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Flickering
When a VFD is driven dynamically, flickering may occur as a result of driving frequency. This phenomenon can be resolved by changing the driving frequency. When designing, please note the following.
(1)The driving frequency is too low
When the display and the observer are not moving, flickering can occur when the driving frequency is below 50 Hz. If the display and/or the observer is moving, flickering can occur even if the driving frequency is above 50 Hz.
(2)Interference between filament frequency and grid scanning frequency
The filament is used as a cathode. When the cathode potential is changed by the AC power supply voltage of the filaments, the effective voltage of the anodes and grids is changed. This can result in interference between the frequency of the AC power supply of the filaments and the grid scanning frequency, which could cause flickering in the display. It is recommended that the filament driving frequency stay above 220 Hz.
(3)Interference between resonant frequency of filaments and the frame frequency
Each filament wire has a natural resonant frequency. When a mechanical shock is applied to the display, the filaments will vibrate. When a filament vibrates, the distance between the filament and the grids change, causing the frequency as that of the filaments. Flickering occurs when the driving frequency of the grids is almost the same as the resonant frequency of the filaments or a harmonic of it.
Un-balanced Lighting
1) Cut off bias voltage is not sufficient
Segments luminesce when a positive voltage as referenced to the filament is applied simultaneously to the anode and grid. When either the grid or anode voltage are lower than the filament potential, segment luminescence ceases. To turn a luminescing segment off the anode or grid must usually have a negative potential to that of the filament. This negative potential is called cut off bias voltage. Partial lighting may occur if the cut off bias voltage is not sufficient for the AC filament drive . The cut off bias voltage called out in the specification is calculated using sine wave filament voltage and filament center tap condition. The side tap bias condition may cause a greater amount of unwanted lighting if too low of a voltage is applied.
2) Blanking time is not sufficient
Blanking time is required between grid on times to prevent unwanted partial luminance of a segment that is off. The bottom of the grid waveform tends to be rounded, particularly when it transitions from on to off. A blanking time of 20 microseconds is typically used.
3) Grid or Anode drive pulse has spike noise
If the driving pulse has capacitive noise, partial lighting may occur. Adding a capacitor or resistor between the negative power supply and the output can solve this problem.