Voltage-regulator tube

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 have a cold cathode; the cathode is not heated with a filament to emit electrons.

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

• They rely on gas ionization, 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 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. 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.
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.