From: Neil <picdude3@narwani.org>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
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Date: Sun, 13 Dec 2020 16:15:43 -0800
Subject: Re: [EE] Eliminating external noise through PS
Thread-Topic: [EE] Eliminating external noise through PS
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That's a definite possibility at this point.  The original circuit=20
(prior version) worked great with 2 layers, but I needed 2oz copper to=20
handle the pump current.
Never version works just as great.  Or  I should say "worked", until=20
they decided to switch from the AC pump to this DC pump.

Cheers,
-Neil



On 12/13/2020 6:58 PM, Jason White wrote:
> Neil, from the pictures it looks like you're using a 2 layer PCB. Have yo=
u
> considered a 4 layer PCB? A continuous dedicated ground plane layer would
> drastically reduce the loop area (and thus susceptibility to EMI) of your
> traces.
>
> Otherwise, it may be worth considering moving the relays to a separate
> board/area.
>
> On Sunday, December 13, 2020, Neil <picdude3@narwani.org> wrote:
>
>> I strongly feel so, because in my early tests, I:
>> (a) disconnected one of the pump wires (so the pump wouldn't run), and
>> it was (expectedly) very clean (on the scope).
>> (b) disconnected both of the pump wires and plugged it into the female
>> side of a PC power cord, so the pump ran, but was connected to 120VAC
>> from a different source.  Very clean power again.
>> In both cases, all else stayed the same including where the wires were
>> draped, etc.
>>
>> In other tests, I moved the scope, power lines, etc and no change to the
>> noise on the line.
>>
>> Either way, I haven't done my other tests yet since I took the pump with
>> 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
>> probe
>>> makes a very effective antenna. Maybe your power rail is cleaner than y=
ou
>>> 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 abo=
ve
>>>> 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 shoul=
d
>>>> run two 3.3V regulators instead?  I would think that the buck converte=
r
>>>> 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. Di=
d
>>>> 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, b=
ut
>>>> there's a bridge rectifier on the wires.  So I'm now convinced that th=
e
>>>> 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 a=
nd
>>>> 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 flyba=
ck
>>>>> 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
>>>> 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
>> 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 happe=
ns
>>>> to
>>>>>> generate a LOT of conducted noise, which passes through the 5V PS, a=
nd
>>>> 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 wi=
th
>>>> me,
>>>>>> but...
>>>>>> Adding a bunch more 0.1uf bypass capacitors all over the board did n=
ot
>>>>> 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 whateve=
r
>>>>>> components I had on hand (33uh inductor and 1000uf capacitor, should
>> be
>>>>>> low ESR), then a Pi filter (added 680 uF capacitor), and those barel=
y
>>>> 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 i=
t
>>>> will
>>>>> take
>>>>>> out most of the noise.
>>>>>> What is the right type of filter to eliminate this noise?
>>>>>>
>>>>>> Cheers,
>>>>>> -Neil.
>>>>>>
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