4.1 Filament Voltage

Fig.7 Luminance and Filament Voltage

Luminance varies with the filament voltage(Ef) as shown in Fig.7.
Since the lifetime of VFD is dictated by the extent of evaporation of oxide materials coated onto the tungsten filament wires, filament voltage is so critical and should only be supplied within the specified ratings. Due to current drain from anode and grid to filament, which can cause ghosting, the filament potential should be set with a cathode bias voltage against ground level.

4.2 AC Filament Drive (50 or 60Hz)
Generally, the transformer is the most popular device utilized to supply filament voltage (Ef) with a 60(or 50)Hz sine wave form, which also has a center-tap for cathode bias as shown in Fig.8. The center - tap technique is used to prevent luminance slant i.e. difference in brightness from one side of the display to the other. Using a transformer without this center-tap can not only cause luminance slant but also ghost illumination due to exceeding the amplitude of the filament voltage in excess of the specified cut-off voltage rating.

Fig.8 Transformer with Center-Tap Fig.9 Transformer without Center-Tap

4.3 Pulse Filament Drive (High Frequency RMS)

Fig.10 DC to AC Converter

In the case of DC or battery power supply, a pulse wave form is available for the filaments from the DC to AC converter. The concept of pulse voltage supply to the filament is the same as AC filament drive. In either case Noritake still recommends the DC to AC converter with a center-tap as shown in Fig.10. Please note that the pulse voltage should be calculated as an RMS (root mean square) value from the wave form as shown in formula (1).
However, a 1/2 duty factor should be set, and the peak to peak pulse wave form should be 1.5 times or less than the RMS value. A frequency range of 10kHz to 200kHz is recommended.

4.4 DC Filament Drive

Fig.11 DC Filament Drive

In DC filament drive, potential difference between the anode and grid voltage will be apparent as a luminance slant across the display as shown in Fig.11.
This shows brighter luminance at one side of the display due to the DC voltage drop. In order to avoid this problem, special measures are applied during display construction and the polarity (+,-) of the filament or grid terminal is specified. However, this is only possible for relatively small VFD's.

Note : Please consult Noritake in advance before designing DC or DC pulse filament drive circuits.