Home - About AR - Learning Material - Exams - Clubs - Posters

Amateur Radio Info & Exams - Electronic Components

Component refresher

Resistors oppose the flow of current in any circuit, including a DC circuit. Inductors oppose current flow in an AC circuit, increasing with the frequency. While it is true that if you put an inductor in a DC circuit, and then apply power, the current will not raise instantly as it would without it. However, the question relates to the steady state, and once this occurs, the inductor has no influence.

White ceramic power resistorA 470 Ω wire-wound power resistor, rated 3 watts; J meaning 5% tolerance.
It was unsoldered from an old CRT TV.
It is in a ceramic case. Lead spacing is 0.2" = 5.08 mm.
Many have a lead at each end. In either case, the element is held in place by cement.

A potentiometer is a specific kind of variable resistor, often used as a volume control.

A capacitor stores energy in the electric field in the insulating layer or air between the plates. They are also handy, as they can pass an AC (audio or RF) signal while blocking DC, and so are often used between stages of an amplifier.

Four capacitors, and a MOV
From left to right: A Ceramic Capacitor, 104 meaning 10 × 10&sup4; = 100,000 picofarads, or 100 nanofarads. A polyester capacitor, also 100,000 pF or 100 nF, J indicating ±5% tolerance. On the orange coated ceramic, 222 means 22 × 10² meaning 2200 pF or 2.2 nF, K indicating ±10%, and rated at 2kV, or 2000 volts. Oops, the blue item is a Metal Oxide Varistor, designed to clamp large voltage spikes on mains supplies. The last is a "safety capacitor", a ceramic designed to be placed directly across the mains, to help prevent noise and interference being conducted in or out of a piece of equipment.
The varistor is around 10 mm in diameter.
Ceramic capacitors are often used in radio frequency circuits, and at higher voltages.
Polyester capacitors, an example of a plastic film capacitor, were traditionally coated with a green epoxy, and were called "green-caps". Brown now appears popular. They work well at audio frequencies.
100 nF is also 0.1 μF, or 0.1 microfarads.

Silver-mica capacitors also work well at radio frequencies.

An inductor stores energy in a magnetic field, within the coil of wire they are normally made from. They are also used when you want to pass DC or low frequency (AC) power into a feed-line, or extract it to run a pre-amplifier at the the top of your mast, while blocking RF from entering the power supply.

A tuned or resonant circuit causes energy to circulate between a capacitor and an inductor.

All but the smallest electrolytic capacitors are gernerally larger in capacity than most ceramic or plastic film capacitors. They are useful for smoothing DC power supplies, and for low frequency oscillators. Unlike the devices above, "electros" are polarised, being marked most often in the negative leg. They range from 5mm in diameter, to 10 cm in diameter, and 20 cm tall. The later are mounted using a clamp, and terminated using screw terminals.

All but the smallest electrolytic capacitors are gernerally larger in capacity than most ceramic or plastic film capacitors. They are useful for smoothing DC power supplies, and for low frequency oscillators. Unlike the devices above, "electros" are polarised, being marked most often in the negative leg. They range from 5mm in diameter, to 10 cm in diameter, and 20 cm tall. The later are mounted using a clamp, and terminated using screw terminals.

Voltage ratings range from under 10 volts, to 900 volts in some forms. Values range from around 0.1 to over 100,000 uF.

Electrolytic capacitors
A fairly small capacitor, of 510 μF, rated at 25 volts DC. It has a temperature rating of 135°C, which is quite high. This is suitable for smoothing a few supply, supplying a few hundred mA at most. Lead spacing is 0.2" or 5.08 mm. The short lead is the negative. This is an RB or radial cased capacitor, designed for PCB mounting. The alternative is the RT or axial case, with a lead at each end, designed for use in old-style point-to-point mounting.


The simplest semiconductor device is the diode, which allows current to flow from the plain end to the banded one, but not in reverse. Flow is Anode (A) to Cathode (K). Both regular and zener diodes vary from tiny to huge stud mounted (affixed with a large nut, and connected with a cable over 5mm thick), and hockey-puck sized devices. Small diodes have a band painted on the cathode end.

Different diode types have different forward voltage drops. Germanium is about 0.2 volts, and are most useful as detectors in simple "crystal radios". Standard silicon rectifier diodes for small to medium sized power supplies drop 0.6 volts, or thereabouts. Good for high current, lower voltage circuits, the Schottky diode only drops 150–450 mV.

The standard transistor, also called a bipolar junction transistor (BJT), consists of three layers of semiconductor material, usually silicon these days, but germanium in the past, or for a few special applications. The exams just calls these a "transistor". Transistors require current to flow into (or from) the Base to cause then to pass a larger current from the collector to emitter. That current is used to operate the load.

Physically, the BJT consists of a main piece of (usually) silicon, the base, with regions doped to be the emitter and collector. That is, there are tree regions.

The transistors ability to amplify is called its gain. Within a single part-number this can vary over a wide range, so surrounding circuitry is often designed to manage this, or at least take it into account.

Transistors can be used as a switch, say operating a lamp or a relay; or in "linear" mode, as a DC, audio or radio frequency amplifier; or as an oscillator.

A small E-line transistor, which is black with light paint, but leaving the "BS 250 P" marking black. This is an "E-line" package which is about 4.5 mm high, and just under 4 mm high.The lead spacing is 1.27 mm, or 0.05", meaning the leads are normally splayed out a few mm below the case, then made parallel 2.54 mm (0.1") apart, to allow PCB mounting. It is somewhat smaller than the common TO-92. It conatins a FET.

In the FET, or field effect transistor, conventional current flows from drain to source (note electrons flow the other way). Voltage on the gate controls the current flow. These are also called unipolar transistors. The gate current is super-low (often picoamps).

The first transistors were Germanium, replaced in the main by silicon, but this may be making a comeback in some applications.

Integrated circuits or "silicon chips" contain tens to billions of transistors.

Passive or Active?

Off the exams, but perhaps handy to know if buying parts, resistors, capacitors, and inductors are termed "passive" components, or "passives". Those with an input which influences current flow in other parts are termed "active" components, including all classes of transistors, and ICs. You will notice that the latter are semiconductors, but valves are also active devices.


Unless defined otherwise, a switch is a mechanical device with physically connects or disconnects the circuit. The big old knife switches in Frankenstein style movies demonstrate this.

A simple toggle or rocker switch is specified by the number of poles and throws it has. The most basic is a light switch at home, where switching only the active or hot line turns on a lamp, fan, etc. This is a single pole switch, as only one line is switched, and single throw, because it has only one output circuit (even if a more than one device comes on when you switch the switch. This is an SPST switch in the diagram on the previous page.

When a switch selects between functions it may be a single pole, double throw switch, one pole, and one of two circuits supplied. An example might be in a model railway, where either the green or red signal must be on.

An SPDT switch selecting between a red and a green LED
This schematic shows an SPDT switch switching a single positive wire to either a red or a green LED, with the return to the negative terminal via a resistor, indicated using the international symbol. The examiner would describe this as connecting one circuit (the power supply) to the one of two circuits, these being either the red or the green LED. (The resistor could instead be placed before the switch, or individual resistors, perhaps of different values, could be placed before (or after) each LED).

A double-pole switch switches two independent parts of a circuit. Consider an older, mechanically tuned AM-FM portable radio. You are not only selecting audio from the AM detector and FM discriminator circuits, but also between receiving signals from two different bands, medium frequency (MW), and VHF. Thus some designs use a DPDT switch for this. The symbol often uses a dotted line to show that the elements are part of the same switch. as in some cases they can appear some distance apart on the diagram.

DPDT switch as polarity reverser
The DPDT switch is also useful for changing the polarity of power feeding a small permanent magnet motor, in order to reverse its rotation. This is used in low cost model trains to reverse the locomtive. Modern layouts use constant power to the track, and digital signals to control multiple trains. With the switch as above, the cerrent flows from teh posative terminal, via the switch into the top of the motor, out the bottom, and to the negative terminal, via the lower element of the switch. With the stitch toggled, current flows into the bottom of the motor, and out the top, reversing its rotation.
One aftermarket car accessory popular in the 1980s and 1990s was an electrically controlled telescopic radio antenna which used a centre-off momentary rocker switch to raise and lower the radio antenna. More expensive radios would wind the antenna up and down automatically when switched on or off. A unit which will rotate a ham antenna between vertical and horizontal to allow a van or SUV to fit into a parking garage is also available today. In the case of a centre-off switch, black rings would be included where the green ones are, to show this off position.

If you are an American building a large linear amplifier, a double pole switch is necessary to disconnect both hots from a split phase 240 volt input.

Meanwhile Australian law requiers Active and Neutral be switched in power sockets in caravans and communications vans, as there is the risk the that neutral input has been cross-connected to active if a dodgy lead or power outlet is used.

Switches have several ratings, especially for voltage and current. The voltage a switch can safely break is often different for AC and DC as DC tends to draw an arc as the switch opens. A switch rated 3 amps an 250 volts AC might only be rated for this current at 28 VDC. You will notice that many switches "click". This is because the mechanism is designed to quickly make the circuit, and to rapidly separate the contacts, to quickly break the circuit, to minimise this arcing.

Cells and Batteries

There are two families of batteries, non-rechargeable (consisting of primary cells) and rechargeable (consisting of secondary cells).

The non-rechargeable in the exam is the carbon-zinc, the standard 1.5 volt cell, actually somewhat rare. An improved version, containing chloride in the electrolyte sold as "heavy-duty", "extra heavy-duty", or "super heavy-duty". Despite the spin, these have considerably less capacity than the alkaline or "alkaline-manganese" product. In the carbon-zinc, the case is the zinc negative electrode, and the acid electrolyte eats away the case as the battery is used. If they are left in a device for an extended period, this will corrode through, and damage the device. Alkaline is much more suitable for using in AA cell cases available for some hand-held radios. Off the exam, some radios use internal lithium coin cells (3 volt), or silver-oxide cells, to maintain the memory.

Used cells should be placed in the bins at Aldi, etc, as the metals, especially zinc, should not go to landfill. Also, lithium batteries can potentially cause fires in garbage trucks, if compacted.

Rechargeable: Motor vehicles use lead-acid batteries to start the car, and to operate accessories when the ignition is off. They also smooth the pulsing or rippled DC from the alternator. Standard automotive batteries are not suitable for running equipment for extended periods, but "deep cycle" and some marine batteries are. The lead-acid gel-cell (also called AGM) is a variation, handy for emergency or field day operation.

Nickel-cadmium and Nickel-metal hydride are often sold in dry-cell formats, or in battery-packs in older hand-helds. There are also liquid Ni-Cad batteries, which can last many decades if looked after.

A more recent addition is the Lithium-ion cell, used as single cells in pocket sized HTs, and as two cell batteries in larger models, providing around 7.2 volts. 14.4 volt four cell ones, designed for radio controlled models, are also being used by the Summits-on-the-Air operators to power 12 volt radios.


Often enclosed in glass, ceramic, or plastic, a fuse consist of a thin wire, or stamped sheet metal, which melts or vaporises when an excessive current flows through it. A mirrored glaze inside the glass means the fuse's end was the result of a serious short or surge; small balls at the ends of the remaining wires means a much less serious overload.

By the way, if you are travelling to the UK make sure the power adapter you use has a fuse. Many sold in Australia do not, even if the drongos at SAA have sold an approval number to go on them. Buying a BS-8546 compliant one in the UK or Ireland is probably the only safe option.

Relevant Questions

These are the actual questions relating to the information above.

What electrical component opposes the flow of current in a DC circuit?
A. Inductor
B. Resistor
C. Inverter
D. Transformer

In a steady-state DC circuit only a resistor opposes (or reduces) current flow, so mark answer B.

What type of component is often used as an adjustable volume control?
A. Fixed resistor
B. Power resistor
C. Potentiometer
D. Transformer

This is the "Pot", or potentiometer, answer C.

What electrical parameter is controlled by a potentiometer?
A. Inductance
B. Resistance
C. Capacitance
D. Field strength

While it may affect another real world function, the electrical parameter is resistance, answer B.

What electrical component stores energy in an electric field?
A. Resistor
B. Capacitor
C. Inductor
D. Diode

This is the capacitor, answer B. Capacitors store energy in an insulator, between plates.

What type of electrical component consists of conductive surfaces separated by an insulator?
A. Resistor
B. Potentiometer
C. Oscillator
D. Capacitor

The symbol for a capacitor is two plates with an air-gap insulator between them, answer D.

We physically see these plates in the large old tuning capacitors, also used in some antenna tuners.

What type of electrical component stores energy in a magnetic field?
A. Varistor
B. Capacitor
C. Inductor
D. Diode

This is the inductor, answer C.

An inductor is a coil, and we relate coils to electro-magnets.

What electrical component is usually composed of a coil of wire?
A. Switch
B. Capacitor
C. Diode
D. Inductor

The symbol for the inductor looks like a coil of wire, answer D. You also saw this in the image above.

What is the function of an SPDT switch?
A. A single circuit is opened or closed
B. Two circuits are opened or closed
C. A single circuit is switched between one of two other circuits
D. Two circuits are each switched between one of two other circuits

A single pole switch has one moving element, and a double throw switch selects between to options, so it is answer C.

What electrical component is used to protect other circuit components from current overloads?
A. Fuse
B. Thyratron
C. Varactor
D. All of these choices are correct

A fuse "blows", or goes open-circuit when excessive, potentially damaging, current flows, answer A.

Which of the following battery chemistries is rechargeable?
A. Nickel-metal hydride
B. Lithium-ion
C. Lead-acid gel-cell
D. All of these choices are correct

All are rechargeable, and all have application in Ham radio, answer D.

Which of the following battery types is not rechargeable?
A. Nickel-cadmium
B. Carbon-zinc
C. Lead-acid
D. Lithium-ion

yͤ aulde carbon-zinc cell is non-rechargeable, and also called a "primary cell". Answer B gets you the Banana!

Diagram T-2

What type of switch is represented by component 3 in figure T-2?
A. Single-pole single-throw
B. Single-pole double-throw
C. Double-pole single-throw
D. Double-pole double-throw

This shows the simplest switch, the SPST, or Single-pole single-throw, answer A.

Which is true about forward voltage drop in a diode?
A. It is lower in some diode types than in others
B. It is proportional to peak inverse voltage
C. It indicates that the diode is defective
D. It has no impact on the voltage delivered to the load

Different diode technologies result in diodes with different forward voltage drops, answer A.

What electronic component allows current to flow in only one direction?
A. Resistor
B. Fuse
C. Diode
D. Driven Element

This is the diode, answer C.

Which of these components can be used as an electronic switch or amplifier?
A. Varistor
B. Potentiometer
C. Transistor
D. Thermistor

The only applicable device here is the Transistor, answer C. There remainder are special resistors.

Which of the following components can consist of three regions of semiconductor material?
A. Alternator
B. Transistor
C. Triode
D. Pentagrid converter

This is the transistor, the only solid state device here, answer B.

A bipolar junction transistor, or BJT, has a Collector, Base, and Emitter.

What type of transistor has a gate, drain, and source?
A. Varistor
B. Field-effect
C. Tesla-effect
D. Bipolar junction

This is the Field Effect Transistor, or FET, answer B.

How is the cathode lead of a semiconductor diode often marked on the package?
A. With the word "cathode"
B. With a stripe
C. With the letter C
D. With the letter K

A diode's cathode is most usually identified with a band, matching the bar on the symbol, answer B.

On big stud mounted diodes a diode symbol may be used to show the orientation, and this has the bar towards the cathode end.

What causes a light-emitting diode (LED) to emit light?
A. Forward DC current
B. Reverse DC current
C. Capacitively-coupled RF signal
D. Inductively-coupled RF signal

For current to flow the diode must be forward biased, so forwards DC current, so answer A.

What does the abbreviation FET stand for?
A. Frequency Emission Transmitter
B. Fast Electron Transistor
C. Free Electron Transmitter
D. Field Effect Transistor

It is the Field Effect Transistor, answer D. The rest of the terms are plain silly.

What are the names of the two electrodes of a diode?
A. Plus and minus
B. Source and drain
C. Anode and cathode
D. Gate and base

These are the anode and cathode (A & K), answer C. You may recognise these from electro-chemistry.

Which of the following can provide power gain?
A. Transformer
B. Transistor
C. Reactor
D. Resistor

A transistor provides gain, answer B.

What is the term that describes a device's ability to amplify a signal?
A. Gain
B. Forward resistance
C. Forward voltage drop
D. On resistance

This is its gain, answer A.

What are the names of the electrodes of a bipolar junction transistor?
A. Signal, bias, power
B. Emitter, base, collector
C. Input, output, supply
D. Pole one, pole two, output

Emitter, base, collector, answer B.

A transistor is a component, not a complete switching unit, or a complete amplifier circuit, and requires other components around it, including resistors. They are not a device which can be described as having supply connections and an input and output.

On to: Electrical Units & Metric Multipliers

You can find links to lots more on the Learning Material page.

Written by Julian Sortland, VK2YJS & AG6LE, January 2022.

Tip Jar: a Jefferson (US$2), A$3 or 20 Kroner. Thanks!