108C1 — Gas-Filled Voltage Stabiliser Tube

The 108C1 (also known by the equivalent designation OB2) is a cold-cathode, gas-filled miniature tube designed for voltage stabilisation. It provides a highly stable nominal maintaining voltage of 108 V over a wide operating current range of 5.0 to 30 mA.

Overview

The 108C1 is a gas-discharge device used primarily for DC voltage regulation. It operates by maintaining a nearly constant voltage drop across its terminals despite variations in the current flowing through it. The tube is housed in a standard 7-pin miniature (B7G) glass envelope and can be operated in any mounting position.

In addition to standard DC shunt regulation, the 108C1 can be used in pairs in an antiparallel configuration to achieve AC voltage stabilisation. Its design features a low dynamic impedance and excellent long-term stability, making it a reliable reference and regulation element in electronic power supplies.

Construction, Base, and Pinout

The 108C1 is a cold-cathode device, meaning it does not contain a filament or heater. The electrical discharge is initiated and maintained solely by the potential difference between the anode and the cathode in the gas-filled envelope.

Physical Characteristics

• Envelope: Glass bulb (DIN 41537, Form A, Nenngröße 50)
• Base: 7-pin miniature (B7G)
• Maximum Seated Height: 60.3 to 60.5 mm
• Maximum Overall Length: 67.3 to 67.5 mm
• Maximum Bulb Diameter: 19 mm
• Weight: Approximately 11 g
• Mounting Position: Unrestricted (any position)

Pin Connections

The B7G base layout provides multiple redundant connections for both the anode and the cathode. These redundant pins are internally connected within the tube. This configuration allows designers to wire the load through the tube socket, ensuring that if the tube is unplugged, the load is safely disconnected from the unregulated supply voltage.

Electrical Characteristics and Operating Points

The 108C1 is designed to maintain a stable voltage across its terminals once ignition has occurred. The electrical characteristics are measured at an ambient temperature of 20 to 30 degrees C unless otherwise specified.

Ignition Voltage and Delay

Before the tube can stabilise, the applied voltage must exceed the ignition threshold to initiate the gas discharge.

• Ignition Voltage in Ambient Light: A minimum supply voltage of 133 V is required to guarantee ignition. The nominal ignition delay time under these conditions is 1.3 seconds.
• Ignition Voltage in Complete Darkness: In the absence of ambient light, the minimum voltage necessary for ignition increases significantly to 210 V due to the lack of photo-ionization.
• Dark Ignition Delay Probability: The time required for the tube to ignite in darkness is statistical. At a supply voltage of 133 V, the probability of ignition within 10 ms is low, but it rises steadily, with a high probability of ignition occurring within 100 ms to 1000 ms. Increasing the supply voltage beyond the minimum threshold reduces this delay.

Maintaining and Regulation Voltage

Once ignited, the tube enters its normal operating glow-discharge region.

• Nominal Maintaining Voltage: 108 V (typically ranging between 106 V and 111 V at a current of 17.5 mA).
• Initial Maintaining Voltage Limits:
  • Maximum at 30 mA cathode current: 112 V
  • Minimum at 5.0 mA cathode current: 105 V
• Regulation Voltage: The change in maintaining voltage over the full current range (5.0 to 30 mA) is exceptionally low, with an average value of 1.5 V and a maximum limit of 3.5 V initially.
• Dynamic AC Resistance: Approximately 100 ohms.

Long-Term Stability and Life Performance

The 108C1 exhibits excellent stability over its operating life. Under continuous operation at an average current of 17.5 mA, the performance parameters are specified as follows:

• Maintaining Voltage over Life (up to 3000 hours):
  • Maximum at 30 mA cathode current: 113 V to 114 V
  • Minimum at 5.0 mA cathode current: 101 V to 104 V
• Regulation Voltage over Life (5.0 to 30 mA): Typically remains at 1.5 V, with a maximum limit of 4.0 V.
• Voltage Drift: The maximum variation in maintaining voltage at a constant current during 1000 hours of life is +/- 3.0%, with a typical variation of +/- 1.0%. The total change in maintaining voltage for any individual tube will not exceed +3 V or -4 V over its lifetime. Over a shorter interval of 500 operating hours, the maximum fluctuation is limited to 4 V.

Absolute Maximum Ratings

Operating the tube outside the absolute maximum ratings can cause immediate failure or severely degrade its operating life.

• Maximum Continuous Cathode Current: 30 mA
• Minimum Cathode Current: 5.0 mA (current must not fall below this value, or the gas discharge will become unstable and extinguish)
• Maximum Surge/Starting Current: 75 mA. This peak current is permitted only during the turn-on phase and must be restricted to a maximum duration of 10 seconds (or up to 30 seconds in each 8 hours of use for long-life applications).
• Maximum Negative Anode Voltage: 75 V (the anode must not be biased negatively relative to the cathode beyond this limit)
• Maximum Parallel Capacitance: 0.1 uF
• Ambient Temperature Range: -55 to +90 degrees C
• Maximum Bulb Temperature: +150 degrees C (for operation)
• Storage Temperature Range: -55 to +70 degrees C

Relaxation Oscillations and Capacitive Loading

A critical design constraint is the maximum parallel capacitance limit of 0.1 uF. If a capacitor exceeding this value is connected directly across the 108C1, the circuit can form a relaxation oscillator. The capacitor charges through the series resistor until the tube's ignition voltage is reached, at which point the tube ignites and rapidly discharges the capacitor until the maintaining voltage falls below the extinguishing threshold. This cycle repeats, causing severe voltage instability and potential damage to the tube.

Circuit Design and Calculations

In a typical shunt regulator circuit, the 108C1 is connected in parallel with the load, and a series resistor (Rs) is placed between the unregulated input voltage (Vin) and the tube's anode.

Series Resistor Calculation

To design a stable regulator, the series resistor must be calculated based on the minimum and maximum input voltages and the load current variations. The resistor must satisfy two conditions:

1. It must allow the anode voltage to rise above the minimum ignition voltage (133 V in light) when the tube is off.
2. It must limit the tube current to between 5.0 mA and 30 mA during normal operation.

Calculation Example: Assume an unregulated input voltage (Vin) of 200 V, a nominal load current (Iload) of 15 mA, and a nominal tube maintaining voltage (Vout) of 108 V. To set the tube current (Itube) to its typical midpoint of 17.5 mA:

Rs = (Vin - Vout) / (Itube + Iload)

Input values:

• Vin = 200 V
• Vout = 108 V
• Itube = 17.5 mA = 0.0175 A
• Iload = 15 mA = 0.015 A

Calculation: Rs = (200 - 108) / (0.0175 + 0.015) = 92 / 0.0325 = 2830 ohms

The designer would select a standard resistor value close to 2.8 kohms with an appropriate power rating to handle the continuous dissipation.

Technical Summary

The 108C1 is a cold-cathode, gas-filled voltage stabiliser tube designed to provide a stable 108 V reference or regulated output. Operating within a current range of 5.0 to 30 mA, it features a low dynamic impedance of 100 ohms and an average regulation voltage of 1.5 V. Housed in a 7-pin miniature B7G envelope, it supports both DC shunt regulation and AC antiparallel stabilisation. Designers must ensure a minimum supply voltage of 133 V (or 210 V in darkness) for reliable ignition, limit parallel capacitance to 0.1 uF to prevent relaxation oscillations, and maintain the operating current strictly within the specified limits to guarantee long life and stable performance.