Cold Cathode Display or Nixie Tube – Operation and Characteristics:
These are the most well known type of alphanumeric displays. They operate on the basis of emission of light in a cold cathode gas filled tube under breakdown condition. These are also known as neon or gas-discharge displays. The Nixie tube is a numeric indicator based on the glow discharge in cold cathode gas filled tubes. It is essentially a multicathode tube filled with a gas such as neon and having a single anode, as illustrated in Fig. 25.97 (a).
Each of the cathodes is made of a thin wire and is shaped in the form of characters to be displayed, for example, numerals 0 to 9. In its normal operation, the anode is connected to the positive terminal of the supply through a suitable current limiting resistor. The value of the supply voltage is greater than the worst-case breakdown voltage of the gas within the tube. The gas in the vicinity of the appropriate cathode glows when the cathode is switched to the ground potential.
In the older versions, each display tube has 10 numeral cathodes placed at different depths. The inherent disadvantage is that the numerals displayed are not on the same plane. Seven segment displays of the cold cathode type are now available in sizes up to 17 mm high. The display typically has a red-orange colour.
Many Nixie tubes also have dot cathodes either on the left or right of the character to serve as decimal points.
Both bar and dot matrix versions of the Nixie tube are available. The bar type Nixie tube have a cathode which forms the segment and operates in a fashion similar to the standard Neon tube. Identical supply voltage and drivers are required. In the dot type Nixies, each dot is in matrix fashion and operates as an individual glow discharge light source.
Characteristics of Nixie Tubes:
- The numerals are large (typically 15-30 mm high). They appear in the same base line for in-line read-out.
- Alphabetical symbols can be introduced.
- They need a dc supply of 150-200 V. The selected cathode carries a current of 1-5 mA.
- The Nixie tube can be pulse operated and hence can be used in multiplexed displays.
- Nixie tubes are single digit devices with or without decimal point.