Digital Multimeters and Ghost Voltages

Posted by Power Probe TEK on December 20th, 2016

The Tech Support line has received a few calls recently about the PPDMM Digital Multimeter. Some customers are noticing, when using the DC Volt mode, that there are fluctuating voltage readings on the meter display even when nothing is connected to the leads or even when the test leads are not plugged into the meter. Some customers believe this is a fault of the meter and that the display should “zero out” if nothing is connected to the test leads.

These floating voltage readings are called “ghost voltages” and are a normal occurrence on any hi-impedance (most digital) multimeters. Hi-impedance meters are very sensitive to any relative voltage between the two test leads and, by a process call capacitive coupling, can pick up voltages from adjacent wiring or even out of thin air.

These are usually very low millivolt (mV) readings but they can jump around a lot especially if the test leads are being moved around. When probing a live circuit the capacitive coupling is cancelled and the meter will read accurately and consistently, so any voltage readings should not be randomly fluctuating (like ghost voltage does) when connected to a consistent voltage source. However, when the test leads are not connected to any circuit it is perfectly normal to see the ghost voltage readings.

Not only is it normal, but it is actually a usable testing indicator. For example, when using a Digital Multi-Meter in the DC Volt mode, you will usually connect the black test lead to battery ground and use the red lead to probe/test for any voltage. Now realize that you will get one of three possible readings —

  1. A voltage reading (usually Battery volts) indicating that wire is connected to power, or
  2. A steady 0.0 reading, which indicates that wire is connected to ground, or
  3. A floating “ghost” voltage, which indicates that the wire being probed is open and/or not connected to anything at all.

So, there is nothing wrong with your meter. “Ghost voltages” are normal and actually a good thing from an electrical testing standpoint. Now with one test, you can tell if the wire or circuit you are testing is connected to power, connected to ground, or not connected to anything (open).

Hope that clears up any confusion.

Dave Barden

Tech Support

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