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'[PIC]: Conducted noise causing program to restart'
2000\08\01@095635 by NDuckworth

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My project is a gas fire controller using a 16F873, during the ignition
sequence the program will regularly restart as if a hardware reset has been
performed, I do not have the watchdog enabled.
The source of the noise is the mains powered spark generator which my
customer insists must remain on the same PCB.

The thyristor gate drive for the ignition circuit is opto isolated and I'm
using a flame rectification technique for flame detection.
The PIC is bypassed by 100nF directly across the supply pins and also has
47uF
soldered to the same point.
All of the PIC ports have at least 330 Ohms in series and its supply is a
78L05
with 100nF and 100uF either side.
MCLR has 10uF to 0V and 10K to +5V but I have tried tying MCLR to +5V
without success.

Generous use of 10 uH chokes is helping but becoming expensive, I'm
experimentally trying to reduce the noise level at source but would be
grateful for any advice on making my PIC circuitry more immune.
Any path into the control circuitry seems to cause the problem.

Thanks in advance.

Nigel Duckworth

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2000\08\01@120058 by Alan B. Pearce

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>grateful for any advice on making my PIC circuitry more immune.
>Any path into the control circuitry seems to cause the problem.

What happens if you have either the start circuit or the PIC circuit enclosed in
a can like happens with TV tuners or RF stages in mobile phones? What I envisage
is essentially a flat plate with the corners turned down to solder to on the
solder side of the PCB, and a deeper can, but similar mounting method on the
component side. This was the sort of shielding the old "wedding cake tins" were
good for.

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2000\08\01@122204 by James Paul

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Nigel,

What about putting Ferrite beads on all signal leads
and power supply to and from the PIC.   They shouldn't
be too expensive, and will act much like the chokes you
are using now.  Also, they're much smaller.   You also might
want to try surrounding the PIC in a metal shield of some sort.
I don't know if this will help, but it certainly couldn't hurt.
I think I'd go with the ferrite beads first, as it's the least
expensive, and most expedient.  Let me know what you find out,
and how you ultimately fix the problem.

                                        Regards,

                                          Jim


On Tue, 01 August 2000, Nigel Duckworth wrote:

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2000\08\01@185855 by Tony Nixon

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Nigel Duckworth wrote:
>
> My project is a gas fire controller using a 16F873, during the ignition
> sequence the program will regularly restart as if a hardware reset has been
> performed, I do not have the watchdog enabled.
> The source of the noise is the mains powered spark generator which my
> customer insists must remain on the same PCB.

I had the same problem with an PIC powered ignition system. The PICs
don't like strong EMI near them, so you'll need to shield the PIC or the
spark generator or both.

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Best regards

Tony

ICmicro's
http://www.picnpoke.com
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2000\08\02@042726 by NDuckworth

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Thanks for the advice Alan I'll give it a try but I'm fairly certain the noise
is conducted.

The prototype consists of two PCBs, one for the mains transformer, 12V PSU,
spark generator, flame detection circuit and four relays to operate gas
solenoid valves. The top PCB has the 16F873, ULN2003 Darlington driver for the
relays,
5V PSU, LED's, switches etc.
The two boards are connected by 30mm flying leads and the only interconnections
not yet protected by 10uH chokes are the low sides of the relay coils.
With the relays in circuit the problem occurs but not when they are
disconnected, this is why I believe the noise is conducted.
The production unit may well use one PCB but I need to understand what's
happening with the prototype first.
The obvious thing for me to do is protect these inputs as well but my customer
is
concerned about the rising cost of the components and I'm hoping to find
another solution.


On Tuesday, August 01, 2000 4:59 PM, Alan B. Pearce [SMTP:KILLspamA.B.PearceKILLspamspamRL.AC.UK]
wrote:

{Quote hidden}

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2000\08\02@044205 by rubenj

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{Quote hidden}

The best solution I have found for these type of problems is a good
groundplane. I usually try to keep my signal and powersupply traces
on the bottomside (of two sided pcb's) and pour copper over the
entire topside as a groundplane. Of course I have to make some gaps
in the groundplane for some traces that I can't put on the
bottomside. Think of it as a one sided PCB with as short jumpers as
possible on the topside. In fact, You could get a (virtual) 3 layer
board by having all the traces on the topside replaced with jumpers
and the middle layer (which is the topside layer of the PCB) with
only groundplane and pads for the components and jumpers. Most of my
(PIC) boards actually needs very few traces (or jumpers) on the
topside.

Another thing is the bypass capacitors, make sure they are of
(multilayer) ceramic types. And of course, as short traces as
possible to the cap (which is very easy if you have the topside
poured with copper).

I have also found it very usefull to put a small ceramic cap (6.8nF)
across MCLR and ground when using an external brownout/reset
chip/circutit.


==============================
Ruben Jvnsson
AB Liros Elektronik
Box 9124, 200 39 Malmv, Sweden
TEL INT +46 40142078
FAX INT +46 40947388
TakeThisOuTrubenEraseMEspamspam_OUTpp.sbbs.se
==============================

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2000\08\02@182915 by Peter L. Peres

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imho place a 5V6 zener directly across the PIC pins and solder 2 sheets of
grounded copper foil (ont on top one on bottom) onto the PIC area. If at
this point, assuming that you have proper grounds throughtout the circuit,
you still have troubles, then I don't know what you can do ... ;) BTW you
probably NEED a guard ground passing between the legs of the OC, connected
to the PIC ground (and a second guard ground connected to the mains ground
passing in the same space). If you use a transformer type power supply you
want to have decoupling across the rectifier diodes and the iron core
grounded too.

hope this helps,

Peter

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2000\08\07@040419 by NDuckworth

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Following a weekend of experimentation based upon advice from list members I
can report that my problem appears to have been solved by screening the spark
transformer with earthed foil.

I would like to thank everyone who offered advice both on and off list.

Nigel Duckworth

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