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What is a Cold Cathode?

M. McGee
M. McGee

A cold cathode is a cathode that is not heated. While that may seem obvious, this difference is very important to the internal functions of the cathode. These cathodes, while not specifically heated like hot cathodes, still have the potential to get very hot through use. A cold cathode is used in several forms of fluorescent and gas-based lighting systems, like neon signs. While cold cathode lights are widely used in many modern systems, their electricity use is often higher than alternative systems.

A cathode is the portion of a diode where electricity flows out during power usage. Most cathodes are hot cathodes or filaments. These cathodes maintain a much higher power density by heating the cathode. This improves electron flow and allows the cathode to maintain a stronger power flow. These types of cathodes are used for everything from light bulbs to cathode ray tubes (CRTs), otherwise known as television (TV) tubes.

Neon signs use cold cathode electrodes.
Neon signs use cold cathode electrodes.

A cold cathode doesn’t use any form of heating to charge its power flow. In this case, a significant voltage is pushed through the cathode, generating an electric field. The cathode then utilizes this electric field to maintain its power flow. This method of generating and maintaining a charge is less efficient than that of a hot cathode, but it also generates less heat.

While the cold cathode isn’t heated, that doesn’t mean it isn’t hot. As the current moves through the cathode, a small amount of power is transferred into heat. That heat will improve the power usage of the cathode, but it will also make the unit hotter. While some cold cathode devices are always cool to the touch, others may become as hot as standard hot cathode devices.

One of the primary uses of cold cathodes is in gas-based lighting, such as those in neon signs. The voltage required to excite the gasses is usually lower than the voltage required for other forms of lighting. This allows the cold cathode to operate at a lower voltage than a hot cathode and generate less excess warmth.

Cold cathodes are also the primary way of making the back light for liquid crystal display (LCD) screens. In this case, a cold cathode is used primarily for the lower heat generation and the stable voltage use. This allows most LCD TVs to work without an active cooling system.

The problem with cold cathodes lies in the voltage required to keep them operational. Hot cathodes can operate in variable voltages very easily due to the thermal induction created by their heat. Cold cathodes are unable to do this, and dips in voltage cause the cathode to lose power. This means that a cold system must maintain a higher voltage and use more electricity than similar hot systems.

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    • Neon signs use cold cathode electrodes.
      By: Karin Hildebrand Lau
      Neon signs use cold cathode electrodes.