Cold-Cathode Thyratron

Cold-Cathode ThyratronYou have definitely heard about such a device as thyratron... Let's try to understand what it is.Thyratron is a gas-discharge rectifier with one or more supplementary electrode grids. The beginning of a current flow through the thyratron is regulated by potentials applied to these grids. Thyratron is a kind of highly sensitive relays that can be switched on (lit), but can not be turned off without de-energizing the anode.There are two basic types of thyratrons: cold-cathode and glow-cathode ones. We'll dwell on cold-cathode thyratrons.Design features, technical parameters and specifications determine the following advantages of cold-cathode thyratrons: high efficiency, simplicity of design, small size and weight, ability to conduct high currents in pulsed mode, relative stability of characteristics, durability, high vibration resistance, wide operating temperature range and low starting currents at high input resistance.We'd like to specify a few features of these devices:The role of the control grid of a thyratron differs significantly from its role in a usual three-electrode tube. In the triode grid the value of the anode current can be changed by voltage at any point of time, and in the thyratron the grid voltage can be controlled only at the moment of discharge ignition. After the moment of discharge ignition the grid loses control action, and current passes through the device as if there was no gridIgnition control in a cold-cathode thyratron may be carried out by two methods: current (current change in the control electrode) and potential (potential change at the control electrode).Current control is characteristic of triode-thyratrons with one control electrode (grid)Potential control is characteristic of tetrode-thyratrons with two control electrodes (grids). In the two-grid thyratrons the first grid provides the trigger spark facilitating and stabilizing the process of discharge in the anode circuit. The current limited with series resistance is constantly flowing through the grid. The second grid is used to control the ignition voltage at a certain constant voltage at the anode. A positive bias voltage is applied to it. This voltage is less relative to the cathode than the voltage at the first grid, but insufficient to unlock the thyratron. In order to unlock the thyratron a positive start pulse of sufficient amplitude and duration is supplied to the second grid through a coupling capacitor. Thus, a simplest pulse-length modulator is obtained. As compared with one-grid thyratrons, two-grid thyratrons have more stable control mode and ignition characteristics.Where do they use these devices?Cold-cathode thyratrons (glow discharge thyratrons) are low-power control devices and are designed to be used in ladders, electronic pulse equipment, computers, relaxation oscillators and other low-power devices. For example, in a number of TV sets produced in our country the glow-discharge thyratron TH4B is used as a vertical scanning master oscillator.

Category:

Science & Technology

Tags:

• anode
• cathode
• circuitry
• discharge
• vacuum tube

[radumotisan]

good explanations, thank you.