Voltage Drop Test
The DC voltage difference measured between the two ends of a connection or a section of line while the circuit runs at its real load. The common interpretation thresholds are: below 0.2 V good, 0.2–0.5 V borderline, above 0.5 V faulty. It reveals a bad connection far more reliably than a continuity test.
The logic of the measurement is simple: no voltage drops across a resistance carrying no current. So you put the probes on either side of the suspect section while the load is running; the value read is the product of that section's resistance and the current. A corroded contact that looks sound on an ohmmeter gives itself away immediately here.
- Practice: set the voltmeter to DC, place the probes immediately either side of the connection and run the load. The reading is taken at the moment the load is running steadily.
- Return path: the same test is also done for the chassis line. Between the device negative terminal and the battery negative post the typical target under load is below 0.2 V; on a good chassis path a figure of the order of 0.05 V is seen.
- Interpretation: a high reading tells you there is resistance inside that section; narrow the section down to find the exact point.
Even in a system with correctly chosen cable cross-sections, a single loose terminal can drop more voltage than the entire run. With field complaints of a slow motor or a weak lift, this is the first measurement to make.
The context in which this term is used: DC motor and driver fault diagnosis matrix