Schematic Symbols

This is the one topic in the subtest that is pure memorization, and it is also the fastest to learn. There are twelve symbols and no reasoning involved.

The twelve

Enlarge

Zoom in

Marks that modify a symbol

The basic symbols gain small additions that change their meaning, and the harder questions ask exactly those:

Switches are named by poles and throws. A pole is a separate circuit the switch controls; a throw is a position it can connect to. A plain on-off switch is single-pole single-throw. A switch with one lever that can reach either of two contacts is single-pole double-throw - the kind that lets two switches control one stairway light. A switch lever drawn touching its contact is closed, so current flows.

The pairs that get confused

Battery and capacitor are the most confused pair, and they are similar: both are parallel lines crossing the circuit.

The battery has plates of different lengths, alternating long and short. The capacitor's two plates are equal. That is the difference, and the long plate on a battery is the positive terminal.

Ammeter and voltmeter differ only by the letter inside the circle, and the practical difference matters more: an ammeter goes in series and a voltmeter goes across. A diagram showing a circled meter in line with the circuit is an ammeter whatever the letter, which is a useful cross-check.

Lamp and a circled letter can look alike at a glance. The lamp's cross reaches the circle's edges; a meter has a letter floating inside.

Inductor and resistor are both wiggly. The resistor's zigzag has sharp points; the inductor's loops are round. Sharp for resistor, round for inductor.

Reading a schematic

A schematic is a map of connections, not a picture of the object.

• Wire length means nothing. A long line and a short line are the same wire.
• Component position means nothing. A resistor drawn at the top is not physically above anything.
• Only the connections carry information.

That is why a schematic of a circuit looks nothing like the circuit, and it is worth stating because a question may describe a physical arrangement and ask what the schematic shows.

Junctions

A dot where two wires cross means they are connected. No dot means they cross over without touching.

That convention decides whether a diagram shows a series or a parallel circuit, which is why it is the most consequential mark on the page. Look for the dots before deciding what the circuit is.

Working out a circuit

1. Find the source - a battery or a supply.
2. Trace from one terminal back to the other.
3. One path means series. A split and rejoin means parallel.
4. Note anything in series with everything else - a master switch, a fuse - because it controls the whole circuit.

What you can skip

Across all 1,358 Electronics Information questions in our bank:

• Logic gate symbols belong to the digital lesson, not here.
• Integrated circuit pinouts and transistor symbol variants are not asked on schematics. The questions stay with the dozen symbols in the table and the modifying marks above.
• Physical wiring diagrams. One question asks how a schematic differs from a pictorial diagram; none asks you to read a pictorial one.

Where people lose points

Confusing battery and capacitor. Unequal plates means battery.

Missing a junction dot, and reading a parallel circuit as a series one.

Reading physical layout into a schematic.

Putting an ammeter across a component in a question about how to measure.

Confusing the resistor's sharp zigzag with the inductor's round loops.

Taking a plain rectangle for a resistor when the American set uses a zigzag and a rectangle is a fuse.

Work one in under a minute

A schematic shows a battery, then a single line to a point where it splits into two branches each containing a lamp, and the branches rejoin before returning to the battery. There are dots at both split points. What kind of circuit is it, and what happens if one lamp fails?

The split and rejoin with dots at both means the two lamps are on separate paths: a parallel circuit.

If one lamp fails open, the other stays lit, because it still has a complete path of its own.

Note that the answer came entirely from the dots and the split. Nothing about the drawing's shape mattered.

Where this leads

Every circuit diagram in the subtest is built from these twelve, and reading the junction dots correctly is what makes the series and parallel lessons applicable to a drawing.