Wednesday, September 16, 2009

THE VACUUM DIODE EXPOSED, Lee Bond N7KC

September 16, 2009 Educational Radio Net, PSRG 69th Session

What a digital summer we experienced at the hands of Curt Black, WR5J! Curt pulled all the stops on a summers survey of free software downloads for your computer which, more or less, turned your radio into an analytical tool. As impressive as this tour was, one must ultimately realize that the most complex task that your computer performs is based on tiny packets of electrons or "charge" being directed here and there by the software commands. Elemental electrons in motion, or moving electric "charge", is the bottom line idea in electrical theory and all electrical devices including radio equipment.

Let's review what we know about "charge". We know that it enjoys the symbol Q in the literature. We also know that it is an assembly of electrons and can be as few a one electron. We know that a collection of electrons numbering 6.14 x 10^18 is known as a Coulomb of charge. We know that charge will move under the influence of an electric field. We know that the original notion, or conventional notion, of charge was based on the erroneous idea that charge carried a positive sign hence moved in the direction of an applied electric field. Modern theory has reversed the original positive charge idea since electrons are negative entities and, in fact, move counter to the direction of any applied electric field. We know that charge can be motionless as in static charge. We know that an electrical current is charge in motion. It would seem that we know a lot about charge.

The study of charge is best represented by the science of Physics with Physical Chemistry running a close second. The study of charge as it applies to the vacuum tube would come under the heading of Classical Newtonian Physics in contrast to the study of charge in semiconductor materials which would come under the heading of Modern Quantum Physics. In my view the vacuum tube represents the most elegant device for demonstrating the behavior of electrical charge influenced by an electric field. Lucky me to be raised in the heyday of the vacuum tube. As late as 1963 the US Navy destroyer to which I was assigned had a single piece of transmitting equipment that had modern solid state diodes in the circuitry. The selenium rectifier preceded germanium devices and modern silicon devices and required no heater but it is a stretch to include it as more than a rudimentary solid state device. The solid state transistor with all its ramifications is a relative newcomer to the field of electronics. Modern radio equipment's are solid state for the most part and only the old timers can relate stories of warming their hands in the glow of those magnificent glass bottles.

No one has actually seen an electron. These entities are very, very tiny and to image them requires wavelengths small in comparison to the size of an electron. The one common instance of, more or less, stationary electrons occurs in the lattice structure of many crystals and x-ray crystallography has demonstrated diffraction images suggesting that these things are real. The fact that we can manipulate these tiny guys to the degree that we can is testimony to the very clever work of early scientists.

We know that conductors have an abundance of so called "free" electrons. This is in contrast to tightly "bound" electrons which are not available to contribute to electric current flow. For example the neutral copper atom has 29 electrons associated with the nucleus in 4, so called, shells with the innermost shell containing 2 electrons followed by the next shell containing 8 electrons followed by the next shell containing 18 electrons followed by the outermost shell with a single, so called, valence electron. The three inner shells are tightly bound to the atomic nucleus but the outer single electron is easily forced out of place and can contribute to the electric current bumping along a copper wire. Most metals are conductors to varying degrees with silver, copper, and gold at the top. A copper atom missing its valence electron is known as a copper ion.

I think we now have enough information to appreciate how a two terminal vacuum diode works so let's move on to some apparatus to demonstrate the effect. Forget the little glass bottle for the moment. We are going to use a laboratory bell jar and good quality vacuum pump as a part of our apparatus. Everyone has seen the "bell" jar on a stand with a mechanical vacuum pump attached. For our purposes the bell jar stand needs some electrical penetrations so that we can supply potentials to the bell jar innards. The first of the two inside devices is the filament or heater which also serves as a cathode. There are several schemes for heaters so let me select the one known as the directly heated filament cathode. This will be a tungsten wire section which will glow a bright red to orange when filament voltage is applied. Adjacent to the filament-cathode wire we will position a "plate" of flat metal such that it does not touch the filament. This metallic plate is, in fact, called the "plate" electrode in vacuum tube terminology and serves as the anode. Note that the plate may be cylindrical and surround the filament in real world devices.

Our demonstration diode is complete. We have a filament (cathode) and plate (anode) plus a means of producing a good vacuum so on goes the bell jar but someone forgot to start the vacuum pump. Not realizing that a good vacuum is missing we switch on the filament voltage and sure enough the filament starts to light. Then there is a bright flash as the oxygen in the bell jar contributes to the destruction of the filament. Oops. Off with the jar and we install another filament wire. Ok, this time we turn on the pump and let it run until it chortles. Now when we flip the switch for the filament the wire glows a cheery orange. The chortling pump indicates that the internal vacuum (or pressure) is in the 1 to 10 micron range and suitable for our demonstration. The low internal pressure means that atoms of oxygen and nitrogen are scarce and will not interfere with the electronic process that we are interested in observing.

Let's think about the filament voltage for a moment. I did not mention it but the source of the filament voltage is a battery. Traditionally the battery used for filament power is known as the "A" battery.

So, here is our situation... we have a nicely glowing filament with an unconnected plate nearby and both are within a reasonably good vacuum. The glowing filament is probably heated to 800 degrees F or thereabouts and the thermal energy of the filament has caused lightly bound electrons to break free and form a cloud in the immediate vicinity of the filament-cathode. The thermal energy for breakaway is known as the work function and metals vary in this regard. Some substances such as barium offer very low work functions and are used in cooler, indirectly heated, cathode structures.

Our cloud of electrons is negatively charged. If we cause the nearby plate element to become positively charged with respect to the filament-cathode then the electrons will move toward the plate anode and, since moving electrons constitute electrical current, we can measure a plate current if we insert some current measuring device in series with the plate. With regard to the plate, the battery traditionally used to supply plate voltage is known as the "B" battery hence follows the term B+ for the plate positive voltage supply.

There is a third battery associated with vacuum tubes which is known as the "C" battery however it is not relevant here since we are using a two terminal device or diode and the C battery is only relevant in triode structures and beyond for grid biasing.

At this point we have demonstrated that electrons will traverse a vacuum if the plate is positive with respect to the source of electrons. If the plate is negative relative to the source then the electrons are repelled and no plate current will flow. Herein lies the secret of the rectifying diode. If the plate is alternately positive then negative with respect to the cathode as would happen if connected to alternating mains then plate current only flows on positive excursions of plate voltage. Bi-directional current from AC mains becomes unidirectional current in the plate circuit. A single diode offers half wave rectification and a dual diode (or two individual diodes) offers full wave rectification.

The high vacuum in the bell jar or little glass bottle performs two functions. First the pumping process removes virtually all oxygen so the filament suffers no oxidation. Secondly, the high vacuum is synonymous with low pressure both which equate to few residual gas particles present to hinder electron flow from cathode to plate anode. In reality small glass bottles with high internal temperatures will out gas damaging particles which will poison the vacuum so special devices known as "getters" are used internally to trap these vacuum destroyers.
Special attention must be given to the metal leads going through the little glass tube envelopes. If the glass and wire conductors do not expand and contract in the same manner with extreme changes in temperature then the seal will be broken and the tube will be rendered useless in short order. Special wire alloys which match the glass thermal characteristics are used to avoid this problem.

In summary, the physics and mathematics associated with the classic vacuum tube is elegant and a fun pursuit for the very curious. The concept of electronic charge flow within a triode vacuum tube is easily grasped and directly applicable to field effect transistors. The natural extension from diode to triode by introducing a control grid between cathode and plate made possible amplification and the rest is history.

This concludes the set up discussion for the Vacuum Diode. Are there any questions or comments with regard to tonight's discussion topic?

This is N7KC for the Wednesday night Educational Radio Net

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