GAS FILLED TUBES

INTRODUCTION 

A Gas filled tube is essentially a vacuum tube filled with small amount of inert gas, these gases includes Argon, Neon etc. Like any vacuum tube it has a Cathode, a Anode, a Heater (present in Hot Cathode Gas Tube) and a Grid (depending on the type of gas tube, whether it is a triode or diode).

THE BASIC DIFFERENCE BETWEEN A GAS TUBE AND A VACUUM TUBE IS  THAT IN A GAS TUBE DUE TO THE PRESENCE OF THE INERT GAS, WHEN THE IONIZATION STARTS IT IS RELATIVELY DIFFICULT TO CONTROL THE RATE OF IONIZATION AND DUE TO THIS IT OFFERS MORE CURRENT FLOW COMPARED TO VACUUM TUBE.

EVERY TYPE OF GAS OR VACUUM TUBES GLOWS DEPENDING ON THE VOLTAGE ACROSS THE ELECTRON PATH AND THE  INERT GAS  IN TUBE OR COATING  ON GLASS. FOR EXAMPLE WHEN THE HIGHLY ENERGIZED ELECTRONS HITS THE PHOSPHORESCENT COATING, IT WOULD CAUSE IT TO GLOW.

CHEAT 1

Question:
I assume you all have seen a tube light, can you identify just by seeing which end is cathode and which end is anode ?

Answer:

Its simple, switch off the tube light and while switching it on, watch which parts of the tube glows first. you might find it's the end of the tube, Yes, this is where the cathodes are located. But where are the anodes, well the electrodes present at the ends acts both as cathode and anode. The explanation will be posted on the working of a fluorescent lamp.

1.0  APPLICATION OF GAS TUBES

• It can be used for voltage regulation.
• It can be used as an electronic switch.

1.1  TYPES OF GAS TUBES

  The gas tubes can be classified as

• Cold Cathode Gas diode
• Hot Cathode Gas diode
• Hot Cathode Gas triode
• Cold Cathode Gas triode

As the name suggest whenever  "Hot" word is used with "Cathode", it signifies, the gas tube uses a heater to energize cathode. And "Cold" would signify that there is no heater and the cathode is solely heated by natural elements, this includes cosmic rays, sun rays, cold gas tube placed near a  heated body etc.

CHEAT 2

Question:
Diode/Triode/Tetrode/Pentode.... How to you simplify these things as a beginner ?

Answer:
Simple... Ode means PATH in Greek... So
DIODE= 2 PATHS
TRIODE= 3 PATHS
TETRODE= 4 PATHS
PENTODE= 5 PATHS

ALL I HAVE TO SAY, JUST REMEMBER THIS, THAT THERE IS JUST ONE CATHODE AND ONE ANODE THUS WE HAVE 2 PATHS FIXED, SO THE EXTRA PATHS THAT ACCOMPANIES ARE THE GRIDS, THESE ARE USED TO ENHANCE THE WORKING OF THE TUBES.

DIODE= (CATHODE+ANODE)
TRIODE= (CATHODE+CONTROL GRID+ANODE)
TETRODE= (CATHODE+CONTROL GRID+SCREEN+ANODE)
PENTODE= (CATHODE+CONTROL GRID+SCREEN+SUPPRESSOR+ANODE)

2.0  COLD CATHODE GAS DIODE SYMBOL

The symbol of Cold Cathode gas diode has been shown. It is to be noted.

1- Cathode
2- Inert Gas Mark
3- Anode

THE DOT WITHIN THE CIRCLE INDICATES THE PRESENCE OF GAS

2.1 COLD CATHODE GAS DIODE WORKING PRINCIPLE

IT IS TO BE NOTED THAT THE ANODE IS ALWAYS AT A HIGHER POTENTIAL THAN ANODE. 

The working of cold cathode gas diode is divided into three stages.

FIRST STAGE: Assume initially there is a free electron in the gas tube that may have been generated by some external effect like due to radioactivity then when an electric field is applied such that the anode is at higher potential than cathode. If the electric field is strong enough to ionize a single gas molecule by this free electron, it will collide with the gas molecule producing two free electrons and a positive ion and this two electrons will further cause collision with other gas molecules to form four free electrons and three positive ions. In this way there is a increase of free electrons due to this cumulative effect, However the amount of electrons generated is very less compared to hot gas tube (which facilitates significant amount of free electrons by using a heater to energize the cathode) and the current produced in this stage is very small (1mA). This process as a whole can be summarized as, due to a low applied voltage which is sufficient to ionize a single gas molecule, which would result in  increase in current due to thickening of electronic path, this is also called TOWNSEND DISCHARGE.
IMPORTANT THING TO NOTE IN THIS STAGE IS THAT THE GLASS TUBE RESISTANCE REMAINS ALMOST CONSTANT AND THE CURRENT PRODUCED IS EXTREMELY SMALL ABOUT (1mA) AS THE CATHODE IS COLD.

SECOND STAGE: In this stage as the voltage across the diode is increased, more free electrons  produced near the cathode (by Townsend discharge) acquire more speed and energy, thus causing more ionization and more positive ions. These positive ions nullifies the space charge (free electrons around the cathode) thereby allows more space for free electrons formation, this causes the ionization to occur at a voltage lower than the previous one i.e. at this applied voltage, breakdown of the dielectric medium (gas) takes place and there is a burst of electrons emitted from cathode and this voltage is called the IONIZATION VOLTAGE and it is lower than the previous operating voltage. At the same time GLOW is seen near the cathode, This stage is called GLOW DISCHARGE.

THE IMPORTANT CHARACTERISTICS OF THIS STAGE IS THAT THE VOLTAGE DROP ACROSS THE DIODE ALMOST REMAINS CONSTANT I.e. IF I WERE TO USE THIS DIODE AS A VOLTAGE REGULATOR, I WOULD BE OPERATING THE DIODE IN THIS STAGE.

The reason behind the voltage drop remaining constant is that, we know that resistance of a path is inversely proportional to its cross-sectional area. So as the ionization voltage is reached there is a burst of electrons emitted from cathode which thickens the current conductive path from anode to cathode, thus as the cross-sectional area of the conductive path increases the tube resistance decreases and current increases keeping the voltage drop constant.

ANOTHER IMPORTANT CHARACTERISTICS OF THIS STAGE IS THE POINT FROM WHICH GLOWING STARTS I.e. SINCE THE CURRENT BEFORE THIS STAGE IS VERY LOW THUS IF THE DIODE IS KEPT AT A LOWER VOLTAGE THAN THAT OF GLOW DISCHARGE VOLTAGE AND IF BY PROVIDING SHORT PULSE THE DIODE CAN BE BROUGHT TO GLOW STATE THEN THE DIODE CAN ACT AS A SWITCH, THUS A SWITCHING DIODE.

THIRD STAGE: This stage is also called the ARC DISCHARGE stage. In this stage the conductive path covers the entire tube space and thus the resistance cannot decrease any further and the current keeps on increasing, increasing the voltage drop and at this high voltage electrons travel rapidly from cathode to anode creating an arc. This voltage at which this occurs is also called  BREAKDOWN VOLTAGE.

Below is the graph showing the discharge characteristics

1- Townsend Discharge
2- Glow Discharge
3- Arc Discharge

3.0 HOT CATHODE GAS DIODE SYMBOL

The symbol of Cold Cathode gas diode has been shown. It is to be noted.

1- Anode/Plate 

2- Inert Gas Mark

3- Cathode

4- Heater

THE DOT WITHIN THE CIRCLE INDICATES THE PRESENCE OF GAS

3.1 HOT CATHODE GAS DIODE WORKING PRINCIPLE

1- Heater
2- Cathode
3- Anode
4- Inert gas

The arrangement is similar to that of a cold cathode diode but the difference lies in the addition of heater. Initially when the voltage applied between the electrodes is low, and simultaneously heating of cathode takes place then a small current flows (since the electron conduction cross-section path is small thus for initial low voltages and up to the point before breakdown voltage of the gas the resistance of the tube remains low). As the voltage increases to that of the breakdown voltage of the gas, at that instant the current rises sharply but the voltage remains constant due to the fact that resistance of the tube decreases simultaneously. Any voltage above the breakdown voltage is useless as the resistance cannot decrease any further as the electron path/glow covers the whole region of the tube and thus as the current increases, the voltage increases.

Below is the graph showing the discharge characteristics

 

 

 

 

 

4.0 COLD/HOT CATHODE TRIODE SYMBOLS

1- Cathode
2- Inert gas mark
3- Anode
4- Grid

 
1- Anode
2- Inert gas mark
3- Cathode
4- Grid
5- Heater

  

4.1 HOT/COLD CATHODE TRIODE WORKING PRINCIPLE

The process, how electrons are emitted and how the discharge  takes place is same as before for cold and hot cathode tube. The new addition of third path a.k.a control grid provides control over the voltage across the tube needed to ionize the gas. The control grid is close to cathode, thus if the grid voltage is made higher with respect to cathode, then for the less distance that the electrons have to travel from cathode to control grid, it will cause the electrons to gain energy quickly(quick acceleration) and it will start bombarding with the gas molecules. Now for a certain grid voltage known as CRITICAL GRID VOLTAGE ionization of gas occurs and this spreads quickly to other end of the anode and cause current to flow. Thus the function of the control grid is to start the current across the ends, once the current flow has started, the tube acts like a normal diode as before.