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Avoltage-regulator tube (VR tube) is anelectronic component used as ashunt regulator to hold avoltage constant at a predetermined level.
Physically, these devices resemblevacuum tubes, but there are two main differences:
Electrically, these devices resembleZener diodes, with the following major differences:
When sufficient voltage is applied across the electrodes, the gasionizes, forming aglow discharge around thecathode electrode. The VR tube then acts as anegative resistance device; as thecurrent through the device increases, the amount of ionization also increases, reducing theresistance 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 externalresistor 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 anarc discharge mode where the voltage will be significantly lower than nominal and the tube may be damaged.
Some voltage-regulator tubes contained small amounts ofradionuclides to produce a more reliable ionization.[1]
TheCorona VR tube is a high-voltage version that is filled withhydrogen at close to atmospheric pressure, and is designed for voltages ranging from 400 V to 30 kV at tens of microamperes. It has acoaxial form; the outer cylindrical electrode is the cathode and the inner one is the anode. The voltage stability depends on the gas pressure.
A successful hydrogen voltage regulator tube, from 1925, was theRaytheon tube, which allowed radios of the time to be operated from AC power instead of batteries.
In America, VR tubes were givenRETMA 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 theprofessional system ("ZZ1xxx") and under adedicated 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, 5–40mA current:[3]
Miniature tubes, 5–30 mA current:
Miniature tubes, 1–10 mA current:
Voltage reference 1.5–3.0 mA current:
Subminiature tubes:
Miniature corona tubes, 5–55 μA current:
Wire-ended subminiature corona tubes:
Some voltage regulator tubes have an internal jumper connected between two of the pins. This jumper could be used in series with the secondary transformer winding. Then, if the tube was removed, rather than leaving the voltage unregulated, the output would turn off.
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 can be easily filtered out by placing a smallcapacitor in parallel with the VR tube or using an RC decoupling network downstream of the VR tube. Too large a capacitance (>0.1 μF for an 0D3, for instance), however, and the circuit will form arelaxation oscillator, definitely ruining the voltage regulation and possibly causing the tube to fail catastrophically.
VR tubes can be operated inseries for greater voltage ranges. They cannot be operated inparallel: because of manufacturing variations, the current would not be shared equally among several tubes in parallel. (Zener diodes show similar behavior when connected in series and parallel.)
In the present day, VR tubes have been almost-entirely supplanted bysolid-state regulators based onZener diodes andavalanche breakdown diodes.
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.
