Anode and Grid Power Supply
Circuits
As shown in Fig.17, the power supply voltage for anode and grid should be Vdisp = ebc+ Ek (Volts) which is the sum of grid/anode voltages (ebc=ec=eb) and cathode bias voltage (Ek). This output voltage must be stabilized otherwise its ripple may coincide with the grid scanning frequency and may cause flickering of the display.
Fig.17 Cathode Bias
Relationship between voltage and brightness
A peculiarity of VFD is that both grid and anode are active when high. If there are any limitations with the power supply and/or driving software, the supply voltage or duty factor may not be available to meet the specified value (typical ratings). In this case, you may re-calculate the ratings using the following formula.
K: constant of each display
ebc: anode and grid voltage (ec=eb)
Du: Duty factor
For example, when the duty factor is beneath the specified rating, a specified brightness rating can be achieved by modifying ebc2.5*Du. If Du(TYP) and ebc(TYP) are as specified, and Du(x) and ebc(x) are modified values than the related expression is as follows:
L = K * ebc(x)2.5 * Du(x) .............. (4)
ebc(TYP)^2.5 / ebc(x)2.5 = Du(x) / Du(TYP) ......... (5)
IMPORTANT !
ebc(x) and Du(x) can be calculated as above, however,
ebc(x) must be within the maximum ratings stated in each particular specification.
Brightness Control (Dimming)
High brightness is the main characteristic of VFD. However, in certain applications, it may be desirable to offer dimming capabilities for operation in dark environments. In such a case, the brightness level can be controlled by reducing the duty factor as shown in Fig.18. The brightness level can be adjusted in proportion to the luminous 'on' time to 'off' time. However brightness dimming by reduction of filament voltage or anode/grid voltage is not recommended because this may cause uneven illumination.
Fig.18 Pulse Width Control
Guide to VFD Operation
