Safety circuit for hand tools, and method for safe operation thereof

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).

Safety circuit used in power tools, particularly hand tools, used in deenergizating the power tool upon contact of a bit with an electrically conductive object (16, 17), said power tool having an electrically conductive bit holder (6) and an electric motor (5), comprising

• a high-frequency electrical energy source providing electrical energy having a frequency substantially above power line frequency and a voltage substantially below power line voltage;
• coupling means (9) applying the voltage from the electrical energy source to the bit holder (6);
• a current measuring circuit (10, 30, 31) connected between the electrical energy source (11) and the coupling means (9);
• a disconnect circuit (14, 15;
• 24, 25, 26, 28, 29) connected to and controlled by the current measuring circuit (10, 30, 31) and disconnecting energization to the motor (5) when the current measuring circuit detects current above a predetermined level, said disconnect circuit including
• a semiconductor switch connected to the motor (5);
• a trigger circuit connected to the semiconductor switch (15) and thereby controlling the motor (5);
• the current measuring circuit (10, 30, 31) being connected to and in turn controlling the trigger circuit;
• and
• a direct current source providing power to the trigger circuit;
• said trigger circuit including a trigger diode (28) and a capacitor (26) charged by direct current from the direct current source, the trigger diode (28) periodically discharging the capacitor (26) to form therewith an oscillatory circuit;
• and connection means coupled to the oscillatory circuit and forming said high frequency energy source.

Safety circuit used in power tools, particularly hand tools, for de-energizing 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 an electrically conductive bit holder (6) and a two-wire power supply connection, comprising

• a source of electrical test energy having a frequency substantially above power line frequency and a test voltage substantially below power line voltage;
• a capacitor (9) applying said test energy from the electrical test energy source to the bit holder (6), said capacitor (9) being capable of effectively blocking power line frequency current;
• a test current measuring circuit connected between the electrical test energy source and a capacitor (9), and sensing change of current to said bit and bit holder, said test current measuring circuit comprising
• a resistor (32) serially connected between the source of electrical test energy and the capacitor (9);
• a transistor (31) having its base-emitter path connected in parallel to the resistor (32);
• and a response circuit (30) including the collector of the transistor (31), the transistor changing conduction state when the voltage across the resistor (32) due to current flow to said bit and bit holders exceeds the base-emitter voltage of the transistor;
• and
• a disconnect circuit (14, 15, 24, 25, 26, 29) connected to and controlled by the test current measuring circuit and disconnecting energization from said two wire power supply connection to the power tool, when the test current measuring circuit detects change in test current above a predetermined level, upon contact of the bit with said metallic object (16, 17).

A power tool, particularly motor-driven hand tool, having

• an electric motor (5);
• two-wire power supply lines (3, 4) supplying electrical power to said motor at network frequency;
• an electrically conductive bit holder (6) driven by the motor;
• a motor controlled triac (15) connected for energization of the electric motor from said power lines;
• and
• a triac trigger for rendering said motor controlled triac conductive,
• in combination with a safety circuit for deenergizing the motor upon contact of a bit in the bit holder with an electrically conductive object (16), said safety circuit comprising
• a source of electrical energy coupled to at least one of the power line conductors, and having a frequency substantially elevated above power line frequency and a voltage substantially below power line voltage;
• capacitor means (9) dimensioned to have a high impedance at power line frequency and a low impedance at said elevated frequency, and applying the electrical energy from the electrical energy source to the bit holder (6);
• a current measuring circuit connected between the electrical energy source and the capacitor means (9);
• and
• a disconnect circuit connected to and controlled by the current measuring circuit and disconnecting energization to the motor when the current measuring circuit detects current at said elevated frequency above a predetermined level,
• said disconnect circuit including an oscillator circuit which serves both as said triac trigger and as the source of said electrical energy having said elevated frequency.