Voltage-regulator tube

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Schematic representation of a cold cathode voltage-regulator tube Ccvrt.svg
Schematic representation of a cold cathode voltage-regulator tube
5651 Regulator tube in operation 5651RegulatorTubeInOperation.jpg
5651 Regulator tube in operation

A voltage-regulator tube (VR tube) is an electronic component used as a shunt regulator to hold a voltage constant at a pre-determined level.

Electronics physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter

Electronics comprises the physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter. The identification of the electron in 1897, along with the invention of the vacuum tube, which could amplify and rectify small electrical signals, inaugurated the field of electronics and the electron age.

Voltage difference in the electric potential between two points in space

Voltage, electric potential difference, electric pressure or electric tension is the difference in electric potential between two points. The difference in electric potential between two points in a static electric field is defined as the work needed per unit of charge to move a test charge between the two points. In the International System of Units, the derived unit for voltage is named volt. In SI units, work per unit charge is expressed as joules per coulomb, where 1 volt = 1 joule per 1 coulomb. The official SI definition for volt uses power and current, where 1 volt = 1 watt per 1 ampere. This definition is equivalent to the more commonly used 'joules per coulomb'. Voltage or electric potential difference is denoted symbolically by V, but more often simply as V, for instance in the context of Ohm's or Kirchhoff's circuit laws.


Physically, these devices resemble vacuum tubes, but there are two main differences:

Vacuum tube Device that controls electric current between electrodes in an evacuated container

In electronics, a vacuum tube, an electron tube, or valve or, colloquially, a tube, is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied.

Gas gaseous object

Gas is one of the four fundamental states of matter. A pure gas may be made up of individual atoms, elemental molecules made from one type of atom, or compound molecules made from a variety of atoms. A gas mixture would contain a variety of pure gases much like the air. What distinguishes a gas from liquids and solids is the vast separation of the individual gas particles. This separation usually makes a colorless gas invisible to the human observer. The interaction of gas particles in the presence of electric and gravitational fields are considered negligible, as indicated by the constant velocity vectors in the image.

Cold cathode Type of electrode and part of cold cathode fluorescent lamp.

A cold cathode is a cathode that is not electrically heated by a filament. A cathode may be considered "cold" if it emits more electrons than can be supplied by thermionic emission alone. It is used in gas-discharge lamps, such as neon lamps, discharge tubes, and some types of vacuum tube. The other type of cathode is a hot cathode, which is heated by electric current passing through a filament. A cold cathode does not necessarily operate at a low temperature: it is often heated to its operating temperature by other methods, such as the current passing from the cathode into the gas.

A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic CCD for Cathode Current Departs. A conventional current describes the direction in which positive charges move. Electrons have a negative electrical charge, so the movement of electrons is opposite to that of the conventional current flow. Consequently, the mnemonic cathode current departs also means that electrons flow into the device's cathode from the external circuit.

Electrically, these devices resemble Zener diodes, with the following major differences:

Zener diode diode that allows current to flow in the reverse direction

A Zener diode is a type of diode that allows current to flow not only from its anode to its cathode, but also in the reverse direction, when the Zener voltage is reached.

Ionization or ionisation, is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecule is called an ion. Ionization can result from the loss of an electron after collisions with subatomic particles, collisions with other atoms, molecules and ions, or through the interaction with electromagnetic radiation. Heterolytic bond cleavage and heterolytic substitution reactions can result in the formation of ion pairs. Ionization can occur through radioactive decay by the internal conversion process, in which an excited nucleus transfers its energy to one of the inner-shell electrons causing it to be ejected.

When sufficient voltage is applied across the electrodes, the gas ionizes, forming a glow discharge around the cathode electrode. The VR tube then acts as a negative resistance device; as the current through the device increases, the amount of ionization also increases, reducing the resistance of the device to further current flow. In this way, the device conducts sufficient current to hold the voltage across its terminals to the desired value.

Negative resistance the property that an increasing voltage results in a decreasing current

In electronics, negative resistance (NR) is a property of some electrical circuits and devices in which an increase in voltage across the device's terminals results in a decrease in electric current through it.

Electric current flow of electric charge

An electric current is the rate of flow of electric charge past a point or region. An electric current is said to exist when there is a net flow of electric charge through a region. In electric circuits this charge is often carried by electrons moving through a wire. It can also be carried by ions in an electrolyte, or by both ions and electrons such as in an ionized gas (plasma).

Because the device would conduct a nearly unlimited amount of current, there must be some external means of limiting the current. Usually, this is provided by an external resistor upstream from the VR tube. The VR tube then conducts any portion of the current that does not flow into the downstream load, maintaining an approximately constant voltage across the VR tube's electrodes. The VR tube's regulation voltage was only guaranteed when conducting an amount of current within the allowable range. In particular, if the current through the tube is too low to maintain ionization, the output voltage can rise above the nominal output—as far as the input supply voltage. If the current through the tube is too high, it can enter an arc discharge mode where the voltage will be significantly lower than nominal and the tube may be damaged.

Some voltage-regulator tubes contained small amounts of radionuclides to produce a more reliable ionization. [1]

The Corona VR tube is a high-voltage version that's hydrogen-filled at close to atmospheric pressure, for voltages ranging from 400 V to 30 kV at tens of microamperes. It has a coaxial form; the outer cylindrical electrode is the cathode and the inner one is the anode. The voltage stability depends on the gas pressure.

Specific models

In America, VR tubes were given RETMA tube part numbers. Lacking a heater (filament), the tube's part numbers began with "0" (zero).

In Europe, VR tubes were given part numbers under the professional system ("ZZ1xxx") and under a dedicated system.

In USSR, glow-discharge stabilitrons were given designation in Cyrillic with serial number of development. [2] For example, "СГ21Б", "СГ204К" and i.e. [2]

VR tubes were only available in certain voltages. Common models were:

Octal-based tubes, 5–40 mA current: [3]

Miniature tubes, 5–30 mA current:

Miniature tubes, 1–10 mA current:

Voltage reference 1.5–3.0 mA current:

  • 5651A – 85.5 volts

Subminiature tubes:

Miniature corona tubes, 5–55 µA current:

Wire-ended, subminiature corona tubes:

Design considerations

The pinout of VR tubes was designed so that power could be forced to flow "through" the VR tubes to the load. That is, the load current would flow in one pin of the VR tube and out through a second pin. In this way, the circuit could be arranged so that unplugging the VR tube would disconnect the load. (Otherwise, unplugging the VR tube would have allowed the voltage to become unregulated, possibly rising as high as the source voltage and damaging the downstream load.)

Because the glow discharge is a "statistical" process, a certain amount of electrical noise is introduced into the regulated voltage as the level of ionization varies. In most cases, this could be easily filtered out by placing a small capacitor in parallel with the VR tube or using an RC decoupling network downstream of the VR tube. Too large a capacitance (>0.1μF for an 0D3, for instance), however, and the circuit will form a relaxation oscillator, definitely ruining the voltage regulation and possibly causing the tube to fail catastrophically.

VR tubes could be operated in series for greater voltage ranges. They could not be operated in parallel: because of manufacturing variations, the current would not be shared equally among several tubes in parallel. (Note the equivalent behavior with series and parallel connected Zener diodes.)

Presently, VR tubes have been almost-entirely supplanted by solid state regulators based on Zener diodes and avalanche breakdown diodes.

VR tube information

Correctly operating VR tubes glow during normal operation. The color of the glow varies depending upon the gas mixture used to fill the tubes.

Though they lack a heater, VR tubes often do become warm during operation due to the current and voltage drop through them.

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