How does a non contact voltage tester work?
they work by capacitive sensing of the AC voltage on the live conductor. they only work with AC.
It's obviously responding to some varying signal on the USB cable, (possibly "ground bounce" due to the varying current draw of the switching regulator in the phone. (USB cables are usually well shielded so that's about all that's likely to be there.
As the pickup is capacitive, the higher the frequency the more sensitive it is (up to the frequency limit if the amplifier it uses), so 100V at 60Hz produces the same output signal amplitiude as 10mV at 600Khz
if the charger has a captive cable (doesn't use a detachable USB cable) then it's probably not shielded and the signal may be from the powersupply itself.
Current clamps are for measuring how much electric current is flowing through a single conductor (eg to get an an indication that the circuit is not only live but is also in-use), as you need to separate out the individual conductors of the cable, you typically use them at a junction box or some other place where the cable is opened up. using a current clamp on a bundled cable will usually give you a zero reading (unless there's some sort of electrical fault)
You need to realize that there are two different things: voltage and current. Voltage is present in an outlet even if nothing is connected to it (therefore you can have a voltage and no current). Alternatively the neutral wire is at 0V earth potential, but you can have a big current flowing through it.
A voltage creates an electric field, while a current creates a magnetic field. Therefore depending on its construction you can have devices sensitive to static electric fields (therefore a DC detector), and devices that are only sensitive to alternate electric fields (an AC detector). The same applies for magnetic field detectors, such as the ones in a current clamp sensor.
Therefore if you only need to measure the presence of a harmful voltage you would use a voltage detector, and not a current clamp (that will indicate 0 A if no equipment is drawing current from the mains at this moment). Actually, if you were to cut a wire, for example, you should first measure both: that it has no harmful voltage in respect to earth, and that there is no current flowing through it.
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)?
They rely on capacitive coupling which limits them to AC and they are generally designed for mains voltages. Your body being a large object has some capacitance to ground. This makes a (very weak) circuit from the item with AC voltage on it, through the tester, through your body and through the capacitance to ground.
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.
Wrong!
In order to supppress electromagnetic interference from switched mode power supplies capacitance must be placed between the input and output sides.
In a class 1 (Earthed) power supply the earth is used as a barrier between input and output either by connecting the output to mains earth or by splitting the capacitance into two parts in series, a part between output and mains earth and a part between mains earth and mains live/neutral.
In a class 2 (non-earthed) power supply the mains earth is not available and so can't be used as a barrier. The result is that the output is often at a significant voltage relative to earth (half the mains voltage is common). This should not be a safety hazard if the power supply is properly designed as the capacitors have a high impedance (low capacitance) and hence the "touch current" is low despite the high open circuit voltage. The capacitors will be special safety types so that short circuit failure of the capacitors is extremely unlikely.
As a general rule PC power supplies are class 1 while smartphone power bricks are class 2. This is why your tester lit up on the cable for charging your phone but not on the cable for your keyboard.
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?
There is no surefire way to detect electricty non-intrusively. Especially when dealing with multicore cables rather than individual wires.