Learning Library · Multimeters, explained · 5
Input impedance, loading, and ghost voltages
A voltmeter has to draw a tiny current to measure anything, so connecting it changes the circuit a little. Usually the change is too small to see. Sometimes it is the whole story: a high-resistance circuit reads low, and a wire that is not connected to anything reads like it is live.
The meter is part of the circuit
On its voltage ranges, a digital multimeter looks like one large resistor across its probes, usually about 10 MΩ. The moment you connect it, that resistor sits in parallel with whatever you are measuring.
Across a 10 kΩ resistor, 10 MΩ in parallel changes almost nothing. Across a 1 MΩ resistor it pulls the combination down by about 9%, and the voltage you read drops with it. The rule of thumb: the meter's input should be at least 100 times the resistance of the circuit at the point you measure.
Ghost voltages
A wire that runs beside a live conductor, in the same conduit or cable, is coupled to it by a small capacitance. If the wire is disconnected at both ends, nothing holds it at zero, and a 10 MΩ meter is a light enough load that the coupled voltage shows up on the display. It is real voltage, but it has almost no current behind it. Load it with a few kilohms and it collapses.
That is what a low-impedance, or LoZ, mode is for. It switches a low resistance across the input, so a ghost reads near zero while a genuinely live conductor, which can supply the current, still reads its full voltage.
Try this
- In the divider, with 10 V and 1 MΩ resistors, the middle should be at 5 V. The 10 MΩ meter reads 4.76 V, 4.8% low.
- Turn the resistors down to 10 kΩ. The same meter now reads 4.998 V, an error of 0.05%.
- Switch to ghost voltage with 200 pF of coupling and a disconnected wire. The 10 MΩ meter reads 72.2 V on a wire that is not connected to anything.
- Switch the meter to LoZ. The ghost falls to 27 mV. Now connect the wire to 120 V: LoZ reads 120.0 V, because the supply can drive the current.
- Go back to the divider with 1 MΩ resistors and use LoZ. The reading collapses to about 30 mV. LoZ is for finding ghosts on power wiring, never for electronics.
Input impedance on UNI-T meters
UNI-T lists DC voltage input impedance at 10 MΩ or more on the UT181A, and LoZ modes on handhelds including the UT171B, UT196, and UT197PV. The UT171B's LoZ input is about 3 kΩ, the value used here.
LoZ draws real current, often tens of milliamps on mains voltage, and can trip a GFCI, turn on a contactor, or disturb a sensitive circuit. Use it to confirm whether power wiring is live, and use the normal high-impedance voltage function everywhere else.
Tell a live wire from a ghost. UNI-T handheld multimeters with LoZ.
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