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Driving Method for Anode and Grid

Driving method for Anode and Grid

Static Drive

For static drive, generally there is one grid that is individually pinned out.
  • All anode segments must also individually be pinned out. 
  • The grid is always supplied a positive DC voltage, and each anode is supplied a positive or negative voltage depending on it's "selected" or "non-selected" status.
  • Each anode requires an individual driver port for this driving method, low voltage drive and high luminance can be achieved with this driving method.
  • The timing circuit which is required for dynamic drive is not needed.
Static Drive

Dynamic Drive

For dynamic drive, there is one individually pinned out grid for each digit.
  • Corresponding anode segments are internally connected and pinned out.
  • With dynamic drive a large number of digits does not significantly increase the number of leads.
  • In order to be luminate a desired segment, it's on time is  synchronized with its grid scanning.
  • Relatively higher voltage is required for dynamic drive than static drive. When the duty cycle is 1/2, it is called duplex drive.
Dynamic Drive

Driving Method for Filament

Filament AC Drive

AC filament drive is the most popular driving method for the VFD. Since the filament potential (cathode potential) has some amplitude, the proper cut off bias voltage has to be applied to make the minimum filament potential higher than the anode and grid off voltage. Generally, a zener diode is used to set the cut off bias voltage. It is recommended to ground center tap of the filament. If one end of the filament is connected to the ground, the amplitude of the filament voltage will become larger, requiring a higher cut off bias voltage. When anode and grid voltage is not sufficiently higher than the filament voltage, the luminance on the grounded side is lower than the other side.
Filament AC Drive

Filament DC Drive

DC filament drive is commonly used for battery driven VFD's. A basic connection diagram and relation of electrode potentials are shown in figures to the left. The directly heated filament has a voltage gradient from the negative side to the positive side. If a typical AC filament designed VFD is connected, luminance gradient along with the voltage drop will be observed. So, filament height of DC filament designed VFD is offset to correct the luminance gradient. When a DC driven filament is used together with dynamic driven anodes and grids, unwanted noise or spikes can occur on the anode or grid driver output. A few volts of cut off bias voltage are required to prevent cross talk.
Filament DC Drive

Drive Voltage

Anode, Grid Voltage

Anode and grid voltages must be higher than the filament voltage in order to illuminate the display image. The voltage applied to the driver is sum of anode or grid voltage and cut off voltage. When the VFD is directly driven by micro-controller (micro-controller with built-in VFD driver), the output becomes negative voltage power supply.

Filament Voltage

Application of a specified voltage raises the temperature of the filament cathode, which causes thermionic emission. Excessive voltage causes evaporation of the coating
and  leads to degradation of display quality and reduces the lifetime of the display. Insufficient voltage causes unstable thermionic emission and leads to degradation of display quality. The rated filament voltage should be carefully maintained.

Cut Off Bias Voltage

If the filament potential is lower than the anode and grid cut off voltage, thermionic electrons can reach the anode and cause illumination of the phosphor. The filament
bias voltage should be increased to prevent this problem. Generally, a zener diode is used to obtain the bias voltage. The cut off bias voltage specified in the specification is calculated from the sine wave filament voltage and filament center tap condition.

Relationship of Electrode Potential of Dynamic Drive VFD

Relationship of Electrode Potential of Dynamic Drive VFD

The figure shows relationship of electrode potential of dynamic driven VFD. Anode and grid supply voltage is the sum of anode or grid voltage and cut off bias voltage. Filament bias voltage(EK) is required to prevent improper illumination.

Timing Chart of Grid (Digit) and Anode (Segment) Signals

Timing Chart of Grid (Digit) and Anode (Segment) Signals
This figure show the timing chart of the grid (digit) and anode (segment) signals. The individually pinned out grids are repeatedly and sequentially driven per the wave forms in figure. Anodes that are to be illuminated are driven synchronously with their corresponding grid. The figure gives an example of which segments to drive for the characters "4,3,2,1" to be illuminated under grids 1,2,3 and 4 (1G, 2G, 3G and 4G).