A technician is troubleshooting a series circuit powered by a 12\text{ V} DC source that n… — Armed Services Vocational Aptitude Battery (ASVAB)

A technician is troubleshooting a series circuit powered by a $12\text{ V}$ DC source that normally operates with three identical $10\,\Omega$ resistive loads ( $R_1$ , $R_2$ , and $R_3$ ). Match each specific multimeter reading or diagnostic symptom on the left to its corresponding circuit fault condition on the right.

Voltmeter across $R_2$ measures $12\text{ V}$ , voltage across $R_1$ and $R_3$ is $0\text{ V}$ , and total circuit current is $0\text{ A}$ .Open circuit fault located internally within $R_2$
Voltmeter across $R_2$ measures $0\text{ V}$ , voltage across $R_1$ and $R_3$ each rise to $6\text{ V}$ , and total circuit current increases from $0.4\text{ A}$ to $0.6\text{ A}$ .Direct short circuit across $R_2$ bypassing its resistance
Voltmeter across each resistor drops from $4\text{ V}$ to $2.5\text{ V}$ , total circuit current decreases, and a $4.5\text{ V}$ drop is measured across a corroded terminal lug.High-resistance series connection in the circuit path
Ohmmeter test with power disconnected measures $0\,\Omega$ from the switched line to the metal chassis ground before reaching $R_1$ .Direct short-to-ground fault upstream of the load network

Answer

The multimeter readings correctly pair with the specific circuit faults as follows: measuring full supply voltage across a single component with zero current corresponds to an open circuit in that component; measuring zero volts across a component while current and remaining voltage drops increase corresponds to a shorted component; reduced voltage across all loads with an unexpected voltage drop at a junction corresponds to a high-resistance connection; and measuring zero ohms to chassis before loads corresponds to a direct short-to-ground fault.

Each fault produces a distinct electrical signature in a series circuit: an open component exhibits infinite resistance and drops the full source voltage with zero current; a shorted component exhibits zero resistance and zero voltage drop while increasing circuit current; a high-resistance connection adds unwanted series resistance that drops voltage and reduces current to all loads; and a short-to-ground creates a direct zero-ohm path to ground before the load network.

Step-by-Step Solution

Analyze the voltage and current behavior for the open circuit symptom

Full source voltage (

12\text{ V}

) across

R_2

with

0\text{ A}

current indicates an open circuit at

R_2

An open circuit creates infinite resistance, stopping all current (

I = 0

) so no voltage drops occur across intact series components, placing total source EMF across the open terminals.

Analyze the voltage and current behavior for the shorted component symptom

Zero volts across

R_2

with increased current from

0.4\text{ A}

0.6\text{ A}

indicates a short circuit across

R_2

A shorted component has

0\,\Omega

, dropping

0\text{ V}

while decreasing total circuit resistance (

R_{total} = 20\,\Omega

) and increasing current (

I = 12\text{ V} / 20\,\Omega = 0.6\text{ A}

Analyze the effect of an unwanted series resistance

Lower voltage drops across all loads combined with an unexpected drop across a terminal indicates a high-resistance connection.

Corrosion or a loose terminal adds resistance to the series string, lowering total current and taking a portion of the source voltage away from the loads.

Evaluate the de-energized ohmmeter reading to chassis

A reading of

0\,\Omega

from the power lead to chassis ground prior to the loads confirms a short-to-ground fault.

An ohmmeter reading continuity (

0\,\Omega

) to chassis ground before the loads shows an unintended low-resistance return path that bypasses the load network entirely.

Key Concept

Circuit Fault Diagnosis and Multimeter Reading Interpretation in DC Series Circuits

Estimated Time:1m 30s

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