Thyratron

Thyratron

Thyratrons are fast acting high voltage switches suitable for a variety of applications including radar, laser and scientific use. Perkin Elmer’s thyratrons are constructed of ceramic and metal for strength and long life. Over 300 thyratron types are available from Perkin Elmer. The types listed in this guide are a cross section of the broad line available. We encourage inquiries for thyratrons to suit your particular application.

construction :


A thyratron ( a hot - cathode gas field triode) consists of the following components :

1. plate : It is a disc of graphite. 
2. Grid : It is a cylinder having diaphragm with one or more holes in it. The grid separates the cathode and the plate. 
3. Cathode : A special construction oxide coated cathode is used so that its surface is not damaged due to bombardment of positively charged particles.
                                                   Normally, a thyratron is filled with a small amount of inert gas such as argon, hydrogen, neon or mercury vapour. the characteristics of hydrogen thyratron are stable and the devices are fast. the mercury has the disadvantage that its characteristics vary with temperature and it takes time in deionization therefor the devices employing mercury vapour are not fast devices. Fig. a shows the construction of thyratron while in Fig. is shows the schematic representation, in which heavy dot indicates the presence of gas. The control grid surrounds the cathode and the plate and it is cylindrical in shape, a baffle or a series of baffle are inserted between plate and grid.
                                                   ( thyratron may be of the cold- cathode or hot- cathode types.)

Operation :

thyratron

  • When cathode of a thyratron is heated electron are emitted. The potential applied to the grid determines whether or not the electron floe to the positively charged plate.
  • Initially let the grid has a high negative potential while the plate has a steady positive potential. if the grid potential is gradually reduced, a point is reached when a very small thermionic plate current commences. further reduction in grid potential causes in plate current and a point is reached at which the glass ionization increases and enormous plate current flows. At this point, when the value is said to have fired, a hot cathode discharged appears, the grid losses control of plate current completely and the plate current is limited by external resistance in the plate circuit.
The negative grid potential for a given plate Fig.b schematic symbol for a thyratron. Potential at which the ionization starts is known as critical grid potential.

  • when the potential is increased, the critical grid potential increases as well. if the grid potential is more negative than the critical grid potential the thyratron  will fire. However, if the grid potential is lesser negative than the critical grid voltage, the thyratron will fire as soon as the plate potential is switched on.
  • when the thyratron fires, the gas gets ionized having equal number of positively charged particles and the electrons. The electron move towards the plate and increase the current. the positively charged particles move toward the cathode where the combine with the electrons and get neutralize ; if the grid is held negative, they gather around the grid making a sheath (of positively charged particles), the total charged of which is just sufficient to neutralized the grid potential. The thickness of this sheath will increase if the grid is made more negative and in case the grid is made less negative the thickness, reduces  as such the flow of electrons is not controlled not by the grid and consequently after the thyratron is fired losses control.
  • To stop the flow of current it is necessary to reduce the plate voltage below  the tube striking voltage, or to open the plate circuit, for a period long enough for deionization  of gas in the tube . The grid then regains control and again prevents the flow of plate current until the grid voltage is brought to the required value.     
Applications of thyratron :

The thyratron has the following field of applications.
  1. As an electric switch. the thyratron when as used as an electronic switch keeps the circuit open from the pate to cathode so long as the grid is kept at high negative potential. As soon as the grid potential is brought to such a value that the valve fires, the circuit is closed.
  2. As a grid controlled rectifier. A thyratron is used as a grid controlled rectifier when voltage control is required . Thyratron rectifiers are also  used in electronic methods of electric motor control for both small and large powers.
  3.  the small argon-filled thyratron is generally used in saw tooth sweep generator for t.v and radar equipment. 


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