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Zorluk: OrtaMultimeter: Resistance and Continuity

While troubleshooting a de-energized printed circuit board, a technician measures the resistance across a 10 kΩ10\text{ k}\Omega resistor that remains fully soldered in place. The digital multimeter displays a steady reading of 2.4 kΩ2.4\text{ k}\Omega. What is the primary cause of this lower resistance reading?

  1. A
    The circuit must be energized so that the multimeter internal sensing circuit receives sufficient operating voltage.
  2. The test current from the multimeter is flowing through alternate parallel paths connected across the circuit board.Cevap
  3. C
    The resistor has experienced an internal breakdown and formed a complete electrical short circuit.
  4. D
    The multimeter leads were placed across the component in reverse polarity without zeroing the test leads.

Cevap

The test current from the multimeter is flowing through alternate parallel paths connected across the circuit board.
A digital multimeter measures resistance by injecting a small internal test voltage and measuring the resulting current. When measuring a component that is still soldered into a circuit, that test current splits across the component and any other interconnected parallel branches. Because parallel resistance combinations always result in an equivalent resistance smaller than the smallest branch (Rtotal<RbranchR_{total} < R_{branch}), the meter registers a lower value (2.4 kΩ2.4\text{ k}\Omega) than the resistor's nominal rating (10 kΩ10\text{ k}\Omega). To obtain an accurate reading, at least one lead of the component must be disconnected or desoldered.

Adım Adım Çözüm

1
Analyze the measurement environment and the component's circuit state.
The resistor is measured while fully connected (in-circuit) to other components on the board.
Components soldered onto a circuit board are rarely isolated and typically connect to adjacent traces, semiconductors, or resistors.
2
Apply parallel circuit principles to the ohmmeter test current.
The ohmmeter supplies a small internal test current across the probes, which divides among all available conductive paths between those two nodes.
According to the laws of parallel resistance, adding parallel paths always reduces the total equivalent resistance below the value of the lowest single branch (Req<R1R_{eq} < R_1).
3
Determine the necessary troubleshooting procedure for an accurate reading.
Isolate at least one terminal/lead of the resistor from the circuit board before measuring.
Desoldering one leg breaks parallel loop paths, ensuring 100% of the meter's test current flows strictly through the component under test.

Anahtar Kavram

In-circuit resistance measurement, component isolation, and parallel path loading
Tahmini Süre:1m 0s
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Multimeter: Resistance and Continuity

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