Redundant Power for Pixhawk

PX4 Autopilot
1

I am building up a DJI S900 but with a pixhawk autopilot and a fairly expensive payload. I’d like it to have a redundant power supply (trying to eliminate single points of failure as much as possible) and from what I can gather from the Copter forums this is possible by supplying power to the pixhawk power port and also to the servo rail. They recommend putting a Zener diode and capacitor across the servo rail to protect against voltage spikes that could cause the pixhawk to reboot (Powering the Pixhawk — Copter documentation). However they don’t say how big the capacitor should be (other than “not ‘too’ big”) The only servos on the S900 are for the landing gear, so I’m not even sure I need this. I am planning on having the primary power sit at 5.3V and the backup around 5.0V to avoid internal switching back and forth between supplies.

Does anyone have any experience with this and could verify is this correct or make a recommendation?

2

I use a 1000uF, there is good explanation here : http://www.icarusfpv.com/2014/11/installing-a-zener-diode-and-capacitor-for-hkpilot32-pixhawk/

3

Yeah I had looked at that post - originally he used a 220uF cap and then it looks like in his version 2 one he upped it to 1000uF, but never really explain why he did so or how it was performing. He said he doesn’t have an electronics background, so I’m guessing he just picked a value? I guess the engineer in me was hoping for some justification on the value used.