초록 ▼

To detect if a tool bit, such as a drill, a saw blade, or the like, upon being worked into a wall, meets a metallic obstruction, such as a conduit, reinforcement rod, hydraulic pipe or the like, an alternating voltage, preferably between 2 and 20 kHz, is coupled through a coupling capacitor (9) to t

To detect if a tool bit, such as a drill, a saw blade, or the like, upon being worked into a wall, meets a metallic obstruction, such as a conduit, reinforcement rod, hydraulic pipe or the like, an alternating voltage, preferably between 2 and 20 kHz, is coupled through a coupling capacitor (9) to the tool bit, and current flow to the tool bit is detected by a current measurement stage (10; 31, 32) to disconnect energy to the drive motor (5), for example by disabling firing of a thyristor (15), or closing a valve of a compressed-air tool. The coupling capacitor should have a capacity small enough to provide high impedance to network power frequency, and to insure a substantial change in current being supplied to the tool bit holder, typically a chuck, if the tool bit meets a metallic object in the wall. The a-c supply circuit can be formed as part of a trigger circuit for the thyristor, by pulse-energizing a trigger diode (28) upon repetitive charge of a capacitor (26) through a charging network (22, 23, 24, 25).

대표청구항 ▼

Safety circuit used in power tools, particularly hand tools, used in deenergizing the power tool upon contact of a bit with a metallic object (16, 17), regardless of whether said metallic object is grounded, said power tool having a two-wire power supply connection and an electrically conductive bit

Safety circuit used in power tools, particularly hand tools, used in deenergizing the power tool upon contact of a bit with a metallic object (16, 17), regardless of whether said metallic object is grounded, said power tool having a two-wire power supply connection and an electrically conductive bit holder (6), comprising a source of electrical test energy having a frequency elevated substantially above power line frequency and a test voltage substantially below power line voltage, said source having two test energy output terminals; a capacitor (9) applying said test energy from one terminal of the electrical test energy source to the bit holder (6), said capacitor (9) having a coupling impedance which is high with respect to power line frequency and an impedance which is low with respect to the elevated frequency of electric test energy; coupling means (34; 22-25, 26) for high frequency coupling the other terminal of the test energy source to at least one wire of the two-wire power supply connection; a test current measuring circuit (10, 30, 31) connected between the electrical test energy source and the capacitor (9) and sensing change of current to said bit and bit holder; and a disconnect circuit connected to and controlled by the test current measuring circuit (10) and disconnecting energization from said two-wire power supply connection to the power tool when the test current measuring circuit (10) detects change in test current above a predetermined level, upon contact of the bit with said metallic object (16, 17).