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'[PIC] pics burn easily?'
2006\01\30@212936 by Cristóvão Dalla Costa

picon face
I've just had a quick power shortage at my house... Ligthning, then the
lights went off for a second and back on again. Only problem is the
PIC18F4525 I was working with is no more. ICD2 reports its device id as 00,
I can program it but it fails. Other chips in the board, including CMOS
logic, CMOS op amps and a VFD display were not harmed.

That's actually the second pic I have that burned in this fashion, the first
went away when a short circuit in the AC part of the circuit board
presumably caused a power surge. And again the PIC was the only component to
fail. A previous board version had problems with relay switching resetting
the pic.

Now, these are hand built prototype boards for a product I expect to have
ready in a month or so. It's a low volume production (20 units per year or
so) but a quite expensive product and it would look bad if customers had to
replace control units every couple months... This is actually my first pic
product so I figure I'm missing something here.

The power supply consists of a transformer, full wave rectifier with 1000uF
electrolytic, a 7805, another 1000uF electrolytic with a 100n disc cap, and
every digital chip has its own 100n disc cap. Board current comsumption is
in the 400mA range. Would a series inductor help with the problem? What else
can I do to make my project surge-proof?

Thanks in advance.

2006\01\30@214031 by Marcel Birthelmer

picon face
Some sort of inductive decoupling on the DC?

On 1/30/06, Cristóvão Dalla Costa <spam_OUTcdallacostaTakeThisOuTspamgmail.com> wrote:
{Quote hidden}

>

2006\01\30@214522 by Xiaofan Chen

face picon face
On 1/31/06, Cristóvão Dalla Costa <.....cdallacostaKILLspamspam@spam@gmail.com> wrote:
>
> The power supply consists of a transformer, full wave rectifier with 1000uF
> electrolytic, a 7805, another 1000uF electrolytic with a 100n disc cap, and
> every digital chip has its own 100n disc cap. Board current comsumption is
> in the 400mA range. Would a series inductor help with the problem? What else
> can I do to make my project surge-proof?

Do you have normal protection device like Varistor (over voltage protection)
and fuse (over current protection) in front of the circuits?

Proper layout is also important with this kind of setup. Do you have proper
creepage and clearance between the high voltage side and low voltage side?
Do you have proper isolation between them?

Regards,
Xiaofan

2006\01\30@215352 by Spehro Pefhany

picon face
At 12:29 AM 1/31/2006 -0200, you wrote:
{Quote hidden}

You problem might be more to do with I/O than with the power supply. It could
be the circuit design or the layout. But, no, PICs are quite robust
generally. Your earlier noise problem suggests strongly that you have layout
or EMC design issues in general.

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speffspamKILLspaminterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
->> Inexpensive test equipment & parts http://search.ebay.com/_W0QQsassZspeff


2006\01\31@012930 by TGO Electrónica SA de CV

flavicon
face
Generally they are quite sturdy. At least in my experience.
I've had some shorts and lost some ports, but never a whole pic.
Recently I zapped a 18F2610 with static when grabbing it and I was sure it
was toast, but to my surprise
it is still ticking after several hundred reprogrammings.
When I was using windowed parts I lost several to the erasing lamp, but
never in use.

Gabriel

PS: As some have suggested, there may be a weakness in your layout or
implementation. And
circuit protection (fuse, varistor, etc) and decoupling is suggested at the
AC input.

{Original Message removed}

2006\01\31@061915 by Cristóvão Dalla Costa

picon face
On 1/31/06, Xiaofan Chen <.....xiaofancKILLspamspam.....gmail.com> wrote:
>
> On 1/31/06, Cristóvão Dalla Costa <EraseMEcdallacostaspam_OUTspamTakeThisOuTgmail.com> wrote:
> >
> > The power supply consists of a transformer, full wave rectifier with
> 1000uF
> > electrolytic, a 7805, another 1000uF electrolytic with a 100n disc cap,
> and
> > every digital chip has its own 100n disc cap. Board current comsumption
> is
> > in the 400mA range. Would a series inductor help with the problem? What
> else
> > can I do to make my project surge-proof?
>
> Do you have normal protection device like Varistor (over voltage
> protection)
> and fuse (over current protection) in front of the circuits?


Only a fuse, but the circuit was connected through an AC filter which was
supposed to have a varistor and protect against surges.

Proper layout is also important with this kind of setup. Do you have proper
> creepage and clearance between the high voltage side and low voltage side?
> Do you have proper isolation between them?


I'm pretty sure, I left at least a fourth of an inch of clearance between
low voltage and high voltage sides and only the transistor-driven relay coil
lines connect both sides.

Thanks for the input.

2006\01\31@193348 by andrew kelley

picon face
Try some 10uF on each side of the regulator and add a tiny cap(470pf)
across the PIC..  I'm not promising but I think that it will help. (it
fixed my problem on 12c508's glitching from opening shutting a
microwave door and other inductive kickback on the line)  We still
don't know how real the signals on the scope were however. (But it was
affecting the operation of the unit)

--
andrew

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