ASVAB Electronics Practice Test 3 — Questions and Answers

Question 1: A transformer has a primary coil with 500 turns and a secondary coil with 100 turns. If the primary voltage is 220V, what is the secondary voltage?

44V (Correct answer)
1,100V
22V
440V

Correct answer: 44V

V₂/V₁ = N₂/N₁ → V₂ = 220 × (100/500) = 220 × 0.2 = 44V.

Transformers work on the principle of electromagnetic induction and follow the turns ratio formula: V₁/V₂ = N₁/N₂, where V = voltage and N = number of turns. Rearranging: V₂ = V₁ × (N₂/N₁) = 220 × (100/500) = 220 × 0.2 = 44V. This is a step-down transformer because the secondary has fewer turns than the primary, so voltage decreases. A step-up transformer has more secondary turns, increasing voltage. Transformers only work with AC (alternating current) — they rely on a constantly changing magnetic field. On the ASVAB EI section, remember: more turns = more voltage, fewer turns = less voltage. Power is conserved (ideally): P = V₁I₁ = V₂I₂, so when voltage drops, current increases proportionally.

Question 2: In a parallel circuit, three resistors have values of 6Ω, 12Ω, and 4Ω. What is the total resistance?

22Ω
2Ω (Correct answer)
7.33Ω
1.5Ω

Correct answer: 2Ω

1/Rt = 1/6 + 1/12 + 1/4 = 2/12 + 1/12 + 3/12 = 6/12 = 1/2. So Rt = 2Ω.

Parallel resistance uses the reciprocal formula: 1/Rt = 1/R₁ + 1/R₂ + 1/R₃. Converting to a common denominator of 12: 1/6 = 2/12, 1/12 = 1/12, 1/4 = 3/12. Sum = 6/12 = 1/2. Therefore Rt = 2Ω. Note that total parallel resistance is always less than the smallest individual resistor (here, 4Ω minimum, and the answer 2Ω is less). This is a key sanity check. In contrast, series resistors add directly: Rt = R₁ + R₂ + R₃ = 22Ω (answer A is the series result — a common trap). The ASVAB EI section frequently tests both configurations. Parallel circuits provide multiple paths for current; adding more parallel resistors always decreases total resistance.

Question 3: What does a diode primarily do in an electrical circuit?

Amplifies voltage signals
Stores electrical charge
Allows current to flow in one direction only (Correct answer)
Converts AC to a stable DC voltage

Correct answer: Allows current to flow in one direction only

A diode allows current to flow in only one direction (from anode to cathode when forward-biased) and blocks it in the reverse direction.

A diode is a semiconductor device made from a p-n junction. When the anode (positive terminal) is at a higher voltage than the cathode (negative terminal), the diode is forward-biased and conducts current. When reversed (reverse-biased), it blocks current. This one-directional behavior makes diodes essential in rectifier circuits that convert AC to pulsating DC. A full-wave bridge rectifier uses four diodes to convert both halves of an AC cycle. Note: a diode rectifies AC but the output is pulsating, not stable DC — a capacitor (answer D would need one) smooths it. Zener diodes operate in reverse breakdown for voltage regulation. On the ASVAB, know that the diode symbol is an arrow (direction of conventional current flow) pointing to a vertical line (the cathode).

Question 4: A capacitor with a capacitance of 50 microfarads (µF) is fully charged to 12V. What charge (in microcoulombs) is stored on the capacitor?

600 µC (Correct answer)
4.17 µC
62 µC
0.24 µC

Correct answer: 600 µC

Q = C × V = 50 µF × 12V = 600 µC.

The fundamental capacitor equation is Q = C × V, where Q is charge in coulombs, C is capacitance in farads, and V is voltage. Using the given values in consistent units: Q = 50 × 10⁻⁶ F × 12 V = 600 × 10⁻⁶ C = 600 µC. Capacitors store electrical energy in an electric field between two conductive plates separated by a dielectric (insulating) material. Energy stored = ½CV². They block DC (once charged, current stops) and pass AC. On the ASVAB EI section, remember: larger capacitance = more charge stored at the same voltage; higher voltage = more charge for the same capacitor. Capacitors oppose changes in voltage (analogous to how inductors oppose changes in current).

Question 5: What is the primary difference between NPN and PNP transistors?

NPN uses AC; PNP uses DC
In NPN, current flows from collector to emitter; in PNP, from emitter to collector (Correct answer)
NPN amplifies voltage; PNP amplifies current
NPN has three terminals; PNP has two terminals

Correct answer: In NPN, current flows from collector to emitter; in PNP, from emitter to collector

In an NPN transistor, conventional current flows from collector to emitter (electrons flow emitter to collector). In a PNP, current flows from emitter to collector.

Bipolar Junction Transistors (BJTs) come in two types based on their semiconductor layer arrangement. NPN: N-type collector, P-type base, N-type emitter — the base is lightly doped P-type sandwiched between two N-type layers. Conventional current flows from collector (C) to emitter (E) when the base is forward-biased. PNP: P-type emitter, N-type base, P-type collector — conventional current flows from emitter to collector. Both types use a small base current to control a larger collector-emitter current (amplification). The symbol for NPN has the emitter arrow pointing outward (away from base); PNP has it pointing inward. On the ASVAB, mnemonic: NPN = 'Not Pointing iN' (arrow points out); PNP = 'Pointing iN Proudly' (arrow points in).

Question 6: If a circuit has a voltage of 120V and a resistance of 30Ω, what is the power dissipated?

4W
3,600W
480W (Correct answer)
360W

Correct answer: 480W

I = V/R = 120/30 = 4A. P = IV = 4 × 120 = 480W. Or P = V²/R = 14,400/30 = 480W.

Power in an electrical circuit can be calculated using three equivalent formulas: P = IV, P = I²R, and P = V²/R. All three come from combining Ohm's Law (V = IR) with P = IV. Using P = V²/R: P = 120²/30 = 14,400/30 = 480W. Or find current first: I = 120/30 = 4A, then P = 4 × 120 = 480W. Common answer trap: 3,600W = 120² = V² without dividing by R; 360W = 120 × 3 (incorrectly using 120/40). Power is measured in watts (W) and represents the rate of energy conversion. In military/industrial applications, knowing power dissipation is critical for sizing fuses, breakers, and heat sinks. The ASVAB EI section frequently tests all three power formulas.

ASVAB Electronics Practice Test 3

A transformer has a primary coil with 500 turns and a secondary coil with 100 turns.
If the primary voltage is 220V, what is the secondary voltage?