From: Manu Abraham <abraham.manu@gmail.com>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
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Date: Tue, 8 Dec 2020 10:19:51 -0800
Subject: Re: [EE] Eliminating external noise through PS
Thread-Topic: [EE] Eliminating external noise through PS
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I guess you need a common mode filter at the PSU 120V in might help.

Other than that:
Are you using a flyback PSU ?
If so, do you have a High voltage capacitor (the one between the
primary and secondary, for the noise return path) ?

On Tue, Dec 8, 2020 at 11:35 PM Neil <picdude3@narwani.org> wrote:
>
> Hi,
>
> I'm working on a device that takes 120VAC, converts to 5VDC with a
> PCB-mounted power supply, then further reduces that to 3.3VDC with a
> linear regulator.
> A 3.3V microcontroller switches a relay to power a 120VAC pump.
> Overview...
> http://orlandorobotbuilders.com/stuff/SMNoise/20201207-SMN-00.jpg
>
> All has been great until we switched to a specific pump, which happens
> to generate a LOT of conducted noise, which passes through the 5V PS,
> and the 3.3V regulator, and causing the microcontroller to crash.
> I did a bunch of tested and found that the noise is being conducted
> through the wires back to the board through the PS, through the linear
> regulator, and this is the 3.3V line...
> http://orlandorobotbuilders.com/stuff/SMNoise/20201207-SMN-01.jpg
>
> I was over at someone else's place so did not have a lot of parts with
> me, but...
> Adding a bunch more 0.1uf bypass capacitors all over the board did not he=
lp.
>
> Changing the power supply from an IRM-10-5 (industrial spec, 200mV
> ripple) to MPM-10-5 (medical spec, 100mV ripple), helped reduce the rate
> of the crashing... but the ripple itself wasn't causing crashing.
>
> Adding (non-specific size) ferrites to the pump wires, and 120VAC input
> wires did not help.
> I then tried to create a crude/uncalculated LC filter (which whatever
> components I had on hand (33uh inductor and 1000uf capacitor, should be
> low ESR), then a Pi filter (added 680 uF capacitor), and those barely
> made a dent.
> Eventually I added another 0.1uf capacitor to the output side of the pi
> filter and that helped enough to prevent the microcontroller crashing.
> But there is still a lot of noise on the 3.3V line...
> http://orlandorobotbuilders.com/stuff/SMNoise/20201207-SMN-02.jpg
> http://orlandorobotbuilders.com/stuff/SMNoise/20201207-SMN-03.jpg
>
> FWIW, this is where it ended up...
> http://orlandorobotbuilders.com/stuff/SMNoise/20201207-SMN-04.jpg
>
> The noise is in the 7Mhz to 30Mhz range (prob even higher).
> I feel like smaller, low-ESR ceramic caps should help, but I doubt it
> will take out most of the noise.
> What is the right type of filter to eliminate this noise?
>
> Cheers,
> -Neil.
>
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