WPC / WPC 95 driver board
Question: Why do “ghost lights” occur when using LEDs instead of normal pinball bulbs?
Answer: The 64 possible controlled lamps are divided into an 8x8 matrix. So 8 columns and 8 rows. The 8 columns are controlled one after the other by the CPU, with one or the other row being switched on for each column. The result is that the corresponding lamp lights up.
At that time, the Williams programmers had made the mistake when programming the columns that shortly before the end of the active column, the following column started and therefore the columns overlapped in time for a brief moment. This means that the following column already starts, even though the previous one is not yet completely finished. With the result that lamps that are normally off are unintentionally activated. At that time only lamps and no LEDs were used.
Question: Why does that make the difference?
Answer: In contrast to LEDs, lamps become brighter slowly. This is because the filament first heats up and then starts to glow after a delay. And that takes a certain amount of time. During this period of time, it doesn't matter whether the control times overlap a little. LEDs start to light up much faster. They are on almost immediately as soon as they are activated. And that's why the overlap in the control of the columns is also time-critical.
Question: Why do LEDs work perfectly with the original driver board and not with the PinLED driver board?
Answer: On the original circuit board, transistors are used for control. These also have a small time delay when switching, which has a rather positive effect when using LEDs. The transistor only switches on properly after the columns have overlapped in time, so that no ghost lighting occurs.
MOSFETs are used for control on the PinLED driver board. These have a much shorter time delay when switching, so that the overlap in the control of the columns comes into its own here. As mentioned before, this doesn't matter much when using incandescent lamps because they are too slow.
Question: What can I do if I experience “ghost glows”?
Answer: With LEDs, use the original driver board if possible. If you use the PinLED driver board, please pay attention to high-quality LEDs. There are different designs and qualities of pinball LEDs. There are so-called “Non Ghost LEDs”. These have an R-C element built in that causes a certain time delay in activation and thus suppresses the "ghost glow".