From: Neil <picdude3@narwani.org>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
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Date: Tue, 8 Dec 2020 10:03:58 -0800
Subject: [EE] Eliminating external noise through PS
Thread-Topic: [EE] Eliminating external noise through PS
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Hi,

I'm working on a device that takes 120VAC, converts to 5VDC with a=20
PCB-mounted power supply, then further reduces that to 3.3VDC with a=20
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=20
to generate a LOT of conducted noise, which passes through the 5V PS,=20
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=20
through the wires back to the board through the PS, through the linear=20
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=20
me, but...
Adding a bunch more 0.1uf bypass capacitors all over the board did not help=
..

Changing the power supply from an IRM-10-5 (industrial spec, 200mV=20
ripple) to MPM-10-5 (medical spec, 100mV ripple), helped reduce the rate=20
of the crashing... but the ripple itself wasn't causing crashing.

Adding (non-specific size) ferrites to the pump wires, and 120VAC input=20
wires did not help.
I then tried to create a crude/uncalculated LC filter (which whatever=20
components I had on hand (33uh inductor and 1000uf capacitor, should be=20
low ESR), then a Pi filter (added 680 uF capacitor), and those barely=20
made a dent.
Eventually I added another 0.1uf capacitor to the output side of the pi=20
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=20
will take out most of the noise.
What is the right type of filter to eliminate this noise?

Cheers,
-Neil.

--=20
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