Raspberry DCC: Voltage supply
I just updated the schematics Rev D… Today I want to describe the power supply section of the scheme.
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The transformer has to be connected to the connector P1. It should supply up to 18V and 5A. Capacitor C1 has the function to regulate the tension coming from the transformer. From this point we supply power directly to the MOSFET bridge (U5).
To the 18V line is also connected the voltage regulator (U1). The output of U1 is the 5V line needed for the electronic.
A dissertation here is necessary… the Raspberry Pi is powered through a USB plug, so 5V. Internally it has various regulator and through its pinout it offers both 5V and 3,3V back it its pinout. It could be possible to use our voltage regulator U1 to supply power to the Raspberry Pi directly through its pin out. Doing this (the jumper JP1 has to be closed) we bypass the internal protection of the Raspberry Pi, so I foresaw an additional USB A port that can be used to supply the Raspberry as it should be. But the jumper J1 is left for personal choice.
The Raspberry through its pinout provide the 3,3V needed for the PIC (U2) and the logic buffers (U3, U4). On the 3,3V pins the raspberry offers up to 50mA. The PIC at it maximum absorb 170mA. We do not use all the function of the PIN, but we cannot assure that we will never overcome the 50mA provided by the Raspberry… so with Rev _D and additional voltage regulation (U6) has been added and will supply the PIC and other devices with 3,3V!
The MOSFET bridge (U5) has at its output pins up to 5A, the MOSFET has 2 bridges connected in parallel (that’s how the 5,6A in output are reached) and includes an internal protection against overcurrent.
More details about the electrical connection of the PIC and the MOSFET bridge in the next episodes 🙂
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