CCFT Tutorial
CCFT’s are glass tubes with electrodes in both ends. The gas is pressurized Neon/Argon mixed with Mercury Electrically excited mercury produces ultra-violet light. The UV radiation reacts with the phosphor coating which glows visible light.
CCFTs use high pressure -1 to 2 atmospheres This creates a more excited reaction and greater light efficiencies. CCFT’s require a high voltage to start the current flowing from cathode to anode. Typically 1000+ VAC This voltage causes the mercury to ionize, releasing electrons from the molecules. Once ionized, the voltage drops to a nominal level, approximately 400-450 VAC. The higher the pressure, the higher the voltage required to strike. The creation of light is due to ionized Mercury emitting ultraviolet light that strikes the phosphor to produce visible light.
Factors Affecting CCFT Life
Mercury Depletion - The ionization of mercury will eventually exhaust the mercury, reducing the light output. (Below 10°C will accelerate this function.) Mercury absorbed by the glass also reduces the transparency of the glass.
Phosphor Depletion - Mercury is absorbed by the phosphor reducing efficiency (Below 10°C will accelerate this function.) Poor ignition of the bulb will also reduce phosphor life.
Electrode Depletion - Sputtering of the the CCFT (firing voltages below kick-on point) will erode the electrodes. (Operation below 10°C will accelerate this function.)
Low Temperature Effects - Operating CFFT’s between 10°C and 50°C has no effect on life. Starting and operating a CCFT below 10°C greatly shortens life. At 0°C, a CCFT will last approximately 1000 hours.
Inverters
Inverters produce the high voltage AC that CCFT’s require. Most inverters have feed-back circuits that ramp up the kick-on voltage from 1000-2000 VAC, until the CCFT strikes. The Inverter can adjust the sustained voltages from 300-550 VAC to meet the voltage demand of the bulb. The total wattage of the inverter is critical - larger length and diameter CCFTs require higher wattages.
Dimming Methods
Current Limiting -By turning down the output current to the display, it will dim. The output voltage will increase with dimming, so it is limited by the range of inverter (lower the current, higher the voltage)
Pulse Width Modulation - Pulsing the input voltage to the inverter will lower the output Frequency of pulse width modulation may cause flicker and/or failure to ignite bulb.
Voltage Limitation - Turning down input voltage will lower inverter output. Wide ranging input inverters will prevent the use of this technique.