A voltage-regulator tube (VR tube) is an electronic

Electronics

Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...

 component used as a shunt regulator to hold a voltage

Voltage

Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...

 constant at a pre-determined level.

Physically, these devices resemble vacuum tube

Vacuum tube

In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...

s, but there are two main differences:

• Their glass envelopes are filled with a gas
  Gas
  Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
  
   mixture, and
• They do not use a heater.

Electrically, these devices resemble Zener diode

Zener diode

A Zener diode is a special kind of diode which allows current to flow in the forward direction in the same manner as an ideal diode, but will also permit it to flow in the reverse direction when the voltage is above a certain value known as the breakdown voltage, "Zener knee voltage" or "Zener...

s, with the following major differences:

• They rely on gas ionization
  Ionization
  Ionization is the process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions. This is often confused with dissociation. A substance may dissociate without necessarily producing ions. As an example, the molecules of table sugar...
  
  , rather than Zener breakdown
• The unregulated supply voltage must be 15–20% above the nominal output voltage to ensure that the discharge starts
• The output can be higher than nominal if the current through the tube is too low.

When sufficient voltage is applied across the electrodes, the gas ionizes, forming a glow discharge

Electric glow discharge

An electric glow discharge is a plasma formed by the passage of current at 100 V to several kV through a gas, often argon or another noble gas. It is found in products such as neon lamps and plasma-screen televisions, and is used in plasma physics and analytical chemistry.-Basic operating...

 around the cathode

Cathode

A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...

 electrode. The VR tube then acts as a negative resistance

Negative resistance

Negative resistance is a property of some electric circuits where an increase in the current entering a port results in a decreased voltage across the same port. This is in contrast to a simple ohmic resistor, which exhibits an increase in voltage under the same conditions. Negative resistors are...

 device; as the current

Electric current

Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...

 through the device increases, the amount of ionization also increases, reducing the resistance

Electrical resistance

The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...

 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.

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

Resistor

A linear resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element.The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's...

 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

Electric arc

An electric arc is an electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive media such as air. A synonym is arc discharge. An arc discharge is characterized by a lower voltage than a glow discharge, and relies on...

 mode where the voltage will be significantly lower than nominal and the tube may be damaged.

Some voltage-regulator tubes contained small amounts of radionuclide

Radionuclide

A radionuclide is an atom with an unstable nucleus, which is a nucleus characterized by excess energy available to be imparted either to a newly created radiation particle within the nucleus or to an atomic electron. The radionuclide, in this process, undergoes radioactive decay, and emits gamma...

s to produce a more reliable ionization.

Specific models

VR tubes were given part numbers similar to those used for vacuum tubes. Because there was no heater (filament) in the tubes, the part numbers began with "0" (zero). VR tubes were only available in certain voltages. Common models were

Octal-based tubes, 5–40 mA

Ampere

The ampere , often shortened to amp, is the SI unit of electric current and is one of the seven SI base units. It is named after André-Marie Ampère , French mathematician and physicist, considered the father of electrodynamics...

 current:

• 0A3 – 75 volts
• 0B3 – 90 volts
• 0C3 – 105 volts (best regulation of these four)
• 0D3 – 150 volts

Miniature tubes, 5–30 mA current:

• 0A2 – 150 volts
• 0B2 – 108 volts (best regulation of these three)
• 0C2 – 72 volts

Miniature tubes, 1–10 mA current:

• 85A2 – 85 volts(equivalents:CV449,CV4048,QS1209,QS83/3 )

Voltage reference 1.5–3.0 mA current:

• 5651 – 87 volts (the most popular voltage reference ever made)
• 5651-a – 85.5 volts

Subminiature tubes:

• Various models that resembled neon lamp
  Neon lamp
  A neon lamp is a miniature gas discharge lamp that typically contains neon gas at a low pressure in a glass capsule. Only a thin region adjacent to the electrodes glows in these lamps, which distinguishes them from the much longer and brighter neon tubes used for signage...
  
  s, but were optimized for more-accurate voltage regulation

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

Capacitor

A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...

 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

Relaxation oscillator

A relaxation oscillator is an oscillator based upon the behavior of a physical system's return to equilibrium after being disturbed. That is, a dynamical system within the oscillator continuously dissipates its internal energy...

, 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

Solid state (electronics)

Solid-state electronics are those circuits or devices built entirely from solid materials and in which the electrons, or other charge carriers, are confined entirely within the solid material...

 regulators based on Zener diode

Zener diode

A Zener diode is a special kind of diode which allows current to flow in the forward direction in the same manner as an ideal diode, but will also permit it to flow in the reverse direction when the voltage is above a certain value known as the breakdown voltage, "Zener knee voltage" or "Zener...

s and avalanche breakdown diode

Avalanche diode

In electronics, an avalanche diode is a diode that is designed to go through avalanche breakdown at a specified reverse bias voltage. The junction of an avalanche diode is designed to prevent current concentration at hot spots, so that the diode is undamaged by the breakdown...

s.

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.