Neon Bulb Oscillators

Unusual Properties of Neon Bulbs:

The voltage-current (V-I) curve of a neon bulb is quite unique and showcases its distinct operating regions. Here's a breakdown of the key features:

Regions:

• Non-conducting (Dark Discharge): This region starts at 0 voltage and extends up to the striking voltage (around 60-100 volts). Here, the current is very low (microamps) and no light is emitted. As voltage increases, some ionization occurs, but not enough to sustain a glow.

• Glow Discharge: Once the striking voltage is reached, the neon gas ionizes, and the bulb starts to glow with a characteristic orange-red color. This region has a nearly constant voltage drop across the bulb (around 80-120 volts), even though the current can increase over a wide range (0.1-10mA). The glow intensifies as the current increases.

• Abnormal Glow/Arc Discharge: If the current rises further, the bulb enters this region. The voltage across the bulb drops slightly, and the glow changes to a bright, bluish-white arc concentrated around the cathode. This region damages the bulb and should be avoided.

Neon Terminology

Striking Voltage: The voltage at which the neon lamp ignites is usually between 45 and 65VAC for standard brightness types and between 70 and 95VAC for high brightness types. This is sometimes called the breakdown or ignition voltage.

Maintaining Voltage: The voltage across the lamp after it has ignited. This voltage is a function of the lamp current and is usually quoted at the design current. Nominal values are 80V for standard brightness and 75V for high brightness lamps.

Extinction Voltage: The voltage at which the lamp extinguishes if the supply voltage is reduced. It is normally a few Volts below maintaining voltage.

Design Current: The current at which the lamp has been designed to operate. It will be determined by the supply voltage and the value of series resistance. Operation at lower currents will result in the glow discharge becoming unstable (i.e. flickering) and operation at higher currents can severely reduce the useful life of the lamp. It is therefore important to use only the recommended value of series resistance.

Dark Effect: All  neon lamps are subject to a condition called dark effect. Dark affect is defined as a drastic increase in the amount of voltage required to make a lamp glow when the lamp is in a dark environment. Because the lamp is photosensitive, it may require many additional volts to start if no light is present. Neon lamps can also become erratic in total darkness.

External Photoelectric Effect:

• Shining light with sufficient energy (ultraviolet or higher) on the electrodes can eject electrons through the photoelectric effect. These ejected electrons can contribute to the overall current flow, particularly when the applied voltage is close to the striking voltage. This can be observed as a decrease in the striking voltage needed to ignite the bulb when illuminated.

• This effect isn't directly related to the neon gas inside the bulb but rather to the electrode material. Common electrode materials like aluminum or iron have work functions within the range of ultraviolet light energies, allowing photoemission.

Use as high value resistors:

When new neon bulbs may contain trace amounts of ionizing material added during manufacture to improve start up behavior. This can allow a dark leakage current giving the bulb a resistance of 500 megaohms+. Over time the dark resistance of the neon bulb increases as the ionizing material is used up.