0A2WA — Cold-Cathode Gas-Filled Voltage Regulator
The 0A2WA (also written as OA2WA or OA 2 WA) is a Special Quality, cold-cathode, gas-filled diode designed for highly reliable voltage stabilization. Developed for the French "Service Technique des Télécommunications de l'AIR" and manufactured by Société Française Radio-Électrique (which later merged with C.S.F.), this tube provides a substantially constant 150 V output over a wide operating current range. It is engineered to withstand severe environmental conditions, including high temperatures, mechanical shock, and intense vibration, making it ideal for stabilized power supplies and high-voltage bias sources in mobile and industrial electronics.
Mechanical Design, Base, and Pinout
The 0A2WA is constructed with a robust glass envelope (T-5½ bulb, outline A 19-4 UTE) and a miniature 7-pin button base (B7G / 7C10). The tube has a net weight of approximately 10.5 grams and can be mounted in any position.
Physical Dimensions
- Maximum Diameter: 19.0 mm
- Height (excluding pins): 50.8 ± 2.3 mm
- Maximum Seated Height: 60.1 mm
- Maximum Overall Length: 66.7 mm
- Pin Diameter: 1.02 ± 0.05 mm (0.040 inches nominal)
- Pin Circle Diameter: 9.53 mm to 11.1 mm
Pinout Configuration
The 7-pin miniature base utilizes the following connections (viewed from the bottom):
The dual anode (pins 1 and 5) and triple cathode (pins 2, 4, and 7) connections provide a built-in safety feature. By wiring the load through the redundant pins, removing the tube from its socket automatically breaks the circuit and disconnects the high voltage from the load, protecting downstream components.
Electrical Characteristics and Operating Limits
The 0A2WA operates as a single-gap shunt regulator. It requires a minimum supply voltage to initiate ionization (striking), after which it maintains a stable voltage drop across its terminals.
Absolute Maximum Ratings
- Cathode Current (Continuous): 5.0 mA minimum to 30 mA maximum
- Starting Current (Transient): 75 mA maximum (for a duration not exceeding 10 seconds)
- Inverse Anode Voltage: 125 V maximum
- Shunt Capacitance: 0.1 µF maximum
- Bulb Temperature: -55 °C minimum to +150 °C maximum
- Ambient Temperature: -55 °C minimum to +90 °C maximum
- Altitude (Unpressurized): 120,000 feet (36 km) maximum
Ionization and Operating Voltages
To ensure reliable starting in both ambient light (50 to 500 lux) and total darkness, the minimum supply voltage must be at least 165 Vdc. Under typical conditions, the ionization voltage averages 152 Vdc in ambient light and 156 Vdc in total darkness. Once ionized, the nominal regulated voltage is approximately 150 Vdc (typically 149 Vdc, with an allowable range of 140 Vdc to 158 Vdc depending on current and tube age).
Quality Control and Typical Performance
Under MIL-E-1 quality control testing, the 0A2WA exhibits highly stable electrical characteristics. Typical performance parameters for new tubes include:
- Tube Voltage Drop (at 5 mA): 144 Vdc minimum, 149.0 Vdc average, 153 Vdc maximum
- Tube Voltage Drop (at 20 mA): 144 Vdc minimum, 149.8 Vdc average, 153 Vdc maximum
- Tube Voltage Drop (at 30 mA): 144 Vdc minimum, 150.6 Vdc average, 153 Vdc maximum
- Regulation (5 to 30 mA): 5.0 Vdc maximum
- Voltage Jump (5 to 30 mA): <100 mVdc average, 600 mVdc maximum (measured by slowly cycling the current)
- Noise Output (at 30 mA, 500 Ω load): <1.0 mVac average, 5.0 mVac maximum (50 to 5000 Hz)
- Leakage Current (at 50 Vdc, 3000 Ω series resistance): <1 µAdc average, 5 µAdc maximum
- Repeatability (at 10 mA, 1 min on / 1 min off): 300 mVdc average, 600 mVdc maximum shift between firings
Long-Term Stability and Degradation
The 0A2WA is designed for extended service life, but its characteristics gradually shift over time. Equipment designers must account for these end-of-life and aging limits:
A 1-hour stability life test at room temperature with a 20 mA operating current permits a maximum individual change in voltage drop of 2.0 Vdc. Over a 100-hour survival test, this change is limited to 5.0 Vdc.
Circuit Design and Operating Guidelines
To achieve stable regulation and prevent premature failure, several circuit design guidelines must be observed:
Series Resistor Selection
A series resistor must always be placed between the unregulated supply voltage and the 0A2WA anode. The resistor value must satisfy three conditions:
- It must allow the anode voltage to rise above the 165 V striking threshold before ionization occurs, even at the minimum supply voltage and maximum load current.
- It must limit the continuous tube current to a maximum of 30 mA when the supply voltage is at its maximum and the load current is at its minimum.
- It must maintain at least 5 mA of tube current when the supply voltage is at its minimum and the load current is at its maximum.
Shunt Capacitance and Oscillation
The total capacitance in parallel with the 0A2WA must not exceed 0.1 µF. Because gas-discharge tubes exhibit negative resistance characteristics, any capacitance above this limit can form a relaxation oscillator, causing severe voltage instability and output noise.
Transient and Thermal Behavior
- Starting Current: The tube can tolerate a brief turn-on current spike of up to 75 mA for up to 10 seconds. However, passing this high current alters the gas equilibrium; it can take up to 20 minutes of normal operation for the tube to return to its nominal regulated voltage.
- Warm-up Period: A minimum warm-up of 3 minutes is recommended after power-on to allow the bulb temperature to stabilize and minimize initial voltage drift.
- Current History and Rest Effects: The exact regulated voltage is slightly dependent on the tube's recent current history and can shift slightly after a long period of rest.
- Storage: Storing the tube at high ambient temperatures without drawing current can cause marked deterioration of its electrical characteristics.
Parallel and Series Configurations
- Parallel Operation: Placing multiple 0A2WA tubes in parallel to handle higher load currents is generally discouraged. It requires a series resistor of approximately 100 ohms in series with each individual tube to force current sharing. This significantly degrades the overall regulation efficiency and can lead to ignition failures.
- Series Operation: Two 0A2WA tubes can be connected in series to form a stable 300 V supply, which also provides a convenient 150 V intermediate tap. Removing either tube from its socket safely disconnects the load from the supply.
Environmental and Ruggedness Ratings
The 0A2WA is built to withstand extreme physical stress in military and industrial environments. These ratings are verified through rigorous quality control testing:
- Shock Resistance: Capable of withstanding impact accelerations up to 900 G. This is evaluated using five successive blows of 450 G applied in four directions along three perpendicular axes, or via standard hammer impact tests.
- Vibration Resistance: Designed to withstand continuous vibration fatigue of 2.5 G peak acceleration. This is tested by applying sinusoidal vibrations at 25 Hz or 50 Hz for up to 96 hours across three perpendicular axes.
Technical Summary
The 0A2WA is a ruggedized, cold-cathode, gas-filled voltage stabilizer designed to provide a constant 150 V reference. Operating over a current range of 5 to 30 mA, it features a miniature 7-pin base with redundant pin connections that act as an automatic load-disconnect safety switch. With its high resistance to mechanical shock (up to 900 G) and vibration (up to 2.5 G), the 0A2WA is a highly reliable component for military, aeronautical, and industrial power supplies.
