draw a few hundred mA.

Directional Control Valves

The flow of fluids and air can be controlled with solenoid controlled valves. An example of a

solenoid controlled valve is shown in the following figure.

The solenoid is mounted on the side. When actuated, it will drive the central spool left. The top

of the valve body has two ports that will be connected to a device such as a hydraulic cylinder.




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The bottom of the valve body has a single pressure line in the center with two exhausts to the

side. In the top drawing the power flows in through the center to the right hand cylinder port.

The left hand cylinder port is allowed to exit through an exhaust port. In the bottom drawing

the solenoid is in a new position and the pressure is now applied to the left hand port on the

top, and the right hand port can exhaust. The symbols to the left of the figure show the

schematic equivalent of the actual valve positions. Valves are also available that allow thevalves to be blocked when unused.




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Theory of Operation




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Directional Control Valves Fundamentals

A directional control valve is a device which connects, disconnects or changes the direction of

air flow in a circuit.

Positions

The first thing that needs to be determined is the

number of positions the valve has. Most valves have

two positions, but some valves do have three positions.

The number of positions a valve has is represented in

its symbol by a series of squares. The symbol in Figure

1A is composed of two squares, which represent the

two positions of this valve. The symbol in Figure 1B is

composed of three squares, which represent the three

positions of this valve.It should be noted that the squares can be drawn side

by side or one on top of the other.

One square indicates how the ports are connected

when the valve is off or de-energized; the other square

indicates how the ports are connected when the valve is on or energized.

Ports

The second thing that needs to be determined is the number of ports the valve has. A port is an

opening through which air can enter or exit a valve. The number of ports can be determined byexamining the valve and counting them, or by looking at the valve's symbol.

Ways

Note that the number of ports equals the number of

ways

Generally speaking, if each square has two ports it's a

2-way valve, three ports is a 3-way valve and four ports

is a 4-way valve. There are, however, a couple of

exceptions to this rule, which will be discussed later.Figure 2 shows the symbol for a 2-position, 2-way

directional control valve. It is a 2-position valve

because it consists of two squares. It is a 2-way valve

because if you look at any one square it has two ports

labeled 1 and 2.




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T-Symbol and Arrows

In Figure 2, the bottom square shows ports 1 and 2 represented by a T symbol. This symbol is

used to represent a port which is closed or blocked off. The top square shows ports 1 and 2

connected by a line with an arrow on it. The line is used to show that the two ports are

connected. The arrow is added to one end of the line to indicate which direction the air flows

through the valve.

Actuators

Before we can determine which square indicates the energized state and which square

indicates the de-energized state, we must know the symbols for actuators. An actuator is the

means by which the valve is energized and de-energized. Actuators are represented by symbols

that are added to the ends of the directional control valve's symbol.

Manually operated valves

Figure 3 shows some actuator symbols which represent

manual operation. Figure 3A shows the symbol that is

used to indicate that a pushbutton is depressed to

energize the valve. Figure 3B shows the symbol that is

used to indicate that a lever is operated to energize the

valve. Figure 3C shows the widely used universal symbol

to indicate manual operation. While this symbol

indicates that the valve can be manually energized, it

does not indicate the specific means such as,pushbutton, lever, foot pedal, etc.

Mechanically operated valves

Figure 4A and 4B show some actuator symbols which

represent mechanical operation. When a mechanical

arm contacts the roller cam it pushes it down

energizing the valve.

Figure 4C shows the symbol used to indicate a spring.

Springs are normally used to return a valve to its rest

position, after it has been energized by another means.




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Electrically operated valves

Figure 5A shows the symbol used to indicate electrical or solenoid operation. These valves are

usually energized by a coil of wire called a solenoid.

Pneumatically operated valves

Figure 5B shows the symbol used to indicate an air

operated valve. These valves are activated by pressure

pushing on a diaphragm. These valves are commonly

called pilot valves.

Sometimes valves can be energized by more than one

actuator. In Figure 6, the symbol for an air pilot and a

solenoid are stacked on top of each other. In this case

it takes an electric solenoid and air pressure to energizethe valve. If either one is missing the valve will not

energize. In Figure 7, the symbol for manual operation

and an electric solenoid are next to each other. In this case the valve can be energized either by

the electric solenoid or by some manual means, usually a pushbutton. Being able to activate the

valve manually can be very desirable because it can make isolating a problem easier.




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Return

When the valve is de-energized, the spring will return it to its rest position

Description of direction control valve

Figure 8 shows the complete symbol for a 2-position, 2-way, solenoid-operated, spring-return

directional control valve. Since the solenoid symbol is attached to the top square, the top

square shows us that ports 1 and 2 are connected when the solenoid is energized. The spring

symbol is attached to the bottom square. When the valve is de-energized, the spring will return

it to its rest position. The bottom square shows us that ports 1 and 2 are disconnected when

the valve is de-energized.

Figure 9 shows the complete symbol for another 2-position 2-way valve, but this one is

manually or solenoid operated and spring returned. The symbols for valves can be drawn in any

position that is convenient for the diagram. In Figure 9, the valve is drawn sideways. Since the

manual symbol and the solenoid symbol are attached to the right square when the valve isenergized, the right square shows us how the ports are connected. The spring symbol is

attached to the left square. When the valve is de-energized the spring will return it to its rest

position. The left square shows us that ports 1 and 2 are disconnected when the valve is de-

energized.

When an actuator is attached to one of the squares that square shows how the ports are

connected or disconnected when that actuator is used to operate the valve.




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PART 2

Relays




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Theory of operation

Instead of this magnet, an electric current through a conductor will produce a magnetic field at

right angles to the direction of electron flow. If that conductor is wrapped into a coil shape, the

magnetic field produced will be oriented along the length of the coil. The greater the current,

the greater the strength of the magnetic field.

Solenoid Armature RelayIf we place a magnetic object near such a coil for the purpose of making that object move when

we energize the coil with electric current, we have what is called a solenoid. The movable

magnetic object is called an armature, and most armatures can be moved with either direct

current (DC) or alternating current (AC) energizing the coil.




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Solenoids can be used to electrically open door latches, open or shut valves, move robotic

limbs, and even actuate electric switch mechanisms.

However, if a solenoid is used to actuate a set of switch contacts, we have a device so useful

it deserves its own name: the relay.




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Poles and ThrowsWhen 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.

Type of relays according to Poles and Throws

The next figure shows one moving contact and five

stationary contacts, this called as "single-pole, five-

throw" switches.

The following figure shows two moving contact and five

stationary contacts, this called as "Double-pole, five-

throw" switches.

Single Pole Single Throw

(SPST)Single Pole Double Throw

(SPDT)

Double Pole Single Throw

(DPST)

Double Pole Double Throw

(DPDT)




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Relays symbols




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Relays application

Relays are extremely useful when we have a need to control a large amount of current

and/or voltage with a small electrical 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 binary (on or off)

amplifier.

In the above schematic, the relay's coil is energized by the low-voltage (12 VDC) source, while

the single-pole, single-throw (SPST) contact interrupts the high-voltage (480 VAC) circuit. It is

quite likely that the current required to energize the relay coil will be hundreds of times less

than the current rating of the contact. Typical relay coil currents are well below 1 amp, while

typical contact ratings for industrial relays are at least 10 amps.

One relay coil/armature assembly may be used to actuate more than one set of contacts. Those

contacts may be normally-open, normally-closed, or any combination of the two. As with

switches, the "normal" state of a

relay's contacts is that state when

the coil is de-energized, just as you

would find the relay sitting on a

shelf, not connected to any circuit.

Shown here are three small relays

(about two inches in height, each),

installed on a panel as part of an

electrical control system at a

municipal water treatment plant:




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The relay units shown here are called "octal-base," because they plug into matching sockets,

the electrical connections secured via eight metal pins on the relay bottom. The screw terminal

connections you see in the photograph where wires connect to the relays are actually part of

the socket assembly, into which each relay is plugged. This type of construction facilitates easy

removal and replacement of the relay(s) in the event of failure.

Relays as electrical isolation

Aside from the ability to allow a relatively small electric signal to switch a relatively large

electric signal, relays also offer electrical isolation between coil and contact circuits. This means

that the coil circuit and contact circuit(s) are electrically insulated from one another. One circuit

may be DC and the other AC (such as in the example circuit shown earlier), and/or they may be

at completely different voltage levels, across the connections or from connections to ground.

Contactors (Power Control Relay)

When a relay is used to switch a large amount of electrical power through its contacts, it isdesignated by a special name: contactor.

Contactors typically have multiple contacts, and those contacts are usually (but not always)

normally-open, so that power to the load is shut off when the coil is de-energized. Perhaps the

most common industrial use for contactors is the control of electric motors.

The top three contacts switch the respective phases of the incoming 3-phase AC power,

typically at least 480 Volts for motors 1 horsepower or greater. The lowest contact is an

"auxiliary" contact which has a current rating much lower than that of the large motor power

contacts, but is actuated by the same armature as the power contacts. The auxiliary contact is

often used in a relay logic circuit, or for some other part of the motor control scheme, typically

switching 120 Volt AC power instead of the motor voltage. One contactor may have several

auxiliary contacts, either normally-open or normally-closed, if required.




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Protective Relays

Overload Heater contactor

The three "opposed-question-mark" shaped 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

normally-closed switch contact (not shown in the diagram) will spring open. This normally-

closed contact is usually connected in series with the relay coil, so that when it opens the relay

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

Three-phase, 480 volt AC power comes in to the three normally-open contacts at the top of the

contactor via screw terminals labeled "L1," "L2," and "L3" (The "L2" terminal is hidden behind a

square-shaped "snubber" circuit connected across the contactor's coil terminals). Power to the

motor exits the overload heater assembly at the bottom of this device via screw terminals

labeled "T1," "T2," and "T3."

The circuit breakers

The circuit breakers which are used to switch large quantities of electric power on and off are

actually electromechanical relays, themselves. Unlike the circuit breakers found in residential

and commercial use which determine when to trip (open) by means of a bimetallic strip inside

that bends when it gets too hot from overcurrent, large industrial circuit breakers must be

"told" by an external device when to open

NO and NC Contacts

The contacts energize and de-energize as a result of applying power to the relay coil(connections to the relay coil are not shown). When the coil is de-energized, the movable

contacts are connected to the upper fixed contact pair.

These fixed contacts are referred to as the normally closed contacts because they are bridged

together by the movable contacts and conductor whenever the relay is in its "power off" state.

Likewise, the movable contacts are not connected to the lower fixed contact pair when the

relay coil is de-energized. These fixed contacts are referred to as the normally open contacts.

Contacts are named with the relay in the deenergized state.

Normally open contacts are said to be off when the coil is de-energized and on when the coil

is energized.

Normally closed contacts are on when the coil is deenergized and off when the coil isenergized.

Those that are familiar with digital logic tend to think of N/O contacts as non-inverting

contacts, and N/C contacts as inverting contacts.




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Relay Symbols

Relay Symbols

The above figure shows the three most common relay symbols used in electrical machine

diagrams. These three symbols are a normally open contact, normally closed contact and coil.

Notice that the normally closed and normally open contacts of Figure 1-2 each have lines

extending from both sides of the symbol. These are the connection lines which, on a real relay,

would be the connection points for wires.

The coil symbol shown in the Figure represents the coil of the relay we have been discussing.

The coil, like the contacts, has two connection lines extending from either side.

These represent the physical wire connections to the coil on the actual relay.

Notice that the coil and contacts in the figure each have a reference designator label above the

symbol. This label identifies the contact or coil within the ladder diagram.

Coil CR1 is the coil of relay CR1. When coil CR1 is energized, all the normally open CR1contacts will be closed and all the normally closed CR1 contacts will be open. Likewise, if coil

CR1 is de-energized, all the normally open CR1 contacts will be open and all the normally

closed CR1 contacts will be closed. Most coils and contacts we will use will be labeled as CR

(CR is the abbreviation for “control relay”).

A contact labeled CR indicates that it is associated with a relay coil. Each relay will have a

specific number associated with it. The range of numbers used will depend upon the number of

relays in the system.

LabelingThe contact arrangement and the terminal numbers are usually marked on the side of the relay,

similar to that shown here which conforms to BS 5583 (EN50011).

Two numbers are used to mark relay contacts:

First number identifies contact positions 1, 2, 3, etc.

Second number identifies contact type.




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For example:

1 and 2 for NC contacts

3 and 4 for NO contacts

RELAYS terminology

Although relays are rarely used for control logic, they are still essential for switching large

power loads. Some important terminology for relays is given below.

Contactor

Special relays for switching large current loads.

Motor Starter

Basically contactors in series with an overload relay to cut off when too much current is drawn.

Arc Suppression

When any relay is opened or closed an arc will jump. This becomes a major problem with large

relays. On relays switching AC this problem can be overcome by opening the relay when the

voltage goes to zero (while crossing between negative and positive). When switching DC loads

this problem can be minimized by blowing pressurized gas across during opening to suppress

the arc formation.

AC coils

If a normal relay coil is driven by AC power the contacts will vibrate open and closed at the

frequency of the AC power. This problem is overcome by adding a shading pole to the relay.

Rated Voltage

The suggested operation voltage for the coil. Lower levels can result in failure to operate,

voltages above shorten life.




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Rated Current

The maximum current before contact damage occurs (welding or melting).

The most important consideration when selecting relays is the rated current and voltage. If

the rated voltage is exceeded, the contacts will wear out prematurely, or if the voltage is too

high fire is possible. The rated current is the maximum current that should be used. Whenthis is exceeded the device will become too hot, and it will fail sooner. The rated values are

typically given for both AC and DC, although DC ratings are lower than AC. If the actual loads

used are below the rated values the relays should work well indefinitely. If the values are

exceeded a small amount the life of the relay will be shortened accordingly. Exceeding the

values significantly may lead to immediate failure and permanent damage.

Solid-state relays

Disadvantage of electromechanical relays

They can be expensive to build, have a limited contact cycle life, take up a lot of room, andswitch slowly, compared to modern semiconductor devices.

These limitations are especially true for large power contactor relays. To address these

limitations, many relay manufacturers offer "solid-state" relays, which use 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 an LED light source inside

the relay. The relay is turned on by energizing this LED, usually with low-voltage DC power. This

optical isolation between inputs to output rivals the best that electromechanical relays can

offer.

Advantage of solid-state devices

Being solid-state devices, there are no moving parts to wear out, and they are able to switch onand off much faster than any mechanical relay armature 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.




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PART 3

Time-delay Relays (Timers)




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Introduction

Some relays are constructed with a kind of "shock absorber" mechanism attached to the

armature which prevents immediate, full motion when the coil is either energized or de-

energized. This addition gives the relay the property of time-delay actuation.

Time-delay relays can be constructed to delay armature motion on coil energization, de-

energization, or both.Time-delay relay contacts must be specified not only as either normally-open or normally-

closed, but whether the delay operates in the direction of closing or in the direction of

opening. The following is a description of the four basic types of time-delay relay contacts.

Time delay relay contact; NOTC

First we have the normally-open, timed-closed (NOTC) contact.

This type of contact is normally open when the coil is unpowered (de-energized). The contact

is closed by the application of power to the relay coil, but only after the coil has been

continuously powered for the specified amount of time.

This type of contact is alternatively known as a normally-open, on-delay:

The following is a timing diagram of this relay contact's operation:




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Time delay relay contact; NOTO

Next we have the normally-open, timed-open (NOTO) contact. Like the NOTC contact, this

type of contact is normally open when the coil is unpowered (de-energized), and closed by

the application of power to the relay coil. However, unlike the NOTC contact, the timing

action occurs upon de-energization of the coil rather than upon energization.

Because the delay occurs in the direction of coil de-energization, this type of contact isalternatively known as a normally-open, off-delay:


Next we have the normally-closed, timed-open (NCTO) contact. This type of contact is

normally closed when the coil is unpowered (de-energized). The contact is opened with the

application of power to the relay coil, but only after the coil has been continuously poweredfor the specified amount of time. In other words, the direction of the contact's motion (either

to close or to open) is identical to a regular NC contact, but there is a delay in the opening

direction.

Because the delay occurs in the direction of coil energization, this type of contact is

alternatively known as a normally-closed, on-delay:




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Time delay relay contact; NCTO


Finally we have the normally-closed, timed-closed (NCTC) contact. Like the NCTO contact, this

type of contact is normally closed when the coil is unpowered (de-energized), and opened by

the application of power to the relay coil. However, unlike the NCTO contact, the timing

action occurs upon de-energization of the coil rather than upon energization.

Because the delay occurs in the direction of coil de-energization, this type of contact is

alternatively known as a normally-closed, off-delay:




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Time delay relay contact; NCTC


Time-delay relays application

Time-delay relays are very important for use in industrial control logic circuits. Some examples

of their use include:

Flashing light control (time on, time off): two time-delay relays are used in conjunction

with one another to provide a constant-frequency on/off pulsing of contacts for sending

intermittent power to a lamp. Engine auto start control: Engines that are used to power emergency generators are

often equipped with "auto start" controls that allow for automatic start-up if the main

electric power fails. To properly start a large engine, certain auxiliary devices must be

started first and allowed some brief time to stabilize (fuel pumps, pre-lubrication oil




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pumps) before the engine's starter motor is energized. Time-delay relays help sequence

these events for proper start-up of the engine.

Furnace safety purge control: Before a combustion-type furnace can be safely lit, the air

fan must be run for a specified amount of time to "purge" the furnace chamber of any

potentially flammable or explosive vapors. A time-delay relay provides the furnace

control logic with this necessary time element.

module 3 digital outputs

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