Thermal emission The diode The triode How do triodes - PowerPoint PPT Presentation

Agenda A brief history • Thermal emission • The diode • The triode • How do triodes amplify? • RATS presentation 2013 Mike Maxey 23/03/2013 G8CTJ 1

In 1802, Humphrey Davy invents the electric lamp - 1  In 1875, American, G.R. Carey invents the phototube.  In 1878, Englishman Sir William Crookes invents the  Crookes tube - 2 … In 1895, German, Wilhelm Roengten invents an X-ray tube.  In 1897, German, Karl Ferdinand Braun invents the cathode  ray tube. - 5 In 1904, John Ambrose Fleming invents the first practical  valve called the 'Fleming Valve‘ / ‘ Kenotron ’. - 3 In 1906, Lee de Forest invents the Audion later called the  triode. - 4 RATS presentation 2013 Mike Maxey 23/03/2013 G8CTJ 2

RATS presentation 2013 Mike Maxey 23/03/2013 G8CTJ 3

 One of the most famous Crookes tubes.  Demonstrates that electrons go in a straight line and are stopped by metal, hence you see the shadow of the cross on the end glass.  After a while due to fatigue of the glass the glow is weaker RATS presentation 2013 Mike Maxey 23/03/2013 G8CTJ 4

RATS presentation 2013 Mike Maxey 23/03/2013 G8CTJ 5

RATS presentation 2013 Mike Maxey 23/03/2013 G8CTJ 6

Important discoveries stemming from the Crookes tube. Wilhelm Röntgen discovered X-rays using the Crookes tube in  1895. In 1897 J. J. Thomson identified “cathode rays” as negatively  charged particles, later renamed “electrons.” Now Edison …  RATS presentation 2013 Mike Maxey 23/03/2013 G8CTJ 7

 Edison was facing a problem with his light bulbs due to their short life.  Although the filament life was a problem, the major limiting factor was that the bulbs quickly became blackened.  Initially this was attributed to atoms of carbon from the filament hitting the glass.  Despite this … RATS presentation 2013 Mike Maxey 23/03/2013 G8CTJ 8

1880 Edison patent U.S. Patent #223898 issued to Thomas Edison on January 27, 1880 for an incandescent lamp Twenty-two other people also devised versions of the light bulb, Edison's outstrip the others because Effective incandescent material, • Higher vacuum • High resistance made power • distribution from a centralized source economically viable. Photos …. • RATS presentation 2013 Mike Maxey 23/03/2013 G8CTJ 9

 Very early Edison lamp  Not very practical  But some were attractive.. RATS presentation 2013 Mike Maxey 23/03/2013 G8CTJ 10

The early lamps were attractive but very inefficient Edison effect… RATS presentation 2013 Mike Maxey 23/03/2013 G8CTJ 11

Particles leaving the filament were known to be negatively  charged, so experiments were carried out to prevent them from hitting the glass. One method that Edison tried involved placing a second  electrode in the envelope. He reasoned that if he placed a positive charge on the second  electrode, particles would be attracted away from hitting the glass of the bulb. Edison experimented with the polarity of the charge on the second  electrode and he noticed that when the second element was made positive with respect to the filament a current flowed in the circuit. When the potentials were reversed this did not happen. This became known as the Edison effect. Used by Flemming …  RATS presentation 2013 Mike Maxey 23/03/2013 G8CTJ 12

 J.A. Fleming was a consultant to the Marconi  Coherers and Magnetic Detectors  In November 1904 whilst he was walking along Gower Street in London’s West End he had what he called “a very happy thought".  He wondered if the Edison Effect could be used to rectify the " feeble to and fro motions of electricity from an aerial wire".  Fleming set up an experiment and was able to prove that the idea worked . Patented in 1905… RATS presentation 2013 Mike Maxey 23/03/2013 G8CTJ 13

Flemmings Patent 1905 The Kenotron … RATS presentation 2013 Mike Maxey 23/03/2013 G8CTJ 14

 Patent original  Others not so attractive…. RATS presentation 2013 Mike Maxey 23/03/2013 G8CTJ 15

Actual hand blown Valves used in Flemming’s 1904 Experiments Then the first practical diode… RATS presentation 2013 Mike Maxey 23/03/2013 G8CTJ 16

Actual diode valve Used by Marconi in 1904 RATS presentation 2013 Mike Maxey 23/03/2013 G8CTJ 17

RATS presentation 2013 Mike Maxey 23/03/2013 G8CTJ 18

There are 3 types of electron Emission  Secondary – High velocity particles – X rays  Photo – Photons of light – photo cells  Thermionic / Thermal Emission… RATS presentation 2013 Mike Maxey 23/03/2013 G8CTJ 19

 At ambient temperature outer shell electrons have insufficient velocity to escape  As temperature increases the molecular vibration increases  Rotational velocity increases  Electrons “Boil” off into surrounding space… RATS presentation 2013 Mike Maxey 23/03/2013 G8CTJ 20

RATS presentation 2013 Mike Maxey 23/03/2013 G8CTJ 21

Material Normal Operating Efficiency in Temperature mA of emission °K per watt heater power Tungsten 2450 to 2600 3 to 15 Thoriated Tungsten 1900 62.5 Oxide coated 1100 to 1170 50 to 125 All materials developed to produce maximum electron flow… RATS presentation 2013 Mike Maxey 23/03/2013 G8CTJ 22

Two requirements for electron flow  Filament heated to correct temperature to achieve thermal emission  Positive anode potential  Require to plot curves… RATS presentation 2013 Mike Maxey 23/03/2013 G8CTJ 23

A dynamic curve must be  plotted to see how the diode works. This requires :- A Fixed heater voltage  A Variable Anode voltage  We get an Ia /Va curve…  RATS presentation 2013 Mike Maxey 23/03/2013 G8CTJ 24

From this curve we calculate the Ra…. RATS presentation 2013 Mike Maxey 23/03/2013 G8CTJ 25

RATS presentation 2013 Mike Maxey 23/03/2013 G8CTJ 26

 The workshop of Alfred Charles Cossor in Clerkenwell London  Latter to become a leading British valve manufacturer.  Photograph taken in 1896  Some of his workers (children!) preparing glass tubes. RATS presentation 2013 Mike Maxey 23/03/2013 G8CTJ 27

RATS presentation 2013 Mike Maxey 23/03/2013 G8CTJ 28

 Lee DE Forest  1906 works to improve diode  1908 patents at three electrode valve the Audion … RATS presentation 2013 Mike Maxey 23/03/2013 G8CTJ 29

 Early Audion 1906  Latter versions.. RATS presentation 2013 Mike Maxey 23/03/2013 G8CTJ 30

 The Audion valve 1908  The Grid… RATS presentation 2013 Mike Maxey 23/03/2013 G8CTJ 31

 Fitted between heater and anode… RATS presentation 2013 Mike Maxey 23/03/2013 G8CTJ 32

Electrically it looks like… RATS presentation 2013 Mike Maxey 23/03/2013 G8CTJ 33

Like the diode we need to plot some curves…. RATS presentation 2013 Mike Maxey 23/03/2013 G8CTJ 34

With this circuit we can plot the Ia/Va and Ia/Vg curves… RATS presentation 2013 Mike Maxey 23/03/2013 G8CTJ 35

RATS presentation 2013 Mike Maxey 23/03/2013 G8CTJ 36

RATS presentation 2013 Mike Maxey 23/03/2013 G8CTJ 37

 Input voltage  Output current RATS presentation 2013 Mike Maxey 23/03/2013 G8CTJ 38

 Input voltage  Output current RATS presentation 2013 Mike Maxey 23/03/2013 G8CTJ 39

 Input voltage  Output current  To output voltage RATS presentation 2013 Mike Maxey 23/03/2013 G8CTJ 40

 Directly heated – Battery powered sets  Indirectly heated – Mains powered sets  Directly heated specials – large rectifiers and transmitting valves RATS presentation 2013 Mike Maxey 23/03/2013 G8CTJ 41

 The R' valve (1922)  The first widely used triode  Note the thin wire filament suspended inside helical wire grid, inside tubular anode. RATS presentation 2013 Mike Maxey 23/03/2013 G8CTJ 42

Ediswan AR (1922)  Cossor P2 (1922)  Cosmos SP18 (1925)  The Cossor device used an  unusual 'hat' construction to avoid infringing patents which specified tubular electrodes. RATS presentation 2013 Mike Maxey 23/03/2013 G8CTJ 43