The common AC tester is the simplest form of tool to test the presence of AC in the mains wiring. The Screwdriver type AC tester has a Neon lamp inside which glows when the current passes from the mains wiring to the ground through the user’s body. It uses the stray capacitance of the body to complete the circuit.
The body capacitance is the physical property of the body to act as a capacitor. Like any other conductor, the human body can also store electric charge if insulated. The capacitance of the body changes according to the surroundings. For example, if we stand on the top of a pole with nothing nearby, the capacitance of the body is very low. But it is very high if we are close to an insulated and grounded electrical appliance. Capacitance of the body is typically ten to few hundreds of Pico farads (pF) in normal conditions.
The screwdriver type AC tester has a metal shaft to make contact with the live wire. One terminal of the Neon lamp is connected to this metal shaft through a metal spring. The other terminal of the Neon lamp is connected to a high value resistor. The resistor is then connected to the metal touch point thus making a complete circuit.
When the metal shaft is touched to the live wire, current passes through the neon lamp to the ground through the body and lamp glows. The resistor inside the tester limits the voltage to a low value so that, we will not get electric shock. Moreover, inside the Neon lamp, the terminals are separated by the neon gas, so that, only very low current passes through the gas to complete the circuit.