From: Brent Brown <brent@eds.co.nz>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
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Date: Tue, 8 Dec 2020 16:19:11 -0800
Subject: Re: [EE] Eliminating external noise through PS
Thread-Topic: [EE] Eliminating external noise through PS
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Hi Neil,

Just to throw another thought or two into the mix...

You say noise is being conducted through wires, though what if the relay co=
ils are=20
picking up magnetic signal from the AC load and coupling it back to the 5V =
line?=20
Shouldn't be difficult to separate and filter there.

Micro crashing (erratic execution) or just resetting? Some micros have flag=
s for=20
reset cause e.g. if reset caused by brownout detector that would confirm su=
spicions=20
about noise on the supply being the primary issue.

Less likely... noise on I/O lines might not be being handled properly by co=
de (for=20
example, generating too many interrupts too quickly and corrupting the stac=
k).

Brent

On 8 Dec 2020 at 13:03, Neil wrote:

> Hi,
>=20
> 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
>=20
> 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
>=20
> 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 he=
lp.
>=20
> 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.
>=20
> 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
>=20
> FWIW, this is where it ended up...
> http://orlandorobotbuilders.com/stuff/SMNoise/20201207-SMN-04.jpg
>=20
> 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?
>=20
> Cheers,
> -Neil.
>=20
> --=20
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> http://mailman.mit.edu/mailman/listinfo/piclist


--=20
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