From: Neil <picdude3@narwani.org>
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Date: Fri, 11 Dec 2020 12:09:13 -0800
Subject: Re: [EE] Eliminating external noise through PS
Thread-Topic: [EE] Eliminating external noise through PS
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Sorry, disappeared for a couple days, but some quick answers to=20
everything for now, and my next steps:

- Yes I have flyback diodes.   Highlighted with yellow here,  just above=20
the relays...=20
http://orlandorobotbuilders.com/stuff/SMNoise/20201207-SMN-06.jpg
- James & Russell, in that diagram also, I labeled the paths for power. =20
The 5V to the relays is pretty much dedicated and 5V is on the top=20
layer, and (dedicated) ground for the relays on the bottom layer.  But=20
yes, it's right up against the AC section, but as many components have=20
been removed now, I will push that far away.
- The 3.3V regulator has been getting a bit warmer than I like, so I=20
have been planning to use a buck converter instead.  But maybe I should=20
run two 3.3V regulators instead?  I would think that the buck converter=20
would be pass less of the noise through.
- I ran over there quickly to pick up stuff and tried a crude=20
(uncalculated) common-mode choke I made up from stuff I could find. Did=20
not make in dent in the noise coming through.
See here...=20
http://orlandorobotbuilders.com/stuff/SMNoise/20201207-SMN-07.jpg
- I feel like Brent may be correct, that the noise may be coming from=20
the relay side.  The processor (ESP32) is resetting.  I know it's not a=20
firmware issue as it had zero crashes with the previous pump.


Adding some info:
The previous pump was a huge AC pump.  This new pump is actually DC, but=20
there's a bridge rectifier on the wires.  So I'm now convinced that the=20
noise is coming from brushes.
I brought the pump with me, and I have a new blank board.  My plan is to=20
reproduce just enough of the circuit to see the noise, and I can cut and=20
re-route traces (the 5V & ground to the relays) to see if helps.
I will also add a ceramic capacitor to the pump wires (right at the=20
pump), and hope that helps.  Plus twist the wires.

I'll be back with some results.

Cheers,
-Neil.



On 12/10/2020 7:28 PM, FTL wrote:
> This is almost too obvious, but in the schematic I do not see a flyback
> diode across the relay to suppress the back EMF when the relay is turned
> off. The lack of diode would cause a serious spike in the 5V supply when =
the
> relay is turned off.
>
> Is the transistor being turned on hard enough to keep the relay on proper=
ly.
> If it is not fully on with no flyback diode, maybe it is occasionally
> releasing and causing big noise on the 5V bus.
>
>> -----Original Message-----
>> From: piclist-bounces@mit.edu <piclist-bounces@mit.edu> On Behalf Of Nei=
l
>> Sent: December 8, 2020 11:04 AM
>> To: Microcontroller discussion list - Public. <piclist@mit.edu>
>> Subject: [EE] Eliminating external noise through PS
>>
>> 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 t=
o
>> generate a LOT of conducted noise, which passes through the 5V PS, and t=
he
>> 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 m=
e,
>> 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
>> 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 ma=
de
> 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 wi=
ll
> take
>> out most of the noise.
>> What is the right type of filter to eliminate this noise?
>>
>> Cheers,
>> -Neil.
>>
>> --
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