An electronics technician performs diagnostic checks on various malfunctioning direct-curr… — Armed Services Vocational Aptitude Battery (ASVAB)

An electronics technician performs diagnostic checks on various malfunctioning direct-current (DC) systems. Match each electrical troubleshooting symptom or meter reading on the left with its most probable circuit fault condition on the right.

An ohmmeter test across a de-energized relay coil rated at $250\,\Omega$ measures $0.0\,\Omega$ .Short-circuited (shorted) coil windings
A voltmeter measures full source voltage ( $12\text{ V}$ ) directly across the terminals of a closed switch in an active, non-functioning circuit.Open circuit caused by failed or pitted switch contacts
A circuit displays normal source voltage under no-load conditions, but voltage at the load drops drastically below specification when operating under full current draw.High-resistance connection or corroded terminal
An ohmmeter connected between an isolated appliance's ungrounded positive power lead and its metallic chassis reads $0.0\,\Omega$ .Direct ground fault (short to chassis ground)

Answer

The symptom of 0.0 ohms across a 250-ohm coil matches short-circuited coil windings; full source voltage measured across closed switch terminals matches an open circuit from failed switch contacts; voltage dropping severely under load matches a high-resistance or corroded connection; and 0.0 ohms between the ungrounded positive lead and metallic chassis matches a direct ground fault.

Each diagnostic reading correlates directly to fundamental circuit failure modes: a shorted coil eliminates component resistance (

0\,\Omega

); an open switch drops the entire source voltage (

12\text{ V}

) because current cannot bridge the open gap; a high-resistance joint creates excessive voltage drop only when current flows (

V = I \times R

); and continuity between a 'hot' power lead and the chassis represents a short to ground.

Step-by-Step Solution

Analyze the relay coil resistance reading of

0.0\,\Omega

Identifies an internal short circuit.

Inductive components such as relay coils are designed with specific continuous wire resistance. A reading of

0.0\,\Omega

means the insulation between windings broke down, creating a direct short.

Analyze the closed switch voltage reading showing full source voltage.

Identifies an open switch fault.

According to Kirchhoff's Voltage Law, an open point in a series circuit drops the entire source voltage because it creates infinite resistance and blocks current, whereas a properly closed switch drops

0\text{ V}

Analyze the loaded vs. unloaded voltage drop behavior.

Identifies a high-resistance connection fault.

When no current flows (

I = 0

), no voltage is dropped across bad connections (

V = 0\text{ V}

). When load current flows, parasitic resistance drops voltage in proportion to current, starving the load.

Analyze the zero-ohm reading between the positive supply rail and metal chassis.

Identifies a direct chassis ground fault.

Conductors intended to carry current must remain insulated from the metal chassis. A

0.0\,\Omega

reading confirms an unintended low-resistance bridge to the chassis frame.

Key Concept

Circuit fault diagnosis using voltage drop and resistance characteristics for opens, shorts, grounds, and high-resistance faults.

Estimated Time:1m 30s

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