Glossary

A circle with an A, connected in series with the current it measures.⁠[Source 1]

See Voltmeter.

A transformer drawn as a single winding with taps. Its input and output share that winding, so they are not electrically isolated.⁠[Source 4]

See Transformer, Tap.

The bipolar-transistor terminal drawn against the flat bar. A small base current controls the device.⁠[Source 2, Source 5]

See Emitter, Collector.

A multi-position contact that leaves one position before it touches the next. A make-before-break contact briefly bridges the two positions instead.⁠[Source 7]

See Throw.

A connection at the midpoint of a winding, drawn as an extra lead from that winding.⁠[Source 2, Source 4]

See Tap, Transformer.

A single-pole, double-throw contact whose normally-open and normally-closed sides share one common terminal. Form A is another name for a normally-open contact and Form B for a normally-closed one.⁠[Source 9, Source 10]

See Common (COM), Normally open (NO), Normally closed (NC).

A winding drawn as loops or humps. On its own it can be an inductor, a solenoid or a relay coil, so what is drawn with it decides: a relay coil is joined to its contacts by a dashed link.⁠[Source 2, Source 14, Source 15]

See Inductor, Relay, Winding.

The bipolar-transistor lead that meets the base bar and carries no arrow.⁠[Source 2]

See Base, Emitter.

The terminal that a switch's or relay's moving arm belongs to. The arm connects it to one of the other contacts: NC or NO on a relay, a throw on a switch. It is not the circuit's common reference.⁠[Source 9, Source 10]

See Normally open (NO), Normally closed (NC), Throw, Signal reference.

The drawing style used to represent a component. This sheet compares common US and IEC-style forms using cited teaching references.⁠[Source 16, Source 17, Source 18]

A diode's triangle and a bipolar transistor's emitter arrow point the way conventional current flows during conduction: through a diode from anode to cathode, and through the emitter, out of an NPN transistor or into a PNP one. Neither mark shows whether the part is conducting. A MOSFET's body arrow does not show current direction: it points toward the N-type side.⁠[Source 2, Source 3, Source 19, Source 20, Source 21, Source 22, Source 23, Source 24]

See Anode, Emitter, MOSFET.

In the drawings here, lines beside a coil or between windings mark a magnetic core: solid lines for iron, dashed for ferrite. No lines means an air core.⁠[Source 2, Source 4, Source 25]

See Inductor, Transformer.

In the transistor switch model, the off case: no base drive, and the collector–emitter path is treated as open.⁠[Source 28]

See Saturation, Bipolar transistor (BJT).

Two SPDT switches worked together, their arms tied by a dashed mechanical link. It has six contact terminals.⁠[Source 9, Source 10, Source 29]

See SPDT (single-pole, double-throw), Mechanical link.

Two single-throw contacts worked by one mechanism, like two SPST switches linked together. It has four contact terminals.⁠[Source 9, Source 10]

See SPST (single-pole, single-throw), Pole.

One of a MOSFET's two channel terminals: the channel runs between the drain and the source.⁠[Source 24]

See Gate, Source (MOSFET terminal).

The bipolar-transistor terminal that carries the arrow. The arrow points the way conventional current flows through the emitter when the transistor conducts: out for NPN, in for PNP.⁠[Source 2, Source 5, Source 21, Source 22, Source 23]

See Base, Collector, Bipolar transistor (BJT).

A relay coil is energised when current flows through it and moves the contacts. De-energised is its resting state, the one its contacts are drawn in: NC closed and NO open.⁠[Source 9, Source 15, Source 29]

See Relay, Normally open (NO), Normally closed (NC).

A MOSFET whose channel does not conduct at zero gate-to-source voltage. The drawings here show it with a broken channel line.⁠[Source 24]

See MOSFET, Gate.

A diode is forward biased when its anode is at a higher potential than its cathode, whichever way the symbol faces.⁠[Source 3, Source 19]

See Reverse bias, Anode, Cathode.

The MOSFET terminal drawn beside the channel with a gap between them. Its voltage, taken relative to the source, controls the channel.⁠[Source 24]

See Drain, Source (MOSFET terminal), MOSFET.

A teaching model in which a diode blocks all reverse current and has no voltage drop while it conducts.⁠[Source 3]

See Offset model (0.7 V), Forward bias.

A relay with no spring-return resting position: it stays in its last position until its coil is signalled again. Its contacts are drawn in the reset state.⁠[Source 31]

See Single-stable relay, Relay.

A dashed line joining parts of a symbol: the parts move together, but the line is not a wire and carries no current.⁠[Source 9, Source 26, Source 29]

See Relay, DPDT (double-pole, double-throw).

A switch whose internal spring returns it to its resting position as soon as it is released. A switch without that spring stays wherever it was last put.⁠[Source 10, Source 34]

See Push button, Switch.

A name written at a wire or symbol, such as +5 V or GND. Within the drawing's scope, every mark with the same label is the same node, with no wire drawn between them; different labels are not assumed to be joined.⁠[Source 12, Source 27, Source 36]

See Node, Supply rail.

In the ideal-wire model, uninterrupted connected wire forms one node at one potential.⁠[Source 27, Source 37]

See Junction, Net label.

A teaching model in which a conducting diode drops a fixed 0.7 V and does not conduct forward below that. Real forward voltage depends on current and temperature.⁠[Source 3, Source 40]

See Ideal diode model.

A dot beside each of two coupled windings, as on a transformer. The dotted ends have the same polarity at the same instant, each relative to the other end of its own winding. The dots do not show current direction.⁠[Source 4]

See Transformer, Winding.

The moving arm in a switch symbol. The number of poles counts the separate circuits the switch controls.⁠[Source 9, Source 10]

See Throw, Switch.

The transformer winding driven by the source.⁠[Source 46]

See Secondary winding, Transformer.

A letter and number naming a part: R1, C2, D3, Q4, L5, T6, S7, K8, F9. In the IPC list, BT is a battery, CB a circuit breaker, DS a lamp or LED and RT a thermistor. Ground and reference symbols carry none.⁠[Source 12, Source 37, Source 47]

A diode is reverse biased when its cathode is at a higher potential than its anode.⁠[Source 3, Source 19]

See Forward bias.

In the transistor switch model, the on case: enough base drive for the load, and the collector–emitter path conducts.⁠[Source 28]

See Cutoff, Bipolar transistor (BJT).

A drawing of how parts connect, not of where they sit. Parts can be redrawn anywhere on the page without changing the circuit, as long as every connection is kept.⁠[Source 27]

See Node, Net label.

The transformer winding that delivers power to the load.⁠[Source 46]

See Primary winding, Transformer.

A non-latching relay: it has one resting contact position and returns to it when the coil is de-energised. The relay drawn on this site is of this kind.⁠[Source 9, Source 15, Source 29, Source 48]

See Latching relay, Energised and de-energised.

The MOSFET channel terminal joined to the body in the three-terminal form, so the gate voltage is taken between gate and source. Not the same thing as a voltage or current source.⁠[Source 24]

See Gate, Drain, Voltage source.

A switch with one moving arm that connects its common terminal to either of two fixed contacts. It has three contact terminals.⁠[Source 9, Source 10, Source 29]

See DPDT (double-pole, double-throw), Pole, Throw.

A switch with one contact that makes or breaks a path, and two terminals.⁠[Source 10]

See SPDT (single-pole, double-throw), Switch.

A supply connection drawn as a short labelled stub, such as +5 V, VCC or VDD, instead of a wire back to the source. Every stub with the same label is the same node.⁠[Source 12, Source 27, Source 36]

See Net label, Signal reference.

An extra lead joined partway along a coil: a connection to part of the winding.⁠[Source 2, Source 4]

See Centre tap, Autotransformer.

A point where a part connects to the circuit. A symbol names each terminal's role, not where it sits on the real part, and turning the symbol does not change the roles.⁠[Source 9, Source 27, Source 50]

See Node.

A fixed contact that a switch's pole can close onto. The number of throws counts the positions each pole can reach; an off position is not a throw.⁠[Source 9, Source 10]

See Pole, SPDT (single-pole, double-throw).

A list of a logic gate's output for every combination of its inputs. The gate's settled output follows it.⁠[Source 32, Source 33]

See Logic gate.

Values written with the multiplier letter where the decimal point would be: 4k7 is 4.7 kΩ, 47R is 47 Ω and 2n2 is 2.2 nF. A way of reading the value on a drawing, not advice on choosing one.⁠[Source 12, Source 52, Source 53, Source 54]

See Reference designator.

A coil of wire in an inductor or a transformer, drawn as loops or humps. A transformer has two or more windings drawn facing each other.⁠[Source 2, Source 4, Source 46]

See Primary winding, Secondary winding, Coil.

A potentiometer's movable third terminal, drawn as an arrow that touches the resistor body. It makes contact wherever it rests along the track.⁠[Source 45]

See Potentiometer, Rheostat.

A diode meant for reverse breakdown, marked by bent ends on the cathode bar.⁠[Source 2, Source 19]

See Diode.