Tag Archives: Thermionic Valves

What are Cold-Cathode Devices?

Some devices, like thermionic valves, contain a cathode that requires heating up before the device can work. However, other devices do not require a hot cathode to function. These devices have two electrodes within a sealed glass envelope that contains a low-pressure gas like neon. With a sufficiently high voltage applied to the electrodes, the gas ionizes, producing a glow around the negative electrode, also known as the cathode. Depending on the gas in the tube, the cathode glow can be orange (for neon), or another color. Since these devices do not require a hot cathode, they are known as cold-cathode devices. Based on this effect, scientists have developed a multitude of devices.

The simplest of cold-cathode devices is the neon lamp. Before the advent of LEDs, neon lamps were the go-to lights. Neon lamps ionize at around 90 V, which is the strike voltage or breakdown voltage of the neon gas within the lamp. Once ionized, the gas will continue to glow at a voltage of around 65 V, which is its maintain or sustain voltage. This difference between the strike voltage and the sustain voltage implies the gas has a negative resistance region in the operating curve of the device. Hence, users often build a relaxation oscillator with a neon lamp, a capacitor, and a resistor.

Another everyday use for the neon lamp is as a power indicator for the AC mains. In practice, as an AC power indicator, the neon lamp requires a series resistance of around 220k – 1M ohms to limit the current flow through it, which also extends its life significantly. Since the electrodes in a neon lamp are symmetrical, using it in an AC circuit causes both electrodes to glow equally.

Neon signs, such as those in Times Square and Piccadilly Circus, also use the same effect. Instead of a short tube like in the neon lamp, neon signs use a long tube shaped in the specific design of the application. Depending on the display color, the tube may contain neon or another gas, together with a small amount of mercury. By applying a fluorescent phosphor coating to the inside of the glass tube, it is possible to produce still more colors. Due to the significant separation between the two electrodes in neon signs, they require a high strike voltage of around 30kV.

Another application of cold-cathode devices is the popular Nixie tube. Although seven-segment LED displays have now largely replaced them, Nixie tubes are still popular due to their effect as a glorified neon tube. Typically, they have ten electrodes, each in the shape of a numeral. In use, the circuit switches to the electrode required for displaying a particular number. The Nixie tube produces very natural-looking displays, hence, people find them beautiful and preferable to the stick-like seven-segment LED displays.

Photographers still use flash tubes to illuminate the scenes they are capturing. They typically use them as camera flashes and strobes. Flash tubes use xenon gas as their filling. Apart from the two regular main electrodes, flash tubes have a smaller trigger electrode near one or both the main electrodes. In use, the main electrodes have a few hundred volts between them. For triggering, the circuit applies a high-voltage pulse to the trigger electrode. This causes the gas between the two electrodes to ionize rapidly, giving off a bright white flash.