That's right, a capacitor will only charge up to the voltage you give it. It's just good to rate them higher than the expected line voltage to prevent them from popping if there are any spikes.
10v or 16v shouldn't make a difference, pick whichever is physically smaller and go with that.
Thanks. I put the capacitor/resistor in today, and flying it gently low around my backyard this evening it seems to be holding altitude perfectly with no noise on the sonar. I took it to the park afterwards and flew a bit more aggressively, but looks like I still got a bit of noise on it there. I still suspect I have some electrical noise somewhere in the system so will have to figure out how to track it down or prove whether I actually do or not. Here are the logs from today anyway.
I ended up going with a 10 ohm resistor and 16v 100uF capacitor.
Yep, I stil have this same behaviour with RC filter installed and shield.
Probably noise or highcurrents somewhere.
But in flight its not noticable.
I would like to track down the source of this noise anyway. If it's enough to affect the sonar, surely it will be affecting other components in the system.
For example my minimOSD has a crosshair that flashes in the middle of the screen when it shouldn't, and also my GPS takes forever to get a 3d fix. Sometimes not even in 10 mins with the quad armed with no obstructions. My LED strips also behave weirdly when in show_leds mode too. Not sure if these are related at all or just red herrings, but I'm not sure where to start really.
I found my GPS to be very sensitive to power supply noise. Using even slightly dirty power caused a HUGE decrease in signal strength and visible sats.
I got OK performance from soldering a capacitor across my power supply, but will implement a proper filter soon.
Which power supply would you solder that across? The main one coming into the APM from the ESC? And would it be the same sort of filter that goes across the sonar power supply? Eg 10 ohm resistor and 16v 100 uF capacitor?
Since my dirty power was coming from a cheap Chinese USB to serial adapter I just put the cap across the 5V wire and the ground right at the USB connector. A resistor also inserted in the 5V line would be a proper filter.
If you can determine the frequency of the noise you can calculate the values to create a low-pass filter for your system like you describe. I wouldn't just choose values at random.
Gave in and pretty much re-wired the whole quad. Twisted all of the power wires and just cleaned out all of the routing in general. I have a pretty much perfect sonar signal now though!
Just wanted to post here the fact that we should be using a RF choke instead of a resistor. They are around the same price and size and will do a better job and won't suffer from voltage drops.
More infor can be found here