Power quality requirements


3 posts
by empty_inside » Mon Jul 02, 2012 8:03 am
Sorry if this is the wrong forum; I couldn't find a better one.

I'm looking to power my Pi from a (very) noisy 12V source. I'm going to build a regulator to generate 5V. However I'm wondering how much filtering I need to do? Are there any specs anywhere on what kind of voltage variance, noise, transients etc the Pi can handle? If it makes a difference I was going to apply the power to the GPIO header (for ease of soldering mostly), but I heard that bypasses some of the uUSB power filtering (namely a big cap)?

Thanks for your help
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by jbeale » Tue Jul 03, 2012 5:18 pm
Interesting question on the ripple. Probably most types of regulator will be ok, unless you're trying for good quality sound from the analog output :-). I'm guessing as much as 200 mV peak-peak of ripple/noise is ok if you are getting 5.0V to the board. If it's only 4.75 V on the board, that amount of noise might be more of a problem. My adaptors typically have only a few mV of ripple and I haven't tested susceptability to that.

As you see in the RPi schematic http://www.raspberrypi.org/wp-content/u ... s-R1.0.pdf
the big 220 uF capacitor (C6) is directly on the +5V0 rail, so you have that helping you regardless how you connect the power. However, if you go the intended route through the microUSB filter, that input polyfuse F3 has some resistance which helps you with noise, forming an RC filter in combination with C6, C3 and C2. If the F3 resistance is 0.3 ohms, that gives you a low-pass filter with a 2.4 kHz shoulder. So it helps cut down 100 kHz switcher noise, but is not any help with 100 or 120 Hz ripple.
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by empty_inside » Wed Jul 04, 2012 10:11 pm
Thanks for the reply. Sorry for the delay, I was expecting these fora to e-mail me about replies but they didn't...

I couldn't find the schematic before, very useful. The Pi is going in a car so I'm expecting all kinds of noise (alternator, spark plugs (50-500Hz), injector solenoids, etc). I wasn't so much worried about 4.75V for "long" periods, more *big* voltages for microseconds. Most components in a car are basically inductors and don't care about quick transients, but I was worried the Pi would. The 2.4kHz filter should take care of those though.

I think the first job is to 'scope the car and see what the power looks like.
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