Electronic – Why one of the multimeter shows a small voltage when only one probe is connected to AC and the other dont


I've recently acquired a Minipa ET-997 multimeter. I noticed that when I connected one probe to 127V phase and let the other disconnected, it will show around 12V.

But I have an older and cheaper multimeter which shows 0V in this same situation. Why is that?

Another thing, the older multimeter shows -127V when the black probe is on the phase wire and the red probe is on the neutral wire. And it shows +127 when the red probe is on phase and the black probe is on neutral. Therefore, I can easily tell which is phase and which is neutral. Minipa ET-997 always shows 127V, no matter how the probes are connected. Is there a way to "fix" this?

Best Answer

What you are seeing is a result of capacitive coupling and the very high input impedance of the multimeter. This is the same effect that causes hum in a audio line when you touch it with your finger. You're body isn't directly connected to the power line, but it picks up enough, which is then fed into the audio lead, so that it is quite audible after the amplifier makes it larger.

Your multimeter is like the amplifier, except that it displays the value instead of driving a speaker with it. There is enough difference in what is coupled to the two inputs of the meter with a wire dangling from one and none from the other such that the difference is measureable.

Such signal derived from capacitive coupling is very high impedance, meaning it diminishes quickly with even a slight load. However, a good voltmeter presents a high impedance, so this capacitively coupled signal is not attenutated to oblivion. Note that it is still significantly attenuated in your example. The power line is 127 V but after the capacitive coupling and then loading by the meters input impedance you only get 12 V.

In short, this is all expected, and actually shows you have a good meter.

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