Glossary

Every term used in these guides, in alphabetical order, with a short explanation and a link to the page that explains it in full. Guides link here when they use a term that was explained on an earlier page.

Amp (A), milliamp (mA)

The unit of current. 1 mA is a thousandth of an amp. See Ohm's law.

Anode

The end of a diode or LED where current goes in. On an LED it is the longer leg. See diodes.

Base

The control pin of a transistor. A small current into the base lets a larger current flow from collector to emitter. See using a transistor as a switch.

Capacitor

A part that stores charge on two plates separated by an insulator. Its capacitance is measured in farads. See how a capacitor charges.

Cathode

The end of a diode or LED where current comes out, marked with a stripe on a diode and a flat edge on an LED.

Closed circuit

A complete loop from one terminal of the source, through the load and back to the other terminal. Current only flows in a closed circuit. See your first circuit.

Collector

The transistor pin the main current flows into, from the load.

Current

The flow of electric charge, measured in amps. By convention it flows from plus, through the circuit, back to minus.

Datasheet

The document a manufacturer publishes for a part, with its limits and typical values. It is the final word on what a part can take.

Diode

A part that conducts in one direction only, from anode to cathode, once the forward voltage is reached. See diodes.

Duty cycle

The share of time a repeating signal is on, given in percent. See the 555 timer.

E12 series

The set of twelve standard resistor values per power of ten: 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82. See reading resistor colour codes.

Emitter

The transistor pin the current leaves through, usually connected to ground in a switch.

Farad (F), microfarad (µF)

The unit of capacitance. 1 µF is a millionth of a farad, 1 nF a thousandth of a microfarad.

Forward voltage

The voltage a diode or LED needs before it conducts, and keeps across itself while conducting: about 0.7 V for a silicon diode, about 2 V for a red LED.

Ground

The reference point of a circuit, usually the minus side of the supply, taken as 0 V. Voltages are measured relative to it.

hFE (current gain)

How many times larger the collector current of a transistor can be than its base current. It varies between parts, so use the lowest value from the datasheet. See using a transistor as a switch.

LED

Light-emitting diode. A diode that gives off light when it conducts. It always needs something to limit its current. See choosing a resistor for an LED.

Load

The part of a circuit that uses the energy, such as a lamp, an LED or a motor.

Ohm (Ω)

The unit of resistance. 1 kΩ is 1,000 Ω, 1 MΩ is 1,000,000 Ω.

Ohm's law

V = I × R: the voltage across a resistor equals the current through it times its resistance. See Ohm's law.

Open circuit

A loop with a gap in it, so no current flows anywhere in it.

Parallel

Parts connected side by side, each across the same two points, so they share the same voltage and the current splits. See series and parallel.

Polarity

Which way round a part must be connected. Batteries, diodes, LEDs and electrolytic capacitors have polarity, resistors don't.

Potentiometer

A resistor with a sliding contact, the wiper, that you move with a knob. See potentiometers.

Power, watt (W)

How fast energy is used, in watts. In a resistor it all becomes heat. P = V × I. See power and heat.

Resistance

How strongly something opposes current, measured in ohms.

Saturation

A transistor that is fully on, with only about 0.2 V left between collector and emitter.

Series

Parts connected one after another, so the same current flows through all of them and the voltage is shared.

Short circuit

A path with almost no resistance that skips the load, so a very large current flows. See your first circuit.

Time constant (τ)

Resistance times capacitance, in seconds. After one time constant a charging capacitor is at about 63% of the supply.

Tolerance

How far a part's real value may be from its marked value, for example ±5% for a resistor.

Transistor

A part in which a small current at one pin controls a larger current between the other two.

Voltage

The difference in electrical pressure between two points, measured in volts (V). It is what pushes current through a circuit.

Voltage divider

Two resistors in series whose middle point sits at a fixed fraction of the supply voltage. See the voltage divider.

Wiper

The middle pin of a potentiometer, connected to the contact that moves along the track.