| A |
In an a.c., or alternating current, circuit, current flows first in one direction, then in the other.
If atoms of an acceptor impurity, such as boron, become incorporated in the crystal lattice of a semiconductor, extra 'holes' become available to carry current. Acceptor impurities give P-type semiconductor.
An ammeter measures current. Ammeters are connected in series in a circuit and must have a low resistance.
Current is measured in amperes (amps), A, milliamps, mA (thousandths of an amp), and microamps, μA (millionths of an amp).
In an analogue circuit, information is represented by continuous changes in voltage. An audio signals is an example of an analogue signal.
An autoranging multimeter adjusts its measurement range automatically to give an appropriate reading.
| B |
A battery consists of two or more cells. The cells may be connected in series or in parallel.
| C |
A cell provides a source of electrical energy. In a circuit, cells provide the 'push' which makes current flow.
A characteristic curve describes the properties of an electronic component, usually by plotting current, I, against voltage, V.
The chassis of an appliance is the framework that supports the internal parts of the appliance. If the chassis of a mains appliance is made of metal, it is usually connected to the earth terminal of the mains supply.
A circuit is a closed conducting path.
A conductor is a material which allows current to flow easily. Most metals are conductors.
In a circuit, current is thought of as flowing from the positive terminal of the power supply towards the negative terminal. This is the direction of flow for a positively charged particle.
The current gain of a transistor is represented by the symbol hFE and is defined as the collector current divided by the base current: 
| D |
A Darlington pair is an arrangement of two transistors which behaves like a single transistor with a very high current gain.
In a digital circuit, information is represented by discrete voltage levels. A high voltage is called logic 1, or 1, while a low voltage is called logic 0, or 0.
If atoms of a donor impurity, such as phosphorus, become incorporated in the crystal lattice of a semiconductor, extra electrons become available to carry current. Donor impurities give N-type semiconductor.
A dot matrix display consists of a 5 × 7 array of LEDs, or other devices, which can be illuminated to represent all the letters of the alphabet, as well as the numbers 0–9.
A DPDT, or double-pole double-throw, switch consists of a pair of changeover switches which operate simultaneously. There is no electrical connection between the switches.
A DPST, or double-pole single-throw, switch consists of a pair of on/off switches which operate simultaneously. There is no electrical connection between the switches.
| E |
The E12 series includes the values: 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82. Resistors are available in multiples of 10 of these values, e.g. 1.5 Ω, 15 Ω, 150 Ω, 1.5 kΩ and so on.
The E24 series includes the values: 10, 11, 12, 13, 15, 16, 18, 20, 22, 24, 27, 30, 33, 36, 39, 43, 47, 51, 56, 62, 68, 75, 82, 91. Resistors are available in multiples of 10 of these values, e.g. 4.3 Ω, 43 Ω, 430 Ω, 4.3 kΩ and so on.
The earth, E, or ground terminal defines 0 V in many circuits. In a correctly-wired mains circuit, current flows to earth only under fault conditions. Safety systems are designed to detect this current.
Electrons move from the negative terminal of the power supply towards the positive terminal. Electron flow is opposite in direction to conventional current.
| F |
The full scale deflection, or f.s.d., is the maximum reading which a switched range multimeter can display on any particular measurement range.
A diode is forward biased when its anode is connected to a more positive voltage than its cathode. This is the direction of easy current flow.
The forward voltage is the voltage across a diode or LED when it is conducting current in the forward bias direction.
A fuse is included in a circuit as a safety device that isolates the circuit if excess current flows. One type of fuse consists of a fine wire which heats up and melts, breaking the circuit if too much current flows.
| G |
A rubber or plastic grommet is used to protect a cable from damage where the cable enters or leaves the enclosure surrounding any electrical or electronic appliance.
| H |
A hole can be thought of as a space in a crystal where an electron ought to be. A hole behaves in the same way as a positively charged particle.
| I |
An integrated circuit, or i.c. contains a group of transistors manufactured together on the same silicon chip, with appropriate interconnections. Some integrated circuits contain just a few transistors. More complex i.c.'s may have hundreds, thousands, or millions of transistors.
An insulator is a material which prevents the flow of current. Most non-metals are insulators.
| J |
| K |
| L |
A latching circuit is a simple memory device. If the latch is set, its output is high, logic 1. When the latch is reset, its output is low, logic 0.
A light-dependent resistor, or LDR, has a high resistance in the dark, and a low resistance in the light.
The leakage current is the tiny current which flows when a diode or LED is connected in the reverse bias direction. In polarized capacitors, a small leakage current flows across from one plate of the capacitor to the other.
An LED, or light-emitting diode, is illuminated when current passes through it in the forward bias direction.
The live, L, line, or 'hot' terminal of the mains supply carries a changing a.c. voltage. In the UK, this is 240 Vrms.
| M |
The light intensity produced by LEDs is often measured in millicandelas, or mcd. The bigger the number, the brighter the light.
A multimeter can be set up to work as a voltmeter, as an ammeter, or as an ohmmeter. These functions are selected by rotating the central control knob to the appropriate position.
The multiplier band of a resistor colour code tells you how many zeros to add to the first and second digit numbers.
| N |
Pure silicon is converted to N-type silicon when atoms of a donor impurity such as phosphorus become incorporated in the crystal lattice. Electrons are the majority charge carriers in N-type semiconductor.
A neon lamp is a high voltage/low current device which is often used as a 'power on' indicator in mains appliances.
The neutral, N, terminal of the mains supply is fixed close to 0 V. In a correctly-wired mains circuit, current flows between live and neutral.
| O |
Resistance is measured in ohms, Ω, kilohms, kΩ (thousands of ohms), and megohms, MΩ (millions of ohms).
An ohmmeter measures resistance. To find its resistance, the component to be tested must be removed from any circuit and connected to the ohmmeter separately.
| P |
Pure silicon is converted to P-type silicon when atoms of an acceptor impurity such as boron become incorporated in the crystal lattice. Holes are the majority charge carriers in P-type semiconductor.
Components are connected in parallel when they are joined side by side in a circuit, so that they provide alternative pathways for current flow.
A photodiode is a sensor device which can be used to detect infrared or other wavelengths of light.
A potentiometer is a resistor with two end terminals and an additional movable contact, called the slider. The resistance between the slider and the end terminals changes as the slider is moved along the resistive track.
Prototype board is used for building temporary circuits. Connections are made by pushing components and wire links into the holes in the prototype board.
| Q |
| R |
An RCD, or residual current device, is a safety device. In a normal mains circuit, the live and neutral currents are exactly equal. The RCD monitors the currents flowing and trips a switch to isolate the appliance if any difference between the live and neutral currents is detected.
Rectification is the process of converting from a.c. to d.c. using one or more diodes.
A rectifier is an electrical component, such as a diode, that permits current to flow in only one direction. Diodes specifically designed for use in circuits that convert from a.c. to d.c. are known as rectifier diodes.
A resistor is an electronic component with a particular resistance values. Resistors limit current.
A diode is reverse biased when its anode is connected to a more negative voltage than its cathode. Provided the maximum reverse voltage is not exceeded, only a tiny leakage current can flow.
| S |
A transistor which is fully on, with maximum collector current flowing, is said to be saturated.
A semiconductor is a material which has a resistance intermediate between that of a conductor and an insulator. Semiconductor elements include silicon and germanium.
Components are connected in series when they are joined end to end in a circuit, so that the same current flows through each.
A smoothing circuit reduces the size of variations from the average d.c. level of any signal. In power supplies, smoothing capacitors help in converting rectified waveforms to a steady d.c. level.
An SPDT, or single-pole double-throw, switch is a changeover switch with a single input terminal and two alternative output terminals.
In a step-down transformer, the secondary a.c. voltage is smaller in amplitude than the primary a.c. voltage.
The central control knob of a switched range multimeter has different positions for each measurement range.
| T |
A thermistor has a resistance which changes with temperature. Negative temperature coefficient, or n.t.c., thermistors have a high resistance in the cold and a low resistance in the heat.
Tolerance describes manufacturing accuracy. If a component is manufactured to a tolerance of ±5%, this means that its actual value is guaranteed to be within 5% of its marked, or nominal value.
A transducer is an electronic component which converts energy from one form to another, where one of the forms of energy is electrical. Input transducers include microphones, thermistors, and LDRs. Output transducers include loudspeakers, motors, and LEDs.
A transformer consists of two coils of wire wound together so that they are magnetically linked, although there is no direct electrical connection between them. Depending on the number of turns in the input, or primary coil, and the output, or secondary coil, transformers can be used to convert from one a.c. voltage to another.
| U |
| V |
The resistance between the terminals of a variable resistor can be altered by rotating a spindle, or similar control. Variable resistors are often made using the slider and one of the end terminals of a potentiometer.
Potential difference, or voltage V is a measure of the difference in energy between two points in a circuit. Charges gain energy in the battery and lose energy as they flow round the rest of the circuit.
A voltage divider consists of two resistors, Rtop and Rbottom, connected in series. Vin is connected across both resistors. Vout is the voltage across Rbottom. Vout is calculated from: 
The voltage rating of a component is the maximum voltage which can be connected across its terminals.
In a power supply circuit, a voltage regulator converts a variable voltage signal to a nearly constant d.c. level. Most electronic circuits require a regulated d.c. power supply.
voltmeter:
A voltmeter measures the voltage, or difference in energy, between two points in a circuit. Voltmeters are connected in parallel and must have a high resistance.
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