Ahhh, back with another PSU filtering question.
I’m having good success testing a battery monitor application for my camper trailer (schematic attached). Under most of the load conditions I’ve tried so far (incandescent lights, fan motor), I’m getting the precision/stability I want from my measurements (stable to the 0.1A and maybe 10ish mV of drift for voltage)
However, I’m noticing a substantial degradation in precision for both voltage and current measurements when I switch on a small fluorescent fixture in the camper. I guess noise from a fluorescent fixture should not be too surprising.
I assume this fixture uses an electronic ballast—from some quick research, seems like these operate in the 20-30 kHz range… So before I got to thinking too hard, I experimented with adding various capacitors to the +12V input rail. I tried 100uF, 0.1uF, and 33pF with no noticeable improvements. (Note: this is all on a breadboard right now and yes, there are long leads a-plenty, so I realize this could be problematic)
So, next I thought about a low-pass RC filter. I have never designed one of these (likely obvious by now), nonetheless, a quick calculation using
cutoff freq (Hz) = 1 / [2(pi)(R)(C)]
would suggest that a 10 ohm resistor and a 100uF cap should give me a fc of about 160Hz, far below the (ostensibly) offending noise. I didn’t have these exact components, but had a 40 ohm resistor and a 68uF low ESR cap. I tried this setup and again, no real improvement. For lack of any better ideas, I threw out the series resistor and left the 68uF cap (in parallel) on the 12V rail, and got a moderate improvement in stability.
Any thoughts? I have seen some mention about using ferrite cores at the source and/or at the input. However, from what I can tell these are most effective when you’re dealing with higher frequency noise, into the RF spectrum. Thanks much in advance to anyone who comments!