Dimmable Differences Between Hot and Cold Cathode Fluorescent Bulbs

When looking for an energy efficient alternative for dimmable lighting applications, end users have many different options. However, one of the biggest sources of confusion is the difference between hot and cold cathode dimmable fluorescent light bulbs. Why do they operate differently on dimming circuits? The answer is in the lamp technology.

The fluorescent tube has two cathodes – one at each end – and they are wired to an electrical circuit, which is hooked up to an alternating current (AC) supply. The cathode is made of a coiled tungsten filament. When the light bulb is turned on, the cathodes are heated to extremely hot temperatures, which causes them to emit, or release, electrons, the negatively charged particles of an atom. The emitted electrons constantly shoot back and forth between both cathodes at the ends of the tube. The electrons react with the mercury in the tube to create the ultraviolet radiation that reacts with the phosphor to create visible light. Because the cathodes are heated to approximately 900 degrees Fahrenheit, this type of fluorescent light bulb is referred to as hot cathode fluorescent. A standard CFL is a hot cathode light bulb.

Unlike the incandescent and halogen light bulbs, which have constant electrical resistance, the fluorescent light bulb’s electrical resistance decreases as more current flows. If a fluorescent light bulb was connected directly to a constant-voltage mains power line, it would rapidly self-destruct due to the uncontrolled current flow. To prevent this, fluorescent light bulbs must use an auxiliary device, commonly called a ballast, to regulate the current flow through the tube.

Fluorescent light bulbs require a ballast to stabilize the light bulb and to provide the initial voltage required to start the light bulb. A ballast is a device that maintains the current in a circuit at a constant value by varying its resistance in order to counteract changes in voltage. It provides a positive resistance, or reactance, that limits the ultimate flow of current to an appropriate level for the fluorescent light bulb to work properly.

Cold cathode fluorescent light bulbs (CCFL) use similar technology as hot cathode fluorescent light bulbs except that in CCFLs, the cathodes are not heated by a filament in order to excite the mercury and cause current flow. Rather, CCFLs use cathodes without filaments, and the voltage potential within the tube is sufficient to excite the mercury and cause current flow. As a result, the voltage of CCFL light bulbs is higher than CFLs, and the CCFL’s electrical current is lower.

Although the technology is referred to as “cold cathode,” the cathodes heat up to about 200 degrees Fahrenheit while in operation, making the term “cold” a misnomer. The cathodes are relatively cooler than the hot cathode filament, which operates at 900 degrees Fahrenheit; therefore, the name “cold cathode” was applied.

The cathodes in the ends of the cold cathode fluorescent tube are made of a solid metal thimble, making them much more rugged in construction compared to the thin tungsten coil in hot cathode light bulbs. The solid metal thimble withstands destruction by shock and vibration much better than the tungsten filament, which makes it more durable and allows CCFLs to withstand different modes of operation that would otherwise damage a hot cathode CFL, such as rapid on-off cycles.

The different types of cathodes cause fluorescent light bulbs to dim in different ways from each other and from incandescent light bulbs. A traditional incandescent light bulb can be dimmed simply by reducing the amount of power that is sent to the bulb. Less power means less heat and less light; hence, the incandescent light bulb will dim. Dimming a fluorescent light bulb, on the other hand, is much more complicated. Reducing power to a fluorescent bulb can keep its filament from getting hot enough to work properly, and the light bulb will shut off. In order to dim a fluorescent bulb, a ballast specifically designed to dim must be used.

Typically, hot cathode dimmable CFLs have limited dimming capabilities and cannot fully dim like an incandescent light bulb because reducing the voltage to the ballast reduces the temperatures of the cathodes. If the cathodes are not hot enough, they will not be able to emit electrons to excite mercury atoms and create light. Because cold cathode light bulbs do not have to heat their cathodes to as hot a temperature, they are able to dim to a lower light output than hot cathode light bulbs. In addition, the CCFL’s solid metal thimble can withstand the dimming function better than the hot cathode’s thin tungsten coil filament, which further enables cold cathode bulbs to dim lower than hot cathode bulbs.

Remember that not all compact fluorescent light bulbs are compatible with dimmer switches. Be sure to check your product’s packaging to make sure that you purchased a dimmable fluorescent light bulb before you install it on your dimming circuit.