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Continuing

What Is a ?

Another major electronic component used in elevators is detailed.

Learning Objectives P-type silicon) are created essentially as they are for , by a process known After reading this article, you should have learned about: ♦♦ How operate by attracting and repelling charge as doping. Crystalline silicon is not carriers electrically conductive. (Other ♦♦ The effect of doping on crystalline silicon semiconducting materials have been ♦♦ Why the doping process is used used, but today the choice is usually ♦♦ How base, collector and emitter are connected in silicon.) But, when it is exposed to trace electronic circuits amounts of certain other elements, the ♦ ♦ How to handle transistors crystalline silicon becomes a . This does not mean it by David Herres has a fixed resistance that lies Like diodes, transistors are major players in somewhere between that of an insulator like the arena, notably in elevator motion glass and a conductor like copper. To the controllers and variable-frequency drives contrary, it means that under some conditions, (VFDs) that have an extensive operator’s Continued interface. Also like diodes, the transistor semiconductor mechanism is very simple in that it involves the attraction and repulsion of charge carriers by the bias voltages applied to the device terminals. The and holes migrate toward or away from the semiconductor junctions, thereby regulating higher flows of current between various terminals and ground. The defining difference between diodes and Value: 1 transistors is that in the there are two layers (N- and P-type silicon, with a single contact junction between them) where the hour semiconductor action takes place. This article assumes the reader has already understood the (0.1 CEU) author’s piece “What Is a Diode?” (ELEVATOR Approved for WORLD, August 2014) or understands how Continuing these devices work on a subatomic level and in Education by NAEC electrical circuits. Compared to diodes, for CET® and NAESA transistors are somewhat more complex, International for because they have three semiconductor layers Figure 1: Transistor packages in order from top to bottom: QEI. with two junctions. The materials (N- and TO-3, TO-126, TO-92 and SOT-23 (photo by Transisto)

December 2014 • ELEVATOR WORLD 93 the crystalline silicon conducts, and under effective as charge carriers and necessary other conditions, it does not conduct. for semiconductor operation. What underlies all of this is that silicon, Crystalline silicon doped with boron or with an atomic number of 14, consists of a gallium becomes P-type silicon. We have nucleus with four electrons orbiting in its seen in the previous article on diodes that valence (outer) shell. (The number of N- and P-type silicon bonded together electrons in the valence shell of any make a junction. With leads attached at the element accounts for its physical far ends, they constitute a useful device properties and the ways in which it that conducts or does not conduct, interacts with other nearby atoms.) Atoms depending upon the . are able to share valence electrons with Transistors also make use of the adjacent atoms. Because it has four semiconducting properties exhibited by electrons in its valence shell, pure silicon N- and P-type crystalline silicon. Today, likes to assume the form of a crystal lattice, bipolar transistors have been largely each atom sharing electrons and, hence, supplanted, first by field-effect transistors tightly bonded to four adjacent silicon (FETs) and, finally, by the ubiquitous atoms. The crystalline silicon is very stable, metal-oxide semiconductor FET Figure 2: circuit, common-emitter and, electrically, it is an insulator. (MOSFET), which has much higher input configuration, with voltage-divider bias circuit: When the crystalline silicon is doped impedances. Nevertheless, there are plenty note that the emitter arrowhead points outward, with phosphorous or arsenic (don’t worry, of bipolar transistors around in older indicating that the transistor is of the NPN type (image by Aflafla1). the amount is infinitesimal), each of which equipment, as well as in contemporary has five electrons in its outer shell, the applications. In this piece, we will stick to some configurations, through external situation changes abruptly. There is then bipolar transistors, since they are more circuitry. one free for each doped silicon basic and provide a good background for Each of these can be atom. The excess free electrons travel in understanding the more exotic varieties. P- or N-type silicon. The emitter and the spaces between the silicon atoms, and As mentioned above, transistors consist collector are always the same type, and the they are known as “charge carriers.” of three semiconductor layers, a base, an base is the other type. Accordingly, Crystalline silicon, doped with emitter and a collector. These terms do not transistors may be either PNP or NPN. The phosphorous or arsenic and thus having have to be taken too seriously, because the PNP version has an N-type silicon base free electrons, is N-type silicon. It is no “base” is not a base, the “emitter” does not with emitter and collector composed of longer an insulator but is capable of emit anything, and the “collector” does not P-type silicon. “NPN” indicates a P-type conducting an electrical current, due to collect anything. Still, that is the base and an N-type collector and emitter. the presence of charge carriers. terminology, and it is quite graphic. The two junctions, like the single junction In a mirror-image process that reverses Each layer has a attached so the of a diode, may be forward or reverse the polarities, crystalline silicon can be transistor can be wired to other biased, and so, will conduct or not conduct. In most circuits, NPN and PNP Even when the transistor terminals are not labeled, it is transistors can be interchanged, but all possible to identify them and ascertain the transistor type by polarities (including those provided by the ) will be reversed. looking at the arrowhead. A transistor schematic consists of a circle with an internal semiconductor doped by exposing it (so that minute components, such as , shown, including leads that permit circuit amounts are absorbed) to boron or and coils, to comprise a circuit. The three connections. The base resembles a flat gallium, both of which have three electrons semiconductors are bonded, always with plate like the cathode of a diode. Collector in the outer shell. This process creates a the base in the middle and the emitter and and emitter are lines that connect to the deficiency of electrons in the space collector on either side. But, do not base at an angle. By convention, the base is between the silicon atoms. These empty conclude that the middle lead goes to the shown at left, the collector at top and spots are called “holes” and may be base, because this is not always the case. emitter at bottom, but this may vary if thought of as positively charged particles Since there are three layers, there are two required by the overall schematic layout. with negligible mass, like protons that have junctions. The base has two junctions, and The base, collector and emitter may be positive polarity but are far more massive. the emitter and collector each have a single labeled on the schematic, but not always. Whether these holes actually exist as junction they share with the base. The You can always distinguish emitter and particles or more nearly resemble emitter and collector have no common collector, because the angle line having the conceptual entities, we can leave to the junction and are not electrically arrowhead is always the emitter. Moreover, metaphysicians. The fact is that they are connected, except through the base and, in if the arrow points inward toward the base,

94 www.elevatorworld.com • December 2014 the transistor is PNP, and if it points waveform of any desired frequency. This that feeds the motor. These frequencies outward, the transistor is NPN. To would apply to a wide range of are supplied by the inverter section of summarize, even when the transistor applications: the VFD. terminals are not labeled, it is possible to ♦♦ Electronic equipment that incorporates The principle types of electronic identify them and ascertain the transistor an audio beep such as a telephone, oscillators are the linear and non-linear type by looking at the arrowhead. or fire-alarm system will (relaxation) oscillator. A linear oscillator Transistors have three leads. With one have this generated within an produces a pure sine wave. It consists of a lead as common, it is possible to have two oscillator circuit. transistor configured as an amplifier with a two- circuits, and this is how transistors are configured. The emitter is It is possible to get a good idea of the condition of the usually the common connection. In this mode, the base emitter is the input, and transistor on a go/no-go basis using just the ohmmeter. the collector emitter is the output. Small ♦♦ A signal generator produces a range of input changes result in identical output feedback loop, meaning that the output is frequencies and waveforms that changes but at a much higher power level. connected back to the input. When technicians inject into defective This stepped-up output receives its initially powered up, any small amount of equipment for purposes of signal electrical energy from the power supply, so noise generated by atomic motion in the it is not a question of getting free power. tracing and viewing (with an circuitry or within the semiconductor is Nevertheless, the input/output ratio makes ) at each stage until the amplified and conducted back through the the devices extremely useful in many types fault is located. input so that continuous amplification and ♦ of electronic equipment. ♦ Clock that regulate filtering produces the desired tone. As an amplifier, the transistor can boost and quartz clocks are generated by Transistors are purposely operated in an incoming signal applied to the input oscillators. the saturated mode in digital circuits, as terminals, so that an amplified version ♦ ♦ Virtually all radio and TV receivers are opposed to the analog circuits discussed appears at the output terminals. The ratio superheterodynes. They drop the previously. They are used in the digital of base-emitter current to collector- radio-frequency (RF) signal as picked () mode in high-power applications, emitter current is known as “gain.” The up by the to a more such as the switching of power supplies, output of one transistor stage can be manageable intermediate frequency and in low-power modes, such as logic coupled to the input of a succeeding stage (IF) and lower frequencies for gates. In both designs, the transistor is to achieve greater amplification. A number successive stages of amplification. The configured as a switch. It is similar to a of stages can be cascaded, so that the very way they do this is by mixing the light switch (not a dimmer switch) in that weak signal picked up by the antenna can higher-frequency signal with an be amplified sufficiently to drive large it may be either on or off, but not in oscillator-generated signal so a sum and speakers, video or other devices. Low-gain between. When it is in the cutoff mode, difference beat signal is synthesized. transistors have a gain of around 30. A the output is off, and this corresponds to The desired lower-frequency signal is high-gain transistor can reach 800 or more. the digital number zero and the logic state selected using a simple coil- As input is increased, the output also “FALSE.” When it is in the saturated resonant circuit. In the early years of increases in a linear . Beyond a mode, the output is on, and this radio transmission, the sine wave was certain point, however, the transistor corresponds to the digital number one and generated at the broadcast station and (together with its power supply) is not the logic state “TRUE.” sent along with the programming capable of further amplification and is said A linear power supply, as we saw in the signal, but today, the tone is locally to be “saturated.” (This is similar to when diode article, converts AC utility power to generated within the receiver. In an an , such as a a lower-voltage DC power used in old-style receiver, the winding, becomes saturated, and no electronic equipment to bias the active (which was tuned to different further increase in electrical current will components. It performs this task by increase the magnetic flux.) Saturation is frequencies) was made up of two placing a single power diode in series with not good in an amplifier. The output devices ganged on a common shaft. This one incoming line (half-wave ) or waveform will be missing its peak levels, was so that, as different stations were by employing four power diodes in a with a flat line at the point of saturation. acquired, there would be a different diamond configuration (full-wave bridge This condition is known as “clipping” and oscillator frequency for each so that a rectifier). Both arrangements are simple manifests as an unpleasant buzzy single IF could be synthesized for the and fairly reliable, considering the fact that component in an audio output. entire broadcast band. the power supply is the location where the Besides amplification, transistors may ♦♦ A VFD used to provide speed control entire system power flows. The downside be designed and configured for other for an AC motor (such as that in an is that a substantial amount of power is purposes. A major use is as an oscillator, elevator drive) does so by supplying wasted when dissipated as heat. This is which can generate a sine wave or other appropriate frequencies in the power because, for a large portion of the AC Continued

December 2014 • ELEVATOR WORLD 95 cycle, the device is neither turned off voltage. In both cases, the switch is a the socket marked common, is negative. (open) nor turned on (conducting all the power transistor of appropriate rating. But this is not universal, so the best current); it is somewhere in between. Failure modes involve shorted or open procedure is to check polarity with a Once again, the Ohm’s Law Wheel is , capacitors, diodes, transistors, known good diode that has the cathode relevant. Power or heat loss is proportional or wiring (including marked with a single band. Then, the to I2 X R, current squared times resistance. terminations). As always, this equipment probes can be permanently labeled. When the rectifier is fully on,R = 0, and can contain hazardous stored voltages that A transistor may be thought of as two when it is fully off,I = 0. In either of these may persist long after the power source is diodes with the two anodes (representing disconnected and locked out. Do not test instances, the power wasted, dissipated as NPN) or the two cathodes (representing or service this equipment unless you know heat, is 0. PNP) connected. This hookup allows for the dangers and can safely disconnect it The switching power supply meets this three test points. Figure 3 serves as a from all sources of power and discharge condition, since it converts the sine-wave learning device to show how to connect an the stored voltage. A bleeder will ohmmeter to a transistor for testing input to a square wave with very fast rise perform this task automatically, but purposes, but it will never work as a and fall times, resulting in minimal heat nothing should be assumed, because the functioning transistor. loss. For this reason, components and the device may not be properly connected, or Going back to the transistor to be tested, overall device can be downsized, reducing it could be open. if you do not have a schematic of the overall cost and space requirements. There are equipment, nor a parts number or some tradeoffs, however. First, switching manufacturer’s data sheet, it is often not power supplies are more complicated, and possible to know the identity of the three more difficult to diagnose and service. leads and the transistor type. But, using the Moreover, the high-speed transitions can multimeter in ohms mode, you can bias and generate harmful harmonics and RF measure the continuity of each pair of leads, interference in nearby equipment. Figure 3: The thereby gathering valuable information. Additionally, if there are many of these cathode faces outward, Each pair of leads can be forward or units in a facility, they may contribute to a indicating this reverse biased, so there are six possible poor power factor. Any of these diode model measurements that can be taken. These disadvantages may be mitigated by the corresponds to a measurements provide the only PNP transistor installation of shielding, judicious (image from information available from ohmmeter placement and isolation from other Electronics+Radio readings. Referring back to the diode conductors, and connection to power- Today). model, you will notice that a good diode factor correction capacitors. will read open across two of the leads, Despite these disadvantages, there has regardless of which way the ohmmeter is been a major move away from linear and connected. So, these two leads are toward switching power supplies, and this collector and emitter, but we do not know which is which. We do, however, know is likely to continue. In a switching power A transistor may be tested using the that the remaining lead is the base. supply, voltage regulation is achieved by multimeter in ohms mode, the multimeter If all three pairs measured both ways varying the relative on and off times. The in diode or transistor test mode, an read low ohms, the transistor is shorted. If on/off switching action is performed by in-circuit transistor tester, a service-type all three pairs measured both ways read one or more power transistors operated transistor tester or a laboratory-standard high ohms, the transistor is open. Discard alternately at below the cutoff and above transistor tester. All of these impose a bias it immediately, before it gets mixed into the saturation levels. voltage as needed. The service-type tester measures beta (forward) gain. It also your inventory. A consists of an If the transistor passes both of these inductor and diode in series between the measures base-collector leakage current with no voltage on the base. Some tests, further investigation is in order. To supply and load. The switching transistor is instruments identify base, emitter and review, the collector-emitter pair has been placed in parallel with the supply and load, collector leads. The laboratory-standard identified, and the remaining lead is after the inductor but before the diode. transistor analyzer applies bias and signal, connected to the base. Connect the probes When off, it is invisible, and when on, it and reads the output. both ways to each of the other leads. When shunts out the diode and load, but not the An ohmmeter can be used to check a forward biased, the base will conduct to inductor. The inductor reacts to current transistor, because the meter’s power either of the two leads. The two hookups changes in the circuit by inducing a supply (intended to provide the current for require different biasing to conduct. From counter voltage, which is added to the measuring resistance) is suitable for biasing these readings, you can identify the power-source voltage under open switch a semiconductor. To perform the test, the non-base leads if you know the transistor conditions. Alternatively, a capacitor can polarity of the ohmmeter probes must be type, or you can ascertain the transistor be used in series configuration, creating an known. Most manufacturers polarize the type if you know the identity of at least one output voltage higher than the DC input probes so that black, when plugged into of the leads. Continued

96 www.elevatorworld.com • December 2014 We see that with no information at all about the transistor, it is not possible to determine everything, but with partial information, the transistor type or the collector-emitter identity, we can fill in the remaining unknowns. In any event, it is possible to get a good idea of the condition of the transistor on a go/no-go basis using just the ohmmeter. Two important attributes of any bipolar transistor are α and β. With base at signal ground, α is the collector current divided by the emitter current, which is the transistor’s dynamic gain. β is the collector current divided by base current when the emitter is at signal ground. These two attributes apply to a transistor wired in two configurations. This relationship is spelled out in the following formulas: α = β/(1 + β) β= α/(1 - β) You can always find α and β in the manufacturer’s data sheets, and these quantities are relevant for design work and for determining a suitable replacement when the original transistor cannot be obtained. In a circuit board, bad transistors (like other discrete components) can be replaced, but special care is required so they are not damaged by heat from the soldering iron. Attach a between the component to be soldered and the heat source. Smooth-jawed needle-nose pliers, held closed by means of a rubber band, will work. Learning-Reinforcement Questions Use the below learning-reinforcement questions to study for the Continuing Education Assessment Exam available online at www.elevatorbooks.com or on p.115 of this issue. ♦♦ How many semiconducting layers are in the simple transistor? ♦♦ What causes the resistance of crystalline silicon to change? ♦♦ Which role do free electrons play in the semiconducting process? ♦♦ What are the two types of charge carriers? ♦♦ What are the base, emitter and collector?

David Herres holds a New Hampshire Master Electrician’s license and has done that work in the northern part of the state for many years. Since 2006, he has become more focused on writing, and has been published extensively. He has written approximately 130 articles for electronics magazines, including ELEVATOR WORLD, Electrical Construction and Maintenance, Cabling Business, Electrical Business, Nuts and Volts, News Record, PV Magazine, Electrical Connection, Solar Connection, Solar Magazine, Fine Homebuilding magazine and others. He has written four books published by McGraw-Hill: 2011 National Electrical Code Chapter By Chapter came out three years ago. Troubleshooting and Repairing Commercial was released the following year and The Electrician’s Trade Demystified was released this year. The Homeowner’s DIY Guide to will be released soon. He holds a BA in English Literature and Composition from Hobart College of Geneva, New York.

98 www.elevatorworld.com • December 2014 ELEVATOR WORLD Continuing Education Assessment Examination Questions Read the article “What Is a Transistor?” (p. 93) and study the learning-reinforcement questions at the end of the article. To receive one hour (0.1 CEU) of continuing-education credit, answer the assessment­ examination questions found below online at www.elevatorbooks.com or fill out the ELEVATOR WORLD Continuing Education Reporting Form found overleaf and submit by mail with payment. Approved for Continuing Education by NAEC for CET® and NAESA International for QEI.

1. What accounts for the migration of charge carriers in a transistor? 8. The ratio of base-emitter current to collector-emitter current is a. Their innate tendency to attract one another. known as: b. Their innate tendency to repel one another. a. Bias. c. The applied bias. b. Gain. d. None of the above. c. Polarity. d. It depends upon polarity. 2. How many valence electrons are in the outer layer of silicon? a. Two. 9. What is the middle layer of a transistor? b. Four. a. The collector. c. Six. b. The emitter. d. Eight. c. The base. d. It depends upon the circuitry. 3. Which of these materials is used for doping? a. Boron. 10. If the input to a transistor is increased beyond a certain point: b. Gallium. a. It becomes saturated. c. Phosphorous. b. There may be clipping. d. All of the above. c. Neither of these. d. Both of these. 4. An emitter and collector have a common junction. a. True. b. False.

5. The emitter and base are always: a. N-type silicon. b. P-type silicon. c. The same type of silicon. d. Different types of silicon.

6. In most circuits, NPN and PNP transistors can be interchanged if the polarities are reversed. a. True. b. False.

7. In a transistor schematic, the angle line with the arrowhead goes to the _____. a. Emitter b. Collector c. Base d. Any of these ELEVATOR WORLD Continuing Circle correct answer. Education Reporting Form 1. a b c d 6. a b 2. a b c d 7. a b c d 3. a b c d 8. a b c d Article title: “What Is a Transistor?” 4. a b 9. a b c d (EW, December 2014, p. 93). 5. a b c d 10. a b c d Continuing-education credit: This arti- cle will earn you one contact hour (0.1 Signature: ______CEU) of elevator-industry continuing-edu- cation credit. Payment options: Directions: Select one answer for each Check one: ❑ $30.00 – Non-subscriber course fee question in the exam. Completely circle the ❑ $25.50 – ELEVATOR WORLD appropriate letter. A minimum score of subscriber course fee 80% is required to earn credit. You can also take this test online at Subscriber #: ______www.elevatorbooks.com. (Six-digit number on your print label or in your digital confirmation) ❑ Payment enclosed Last name: ______(check payable to Elevator World, Inc.) First name: ______Middle initial:______CET®, CAT® or QEI number:______Charge to my: ❑ VISA State License number: ______❑ MasterCard Company name: ______❑ American Express Address:______City:______State: ______ZIP code: ______Card number: ______Telephone:______Fax: ______E-mail: ______Expiration date: ______

Signature: ______This article is rated for one contact hour of continuing-education credit. Certification regulations require that we verify ­actual study To receive your certificate of completion using the mail-in op- time with all program participants. Please answer the below question. tion, send the completed form with questions answered and pay- ment information included to: Elevator World, Inc., P.O. Box 6507, How many hours did you spend reading the article and studying Mobile, AL 36660. the learning-reinforcement questions? To receive your certificate of completion online, visit www.elevator hours ______minutes ______books.com and follow the instructions provided for online testing.

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