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Date: Thu, 10 Dec 2020 16:28:56 -0800
Subject: RE: [EE] Eliminating external noise through PS
Thread-Topic: [EE] Eliminating external noise through PS
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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 th=
e
relay is turned off.=20

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
>=20
> Hi,
>=20
> 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
>=20
> 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 th=
e
> 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
>=20
> 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.
>=20
> 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.
>=20
> 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 mad=
e
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
>=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 wil=
l
take
> out most of the noise.
> What is the right type of filter to eliminate this noise?
>=20
> Cheers,
> -Neil.
>=20
> --
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--=20
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