Instr 12209 Switches and Photoelectric Devices

--Switches and Photoelectric Devices

Module 12209

Instrument Trainee Task Module 12209

SWITCHES AND PHOTOELECTRIC DEVICESObjectives

Upon completion of this module, the trainee will be able to:

1. State the purpose of a switch.2. Given pictures, samples or specification sheets, identify commonly

used switches.3. Describe the major parts of a switch.4. Classify switches using wiring symbols according to the number of

poles and the number of throws.5. Define a photoelectric device.6. Given pictures, samples or specification sheets, identify commonly

used photoelectric devices.7. State the electrical characteristics of a photocell.8. State the form of electromagnetic radiation that triggers motion

detectors. 9. State the electrical characteristics of a solar cell.10.State the purpose of an SCR.

Prerequisites

Successful completion of the following Task Module(s) is required before beginning of this Task Module: NCCER Core Curricula; NCCER Task Module 12201, Craft-Related Mathematics; NCCER Task Module 12202, Instrumentation Drawings & Documents II; NCCER Task Module 12203, Principles of Welding; NCCER Task Module 12204, Process Control Theory.

Required Student Materials

1. Student Module2. Required Safety Equipment

Instrument Trainee Task Module 12209 2

Course Map Information

This course map shows all of the Wheels of Learning task modules in the second level of the Instrument curricula. The suggested training order begins at the bottom and proceeds up. Skill levels increase as a trainee advances on the course map. The training order may be adjusted by the local Training Program Sponsor.

Course Map: Instrument, Level 2

LEVEL 2 COMPLETE

Switches and Photoelectric Devices — Module 12209 3

TABLE OF CONTENTS

Section Topic ………………………………………………………………….Page

1.0.0 Introduction to Switches and Photoelectric Devices ………………….. 5

2.0.0 Definition, Classification, and Description …………………………………5

2.1.0 Switch Definition …………………………………………………………………… 52.2.0 Switch Classifications …………………………………………………………….. 62.2.1 Switch Contacts …………………………………………………………………….. 62.2.2 Pole of a Switch …………………………………………………………………….. 72.2.3 Throws of a Switch ………………………………………………………………… 72.3.0 Switch Descriptions ……………………………………………………………….. 82.3.1 Single-Pole Switch …………………………………………………………………. 82.3.2 Three-Way Switch …………………………………………………………………. 92.3.3 Double-Pole Switch ……………………………………………………………….. 92.3.4 Float Switch ………………………………………………………………………….. 102.3.5 Pneumatic Switch ………………………………………………………………….. 102.3.6 Limit Switch ………………………………………………………………………….. 112.3.7 Electronic Switch …………………………………………………………………… 133.0.0 Photoelectric Devices ……………………………………………………………… 143.1.0 Photocell Switch …………………………………………………………………….. 143.2.0 Solar Cells …………………………………………………………………………….. 173.2.1 Infrared Devices ……………………………………………………………………. 183.2.2 Motion Detectors …………………………………………………………………… 184.0.0 Proximity Switch ……………………………………………………………………. 20


Trade Terms Introduced In This Module

Anode: The positive terminal of an electrical device.

Cathode: The negative terminal of an electrical device.

Circuit: The complete path of an electric current.

Diode: An electronic device which allows current to flow in one direction only.

Eddy Curents: Currents induced into a metal by transformer action.

Infrared: Invisible heat waves having wavelengths longer than those of red light.

Overload: A circuit operating at greater than rated current.

Oxidation: The process where oxygen combines with another element.

Points: A set of electrical contacts.

SCR: Silicon controlled rectifier.

Short Circuit: A circuit which is accidently or intentionally placed in operation with very little resistance.

1.0.0 INTRODUCTION TO SWITCHES AND PHOTOELECTRIC DEVICES-

One of the most common electrical devices in commercial installations is


the switch. Virtually every piece of electrical equipment or system has switches of some type. Photoelectric devices such as motion sensors and photocells are also becoming very popular.

Because there are so many types and name brands of switches and photoelectric devices, the trainee is encouraged to learn the principals of operation of the most popular types of switches and photoelectric devices and then apply this knowledge to other devices encountered in the field.

The aim of this module will be to describe in sufficient detail the characteristics, description, and operation of the most popular types of switches and photoelectric devices such that proper equipment can be selected and installed for satisfactory service in commercial installations.

2.0.0 DEFINITION, CLASSIFICATION, AND DESCRIPTION-

2.1.0 SWITCH DEFINITION

An electrical switch is a device used to make or break an electrical circuit that is operating within the rated current and voltage of the switch. Standard electrical switches should not be used to interrupt a circuit under short circuit or overload. Fuses or circuit breakers should be used to prevent such conditions.

NOTE: When selecting and installing switches, pay special attention to the voltage and current ratings.

2.2.0 SWITCH CLASSIFICATIONS

Switches can be classified in many different ways.1. According to the number of poles.

a. Single poleb. Double pole c. Triple pole

2. According to the number of closed positions.a. Single throwb. Double throw (two position)c. Three position

3. According to the method of insulation.a. Airb. Oil c. Gas

4. According to the method of operation.a. Manual (push button, knife)b. Pneumatic (air)c. Hydraulic d. Magnetic

5. According to function. a. Timerb. Dimmerc. Three way d. Four waye. Momentary contactf. Safety disconnect


g. g. Electronic/solid state

2.2.1 Switch Contacts

The contacts of a switch are used to make or break the electrical circuit. One set of contacts are stationary and the other set are movable. In all switches, the electrical contact is formed by pressing the movable contact against the stationary contact. Usually a spring is employed to help ensure constant pressure is maintained between the two contacts.

The two most common types of contacts are the "knife blade contacts" and the "butt contacts", see Figure 1. Many times the contacts will be silver coated to prevent oxidation and to ensure a low resistance contact point.

Figure 1. (A) Knife Switch Contacts (B) Butt Contacts

The contacts of a switch, especially burnt or charred contacts, will need to be burnished, filed, or even replaced periodically to maintain a low resistance contact point.

2.2.2 Pole of a Switch

The pole of a switch is where the contacts are attached. The poles in conjunction with the contacts are used to make and break the electrical circuit. Each pole is electrically insulated from one another.

A single pole switch will make and break only one conductor or leg of a circuit, whereas a two pole switch will make and break two legs of a circuit, and so on. See the schematic diagram in Figure 2.


Figure 2. Poles of a Switch

2.2.3 Closed Positions or Throws of a Switch

A single-throw switch will only make a closed circuit when the switch is in one position. A double-throw switch will make a closed circuit when placed in either of two positions, Figure 3. When more than two closed positions are needed, special multiple throw switches should be employed.

Figure 3. Single and Double-Throw Switches

2.3.0 SWITCH DESCRIPTIONS

2.3.1 Single-Pole Switch

Toggle switches or wall switches are used to control lights, appliances, and fractional horsepower motors. The most common switch is the single-pole single-throw switch which is used to control lights or equipment from one location only. This switch has two brass colored screw terminals for wire connections, see Figure 4.


Figure 4. Single-Pole Single-throw Switch

2.3.2 Three-Way Switch

The three-way switch is used in pairs to control a light or piece of equipment from two locations. It has three screw terminals for wire connections; one black or copper and two brass or silver colored. The three way switch is a single pole double-throw switch, see Figure 5.

Figure 5. Three-Way (Single-pole Double-throw)

2.3.3 Double-Pole Switch

The double-pole switch shown in Figure 6 is normally used to control 240 volt equipment. This switch has four brass colored screw terminals for wire connections. The terminals on the left side of the switch are connected


through one set of contacts and the terminals on the right side of the switch are connected through another set of contacts.

Figure 6. Double-Pole Single-Throw Switch

2.3.4 Float Switch

The raising and lowering of a float which is mechanically attached to electrical contacts may start motor driven pumps to empty or fill tanks. The float switch contacts may be either normally open contacts (NO) or normally closed contacts (NC) and they cannot be submerged.

These rod-operated float switches are also commonly used to open or close solenoid valves to control fluids, Figure 7.

Figure 7. Rod-Operated Float Valve With Wiring Symbol

2.3.5 Pneumatic Switch

Pneumatic or pressure switches are used to control the pressure of liquids and gases by starting and stopping motors and compressors at the proper time. Air compressors, for example are started directly or indirectly on a call for more air by a pressure switch.


NOTE: When selecting pressure switches, pay particular attention to the pressure rating of the switch and the operating pressure of the system. Never exceed the switch ratings.

Pressure switches use mechanical motion from pressure changes to operate one or more sets of contacts. A typical pressure switch using a bellows as the pressure sensing element is shown in Figure 8. Other common sensing elements are the diaphragm and bourdon tube. Most pressure switches have an adjustment screw to adjust the setpoint.

Figure 8. Typical Pressure Switch

Many times pressure switches come with two sets of contacts, one NO set and one NC set. When the pressure reaches the setpoint, one set of contacts will open and the other will close. In selecting a pressure switch for a particular application, each of the following should betaken into consideration.

The adjustable range. This is the span of pressures within which the pressure sensing element can be set to actuate the contacts of the switch. For example, a pressure switch may have a range of 20 to 100 psi.

The adjustable differential pressure. This is the span of pressure between the high pressure limit which actuates the contacts, and the low pressure limit which resets the contacts to their normal position. A pressure switch may have an adjustable range between 20 and 100 psi with a differential adjustable pressure of only 5 to 15 psi.

The rating of the switch and contacts. The rating includes the type of switch used such as single-pole single-throw, single-pole double-throw, etc. It also includes the amount of current and voltage that the contacts can safety switch.The accuracy of the switch refers to the ability of the switch to repeatedly


actuate at the setpoint. This value is typically stated as a percent (%) of the maximum operating pressure of the switch (i.e. ± 2% of maximum operating pressure).

2.3.6 Limit Switch

Limit switches are designed such that they can be activated by the motion of machinery. The repeat accuracy of the switches must be reliable and the response virtually instantaneous.

In general, the operation of a limit switch begins when the moving machine or moving part of a machine strikes an operating lever which activates the limit switch, Figure 9.

Figure 9. Limit Switch With Wiring Symbols

Limit switches can be used either as control devices for regular operation or as emergency switches to prevent the improper functioning of equipment. The contacts may be normally open (NO) or normally closed (NC) and they may be momentary (spring returned) or maintained contact types.

The installation of limit switches involves picking the best actuator and then mounting the: limit switch in the correct position in relationship to the moving part. It is important that the limit switch not be operated beyond the manufacturer's recommended specifications of travel. A mechanical stop as shown in Figure 10 should be used to ensure the switch is not damaged by the motion of the machine it is supposed to be controlling.


Figure 10. Limit Switch Installations

2.3.7 Electronic Switch (SCR)

Silicon Controlled Rectifiers or SCR's are electronic devices that normally operate in the ON or OFF state very much like a switch does. SCR's are used extensively in motor controls and power supplies, see Figure 11.

Figure 11. Silicon Controlled Rectifier (SCR)

The basic purpose of the SCR is to function as a switch that can turn on and off large amounts of power. It performs this function with no moving parts that wear out and no points or contacts that require replacing.

The SCR can be used in AC and DC circuits but it will only allow current to pass in the forward direction (from anode to cathode). It has an anode (positive) lead, a cathode (negative) lead, and a gate lead. No current will flow through the SCR until the gate lead receives an electrical signal. Once the gate receives a signal, the SCR is like a closed switch and it conducts current easily in the forward direction (anode to cathode). The gate signal


can now be removed but conduction will not stop until:

the current tries to reverse direction, or the current drops to near zero.

Once conduction has stopped, the SCR will be like an open switch and no current will flow. A new gate signal must be applied before the SCR will conduct again. The SCR is similar to a diode in that it will only allow current to flow in the forward direction (anode to cathode) and never in the reverse direction (cathode to anode).

3.0.0 PHOTOELECTRIC DEVICES-

Light sensitive devices, sometimes called photoelectric transducers, alter their electric characteristics when exposed to visible or infrared light. Light sensitive devices include photocells, solar cells, motion detectors, and phototransistors.

Such light sensitive devices can trigger many different kinds of circuits for the control of alarms, lights, motors, relays, and other actuators.Some of the more common photoelectric devices and their principal of operation will be discussed in this section.

3.1.0 PHOTOCELL SWITCHES

Photocells are also called many other names including photoconductive cells, light dependent resistors (LDR's), and photoresistors. Basically, photocells are variable resistors with an extremely wide range of resistance values which depends upon the amount of light that falls on them.

The resistance of the photoconductive material shown in Figure 12 varies inversely with the amount of light that shines on it. In other words, the more light that hits the photocell, the lower the resistance of the photoconductive material. The photoconductive material is usually made of Cadmium Sulfide (CdS) or Cadmium Selenide (CdSe) and is placed directly between the two terminals of the photocell. In bright light, the resistance between the terminals can be as low as 50Ω, and in darkness as high as 5 MΩ.


Figure 12. Cutaway View of a Photocell or Light-Dependent Resistor (LDR)

The most common use for photocells is the photocell switch used to control lights between dusk and dawn. Essentially, the photocell switch shown in Figure 13, acts as a single-pole single-throw (SPST) switch that closes when it gets dark and opens when it gets light.

Photocell switches as in Figure 13 can be rated for up to 300 watts and can directly control a set of floor lights rated 300 watts or less.

Figure 13. Photocell Switch and Typical Application

Photocell switches can also be used in control circuits by using a light beam and a photoreceiver to scan across an area to detect the presence of an object. When the light beam is broken, the photocell or photoreceiver responds by sending a DC signal to the control circuit. Figure 14 shows the direct scan method of photoelectric detection. The light source and the photoreceiver are positioned directly opposite each other, so that light from the source shines directly at the receiver. When the roll of paper gets big enough to block the light beam, the photoreceiver sends a signal to the controller which will load a new spool on the paper spindle.


NOTE: Keep the sensor and reflector lenses clean to ensure proper operation of detectors.

Counting products is a common application of photoelectric controls. Counting batches or groups of cans or other items prior to packaging or group processing is also common.

The size of a paper or fabric roll can be controlled by positioning a light source and a photoreceiver so the roll diameter blocks the beam.

Figure 14. Direct Scanning Methods

Reflective scan is another popular scanning method used to detect moving objects. In '' reflective scan, the light source and the photoreceiver are mounted in the same housing and on the same side of the object to be detected, see Figure 15. This method is used when there is limited space or when mounting restrictions prevent aiming the light beam directly at the photoreceiver. As shown in Figure 15, the light source reflects off of the reflector and is absorbed by the scanner/receiver. When the light beam is broken by the bottles, there is no reflection and the photodetector signals the capper to apply the bottle cap.


Figure 15. Reflective Scanning Method Used to Count and Cap Bottles

3.2.0 SOLAR CELLS

Solar cells, also called photovoltaic detectors, generate a DC voltage which is proportional to the amount of light that is absorbed by the cell.

A typical cell, as shown in Figure 16, only generates about 1/2 volt DC. Therefore, cells are often arranged in series and parallel combinations to obtain the desired output voltage and current levels. Cells are joined together to make: a solar module; a solar panel; and, a solar array. Figure 17 shows the components of a photovoltaic array system that could be used to charge a battery bank or power a piece of DC equipment.

Figure 16. Typical solar Cell


Figure 17. Photovoltaic Array Components

3.2.1 Infrared Devices

Any object not at a temperature of absolute zero (-460º Fahrenheit) will emit invisible electromagnetic radiation in the form of Infrared Radiation (IR). Since infrared radiation is a form of energy, a detector can pick up this transmitted energy and produce an electrical signal proportional to the amount of IR radiation detected. This signal can then be used to control other devices.

IR detectors are commonly used for motion/people detectors, TV remote control detectors, IR scanners which detect hot spots in electrical equipment, etc. By far, the most common application is the motion/people detector.

3.2.2 Motion Detectors

Motion detectors are typically used in a security system of some type. The security system may be an elaborate zone controlled system that incorporates motion detectors, door switches, smoke detectors, moisture sensors etc., or it may be as simple as a set of motion detector floodlights mounted above the garage. The basic operation of the detector is the same in all cases.

Figure 18 shows a set of floodlights controlled by an IR motion detector switch. Many times a photocell switch is added to the light controller which automatically turns the lights off during daylight hours. Also, the light


controller usually comes equipped with a manual override method which can be used to turn the lights on at any time.

Figure 18. Motion Detector Flood Lights

When the light controller contains both the photocell switch and an IR detector switch, it operates as if there were three SPST (single-pole single-throw) switches involved, see Figure 19. The photocell switch and the motion detector switch are in series such that have to be closed for the light to come in. The third switch is the manual bypass switch which is in parallel with the series motion detector and photocell switches. Normally for the light to come on, there must be motion detected and it must be dark.

Figure 20 shows a typical application and wiring diagram for a popular IR motion detector with photocell light controller. Besides controlling lights, the same detector could be used to sound a siren, open an automatic gate, or perform some other automatic function.

Figure 19. single-Line diagram of Motion Detector With Photocell Light Controller


Figure 20. typical application of an IR Motion Detector Light Controller

4.0.0 PROXIMITY SWITCH-

Proximity switches are used to detect the presence or absence of an object without ever touching it. They are becoming increasingly popular for industrial applications because they are versatile, safe, reliable, and can be used in place of mechanical limit switches. Typical proximity switches are shown in Figure 21.

Figure 21. Typical Proximity Switches

The most popular type of proximity switch is the inductive RF oscillator type which operates on the Eddy Current Killed Oscillator (ECKO) principle. The oscillator consists of an inductor/capacitor (LC) tuned tank circuit which generates an AC magnetic flux as it oscillates and emits this field from the face of the switch, see Figure 22. When a metal object is placed near the magnetic lines of flux, eddy currents are induced in the metal object which in turn disrupts the tuning of the LC tank circuit and the oscillations die out or are killed. When the oscillations die the integrator signals the trigger to switch the output of the proximity switch.


Figure 22. Eddy Current Killed Oscillator (ECKO) Sensor

SUMMARY

Understanding the classifications and operation of switches and photoelectric devices is very important to anyone working in the electrical field. Without such knowledge, the selection, installation, or maintenance of electrical equipment and circuits will be very difficult if not impossible.

Switches are classified according to the number of poles, the number of throws (SPST, SPDT), and according to their function (i.e. pressure, temp.). Their operation depends mostly upon the type of contacts involved and the method of operating the contacts. Overloading is the most frequent cause of switch failures.

Photoelectric devices on the other hand are generally classified only according to their function (photocell switch). The operation of these devices usually depends upon complicated electronics which are not readily accessible to the user. For photoelectric devices, overloading and improper wiring connections are the most common causes for failure.

References

For a more advanced study of topics covered in the Task Module, the following works are suggested:

Electric Motor Control, Third Edition, Walter N. Alerich.

American Electricians' Handbook, Twelfth Edition, Terrell Croft, Wilford I. Summers.

National Electric Code - 1990 Handbook, Fifth Edition, National Fire Protection Association.


SELF-CHECK REVIEW / PRACTICE QUESTIONS

1. Switches are normally used to make or break an electrical circuit that is operating:a. Within the rated voltage of the switch.b. Under an overload condition.c. Within the rated current of the switch.d. Within rated voltage and current ratings of the switch.

2. Switch contacts are silver coated to prevent _______________.

3. The ______________of a switch, in conjunction with the contacts, are used to make and break the electrical circuit.

4. A double pole switch is typically used to control: a. 120 volt circuits or equipment. b. 240 volt circuits or equipment. c. Three phase circuits or equipment. d. All of the above.

5. Label the switches below according to the number of poles and the number of throws.

6. ____________ switches are designed such that they can be activated by the motion of machinery.a. Pressure.b. Limit.c. Float.d. Photoelectric.


7. An SCR is an electronic device that _____________.a. Is triggered by infrared radiation.b. Can turn on and off large amounts of power.c. Uses four leads during normal operation.d. Will conduct current in the forward and reverse direction.

8. The ___________of a photocell varies depending upon the amount of light that shines on the cell.

9. A solar cell uses converts ______________into electrical power.

10. A motion detector switch uses a detector to sense motion. a. Vibration.b. Temperature.c. Infrared.d. All of the above.

PERFORMANCE / LABORATORY EXERCISES

Identify and select various types of switches which are on display.

Step 1 Identify by name each switch that is displayed on a table.Step 2 Classify each switch according to the number of poles and throws. Step 3 Explain the operation of selected switches.

Identify and select various types of photoelectric devices which are on display.

Step 1 Identify each device on display. Step 2 Explain the operation of each device.

ANSWERS TO SELF-CHECK REVIEW / PRACTICE QUESTIONS


1. a

2. oxidation

3. poles

4. b

5. a. Single-pole Double-throw (SPDT) b. Double-pole Single-throw (DPST) c. Single-pole Single-throw (SPST)

6. b

7. b

8. resistance

9. light

10. c
