From: David Van Horn <david.vanhorn@backcountryaccess.com>
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Date: Tue, 8 Dec 2020 11:48:22 -0800
Subject: Re: [EE] Eliminating external noise through PS
Thread-Topic: [EE] Eliminating external noise through PS
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Basically incoming and outgoing current pass through the core in opposite d=
irections, so the core has very high impedance to HF energy, and almost no =
effect down at line frequency.  The energy from one side cancels out the ot=
her side.

________________________________
From: piclist-bounces@mit.edu <piclist-bounces@mit.edu> on behalf of Neil <=
picdude3@narwani.org>
Sent: Tuesday, December 8, 2020 12:41 PM
To: piclist@mit.edu <piclist@mit.edu>
Subject: Re: [EE] Eliminating external noise through PS

Looking at how common mode chokes work.  Was not aware of these.

This is the power supply I'm using (not sure if it's a flyback), and I
do not have any other caps...
https://www.meanwell.com/Upload/PDF/MPM-10/MPM-10-SPEC.PDF

Cheers,
-Neil.



On 12/8/2020 1:19 PM, Manu Abraham wrote:
> I guess you need a common mode filter at the PSU 120V in might help.
>
> Other than that:
> Are you using a flyback PSU ?
> If so, do you have a High voltage capacitor (the one between the
> primary and secondary, for the noise return path) ?
>
> On Tue, Dec 8, 2020 at 11:35 PM Neil <picdude3@narwani.org> wrote:
>> 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
>> 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 h=
elp.
>>
>> 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
>> 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 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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