Thermionic emission 2 Flashcards
What is a triode?
A vacuum tube with three electrodes: a thermionically emitting cathode, an anode, and a control grid in between.
What is the function of the control grid in a triode?
The control grid acts as an electrostatic screen to control the flow of electrons from the cathode to the anode by applying a potential to it.
How does the grid potential affect electron flow in a triode?
When the grid potential is strongly negative with respect to the cathode, it repels electrons and no current flows. As the grid potential increases in the positive direction, more electrons can pass through and reach the anode.
What is the solid-state equivalent of the triode?
: The transistor.
What is the practical application of a triode in X-ray switching circuits?
The ability to control the current passing through the triode by altering the potential on the grid is useful for switching the valve on and off and for signal amplification.
What is a thyratron?
A thyratron is a gas-filled triode valve used as a high-power electrical switch and controlled rectifier, often filled with argon gas.
How does a thyratron permit the flow of current?
It permits the flow of current in one direction, controlled by a small DC control voltage connected between the grid and the cathode.
What is the function of the grid in a thyratron?
The grid controls the initiation of current flow, providing a trigger effect.
How does the grid potential affect electron flow in a thyratron?
A strongly negative grid potential prevents electron flow, while raising the grid potential allows electrons to flow from the cathode to the plate, causing a discharge.
What happens during the discharge in a thyratron?
Free electrons ionize the gas, leading to a rapid increase in current flow from the cathode to the plate, which can occur in a few hundred-millionths of a second.
How is a thyratron switched “OFF”?
By reducing the anode potential difference with respect to the cathode to almost zero, preventing gas multiplication.
What is the solid-state equivalent of the thyratron?
The thyristor.
How does the structure of the X-ray tube conform to a vacuum diode?
The X-ray tube has a cathode (thermionic emitter) placed close to a positively charged anode (collecting plate).
How does the X-ray tube function as a self-rectified circuit?
Due to its diode action, the X-ray tube can operate in the saturation region, allowing the mA (milliampere) and kVp (kilovolt peak) to be set independently.
