Certain situations call for a simple way to find out if a line conductor is “hot” or not. If the conductor isn’t insulated, a popular method is to use a screwdriver with a built-in high-value resistor and a neon tube. The sensitivity of such a device is too low to work with an insulated conductor. The circuit shown in the figure can identify a “hot” conductor even if it’s insulated.

A clip put around the conductor under test acts as a small capacitor between the conductor and the circuit, and the path to the ground conductor is closed, exactly as in the case of the popular screwdriver test, through the operator. If the conductor is “hot” and the switch is open, a diode charge pump raises the voltage across the gate-tosource capacitance (not shown) of a MOSFET, so that eventually the MOSFET starts conducting.

The MOSFET’s conduction can be monitored either by a built-in ohm-meter, or a buzzer of an inexpensive handheld multimeter. Another possibility is to make the MOSFET turn on a LED indicator powered by a separate battery. In that case, the capacitive “hot”-line-conductor sensor is independent of the multimeter.

A switch is included to quickly discharge the gate-to-source capacitance in case of repeated tests. A resistor is added to protect the operator and the circuit in the event the test clip is brought in direct contact with a non-insulated “hot” line conductor. The shield is insulated and can’t touch the line conductor. Note that the sensor works equally well with insulated and non-insulated line conductors. Finally, a Zener diode prevents the gate-to-source voltage from exceeding the MOSFET’s ratings.

The entire circuit requires so little space that it could be incorporated into handheld multimeters. The author upgraded his multimeter by inserting a small PCB with the entire sensor circuit between the case of the multimeter and its flexible protective sheath. The test clip is a slightly modified mounting clip from one side of a 20-by-5-mm pc-board fuse. The length of the cable between the clip and the first protection resistor of 1.5 M is approximately 50 cm. Both diodes are 1N4007s, the Zener diode is a BZX55C15, and the second protection resistor also is 1.5 M. The MOSFET isn’t critical, but its case should be flat if the circuit is to be mounted between a multimeter and its sheath.

Note that if a digital multimeter (DMM) with an extremely high input impedance were used with a similar clip, it could, in principle, also detect a “hot” conductor. However, because of the omnipresent line-power fields, it would display some reading also with the clip being anywhere in the space—not only when it’s placed on the insulation of a “hot” conductor. Depending on local environment, a DMM would even display a reading with the clip around a “cold” conductor.

The merit of the circuit shown is that it substantially increases the signal-to-noise ratio and thus decreases a probability of false readings. In addition, any of the described versions of this sensor could be used to locate an ac line circuit break.