From: Neil <picdude3@narwani.org>
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Date: Sun, 13 Dec 2020 15:16:44 -0800
Subject: Re: [EE] Eliminating external noise through PS
Thread-Topic: [EE] Eliminating external noise through PS
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I strongly feel so, because in my early tests, I:
(a) disconnected one of the pump wires (so the pump wouldn't run), and=20
it was (expectedly) very clean (on the scope).
(b) disconnected both of the pump wires and plugged it into the female=20
side of a PC power cord, so the pump ran, but was connected to 120VAC=20
from a different source.  Very clean power again.
In both cases, all else stayed the same including where the wires were=20
draped, etc.

In other tests, I moved the scope, power lines, etc and no change to the=20
noise on the line.

Either way, I haven't done my other tests yet since I took the pump with=20
me, so I'll get to those this week.

Cheers,
-Neil.


On 12/11/2020 7:37 PM, Jason White wrote:
> Hi Neil,
>
> This may be a silly question: are you sure that some of these voltage
> spikes that you've captured on the scope are not artifacts of your
> measurement setup? The loop formed by the ground wire/clip on a scope pro=
be
> makes a very effective antenna. Maybe your power rail is cleaner than you
> think - in which case the true problem might be elsewhere.
>
> -Jason White
>
> On Fri, Dec 11, 2020 at 3:11 PM Neil <picdude3@narwani.org> wrote:
>
>> Sorry, disappeared for a couple days, but some quick answers to
>> everything for now, and my next steps:
>>
>> - Yes I have flyback diodes.   Highlighted with yellow here,  just above
>> the relays...
>> http://orlandorobotbuilders.com/stuff/SMNoise/20201207-SMN-06.jpg
>> - James & Russell, in that diagram also, I labeled the paths for power.
>> The 5V to the relays is pretty much dedicated and 5V is on the top
>> layer, and (dedicated) ground for the relays on the bottom layer.  But
>> yes, it's right up against the AC section, but as many components have
>> been removed now, I will push that far away.
>> - The 3.3V regulator has been getting a bit warmer than I like, so I
>> have been planning to use a buck converter instead.  But maybe I should
>> run two 3.3V regulators instead?  I would think that the buck converter
>> would be pass less of the noise through.
>> - I ran over there quickly to pick up stuff and tried a crude
>> (uncalculated) common-mode choke I made up from stuff I could find. Did
>> not make in dent in the noise coming through.
>> See here...
>> http://orlandorobotbuilders.com/stuff/SMNoise/20201207-SMN-07.jpg
>> - I feel like Brent may be correct, that the noise may be coming from
>> the relay side.  The processor (ESP32) is resetting.  I know it's not a
>> 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
>> there's a bridge rectifier on the wires.  So I'm now convinced that the
>> noise is coming from brushes.
>> I brought the pump with me, and I have a new blank board.  My plan is to
>> reproduce just enough of the circuit to see the noise, and I can cut and
>> 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
>> 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 turne=
d
>>> off. The lack of diode would cause a serious spike in the 5V supply whe=
n
>> the
>>> relay is turned off.
>>>
>>> Is the transistor being turned on hard enough to keep the relay on
>> properly.
>>> 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
>> Neil
>>>> 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 linea=
r
>>>> 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
>>> 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 ra=
te
>>> of
>>>> the crashing... but the ripple itself wasn't causing crashing.
>>>>
>>>> Adding (non-specific size) ferrites to the pump wires, and 120VAC inpu=
t
>>> 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 b=
e
>>>> 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 p=
i
>>> 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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