These are the most well-known type of alphanumeric displays. Their operation is based on the emission of light in a cold cathode gas filled tube under breakdown condition. These cold cathode numerical indicators are called Nixies (Numicators and Numbertrons). This Nixie tube is a numeric indicator based on 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 shown in Fig. 2.14.
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. The anode is also in the form of a thin frame.
In its normal operation, the anode is returned to positive supply through a suitable current limiting resistor, the value of the supply being 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 ground potential.
(The characteristic orange-red glow in the case of neon covers the selected cathode completely, thereby illuminating the character brightly.)
Since 10 cathodes have to be associated with a single anode inside the glass bulb, they have necessarily to be stacked in different planes. This requires different voltages for different cathodes to enable the glow discharge.
Many Nixie tubes also possess dot-cathodes either on the left or right of the character to serve as decimal points.
The standard Nixie is not the only format used with cold cathode technology—both bar and dot matrix versions are available. The bar types 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 display, each dot is in matrix fashion and operates as an individual glow discharge light source. The required dots are selected by an X- Y addressing array of thin film metal lines, as shown in Fig. 2.15 (a).
Nixie tubes have the following important characteristics.
- The numerals are usually large, typically 15-30 mm high, and appear in the same base line for in-line read-out.
- Nixie tube are single digit devices with or without a decimal point.
- They are either side viewing or top viewing (as shown in Figs 2.15 (b) and (c)).
- Most Nixie tube require dc supply of 150-220 V, and the selected cathode carries current in the range of 1-5 mA.
- The Nixie tube can be pulse operated and hence can be used in multiplexed displays.
- Alphabetical symbols can also be introduced in the Nixie tube.