Why is a pinball leaf switch diode required?

arcaderob

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Help me understand... Isn't it just a switch? I googled for this but couldn't find a reason why, just a lot of test them and replace if bad.

Thanks
Robert
 
The diodes are only present on solid state machines, and they are there to prevent voltage spikes from messing with the circuitry on the CPU board.

-Ian
 
Help me understand... Isn't it just a switch? I googled for this but couldn't find a reason why, just a lot of test them and replace if bad.

Thanks
Robert

Pinball machines use a "switch matrix". The matrix is set up in columns and rows. One wire is daisy chained to 8 potential switches....the other wire of the switch is also daisy chained to 8 potential switches. None of these wires are a ground (as you find in video game switches). The diode keeps the matrix from getting "confused".....since one wire runs to 8 separate switches.

Edward
 
The diodes are only present on solid state machines, and they are there to prevent voltage spikes from messing with the circuitry on the CPU board.

-Ian

That's true in the case of solenoids but in the switch matrix they keep the current flowing in the right direction.
 
Help me understand... Isn't it just a switch? I googled for this but couldn't find a reason why, just a lot of test them and replace if bad.

Thanks
Robert

In order to understand why the diodes are in there you have to understand how the switch matrix works and remember that diodes will only let current flow in one direction.

The switches in a matrix are arranged in a grid of rows and columns. When the MPU wants to check the switches it applies a ground to the rows one at a time and checks all of the columns each time. Then it moves on to the next row until it has checked all of them. Without the diodes it would be possible for the MPU to "see" a switch closed on a row other than the one it's checking at that time. The diodes prevent this by allowing current to flow in only one direction. It will make more sense if you look at a picture of a switch matrix with this in mind.
 
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Thanks Lindsey and Elutz, although I don't quite understand yet, the splinter has been removed from my mind. I will take a look at the Williams Switch matrix schematics and pinrepair docs while on the bowl. Hope thats not TMINFO

Thanks
Robert
 
The subject could probably use a few more paragraphs. Sometimes it's hard to find the motivation to go into extreme detail.

You should read the Bally Theory Of Operation manual:

http://www.pinball4you.ch/okaegi/Bally Theory of Operation.pdf

You might have to read through it a few times for it to make sense but there is some great information in there. Specifically the section on interrupts. You can skip over the section on the 7 flashes, etc... if you're not working on a Bally game.

If I feel ambitious I might do a little better description of how the switch matrix works on my website.
 
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