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Figure 8: Mosfet Switch Schematic
The complete circuit contains five high side switches, for each of the digits; and
seven low side switch, for each of the segments. When the MOSFET switch was initially
designed, a 5 k ohm resistor was used for the resistance between the +24 volts and the
PMOS gate. The only problem with this was that a large resistor along with the internal
capacitance was creating a large (RC) time constant. This large time constant would lead
to a ghosting effect on the digits as the Maxim driver cycled through the digits. The
following figure (Figure 9) shows the voltage across the 5k resistor.
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