LCD segments are triggered into operation by applying an alternating current. It must be a frequency of over 30 Hz (to prevent the display from flickering.)
This is essential and it makes no difference whether the electrodes have been insulated against the effects of the liquid crystal or not. lf they have not been insulated, the application of a direct voltage will result in electrolysis thereby destroying the electrodes.
If the electrodes are in fact insulated, the ions in the liquid crystal are shifted. This breaks down the electrical field and the display fades at once.
lf the supply is DC (as inbattery powered equipment), an AC waveform will have to be generated by means of an oscillator. To prevent the display from visibly flickering, the frequency range is limited at its lower end.
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The upper end is limited by the resistance of the electrodes and the capacitance ` (the RC time constant) of the segments in the display. In an equivalent circuit an LCD segment represents the parallel connection of capacitor C and of a high—vaIued resistor R.
The capacitance is primarily determined by the size ofthe segment surface. For instance, its capacitance per digit depends on the height of the digit and on the LC material and will be between 150 pF (8 mm digit size, high quality LC) and 4nF (maximum value for 25 mm digit, standard LC).
The resistance is dependent, among other things, on the segment’s surface and on the quality of the electrodes' insulation. In the above examples the corresponding values for the direct voltage resistance would be 1400 MQ (8 mm) and 8 MQ. (25 mm high).
If only alternating current is applied, the resistance in the segment may be disregarded. The current consumption will then rely on capacitance and frequency (figure 7). ln the case of a display with a very small surface area, it is possible to reach a working frequency of up to 1<Hz; with larger displays, however, there is little point in having an operating frequency of over 100 Hz. Usually, manufacturers indicate an operating frequency of 32 Hz at 50 mV.