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'[EE]: EMI causing 12c508 glitches'
2006\01\25@125730 by andrew kelley

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Hello all,

I have a solid state relay controlled by a 'c508 which is powered from
a simple power supply (transformer, rectifier, regulator with 470uF
cap on input and 10uF on output).  We have a microwave which when you
open or close the door generates +/- 0-8v spikes on the power *to the
micro*..  We think that these power glitches are causing RAM data to
change and the PC to change and alter the execution of further code.
Any suggestions on filtering these spikes?

--
andrew

2006\01\25@150852 by Shawn Wilton

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More inductive or capacitive filtering elements on the power line *near* the
chip.

On 1/25/06, andrew kelley <spam_OUTleetslackerTakeThisOuTspamgmail.com> wrote:
{Quote hidden}

> -

2006\01\25@152648 by Bob Axtell

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How did you decide how large the spikes were? with a scope
probe? I'm suspicious of the size of the spikes; power supply
filters should crush a spike that size...Can't you clamp the
regulator output with a transzorber or TVS?

Maybe  you need to  look at the PIC port lines and see if you can
install a few RC filters, cause perhaps the RF is radiating into
the PCB past the power supply (my guess).

--Bob

Shawn Wilton wrote:

{Quote hidden}

>>-

2006\01\25@183036 by Dwayne Reid

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At 10:57 AM 1/25/2006, andrew kelley wrote:
>Hello all,
>
>I have a solid state relay controlled by a 'c508 which is powered from
>a simple power supply (transformer, rectifier, regulator with 470uF
>cap on input and 10uF on output).  We have a microwave which when you
>open or close the door generates +/- 0-8v spikes on the power *to the
>micro*..  We think that these power glitches are causing RAM data to
>change and the PC to change and alter the execution of further code.

Hi there, Andrew.

The spike magnitude suggests strongly that you have a measurement
problem.  I believe the spikes are causing your problem but I'd like
to ask exactly *how* you connected the scope probe when measuring them.

I'm assuming that you used the standard (long) ground from the scope
probe?  If so, try this: your scope probe should have a bare shield
area right near the probe tip.  Wrap a couple turns of bare #22 or
#24 wire around that shield area and twist the free ends of the wire
loop together TIGHTLY so as to make a short, stiff lead.  Use that
wire to make the shortest possible connection to your 0V rail right
at the 12c508.  Solder the free end of your new ground lead to the 0V
pin if you can.

Assuming a standard Tek scope probe, that means that the ground lead
should be no longer than 3/16" and still allow the probe tip to reach
the +5V rail at the PIC.

Now measure the spike amplitude at each of the PIC pins.  Keep that
ground lead as short as is possible.

The spikes might be coming in on the VDD / ground leads - or they
might be coming in somewhere else.

Susceptibility to sharp, fast transients can be the devil to track
down but it is almost always possible.

Good luck!

dwayne

--
Dwayne Reid   <dwaynerspamKILLspamplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax

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2006\01\27@141529 by Howard Winter

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On Wed, 25 Jan 2006 12:57:30 -0500, andrew kelley wrote:

> Hello all,
>
> I have a solid state relay controlled by a 'c508 which is powered from
> a simple power supply (transformer, rectifier, regulator with 470uF
> cap on input and 10uF on output).  We have a microwave which when you
> open or close the door generates +/- 0-8v spikes on the power *to the
> micro*..  We think that these power glitches are causing RAM data to
> change and the PC to change and alter the execution of further code.
> Any suggestions on filtering these spikes?

Glitches like that will certainly cause random malfunctions.  Are those the *only* capacitors involved?  At
that size they are good as reservoirs, but no good at spike filtering or decoupling.  For a start, put a 0.1uF
ceramic on each side of the regulator, and if there's more than a short distance from the regulator to the
PIC,  another across the power connections close to the PIC itself.  

And as an aside, with significant capacitance on the output side, it's a good idea to put a reversed diode
from the output to the input of the regulator - 78-series regulators (actually you don't mention what you're
using) don't like to have a higher voltage on their output than input, and the diode will stop this from
happening after you power-down.  This won't affect the spike problem you have but it might improve the life
expectancy of the regulator!  :-)

Cheers,


Howard Winter
St.Albans, England


2006\01\27@181712 by andrew kelley

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Well, I tried powering the circuit off a battery and without having
the mains plugged in, or the switch cord(6ft) plugged in, and unit
STILL is affected by the microwave and fridge kicking on/off.  I was
even using a scope on a different ac line from the microwave and
fridge, and I saw the EMI/RFI on the scope input(while looking at a
captured waveform on a entirely different project). I also tried a
12f675 instead and it still exhibited the same problems.

andrew

2006\01\27@182243 by Shawn Wilton

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Anyway you could post a picture of your physical setup?


On 1/27/06, andrew kelley <.....leetslackerKILLspamspam.....gmail.com> wrote:
{Quote hidden}

> -

2006\01\28@004723 by andrew kelley

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On 1/27/06, Shawn Wilton <EraseMEblack9spam_OUTspamTakeThisOuTgmail.com> wrote:
> Anyway you could post a picture of your physical setup?

Sure on Monday.. I can't get into the lab this weekend even though I'd
love to. Now.. where are batteries for my camera.

--
andrew

2006\01\28@053901 by Shawn Wilton

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On 1/27/06, andrew kelley <leetslackerspamspam_OUTgmail.com> wrote:
>
> On 1/27/06, Shawn Wilton <@spam@black9KILLspamspamgmail.com> wrote:
> > Anyway you could post a picture of your physical setup?
>
> Now.. where are batteries for my camera.


I wish I could tell you, but I honestly have no idea.

--


Shawn Wilton (b9 Systems)
http://black9.com

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