MCU reset during relay switches ON...

N

Niresh

Guest
Hi,
Could any please suggest how to avoid MCU reset with voltage glitches/noise during relay switches ON. I have protected the relay coil with free wheeling diode and MCU with 2x10uF parallel with 100nF near VDD of MCU and 1uF across RESET pin.


Regards,
NiK
 
On 2020-09-03 08:34, Niresh wrote:
Hi, Could any please suggest how to avoid MCU reset with voltage
glitches/noise during relay switches ON. I have protected the relay
coil with free wheeling diode and MCU with 2x10uF parallel with 100nF
near VDD of MCU and 1uF across RESET pin.


Regards, NiK
Click to expand...

I\'m hesitating to get involved, because I have no idea of your
level of knowledge and this is probably a can of worms.

It isn\'t hard to drive relays from an MCU, but you have to know
what you are doing. A schematic diagram, or even a picture of
your setup would have helped.

First of all, I suppose you are aware that the MCU cannot drive
the relay directly. You must use some sort of driver or buffer
stage. This can be as simple as an NPN transistor with a series
resistor in the base lead, but it could be more involved, depending.
I also assume that you know why and when a freewheeling diode is
needed.

Then there are grounding rules. Choose *one* convenient grounding
point. This may be the ground plane of the MCU board, if it has
one, or the negative terminal of the power supply, or some other
point. Every device should have its own connection to that point,
so that currents flowing back do not share the same conductor.
If your common ground point is some heavy large piece of metal,
you can relax that rule. The only real exception to this would
be the negative lead to the power supply.

Next are crosstalk issues. Bear in mind that current flows in
closed loops. Keep those loops tight, so that current flowing
towards a device follows as closely as possible the same path
as current flowing back. This is most easily assured by twisting
wires together. Do /not/ twist together wire pairs feeding /different/
devices.

Finally, sudden step changes in current may cause short drops
or surges in the power supply voltage. MCUs are often not very
forgiving on that point. Make sure that doesn\'t happen. Give
the MCU its own voltage regulator and provide enough capacitance
to keep the voltage within whatever it needs, even if the voltage
on the input side of the regulator jumps about a bit for a while.
About 10ms of hold-up time is usually enough.

Let us know how you fare.

Jeroen Belleman
 
On 9/3/2020 12:04 PM, Niresh wrote:
Hi,
Could any please suggest how to avoid MCU reset with voltage glitches/noise during relay switches ON. I have protected the relay coil with free wheeling diode and MCU with 2x10uF parallel with 100nF near VDD of MCU and 1uF across RESET pin.
Click to expand...
Is this on a breadboard or on a carefully designed PCB? If it\'s
the former, the jumper wires could be picking up stray fields.
 
On Friday, September 4, 2020 at 12:34:56 AM UTC+5:30, Pimpom wrote:
On 9/3/2020 12:04 PM, Niresh wrote:
Hi,
Could any please suggest how to avoid MCU reset with voltage glitches/noise during relay switches ON. I have protected the relay coil with free wheeling diode and MCU with 2x10uF parallel with 100nF near VDD of MCU and 1uF across RESET pin.


Is this on a breadboard or on a carefully designed PCB? If it\'s
the former, the jumper wires could be picking up stray fields.
Click to expand...

Yes, you are correct. The relay is driven through a NPN transistor with a series resistor. I understand the concept of free wheeling diodes.
It is a well developed PCB board with isolated ground common for relay, MCU and other circuits as well.

I don\'t why I could not attach or copy paste the schematic and noise capture in these group.


Regards,
NK
 
On 9/7/2020 9:53 AM, Niresh wrote:
On Friday, September 4, 2020 at 12:34:56 AM UTC+5:30, Pimpom wrote:
On 9/3/2020 12:04 PM, Niresh wrote:
Hi,
Could any please suggest how to avoid MCU reset with voltage glitches/noise during relay switches ON. I have protected the relay coil with free wheeling diode and MCU with 2x10uF parallel with 100nF near VDD of MCU and 1uF across RESET pin.


Is this on a breadboard or on a carefully designed PCB? If it\'s
the former, the jumper wires could be picking up stray fields.

Yes, you are correct. The relay is driven through a NPN transistor with a series resistor. I understand the concept of free wheeling diodes.
It is a well developed PCB board with isolated ground common for relay, MCU and other circuits as well.

I don\'t why I could not attach or copy paste the schematic and noise capture in these group.
Click to expand...
This is a text-only group. If you want others to see your images,
upload them to an image hosting site and post the link here.
 
Please check the below link for the relay schematic, PCB layout and Noise capture in oscilloscope.

https://www.dropbox.com/s/fyg97b91k36xjto/Glitches%20or%20noise%20capture%20oscilloscope.png?dl=0

https://www.dropbox.com/s/j0hqqfwh7p4ce6w/Relay%20schematic.png?dl=0

https://www.dropbox.com/s/p0s3gismw6sbhzo/PCB%20layout.png?dl=0


Regards,
NK
 
Pimpom wrote:
On 9/7/2020 9:53 AM, Niresh wrote:

I don\'t why I could not attach or copy paste the schematic and noise
capture in these group.
This is a text-only group. If you want others to see your images,
upload them to an image hosting site and post the link here.
Click to expand...

alt.binaries.schematics.electronic still seems to work but it\'s rarely
used.
 
On 07.09.2020 08:17, Niresh wrote:
Please check the below link for the relay schematic, PCB layout and Noise capture in oscilloscope.

https://www.dropbox.com/s/fyg97b91k36xjto/Glitches%20or%20noise%20capture%20oscilloscope.png?dl=0

https://www.dropbox.com/s/j0hqqfwh7p4ce6w/Relay%20schematic.png?dl=0

https://www.dropbox.com/s/p0s3gismw6sbhzo/PCB%20layout.png?dl=0


Regards,
NK
Click to expand...
I can\'t tell which trace is what on the scope capture (it overshoots at
700mV/div and both are 700mV/div - which one is what signal?). But I can
see that the LDO and relay share the same 12V rail. The 100nF cap on the
relay coil seems way too small to be doing anything substantial. I would
replace it with, or add in paralell, a 10uF-100uF cap.

Can you post the scope traces of the 12V and 3v3 rail? One where it does
not overshoot the screen and tell us which one is which.

regards
Nife Sima

--
Nife Sima - the lower earth crust
 
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