Features of Dual-Impedance Digital Multimeters
Basic Knowledge about Impedance
Nowadays, most digital multimeters (DMMs) available on‑market for testing industrial, electrical and electronic systems feature very high input‑circuit impedance, typically greater than 1 MΩ. Simply put, when a DMM is connected to a circuit under measurement, it barely affects circuit performance. This is desirable for most measurement applications, especially for sensitive electronic or control circuits. Older troubleshooting tools such as analog multimeters and testers generally have low input‑circuit impedance, around 10 kΩ or less. Although such tools are immune to stray‑voltage interference, they are only suitable for power‑circuit measurements or other scenarios where low input impedance will not impair or alter circuit behaviour.
A Prime Example Combining Two Types of Input Impedance
With dual‑impedance meters, technicians can reliably troubleshoot faults in sensitive electronic or control circuits as well as circuits where stray voltages may occur, and determine the presence or absence of voltage in a circuit with higher confidence.
High‑impedance meters are preferred for standard electrical measurements, except when stray voltages are present.
For Fluke 114, 116 and 117 DMMs, the regular Vac and Vdc switch positions provide high‑impedance inputs, which cover most troubleshooting tasks, particularly for highly‑sensitive electronic loads. Fluke's low‑impedance feature is named Auto‑V/LoZ. Auto‑V stands for Auto‑Voltage. This capability automatically identifies whether the measured signal is AC or DC voltage, selects the correct function and range, and displays valid readings accordingly. LoZ means Low Impedance (Z). It provides a low‑impedance input to the circuit under test. This reduces false readings caused by stray voltages and improves accuracy when verifying whether voltage is present. Use the Auto‑V/LoZ switch position on the DMM whenever readings are questionable (suspected stray‑voltage conditions) or when checking for the presence of voltage.
