6.1 Circuits

Fig.14 Cathode Bias

As shown in Fig.14, 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).
The output voltage must be stabilized otherwise its ripple may coincide with the grid scanning frequency, and it may cause flickering of the display.

6.2 Relation between voltage and brightness
Depending upon the type of driver chips utilized, either positive or negative voltage can drive the display. However, regardless of this, a positive voltage must be supplied to the grid and anode electrodes in order to supply current. A peculiarity of VFD is that both grid and anode are active when high.
If there are any restrictions with the power supply and/or driving software, the supply voltage or duty factor may not meet with the specified value (typical ratings). In this case, you may re-calculate the ratings using the following formula.

L = K * ebc^2.5 * Du ....... (2)

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 ebc^2.5*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(TYP)^2.5 * Du(TYP) ...... (3) 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.

6.3 Brightness control (Dimming)

Fig.14 Cathode Bias

High brightness is the main characteristic of VFD, however, in certain applications, it may be desirable to offer dimming capabilities for the operation in dark environments.
In such a case, the brightness level can be controlled by reducing the duty factor as shown in Fig.15. The brightness level can be adjusted in proportion to the luminous duty cycle as previously mentioned.
However the dimming of brightness by reduction of filament voltage or anode/grid voltage is not recommended at all, because this may cause uneven illumination.