How does a non contact voltage tester pen detect voltages and/or currents? Are they limited to voltages of a certain range or type (AC or DC)?
Here's some experiments I have conducted that lead to this question: Using a cheap pen I bought for a couple bucks, I can detect the usual 120V AC in an American outlet, but I also have been able to detect voltage in a USB cable connecting a switching power supply to a smartphone (this setup is usually referred to as charging one's phone). Here, of course, the voltage in question is DC with a negligible ripple. I also noticed that while the detector can detect the 5V of a phone charger cable, it cannot detect the voltage in a USB keyboard cable. The only difference between these two scenarios is the current level and perhaps some minor signaling differences.
A final question: In what voltage/current scenario would you have to use a current clamp sensor and not a non contact voltage detector for merely detecting the presence of power non-intrusively?