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The Examination requires candidates whose vision allows them to do so, to identify specific component symbols on three sample circuit diagrams or "schematics". Several of the questions below do not refer to the supplied diagrams, so apply to all candidates.
Schematic diagrams show the interconnections of components, but not the physical layout; and are not to scale. While Americans and Europeans use different symbols for some components, and vary in structure, although all now tend to be drawn with the positive power at the top, ground at the bottom, or centre if there is a negative rail below it. Signals (or power in a supply circuit) tend to flow from left to right. These diagrams do vary from European standards in that instead of having a line for the negative return of power, this is indicated by the connection to a common or ground (not necessarily to earth).
Each component type has a specific symbol, called "Schematic symbols" in the exam.
When reading a magazine about a circuit design or "project", there may be, in addition to the schematic diagram, either a PCB (printed circuit board) layout, a cutting and placement drawing for stripboard, or in the oldest, an arrangement using tags on valve (tube) sockets, tag strips, and other chassis mounted components. Some leave the layout up to the reader.
In this circuit a small current, with the positive on the upper terminal, will cause the transistor to switch on a lamp.
1 - Resistor - This controls, or rather limits, the current flowing into the transistor.
2 - Transistor It is short for "transfer of resistance", evidently. Transistors are current operated devices, in that if a small current flows into the Base, shown at the left, and out the Emitter, at the bottom, a larger current can flow from the Collector at the top. Transistors operate as both an amplifier and a switch, so if the current into the base is small, then the collector current will be some unknown multiple (a factor of tens or hundreds, varying from device to device, even of the same type) of the input. In this mode the voltage drop across the transistor will be perhaps several volts, and this multiplied by the current generates possibly significant heat. If we ensure the current into the base is adequate, the transistor will go into saturation, with a low voltage drop, reducing the heat wasted. We can tell that the transistor is of the NPN variety, with a positive base to collector current. The clue to this is that the arrow is "Not Pointing iN". The symbol does not indicate the size or capacity of the device, it could be a tiny, low current device, or a large, high current one. Current carrying capacity, and voltage blocking capability are among a transistor's specifications; and there are thousands of variants. Useful ones might be BC-337, or 2N2222A, both in the classic half-cylindrical TO-92 packages. (2N is the US JEDEC system, indicating 2 junctions, BC a European "Proelectron" system indicating that it is a "signal" (lower power) silicon bipolar transistor). Note that the ring is not always used. Old school Germanium transistors use the same symbol, but have either AC, AD, or AF prefixes, or in the earliest cases OC, coded as a triode valve with a zero volt heater / filament, that is, none. Power transistors have a metal case, or a plastic case with a metal tab, either allowing heat to be transferred to an aluminium heatsink where necessary.
There are a range of other transistors, such as Field Effect Transistors (FETs), also called unipolar transistors, with high input impedance; and Insulated Gate Bipolar Transistors; plus Silicon Controlled Rectifiers and Triacs.
3 - Filament Lamp - This symbol does not make it clear whether this is an indicator sized lamp, or designed for illumination. Current flowing flowing through the filament generates a lot of heat, and some visible light, over a broad spectrum. Required current varies from tens of milliamps to 10 amps or more.
4 - Battery - A battery is actually a "battery of cells", meaning a number of electrochemical cells connected in series to provide a greater voltage, such as six 1.5 volt cells to make a 9 volt battery, sometimes called a transistor battery, as they will power a transistorised radio. There is no such thing as an AA battery, but an AA cell. Likewise, six 2 volts lead-acid cells makes a 12 volt car battery. Lithium "coin cells" provide 3 volts, various rechargeable lithiums provide around 3.7 volts, with many modern handheld radios using one or two (7.4 volts). NiCad and NiMH provide around 1.2 volts each, six giving 7.2 volts. There are two cell symbols in the symbol, but this does not necessarily indicate that there are two cells in the battery, and the chemistry is also not indicated. The longer line is the positive side. In some drawings the short negative electrode line is made thicker.
5 - Ground Connection - This shows that the terminal or lower connection of the component is connected to what in this case is the negative rail, which may or may not be connected to the case of the device, to earth, or the vehicle chassis. If a negative rail is shown, and such a symbol is used, then it is definitely a connection to the chassis of the device. While symbols are used interchangeably, this one is for a chassis connection, with the earth one being a horizontal line with two shorter ones below it, and maybe a dot.
Specifying the parts would typically work backwards, with either the battery chosen to suit the lamp, or the lamp to suit the supply. Depending on the current, the transistor is then selected, but most silicon NPNs are fine. Unless the transistor is a specialised type, it is best to assume gain of only 20. The junction has a forward voltage drop of 0.6 volts, so apply Vin - 0.6 volts to the R = E / I formula. So, for a 120 mA, 12 volt globe would want 6 mA of drive into the transistor. If we have a 5 volt signal we need R = 4.4 v / 0.006 A = 733.33 ohms, so use a 680 ohm, or 750 ohm, resistor, or a 715 ohm one if you can get it. P = 4.4 x 0.006 = 0.0264 W so a eighth or quarter watt one is fine. Darlington, Field Effect, or Insulated Gate Bipolar Transistors can require a much lower drive current. If the input current is reduced below that required for full brightness, then the transistor will not pass the full current, and so the lamp will glow dimly. This does mean that there is a larger voltage across the transistor, and so it will generate more heat. This is a crude method, as the gain of the transistor varies from device to device, even with the same part number. A better way to vary lamp brightness would be to turn the drive current on and off hundreds of times per second, and vary the duty cycle, something called PWM - Pulse Width Modulation.
This circuit takes a mains voltage input and generates a regulated DC output capable of running a fairly low power device.
1 - Power Connector - This symbol attempts to replicate the parallel terminals of a US mains plug. The solid rectangles indicate a plug, and by inference it is a non-polarised NEMA 1-15 plug, thus without an earth connection. This is used for 120 volt AC connections at up to 15 amps.
2 - Fuse - Placed as the first component within the device, the symbol emulates a glass or ceramic encased fuse with a metal cap at each end, such as a 3AG sized fuse.
3 - Switch - This turns the device on or off. Specifically, it is a Single-Pole, Single Throw switch, in that it connects or disconnects a single wire, and only selects 1 output, rather than changing between two or more outputs or inputs. As per convention, the switch operates by the bar moving in a clockwise direction. The type of switch - toggle, rocker, slide, etc is not indicated.
4 - Transformer - The symbol consists of two inductor symbols with a steel laminated core, which is basically what a transformer is, two coils of wire in an iron or steel core. This transforms the 120 volt input to what would usually be a lower voltage. Both the input and output of this transformer is a sinusoidal waveform. Small transformers are also use for electrical isolation between different circuits, while passing signals, useful for things such as connecting PC soundcards to radios for data modes.
| This is a large audio transformer, about the size of the smallest mains ones. The gunk is wax, used to stabalise the windings. It was made by Ferguson for a 1963 Radio, TV, and Hobbies all-transistor amplifier. It appears that the two outputs are to drive the two output transistors. |
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| This shows the laminated steel from which the core is made, making it more efficient. It also shows the lighter wire if the inner windings, which are the primary, or intput side, and the heavier secondary ones, on the output side. | |
5 - Power Diode or Rectifier Diode - A diode allows current to flow in one direction, but not the other. Flow is in the direction of the arrow, and blocked in the other. Small cylindrical diodes are marked with a band at the negative or cathode (K) end. The positive end is the anode (A). The term power means it is designed to carry significant current, rectifier meaning that it rectifies the AC power to DC. Note that this is a simple half-wave rectifier circuit, which would generate a pulse lasting for one 120th of a second (8.333 milliseconds), with a gap of the same duration; the 60 pulses per second are also called 60 Hertz, the US mains frequency. Diodes are rated with the current they can safely carry, and the reverse voltage they can block in the reverse direction. The forward voltage drop of a standard silicon diode is around 0.6 volts. Diodes are marked with either an industry standard number, or a manufacturer's number. This may be a 1N4001, 1N meaning a diode, having a single semiconductor junction, 400x series being the 1 ampere series, and the 1 a peak reverse voltage of rating of 50 volts (35 volts RMS). A more significant supply would use an arrangement of 4 diodes, called a "bridge rectifier" or "full wave bridge", either made from discrete diodes, or within a package. There are also rectifier valves / tubes, generally no longer used, even in valve based equipment, using a totally different symbol.
6 - Capacitor - These devices store energy as an electric charge in the "dielectric" or insulating material between two metal "plates". The implication is that this is an electrolytic capacitor, used to smooth the "lumpy" output of the rectifier to a smooth DC current. The voltage to which it is charges is the peak voltage of the sine wave, which is &sqrt; &root; √2, or 1.4142 times the RMS voltage (less the diode drop). During the reverse mains cycle the capacitor continues to supply current to the load, but discharges to a certain degree, this being the "ripple voltage".
 | Two standard "electro" capacitors from the same series, pulled from the switch-mode power supply of an HD DVB set-top box. They show that there is an inverse relationship between voltage rating and capacity. Especially the lower "cap" appears to have over-pressured, and be forcing the bottom bung out, and appears to have leaked. It is from the input side of the supply, with the rectified mains applied to it, this being 350 volts or more, and probably spent several years operating 24/7, at close to its rated voltage; and in a relatively hot enclosure. The upper one is one of several on the low voltage (output) side. Electrolytic capacitors drying out, and thus loosing capacity, resulting in excessive ripple is apparently why many such digital TV boxes fail. |
7 - Resistor - As discussed on the last page, these resist the flow of current in accordance with Ohm's Law. The squiggly line is an older symbol, still used in the US. Europeans use a rectangular symbol, more like the fuse one. This would likely be a low powered device of several hundred to a thousand or so ohms. In this case its job is to limit the current through the LED.
8 - Light Emitting Diode (LED) - The flow of current through this device causes the emission of photons of a specific wavelength; in other words, light of a specific colour. The typical LED used as a power indicator, which is the presumed function in this circuit, has a forward current rating to 20 mA, or a little more. They can however be operated at somewhat less than this. The voltage drop at this current is between 1.8 volts (red) to 2.2 volts for yellowish green to 3 or 4 volts for blue and some emerald greens. The permissible reverse voltage is quite low, sometimes around 6 volts. The typical package is a small domed plastic "bulb", 3mm or 5mm in diameter, but there are many others. An alternative symbol is a diode with a lower-case Greek character λ beside it. In many diagrams the component is circled, as the transistor in Figure T-1 is. There are much higher powered LEDs, intended for illumination. A range of standard LEDs are shown below. Click the image to get a bigger photo.
Most LEDs here are 5 mm in diameter. The red one second from the left is 3mm. The metal cased ones are very old. The horizontal green one, and rectangular one are both Soviet in origin. The longest leg on the 4 legged one is a common anode or common cathode, the others Red, Green, and Blue, allowing a wide range of colours to be emitted. The LED just left of the coin is a higher current device, the metal helping to remove heat from the diode junction. The yellow star contains a regular 3 mm LED. An Aussie 5 cent coin is 19.41 mm (a bit over ¾").
9 - Variable Resistor - This device consists of an open carbon or wirewound resistor element, with a wiper which moves along it connected to the right end. This restricts the current flowing beyond this point, to the Zener and the load. While this one might be a larger device, similar ones are used to set various parameters, such as volume in a radio.
10 - Zener Diode - This is a special diode, in that while it conducts in the forward direction like a normal diode, if a reverse voltage is placed across it, it regulates the voltage to its nominal voltage. They also have a power rating, such as 1 watt, 5 watts, etc. From this it is possible to determine the maximum current which should be allowed to flow through it. A 5V1 (5.1 volt) device rated at 1 watt can have a maximum of I = P / V = 1 / 5.1 = 196 mA flowing through it. The final element is the output terminal, to which the load is attached.
This simple regulator circuit is a "shunt regulator", as it functions through the Zener consuming power which would otherwise result in the load receiving too great a voltage. As such, these are the most inefficient regulators. If the load were, say an LED based clock, the current drawn would vary between that used displaying six segments at 1:11 and 21 at 18:58, and this unused current would be dissipated as heat in the zener. A modern circuit would use a low cost three terminal IC regulator, such as a 78xx series device. This uses a series pass transistor so the current drawn from the supply is only just over that required by the load, with only a small current used by the internal reference. Regulators also deal with the variation in the supply to the device, be it due to mains variations, or variation from a low car battery, to an overcharging one, around 11 to 15 volts, where a 7808 (8 volt regulator) can be used to supply various audio, frequency generating, or logic parts of the radio with a constant voltage. They also reduce hum due to ripple the ripple mentioned above, when running audio sections of a radio. The other improvement would be to spend under a dollar to use a bridge rectifier. This would reduce the time between charging pulses from the transformer, and thus the ripple voltage significantly. I suppose back in 1968 the three additional brand new 1N4001 diodes cost more than buying a larger "condenser", as capacitors were called back then. Old ways die hard with some people...
Beyond the exam, as well as making a bridge from 4 diodes, you can buy bridge rectifiers pre-build, ranging from the size of an 8-pin IC and up, some bolting to a heatsink.
Exactly what this is is not stated, but it appears to be an antenna matching unit, also called an "Antenna Tuner". Its purpose is to make a transmitter see a 50 ohm impedance, even if the antenna is of a different impedance, or is being used at other than its resonant frequency.
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| While the tuner on the right has a fixed inductance to ground, and a somewhat difference layout, it has a similar function, with one less knob. The internals are below. | |
1 - Connector - This is either a physical connector, or perhaps diagrammatic connection from a different part of the diagram, where the designer decided not to draw a line, including if it was from another page.
2 - Variable Capacitor - These usually consist of a metal frame with interleaved vanes, allowing the area interleaved to be varied by rotating a shaft, thereby varying the capacitance. Those used in tuners are similar to those in valve and early transistor radios, but with larger spacing to handle higher voltages.
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| This shows the variable capacitors inside the small antenna switch and tuner, which was designed for CB use, although it does work OK on HF ham bands at 100 watts, if the mismatch is not excessive (which causes high voltages or currents). The one on the left is fully inter-meshed, for maximum capacitance, and on the right, fully un-meshed, for minimum capacitance. You can see a small silver-coloured coil, and two parallel ceramic caps. The purple is cheap nail-polish, which I added as it acts as an insulator. |
3 - Variable inductor - An inductor is a coil of wire, tubing or plate, and this is shown in the emulation of a spiral in the symbol. The arrow indicates that there is some means to tap along the coil. Real life coils tend to avoid shorting the tap to one end, as this interacts with the rest of the coil, unlike shorting the portion of a variable resistor not carrying current. The best variable inductors have the heavy silver-plated coil on a ceramic former, with the whole thing rotating freely, with a pulley-like metal wheel over the thick wire or tube, mounted on a shaft, so the wheel travels the length of the coil, as the coil is turned multiple times. This allows precise selection if the inductance, rather than simply selecting from a few taps. Other variable inductors ware adjusted by screwing a slug of ferrite or similar material in and out of the coil. Beyond the exams, passing DC through a second winding will "saturate" the core material, and alter the inductance.
4 - Antenna - No matter the type of antenna used, this symbol is generally used. This one appears to emulate an old-fashioned one with fanned-out wires. There are also symbols for dipole antennas, and yagis, etc. Clearly, when laying out the physical design of the antenna, this is drawn to scale.
The device also functions as a tuneable high pass-filter, as the higher the frequency, the less the capacitors impede the flow of the signal (lower reactance and thus impedance), and the less the inductor shunts the signal to ground (the higher its reactance, and this impedance).
Various series or parallel combinations of an inductor with a capacitor are called tuned circuits. These can become resonant at a certain frequency. Transmitters use a similar arrangement, but with the inductors in series with the signal, and capacitors going to ground, so as to act as a low pass filter, reducing harmonics.
Meters - Typically using electromagnetic deflection, a parameter such as voltage, current, or power, can be displayed via the sweep on a meter needle. These can be built into a device, or be stand-along. A device with such a meter, and one or more dials to select the function and range is called a multi-meter. These also measure resistance. In modern equipment a integrated circuit (IC or chip) digitises the voltage, etc, and displays it using seven-segment or other display technology. On radios parameters displayed can include received signal strength, power, supply voltage, SWR (antenna mismatch), and for FM, deviation; in a valve transmitter, plate current may be displayed. The signal strength, or S-meter is an exam item.
Relays - While there are a range of relay technologies, the classic version is a "switch controlled by an electromagnet", or rather, operated by one. When current is applied to the coil, the magnetic field attracts an iron armature which operates switch contacts. One benefit is that they isolate a low voltage control circuit from a high voltage power one, or occasionally, a high voltage input from a low voltage circuit; in other cases a small current from a switch in the car's cabin operates a relay to turn on high current lamps, or a high-current horn, reducing the length of heavy cable, and I²R losses. Why are they a "relay", not a magneto-switch, or whatever? The original use was in land-line telegraph, where there was a limit to how far one telegraph station could send a signal to operate the electromagnetic click-clunk sounder at the far station, so initially some poor bunny had to write down the message at a "relay station", perhaps in the middle of the desert, then resend it to the next station. Instead of a sounder, how about making the electromagnet operate a switch automatically? Not in the test, but large mains relays, often with 240 volt AC coils, are called "contactors".
 | A large relay. When current flows through the coil, the metal armature below the arrow head moves in to the metal core of the relay, pushing the phenolic plastic yoke, pushing the currently closed contact away from the centre one, and allowing the opened one to contact the centre one. This varies from designs which instead move the centre contact, probably more common. The silvery metal at the other end of the yoke acts as a spring to ensure the relay returns to the off position when power is removed from the coil. There is a plastic cover normally fitted to this relay. |
 | A tiny relay. The pin spacing appears to be 2mm, too small to fit the 2.54mm (0.1") spacing on the matrix board, designed for normal ICs. |
Integrated Circuits - ICs or silicon chips are devices which combine multiple semiconductor devices, such as diodes and transistors / FETS into one piece of silicon, with resistors, etc, to provide complex functions in one small package. These can be timers, voltage regulators, near complete radio receivers, audio amplifiers, etc, these being "analogue ICs". Digital ICs include logic ICs, such as AND and OR gates, inverters, counters, going up to microprocessors and microcontrollers - computers on a chip. While package for a DIL chip (below) might be large, the die is often only, say, a few millimetres wide.
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| On the left, a standard IC package, the Dual-In-Line, or DIL, in this case the civilian black plastic version. There are also white ceramic packages for military specification versions; plus a range of others. The centre of each pin is 0.1", or 2.54mm apart. The stylised N stands for National Semiconductor, the maker, the 9530 is week 30 of 1995, and CD4030CN is the part number, 4000 being the CMOS logic series, and 4030 a quad Exclusive-OR gate (Google the part number for more info). Power can be from 3 to 15 volts. The foil bonded over the foam into which the IC is pressed prevents static discharge damaging the chip. As the IC is much denser than the foam, the slightest bump often makes them "turn-turtle" on your desk, not a problem, but a bit weird when you first see it. On the right is an IC socket, pressed into black carbon-loaded foam, usually used for IC packaging, which dissipates static. The socket is soldered to a PCB, and allows easy replacement of ICs, including programmable devices. | |
The devices above left exhibits the three markings, for orientation, the dimple at pin 1, the notch, and a white stripe. Mentally rotate the image so the dot and notch are at the top; the numbers start at the dimple position, this being 1, and number down the device (to 7 in thsi case, then form 8 to 14 up the other side, that is anti-clockwise. In some ICs the relation to logic values follows the pin count in some way, on others it is not, due to the internal logic. Note that is some case the output numbers count from zero, as 1 = 20, 2 = 21, 4 = 22, 8= 23, and so on. More re pins on Wikipedia: DIL Package - Numbering
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| An IC in "Skinny DIP" packaging, referring to the fact that the IC has 24 pins, but instead of being in a package with 0.6" between rows, like Z-80 processors, micro-controllers, and EPROPMS, it has only 0.3", as the 14 pins devices above. DIP is "Dual In-line Package", and yes, the term skinny, rather than thin or slender was probably chosen as a cheeky reference to swimming sans attire... The 74HC154 takes a hexadecimal digit as 4 bits on its 4 inputs, and selects one of 16 outputs. The ti overlay and map of Texas indicates it is made by Texas Instruments. 74 logic is 5 volt TTL, but the HC means it is a CMOS version, running on 2 to 6 volts. I am not sure if this plastic foam is static dissipative. Pink plastic is a colour code for static dissipative plastic foams and bags. For bags the alternatives are metallisation with nickel, or black, carbon-loaded plastic. The even larger Motorola 68000 processor (from when there was a reason to buy a Mac) had 64 pins, with rows spaced 0.9" apart. |
 | Note that Darlington transistor arrays, arrays of resistors, 7-segment displays, LED bargraphs, and even switches can be mounted in DIL packages. Opto-couplers, also called opto-isolators, may also be in DIP cases, often in white. While only 4 terminals are used, some are in 6 pin packages, and there are also larger ones with several devices. Internally, these contain an LED; and a photo-diode, photo-transistor, or photovoltaic cell; and allow a signal to pass between circuits which may be at radically different voltages, or which need to be isolated to prevent ground loops, or where a device is connected to the human body. A 4 pin photodiode version is shown to the left. |
Shielded Wire - You may have seen wire, say connecting audio or video equipment which is round, rather then being like some charger cables which are a figure-8. You may have noticed such cables sometimes have RCA connector(s) with a pin surrounded by a ring. In these wires a thin central wire is surrounded by plastic insulation, then a layer of bare copper or tinned copper wire "shield", then more insulation (or "sheath"). The idea of this is that the outer layer of wire provides "Faraday" shielding for the inner wire which carries the signal. This style of connection is called "unbalanced", as the signal travels through the single inner, then returned via the shield, which may be earthed. Professional audio systems use "balanced" cable, where the signal is the difference between the two internal wires, and while there is a shield, it is the balance which rejects "common mode" noise. Related is coaxial cable, but the difference is that the ratio of diameters of the inner conductor and the inside of the outer is carefully controlled, as it, along with the type of dielectric material (insulation) affects the characteristic impedance. 75 ohm coax is certainly preferable to thin shielded wire for video signals. Shielded wire also helps stop things such as video signals getting out into audio gear, or causing interference to AM radios. In some cables with several shielded lines, aluminiumised mylar is used for the individual shields, with a "drain wire" in each. Some shielded or coaxial cables also include a foil layer under the braided shield.
European "CY" power cables have a shield, which, as well as offering physical protection, they also help to contain radio frequency noise or "hash" from variable frequency drives for industrial motors, but this does not improve the current carrying capability of the cable.
Note that high priced "audiophile" cables do only one thing: transfer the money of the gullible to the accounts of the retailers and manufacturers, with a big ditto applying to fancy HDMI cable: the signal is digital, and will get through a $5 just as well as a $500 one.
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| Unbalanced audio or video cable, used with RCA connectors. As you can see, there is a single wire for the signal in each, with the shield as return. | |
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| This cable is "balanced" as the signal and return are identical wires (except the colour), and the shield's only function is as a shield. | This shows multiple wires in a cable, with a single shield. In this case is is the three signals (RGB), plus sync lines in thin, dodgy VGA cable. The real-deal contains three separately shielded conductors for the RGB, plus plain wires for sync, and an over-all shield. |
These are the actual questions relating to the diagrams above.
T6C01
What is the name for standardized representations of components in an electrical wiring diagram?
A. Electrical depictions
B. Grey sketch
C. Schematic symbols
D. Component callouts
Schematic symbols are art of a schematic diagram, so C. The rest are not sensible terms (but Grey Code is pretty cool).
T6C02
What is component 1 in figure T1?
A. Resistor
B. Transistor
C. Battery
D. Connector
Two terminals, squiggly line, but not a coil, must be a resister. Answer A. Not a transistor, they have three terminals. The battery is to the right, powering the lamp. The small rings or terminals to the left of the resistor are one way to show a connector.
T6C03
What is component 2 in figure T1?
A. Resistor
B. Transistor
C. Indicator lamp
D. Connector
Three terminals, bar with three connections made to it, so a bipolar transistor, answer B.
T6C04
What is component 3 in figure T1?
A. Resistor
B. Transistor
C. Lamp
D. Ground symbol
This Symbol looks like a small filament lamp, so must be C. Ground symbol? Nope, they are at the bottom of the diagram.
T6C05
What is component 4 in figure T1?
A. Resistor
B. Transistor
C. Battery
D. Ground symbol
The plates of different sizes indicate that this is a battery. The position also suggests that it is the power source for the circuit.
T6C06
What is component 6 in figure T2?
A. Resistor
B. Capacitor
C. Regulator IC
D. Transistor
The symbol with a curved bottom plate is a capacitor. Energy is stored as a charge in the dielectric between the plates. In this circuit the capacitor's job is to smooth the rectified AC to make a smooth DC supply. Straight lines may also be used for the plates. Answer B. Except maybe the component used to make the variable resistor at 9, there are no three terminal devices, so not a transistor.
T6C07
What is component 8 in figure T2?
A. Resistor
B. Inductor
C. Regulator IC
D. Light emitting diode
The outward pointing arrows indicate that something is leaving this diode. This something is photons, or light, so Light emitting diode - D.
T6C08
What is component 9 in figure T2?
A. Variable capacitor
B. Variable inductor
C. Variable resistor
D. Variable transformer
The symbol looks like a resistor, but with an arrow to represent the wiper used to tap off a portion of the resistance. A variable resistor, answer C.
T6C09
What is component 4 in figure T2?
A. Variable inductor
B. Double-pole switch
C. Potentiometer
D. Transformer
There is the interaction of two inductors, but no, NOT a variable inductor. Do you see switch poles or contacts? No! A resistor with a wiper? No, not a "pot"! Transformer! Yes, it reduces the AC input voltage to a lower, safer voltage. Others step up to deadly voltages for large valve "linear" amplifiers. Answer D. A large voltage step-down transformer also makes very large currents available, and this is exactly what a spot welder or arc welder (stick welder) is.
T6C10
What is component 3 in figure T3?
A. Connector
B. Meter
C. Variable capacitor
D. Variable inductor
You see the inductor symbol with an arrow, allowing the inductance to be selected / the coil to be tapped, answer D. While meters use a coil of wire, they are normally represented by a circle with a letter, such as V for volts, or A for amps.
T6C11
What is component 4 in figure T3?
A. Antenna
B. Transmitter
C. Dummy load
D. Ground
This looks like an old antenna, standing up, so A. A transmitter is a complex collection of components, but if built in a small module, would be represented by a rectangle. A dummy load is a non-inductive resistor designed to emulate an antenna, without radiating signals, used for testing radios, etc.
T6C12
What do the symbols on an electrical circuit schematic diagram represent?
A. Electrical components
B. Logic states
C. Digital codes
D. Traffic nodes
Electrical (or electronic) components! Answer A. Logic states and digital codes are terms which might apply to logic / digital electronics, and I presume traffic nodes relates to a process of relaying messages between operators, which might use maps? Or maybe some are just silly terms.
T6C13
Which of the following is accurately represented in electrical circuit schematic diagrams?
A. Wire lengths
B. Physical appearance of components
C. The way components are interconnected
D. All of these choices are correct
Diagrams show the way in which components are interconnected (answer C), rather than the Printed Circuit Board layout, or their appearance. The same resistor symbol could be a component the size of a large sugar crystal, or bigger than a refrigerator, used at the bottom of mountains to dissipate regenerative braking energy from descending electric trains when there is no climbing train to use it. There are however Children's training books which involve setting out components following the circuit diagram, using screws and washers into a drilled timber or plastic block, and this works fine at safe voltages and low frequencies.
T6D01
Which of the following devices or circuits changes an alternating current into a varying direct current signal?
A. Transformer
B. Rectifier
C. Amplifier
D. Reflector
The process of converting AC to DC is called rectification, so the component is a rectifier (or rectifier diode), answer B. (In Telecom-land, the whole transformer and rectifier cabinet is the "rectifier", but not what the examiner is looking for. ICs called operational amplifiers, as they were originally used to perform mathematical functions in analogue computers, can be used to in "precision rectifiers" in measuring equipment, rather than for power. If you look at an old-school multimeter, there will often be a separate offset scale, often red, for low AC voltages, due to the voltage drop in the rectifier diode(s) feeding the meter movement.)
T6D02
What best describes a relay?
A. A switch controlled by an electromagnet
B. A current controlled amplifier
C. An optical sensor
D. A pass transistor
The classic relay consists of an electromagnetic coil, which pulls in an iron armature, in turn moving (via insulated material) the switch contacts, typically made from springy copper alloys, or springy steel. Answer A. The modern alternative in some applications is the "solid state relay", typically using an internal LED and optical device to electrically isolate the input from the mains or other high voltage circuit being controlled, the switching being via silicon switching devices (triacs or fancy insulated gate transistors), so while the last two answers allude to these, they are not the answer sought.
T6D03
What type of switch is represented by component 3 in figure T2?
A. Single-pole single-throw
B. Single-pole double-throw
C. Double-pole single-throw
D. Double-pole double-throw
This switch has a single moving element, or pole; and this pole can only make or break a connection from the terminal on the left to a single terminal on the right, so a single throw. Thus the answer is A, Single-pole single-throw (SPDT). In some circuits it is necessary to control two circuits at once, such as disconnecting both hots in a split phase (American) 240 volt power input to a big amplifier, or both Active and Neutral in an Australian caravan, where there is a risk the neutral input has been cross-connected to Active by a dodgy lead. These are double-pole switches. In others you want to select between two or more signals. These are double-throw switches. Select between two stereo devices, and you need a DPDT switch.
T6D04
Which of the following can be used to display signal strength on a numeric scale?
A. Potentiometer
B. Transistor
C. Meter
D. Relay
A meter, whether an electromagnetic movement moving a physical meter needle, or a bargraph or similar arrangement of pixels on an LCD display, can be used to present a range of parameters, including the received signal strength, usually against a scale from S1 to S9, then in decibels above this. For the HF bands IARU Region 1 Technical Recommendation R.1 defines S9 to be a receiver input power of -73 dBm, meaning 50 microvolts at the receiver's antenna input, assuming the input impedance is 50 ohms. For VHF S9 its -93 dBm, or 5 microvolts into 50 ohms. One S-unit corresponds to a difference of 6 decibels (dB), a power ratio of four. However, many "S-meters" are not calibrated, hence the term "Guess-meter".
T6D05
What type of circuit controls the amount of voltage from a power supply?
A. Regulator
B. Oscillator
C. Filter
D. Phase inverter
Or better, something like "What controls the voltage output of a power supply?" "Controls" has a similar meaning to "Regulates", and indeed, the answer is voltage regulator, A. This can be a 3 terminal device, often in a TO-220 case with a metal tab which can be bolted to a heatsink; or it can be discreet voltage reference and a pass transistor. More complex "switch-mode" supplies do use an oscillator, and various filter components, as well as a voltage reference, and controller ICs, but this is beyond the scope of the question.
T6D06
What component is commonly used to change 120V AC house current to a lower AC voltage for other uses?
A. Variable capacitor
B. Transformer
C. Transistor
D. Diode
Note the question asks for a device which outputs an AC voltage, in this case lower than the 120 volt input. This device is the transformer, typically consisting of two (or more) copper coils and some form of laminated steel core. Thus answer B. "Switch-mode" supplies initially rectify the mains (using diodes), then use signal and power transistors to generate a high frequency current which are passed through inductors or small transformers, then rectified and filtered to DC. A somewhat dangerous circuit doing the rounds in Australia used the reactance of a fixed capacitor to limit the current (or drop the voltage) going to a light-emitting diode, with a diode in reverse parallel. Note that the plastic package of an LED does NOT provide safe isolation from the mains voltages!
T6D07
Which of the following is commonly used as a visual indicator?
A. LED
B. FET
C. Zener diode
D. Bipolar transistor
Field Effect Transistor, don't see the word light in there. Zener diode, nah, its a voltage reference component. Bipolar transistor: cut the top of a metal can transistor, point an IR sensitive camera at it in a dark room, nah, too hard. Must be the Light Emitting Diode! Answer A. Actually, there are Light-Emitting Organic Field-Effect Transistors, aka Organic light-emitting transistors and blue emitting unipolar transistors; which may be used in a range of displays in the future, as they don't need additional transistor arrays like OLED displays. Oh, and IC developers do use optical and IR macro cameras to analyse problems in their designs. But LEDs are the only ones listed intended for use as a visual indicator.
T6D08
Which of the following is used together with an inductor to make a tuned circuit?
A. Resistor
B. Zener diode
C. Potentiometer
D. Capacitor
Resistance in a tuned circuit is generally bad, as it reduces the Q (quality factor or sharpness) of the resonant circuit, so ignore resistor or potentiometers (variable resistor). A zener diode is a voltage reference or regulation device, so nah. Must be the Capacitor, D. The circulation of energy between the capacitor and inductor is resonance, which happens in a tuned circuit.
T6D09
What is the name of a device that combines several semiconductors and other components into one package?
A. Transducer
B. Multi-pole relay
C. Integrated circuit
D. Transformer
Combined and integrated are similar concepts - so could be C. Relays are normally electromechanical, but if we were discussing a semiconductor one, we would say "Solid State Relay", so no; a transducer can use a variety of technology, such as piezo-electric material, and such devices containing transducers can have signal conditioning electronics built in, but it's not the answer they are looking for; and a transformer is an electromagnetic device made from copper and iron. Yep, C.
T6D10
What is the function of component 2 in Figure T1?
A. Give off light when current flows through it
B. Supply electrical energy
C. Control the flow of current
D. Convert electrical energy into radio waves
This is a three terminal device, a transistor, connected via the resister to an external voltage / current source of some kind, and the current from this, when present, causes the device to allow current to flow from the battery via the lamp; and if the input current is low, it could cause the flow to be restricted, so the lamp glows dimly. C is the answer they are seeking, even if if should continue with "under the influence of the input signal". Generating radio waves is certainly not the intention of the device, and is unlikely in this configuration. Any Infra-Red emitted from the transistor die is not its function.
T6D11
What is a simple resonant or tuned circuit?
A. An inductor and a capacitor connected in series or parallel to form a filter
B. A type of voltage regulator
C. A resistor circuit used for reducing standing wave ratio
D. A circuit designed to provide high fidelity audio
Resonant circuits consist of an inductor and a capacitor, and these can be in series or parallel, so A is the answer. Such arrangements can form notch (band cut) or band-pass filters. This is not how regulators work. Resistance in antenna circuits is often a bad thing, and actually reduces the quality of the resonance, but this is how "No tuner needed" folded dipole antennas function, with energy which would otherwise be reflected back to the transmitter (causing high standing wave ratio (SWR) readings) is converted into heat - these antennas' primary function is the transfer of funds from the buyer's bank account to the seller's. Resonance in audio systems is a BAD thing, such as when a speaker cabinet rattles at at particular frequency, so definitely not "hi-fi"!
T6D12
Which of the following is a common reason to use shielded wire?
A. To decrease the resistance of DC power connections
B. To increase the current carrying capability of the wire
C. To prevent coupling of unwanted signals to or from the wire
D. To couple the wire to other signals
Shielded wire is often thin, used for carrying low powered signals, so the reducing resistance or increasing current carrying capability ones are a little silly. Even if it were shielded power cable, the shield does not do either of these things. The intention is to stop noise, other signals, etc getting into cable, and to stop them escaping, and interfering with other circuits or systems, so C. Leaky feeders (coax with slots in the shield), used to distribute signals around tunnels and mines; and using really crappy (poorly shielded) coax between a technical college's transmitter only licensed for operation into a dummy load, and said dummy load, so the signal can be heard around the campus may be things, but these are not in the scope of the question.
On to: More on Components
You can find links to lots more on the Learning Material page.
Written by Julian Sortland, VK2YJS & AG6LE, October 2017.
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