Voltage regulator tube
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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.
Physically, these devices resemble vacuum tubes, but there are two main differences:
- Their glass envelopes are filled with a gas mixture, and
- They do not use a heater.
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.
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.
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 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 — 75 volts
- 0B2 — 90 volts
- 0C2 — 105 volts (best regulation of these three)
Subminiature tubes:
- Various models that resembled neon lamps, but were optimized for more-accurate voltage regulation
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.
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.
[edit] VR tube trivia
Correctly-operating VR tubes glowed during normal operation. The color of the glow varied 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; in working around them, assume their envelopes will be hot.
[edit] See also
- Corona regulator tubes
