The grid loses its control as soon as the electrons begin to flow, and the only way to stop the flow is to reduce the anode voltage to a value that no longer attracts the electrons.
thyrector (electronic component). A special type of semiconductor device used to protect a piece of electronic equipment from being damaged by spikes of voltage. A thyrector contains two zener diodes, connected together back to back. When the circuit voltage is below that for which the diodes are rated, no current flows through them. But if a spike of voltage occurs in the circuit, one of the diodes breaks down and conducts the spike to ground. Since the zener diodes are connected back to back, the thyrector protects against spikes of voltage of either polarity.
thyristor (electronic component). A semiconductor device used as an electrically controlled switch. Two of the most widely used thyristors are silicon controlled rectifiers (SCRs) and triacs. An SCR is a semiconductor device that does not normally allow electrons to flow through it in either direction. However, if a pulse of voltage of the correct polarity is applied to a third electrode, the gate, the SCR conducts in its forward direction.
The SCR continues to conduct until the voltage across it drops to zero. Once electrons stop flowing through it, the SCR remains shut off until another pulse of voltage is applied to its gate. The triac begins to conduct when a pulse of voltage of either polarity is applied to its gate. Once conducting, it can be turned off only by reducing the voltage across it to a value low enough to stop the current flowing through it.
tickler coil (electronic component). A small coil in series with the collector of a transistor in an oscillator circuit. The tickler coil is part of a feedback circuit. Current flowing through the tickler coil induces a voltage in an inductor in the base circuit.
tie rod (structural member). A high-strength steel rod used to tie parts of a structure together. Tie rods carry only tensile loads.
tight-drive fit. An interference fit between mechanical parts in which the parts can be assembled only by driving them together with a sharp blow from a 12- to 14-ounce hammer.
TIG (tungsten inert gas) welding. A form of electric arc welding in which the electrode is a nonconsumable tungsten wire. TIG welding is now called GTA (gas tungsten arc) welding.
time constant (capacitive circuit value). The amount of time, measured in seconds, needed for the voltage across a capacitor to rise to 63.2% of the voltage applied to the circuit. Time constant is determined by the amount of both resistance and capacitance in the circuit.
time constant (inductive circuit value). The amount of time, measured in seconds, needed for the current flowing through an inductive circuit to reach 63.2% of its final value. Time constant is determined by the amount of both resistance and inductance in the circuit.
time constant of an electric motor. The amount of time, measured in seconds, needed for an electric motor to accelerate from standstill to its final no-load speed. Time constant is measured when the motor is connected to a power source having the correct voltage, phase, and frequency.
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