I purchased a Pi controller board and an expansion board. Now I’m considering laying out my own combined board to better suit my application. In reviewing the circuitry I identified an inconsistency in the TVS connections on the Pi controller board vs. the expansion board. Here’s what I see:
I was curious why the 24VAC rail was chosen for the protection path on the Pi controller board. My first thoughts knowing little about the design is that having an un-fused, low independence path to ground for the protection circuit would be an optimal choice. Am I missing anything?
You are right that the TVS’s are connected differently on OSPi vs expansion board. This was not a mistake. When OSPi was designed, the TVS’s are designed to be connected to the COM wire. Later, when the expansion board was designed, they have to be connected to GND, because the expansion board has no access to the COM wire (that runs outside of the expansion board). However, since there is a TVS (TVS0) between COM and GND on OSPi, the two connections are basically equivalent — in the event of transient voltage, the TVS0 will provide a short path. If I am to redesign OSPi, I can change the TVS’s to be connected to GND, which makes it consistent with the expansion board. Hope this makes sense.