Components, Symbols and AC vs DC

🔧 Components, Symbols and AC vs DC

Part 1 of 7 — Components on the ASVAB

Topic Overview

What These Questions Look Like

Electronics Information has 15 questions in 10 minutes on the CAT-ASVAB (about 40 seconds each). Many of them are quick identification questions:

• "What does this component do?" (A diode lets current flow in only one direction.)
• "Which symbol is a ...?" (a small schematic drawing with four choices)
• "What is the unit of ...?" (capacitance is measured in farads)
• "What is AC?" (alternating current: it reverses direction many times a second)

These are recall questions: you either know the part or you don't, so they are fast points once you learn the list below.

The Component Cheat Sheet

Warm-up. 🎯 Part X in the figure uses one of the standard symbols for a protection device.

Match the component to its job. 🔽

Key Takeaways — Part 1

• Component questions are mostly fast recall: learn what each part does, its unit and its symbol
• Resistor limits current; capacitor stores charge; inductor opposes changes in current; diode is one-way; transistor amplifies or switches; transformer changes AC voltage
• Fuses melt and are replaced; breakers trip and are reset; both go in series

Up Next: Part 2 — the schematic symbols, switches and protection devices you need to recognize on sight.

🔧 Components, Symbols and AC vs DC

Part 2 of 7 — Schematic Symbols, Switches and Circuit Protection

Why Symbols

A schematic is a wiring map drawn with standard symbols instead of pictures. Anyone trained in electronics can read one, and the ASVAB expects you to recognize the common symbols at a glance.

The gallery on the next screen shows the symbols for this part. Study the shape of each one: on the test, the label will not be there.

The eight symbols of Part 2. Cover the labels with your hand and name each one.

Name that symbol. 🎯 Use Figure 2.

Switches and Relays

A switch opens (OFF) or closes (ON) a circuit. Switches are named by poles (how many separate circuits they control) and throws (how many positions connect).

A relay is a switch flipped by an electromagnet. A small current through the relay's coil pulls its contacts closed, letting a low-power circuit switch a high-power one (a car's tiny starter-button circuit controlling the big starter current, for example).

Protection: Fuses, Breakers and Ground

Fuses and breakers both go in series with what they protect. Their main job is to protect the wiring from overheating; a GFCI is what protects people from a shock.

Key Fact: Never replace a blown fuse with a higher-rated one. The fuse is matched to the wire; a bigger fuse lets the wire overheat before it blows, which is a fire hazard.

Wires on a schematic

Where two wires cross with no dot, they are not connected (one passes over the other). A dot at the crossing means they are connected.

Read the crossing. 🎯 Use Figure 3.

Switches and protection. 🎯

Key Takeaways — Part 2

• Know the symbols by shape: zigzag resistor, long/short-line battery, hinged switch, X-in-circle lamp, rectangle fuse, arc breaker, three-bar ground, M-in-circle motor
• A dot where wires cross = connected; no dot = not connected
• SPST = simple on/off; SPDT = one input to two outputs; a relay is a switch worked by an electromagnet
• Fuse melts (replace with the same rating); breaker trips (reset); both go in series; ground and GFCI protect people

Up Next: Part 3 — resistors (and their color code), capacitors and inductors.

🔧 Components, Symbols and AC vs DC

Part 3 of 7 — Resistors, Capacitors and Inductors

Resistors

A resistor limits current and drops voltage. Most are fixed (one value). Some are variable:

The resistor color code

Small resistors are marked with colored bands instead of printed numbers. For a four-band resistor, start from the end where the bands are bunched together:

• Band 1: first digit
• Band 2: second digit
• Band 3: multiplier (how many zeros to add)
• Band 4: tolerance: gold = ±5%, silver = ±10%

A memory sentence for the order Black, Brown, Red, Orange, Yellow, Green, Blue, Violet, Gray, White: "Better Be Right Or Your Great Big Venture Goes West."

Worked example (Figure 4). Yellow (4), violet (7), red (2 zeros), gold: 47 followed by two zeros = 4,700 Ω (4.7 kΩ), ±5%.

Another. Brown (1), black (0), orange (3 zeros): 10,000 Ω = 10 kΩ.

Symbols for this part. A diagonal arrow through any symbol means "adjustable."

Capacitors

A capacitor is two metal plates separated by an insulator (called the dielectric). Connected to a battery, charge piles up on the plates: one plate becomes positive, the other negative, and the capacitor stores electric charge (energy in an electric field).

• Unit: the farad (F). A farad is huge, so real capacitors are rated in microfarads (µF) or picofarads (pF).
• DC: current flows only briefly while the capacitor charges, then stops. A capacitor blocks steady DC.
• AC: the voltage keeps reversing, so the capacitor keeps charging and discharging and current keeps flowing. A capacitor passes AC, and passes it more easily at higher frequencies.
• Uses: smoothing ripple in power supplies, timing circuits, tuning radios, starting motors, camera flashes.
• Combining: capacitors in parallel add (the opposite of resistors). In series, they combine the way resistors do in parallel.

Safety: A capacitor can hold its charge after the power is off. Large capacitors in microwaves, TVs and power supplies can give a dangerous shock even when unplugged.

Inductors

An inductor is a coil of wire. Current through it creates a magnetic field, and the coil opposes any change in current: when current tries to rise or fall, the changing field pushes back.

• Unit: the henry (H).
• DC: once the current is steady, an inductor is just a wire. It passes DC easily.
• AC: the current is always changing, so an inductor opposes AC, and opposes it more at higher frequencies (the opposite of a capacitor).
• An iron core inside the coil makes the effect much stronger.
• Inductors in series add, like resistors.

The same idea makes an electromagnet: a coil around an iron core becomes a magnet only while current flows. Electromagnets drive relays, solenoids (a coil that pulls a plunger, like a car's starter solenoid or a door lock), speakers and motors.

Check the ideas. 🎯

Capacitor or inductor? 🔽

Read the bands and combine. ✍️ Numbers only.

1) A resistor reads green, blue, red, gold. What is its value in ohms?

2) A resistor reads orange, orange, orange, silver. What is its value in ohms?

3) A 10-µF and a 22-µF capacitor are connected in parallel. What is the total capacitance in µF?

Key Takeaways — Part 3

• Potentiometer (3 terminals) = adjustable voltage divider; rheostat (2 terminals) = adjustable resistance; thermistors and photoresistors respond to heat and light
• Color code: Black 0, Brown 1, Red 2, Orange 3, Yellow 4, Green 5, Blue 6, Violet 7, Gray 8, White 9; band 3 = number of zeros; gold ±5%, silver ±10%
• Capacitor: plates + insulator, stores charge, farads, blocks DC, passes AC, adds in parallel, can stay charged after power-off
• Inductor: coil, magnetic field, opposes changes in current, henrys, passes DC, opposes AC
• Electromagnets drive relays, solenoids, speakers and motors

Up Next: Part 4 — diodes, transistors and transformers.

🔧 Components, Symbols and AC vs DC

Part 4 of 7 — Diodes, Transistors and Transformers

Semiconductors in One Paragraph

Diodes and transistors are made of semiconductors, usually silicon (sometimes germanium). Adding tiny amounts of other elements (doping) makes N-type material (extra free electrons, negative carriers) or P-type material (missing electrons, called holes, which act as positive carriers). Where P and N meet, a junction forms that lets current cross easily in one direction only.

Diodes: One-Way Valves

A diode is one P-N junction. It conducts when its anode is more positive than its cathode (forward bias) and blocks current when connected the other way (reverse bias).

• In the symbol, the triangle points the way conventional current flows, and the bar is the cathode. (On a real diode, a painted stripe marks the cathode end.)
• A rectifier uses diodes to change AC into DC. One diode lets through only half of each AC cycle (half-wave); four diodes in a bridge use both halves (full-wave).
• An LED (light-emitting diode) gives off light when current flows forward.
• A zener diode is designed to conduct backwards at a set voltage, so it is used to regulate (hold steady) a voltage.

History note: Before semiconductors, the same jobs were done by vacuum tubes (electron tubes): a tube diode rectified, a tube triode amplified. Tubes are larger, run hot and burn out; diodes and transistors replaced them in almost everything.

Read the diode. 🎯

Transistors: Amplifiers and Switches

A transistor has three leads: the emitter, the base and the collector. A small current into the base controls a much larger current between the collector and the emitter. That gives it two jobs:

The two types are NPN and PNP. In the symbol, the arrow is on the emitter:

• NPN: the arrow points out of the transistor ("Not Pointing iN")
• PNP: the arrow points in ("Pointing iN Proudly")

An integrated circuit (IC), or chip, packs many transistors, resistors and other parts onto one small piece of silicon.

Transformers: Changing AC Voltage

A transformer is two coils wound on a shared iron core (Figure 6). AC in the primary coil makes a changing magnetic field, and that changing field induces a voltage in the secondary coil. There is no wire connection between the coils.

$\frac{V_{secondary}}{V_{primary}} = \frac{N_{secondary}}{N_{primary}} \qquad (N = \text{number of turns})$

• More turns on the secondary → step-up (higher output voltage).
• Fewer turns on the secondary → step-down (lower output voltage).
• Power in ≈ power out, so when voltage goes up, current goes down by the same factor.
• A transformer works only on AC. Steady DC makes no changing field, so nothing is induced.

Worked example 1. A primary with 200 turns gets 24 V AC. The secondary has 1,000 turns (5 times as many). Output: 5 × 24 = 120 V. Step-up.

Worked example 2. A doorbell transformer steps 120 V down to 12 V, a 10 : 1 ratio. If the secondary has 50 turns, the primary has 10 × 50 = 500 turns. If the secondary supplies 1 A, the primary draws only about 0.1 A.

Diode-family symbols, an NPN transistor and a transformer. A PNP transistor looks like the NPN one with its emitter arrow pointing inward.

Check the ideas. 🎯

Transformer ratios. ✍️ Numbers only.

1) A transformer has 100 turns on its primary and 400 turns on its secondary. The primary gets 30 V AC. What is the secondary voltage, in volts?

2) A step-down transformer changes 240 V to 12 V. Its primary has 2,000 turns. How many turns are on the secondary?

3) For the transformer in question 1, the secondary supplies 2 A. About how much current does the primary draw, in amperes? (Assume no losses.)

Key Takeaways — Part 4

• Diodes and transistors are made of semiconductors (silicon, germanium) doped into P-type and N-type
• Diode = one-way valve; conducts anode → cathode (triangle points the way); the bar/stripe is the cathode
• Rectifier = AC → DC; LED = light; zener = voltage regulation; vacuum tubes came before semiconductors
• Transistor: emitter, base, collector; amplifier or switch; NPN arrow points out, PNP in
• Transformer: $\frac{V_s}{V_p} = \frac{N_s}{N_p}$; more secondary turns = step-up; current changes the opposite way; AC only

Up Next: Part 5 — AC vs DC, frequency, and how a house is wired.

🔧 Components, Symbols and AC vs DC

Part 5 of 7 — AC vs DC, Frequency and Household Wiring

Two Kinds of Current

AC won the battle to power homes because transformers can step it up to very high voltage for long-distance transmission (high voltage means low current, so less energy is lost as heat in the lines) and back down to safe levels near your house.

Many devices run on DC inside but plug into AC: their power supply uses a transformer to lower the voltage and a rectifier to turn it into DC.

Frequency and Period

One complete back-and-forth of AC is a cycle.

• Frequency ($f$) is the number of cycles per second, measured in hertz (Hz). 1 Hz = 1 cycle per second.
• Period ($T$) is the time for one cycle, in seconds.
• They are reciprocals: $f = \frac{1}{T}$ and $T = \frac{1}{f}$.

Prefixes work as usual: kHz = thousands of hertz (AM radio), MHz = millions (FM radio), GHz = billions (Wi-Fi, cell phones).

Motors, Generators and Induction

Moving a magnet near a coil (or a coil near a magnet) induces a voltage in the coil: electromagnetic induction.

• A generator turns mechanical energy into electrical energy (spin a coil in a magnetic field → AC).
• A motor does the reverse: electrical energy into mechanical energy (current in a coil inside a magnetic field → spinning).
• A car's alternator is a generator; it makes AC, and built-in diodes rectify it to DC for the car's 12-V system.

Household Wiring (United States)

• Standard outlets: 120 V AC, 60 Hz. Large appliances (electric dryers, ranges, central air conditioners, water heaters) use 240 V.
• Circuits are wired in parallel, each protected by a breaker in the panel.

On a standard outlet (Figure 8), the narrow slot is hot, the wider slot is neutral, and the round hole is ground.

Read the graphs. 🎯

Electrical Safety

• It is the current through your body that injures, and it does not take much: a fraction of an amp across the chest can stop the heart. Voltage matters because it is what drives that current through your skin's resistance.
• Water (sweat, wet floors, damp gloves) lowers your body's resistance. Keep electricity and water apart.
• Turn off and lock out the breaker before working on a circuit, and test with a meter to confirm it is dead.
• Work with one hand when you must touch equipment that could be live, so current cannot cross your chest from hand to hand.
• Use tools with insulated handles, and never stand in water near electrical equipment.
• Capacitors can stay charged after power is removed; discharge them before working.
• Electrical fires: cut the power and use a Class C (or ABC) extinguisher. Never use water on an energized electrical fire.

Wiring and safety. 🎯 The first question uses Figure 8.

Frequency and period. ✍️ Numbers only.

1) An AC signal has a period of 0.05 second. What is its frequency, in hertz?

2) A signal has a frequency of 200 Hz. What is its period, in seconds? (decimal)

3) How many complete cycles does 60-Hz household power go through in 5 seconds?

Key Takeaways — Part 5

• DC flows one way (batteries, cars at about 12 V); AC reverses (generators, outlets)
• Frequency = cycles per second, in hertz; U.S. power is 60 Hz; $f = \frac{1}{T}$
• Generator = mechanical → electrical; motor = electrical → mechanical; both rely on electromagnetic induction
• U.S. homes: 120 V outlets, 240 V large appliances; black/red hot, white neutral, green/bare ground
• It is current through the body that injures; no water near electricity; Class C for electrical fires

Up Next: Part 6 — strategy and traps for component questions.

🔧 Components, Symbols and AC vs DC

Part 6 of 7 — Component Test Strategy and Common Traps

Timing

CAT-ASVAB EI: 15 questions, 10 minutes, about 40 seconds each (paper: 20 questions in 9 minutes). Component and symbol questions are pure recall, so aim for 10 to 15 seconds each and save the extra time for circuit calculations.

Remember the CAT rules: you can't return to a question, you must answer to move on, and any question could be an unscored tryout, so treat each as scored. If time is short, keep answering genuinely; a burst of random answers at the end is penalized heavily.

A recall routine

1. Name what the part does before you look at the choices (diode → one way).
2. Check the unit if the question asks for one (capacitor → farads).
3. Picture the symbol if one is shown: shape first, then details (triangle + bar = diode; which way is the triangle pointing?).
4. Eliminate by job: a choice that describes another part's job is wrong even if it sounds technical.

Pairs the test likes to swap

The Traps

Spot the trap. 🎯

Swap check. 🔽 Pick the part that matches each description.

Key Takeaways — Part 6

• Component items are fast recall: about 10–15 seconds each, to bank time for calculations
• Name the job first, then check units and symbol shape, then eliminate choices that describe a different part
• Know the swapped pairs: fuse/breaker, capacitor/inductor, rectifier/transformer, motor/generator, pot/rheostat, NPN/PNP, neutral/ground
• Transformers need AC; voltage up means current down; same-rating fuses only; band 3 = zeros

Up Next: Part 7 — a mixed mastery set.

🔧 Components, Symbols and AC vs DC

Part 7 of 7 — Mixed Mastery Set

Ten questions covering the whole topic at real EI difficulty. Aim for about 40 seconds each, and much less on the recall items.

Questions 1–3 use Figure 9. 🎯

Questions 4–7. 🎯

Questions 8–10. 🎯

Key Takeaways — Components, Symbols and AC vs DC

• Learn each part by job, unit and symbol: resistor (Ω, zigzag), capacitor (farads, two plates), inductor (henrys, coil), diode (one-way, triangle + bar), transistor (amplify/switch; emitter, base, collector), transformer (AC voltage; two coils and a core)
• Fuse melts and is replaced with the same rating; breaker trips and resets; GFCI protects people near water; ground is the safety path
• Color code: digit, digit, zeros, tolerance (gold ±5%, silver ±10%)
• AC reverses (60 Hz in the U.S.); DC flows one way; $f = \frac{1}{T}$
• U.S. homes: 120 V and 240 V; black/red hot, white neutral, green/bare ground

Next unit: Auto Information.