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'16F84 near brush with death!'
1998\03\14@214053 by Dan Larson

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I thought I would pass this along to all...

Two nights ago I was experimenting with an old resurected
O'Scope (it is an old Bell & Howell Schools kit scope).  It
had a shorted focus voltage cap which I replaced.  It seems
to work fine now..  But that where the fun starts!

I had a serial PIC programmer breadboarded up and I thought I would
poke around a bit and see what the scope could see. While doing this,
I accidentally shorted the ground against the metal enclosure of
the old PC power supply I was using.  This created sparks because
of some kind of ground loop. Immediately the LED that the PIC
was flashing quit.  I could smell that infamous burnt electronics
smell.  I cycled the power and the PIC started working again, but
then faded and quit again.  I touched the PIC and it felt very hot!
The burned smell was coming from the PIC!  After powering down and
letting things cool I plugged it back in and everything was fine
again!  My PIC was spared (actually two because the programmer used
one too!).

Two questions:

Does the PIC have some kind of thermal shutdown that was triggered
by the ground loop?  Why did it get so hot even though the ground
loop was very brief.  The only thing I can think of is some kind of
VDD clamp that was triggered shunting VDD to VSS and heating up.
Letting it cool down allowed the shunt to reset.  If a PIC can survive
a disaster like this, I am impressed.  BTW, I attempted programming it
again and all is well!

Is it normal for switchers from old PC's to have a hot chassis?
I can't figure out why the  ground shorting against the case of
the power supply would do this.  This wouldn't be very good if
a ground loop like this existed inside the chassis of the PC from
which the supply was removed.  Perhaps it was a hot ground from the
switching supply disaggreeing with the ground of the scope....  the
scope has a transformer based supply and the primary ciruit is
totally isolated from the chassis.

Dan


*******************************
* Dan Larson                  *
* Software Engineer           *
* Micro Control Company       *
* email: spam_OUTdlarsonTakeThisOuTspamcitilink.com *
*******************************

1998\03\15@093928 by H.P.d.Vries

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Dan Larson wrote:
{Quote hidden}

I don't know how or what went on, but i've had a similar experience.
Two p16C84's were a little mistreated here, and got really hot. But
after reprogramming, they worked fine again! (I'm not complaining, just
surprised...)

Hans

1998\03\15@142233 by Nigel Goodwin

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In message <.....199803150238.UAA05175KILLspamspam@spam@homebase.citilink.com>, Dan Larson
<dlarsonspamKILLspamCITILINK.COM> writes
>Is it normal for switchers from old PC's to have a hot chassis?
>I can't figure out why the  ground shorting against the case of
>the power supply would do this.  This wouldn't be very good if
>a ground loop like this existed inside the chassis of the PC from
>which the supply was removed.  Perhaps it was a hot ground from the
>switching supply disaggreeing with the ground of the scope....  the
>scope has a transformer based supply and the primary ciruit is
>totally isolated from the chassis.

The chassis in a PC power supply should be connected to earth, as
(usually) is the ground connection on a scope. However it's common
practice to disconnect the earth on a scope for safty reasons in the TV
service trade. Many older TV's have a live chassis (or half live),
touching the chassis and the scope, as you tend to when you are clipping
the earth lead on, results in a severe electric shock - probably from
hand to hand, directly across the heart!. Also assuming you don't touch
the chassis as you fit the earth lead, there is an almighty BANG!!, and
parts of the TV disappear!.

For this reason it's common practice to remove the earth conection to
scopes, I usually do it by leaving the earth lead dangling outside the
plug, in this way it's very visable and obvious.

One small problem is you still get 'small' shocks due to current passing
through the RF bypass components, there are usually capacitors from
chassis to both live and neutral - this results in the scope floating at
half mains potential, but with only a tiny current capacity.
--

Nigel.

       /--------------------------------------------------------------\
       | Nigel Goodwin   | Internet : .....nigelgKILLspamspam.....lpilsley.demon.co.uk     |
       | Lower Pilsley   | Web Page : http://www.lpilsley.demon.co.uk |
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       \--------------------------------------------------------------/

1998\03\15@144407 by Morgan Olsson

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...
>This created sparks because
>of some kind of ground loop. Immediately the LED that the PIC
>was flashing quit.

Sounds like latch-up

I could smell that infamous burnt electronics
>smell.  I cycled the power and the PIC started working again, but
>then faded and quit again.

Hmm.  Maybe the chip was so hot that it used exessive current (or maybe
some part still in latchup?? No...) And after a vhile vas so hot it stopped
working.

...

> If a PIC can survive
>a disaster like this, I am impressed.

Me too.

>Is it normal for switchers from old PC's to have a hot chassis?

There is normally small capacitance high voltage caps from AC in to
chassis, parallelled with one-meg resistor.
Maybe the resistor was broke.

In the oscilloscope there may not be anything like this in order to not
hace noise "from behind", and in that case the osc should be grounded the
best way by the user, because A) noise rejection, B) disable charge
building up between chassis and oscilloscope inner cirquit.

It was an old scope, so maybe there is a resistive path of dust that
charges the chassis/inner cirquit capacitance?
...

>Dan

/Morgan
/  Morgan Olsson, MORGANS REGLERTEKNIK, SE-277 35 KIVIK, Sweden \
\  EraseMEmrtspam_OUTspamTakeThisOuTiname.com, ph: +46 (0)414 70741; fax +46 (0)414 70331    /

1998\03\15@172112 by Mike Keitz

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On Sun, 15 Mar 1998 08:45:25 +0000 Nigel Goodwin
<nigelgspamspam_OUTLPILSLEY.DEMON.CO.UK> writes:

>The chassis in a PC power supply should be connected to earth, as
>(usually) is the ground connection on a scope. However it's common
>practice to disconnect the earth on a scope for safty reasons in the
>TV
>service trade. Many older TV's have a live chassis (or half live),
>touching the chassis and the scope, as you tend to when you are
>clipping
>the earth lead on, results in a severe electric shock - probably from
>hand to hand, directly across the heart!. Also assuming you don't
>touch
>the chassis as you fit the earth lead, there is an almighty BANG!!,

The *proper* common practice is to connect such a TV to an isolation
transformer before operating it with the back off.  Then a properly
grounded scope can be connected to the TV chassis without incident, and
everything is grounded.  Lifting the scope ground is *not* safe.

Another good safety practice is to never touch a live TV chassis with
both hands (hold one hand behind your back).  Then if you do touch a live
wire and get shocked the current isn't as likely to go through your
heart.

New TVs also often are of live-chassis design.  Higher-end models usually
use a switching power supply that isolates the rest of the TV, but of
course the power supply circuit is live.

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1998\03\16@181204 by paulb
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Mike Keitz wrote:

> New TVs also often are of live-chassis design.  Higher-end models usually
> use a switching power supply that isolates the rest of the TV, but of
> course the power supply circuit is live.

 Stick to buying those with Audio-visual input-output connectors.  This
is a
guarantee that they do *not* have a live chassis, since isolation of
signal
inputs is ridiculously expensive compared to isolation in the
(switchmode)
power supply transformer.

 Mind you, none of these sets have an earth lead any more, they are
"double-
insulated".  I'm inclined to be suspicious of how well they
double-insulate
the switchmode components but ... Who am I to question the authorities?
Once
you connect a decent earth (via the I/O plugs) they should be quite safe
though.  Their lack of an earth lead does avoid signal hum loops though
-
maybe! ( subject of one of my little muses:
 http://www.midcoast.com.au/~paulb/faq_loop.html )

 Cheers,
       Paul B.

1998\03\16@184604 by Eric Smith

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"Paul B. Webster VK2BZC" <@spam@paulbKILLspamspamMIDCOAST.COM.AU> wrote about television sets::
> Stick to buying those with Audio-visual input-output connectors.  This
> is a guarantee that they do *not* have a live chassis, since isolation of
> signal inputs is ridiculously expensive compared to isolation in the
> (switchmode) power supply transformer.

No, it isn't a guarantee.  My Sony KV1380 has AV inputs and audio outputs,
and it uses optoisolators on all of them.

The only way to be completely sure is to study the schematic of the set.

Or just use an isolation transformer.  Even if the set is isolated, running
it on an external isolation transformer can't hurt.

Cheers,
Eric

1998\03\17@034925 by Nigel Goodwin

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In message <KILLspam19980315.172631.3254.0.mkeitzKILLspamspamjuno.com>, Mike Keitz
<RemoveMEmkeitzTakeThisOuTspamJUNO.COM> writes

Hi Mike,

>The *proper* common practice is to connect such a TV to an isolation
>transformer before operating it with the back off.  Then a properly
>grounded scope can be connected to the TV chassis without incident, and
>everything is grounded.  Lifting the scope ground is *not* safe.

We use isolation transformers on the benches at work, but I still
consider it safer NOT to have a scope grounded. It's all well and good
while you are working on an isolated supply - but it tends to give you
sloppy habits for the odd times when an isolated supply isn't available.
For instance, I don't have an isolated supply at home, so it's far
better to work in an earth-free environment.

>Another good safety practice is to never touch a live TV chassis with
>both hands (hold one hand behind your back).  Then if you do touch a live
>wire and get shocked the current isn't as likely to go through your
>heart.

Never touch any metal work without first testing it, just rub it gently
with the back of your finger, it will feel 'rough' if it's live.

>New TVs also often are of live-chassis design.  Higher-end models usually
>use a switching power supply that isolates the rest of the TV, but of
>course the power supply circuit is live.

Almost all new TV's in the UK are isolated chassis now, it fact I seem
to remember those EEC idiots passing a law that all new TV's have to
have a SCART socket fitted - this really requires the use of an isolated
PSU.
--

Nigel.

       /--------------------------------------------------------------\
       | Nigel Goodwin   | Internet : spamBeGonenigelgspamBeGonespamlpilsley.demon.co.uk     |
       | Lower Pilsley   | Web Page : http://www.lpilsley.demon.co.uk |
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1998\03\21@092944 by Martin McCormick

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Nigel Goodwin writes:
>Never touch any metal work without first testing it, just rub it gently
>with the back of your finger, it will feel 'rough' if it's live.

       I have noticed this effect on equipment that was
transformer-isolated, also.  I think it may be either due to capacitive
coupling between the primary of the transformer and the chassis.  It is a
valid warning sign and lets you know that it could bite.

       When I was a technician with the Audio Visual Center, here, I once
got hold of a film strip projector that had been equipped with an after
market motor drive which advanced the film at an adjustable rate for
paced reading exercises.  The projector had a grounded plug but the motor
drive did not and the connection between it and the metal body of the
projector was a plastic coupling.  For some reason, the mounting that held
the motor assembly to the projector was also non conductive.  The motor
developed a short between its field and the metal case around it.  The short
was somewhere between one end of the field coil and the other so that the
mains, the short, and neutral were like an autotransformer.  The body of the
rest of the projector was Earthed and the little pigtail that plugged in to
the rest of the projector from the motor was not polarized.  This meant that
one got a nasty shock between the film advance motor and the rest of the
projector no matter which orientation of the motor plug was used.  The
operator was 100% assured of a potentially fatal shock just by laying a hand
on the housing of the motor and touching the body of the projector at the
same time.  I have never since seen anything in public use as dangerously
designed as that projector.  We basically condemned it and I cut the pigtail
off flush with the case before returning it to the school system which
had sent it in.  The motor assembly had been professionally installed, but
somebody never thought of the fact that its metal case was electrically
separate from everything else.

Martin McCormick WB5AGZ  Stillwater, OK 36.7N97.4W
OSU Center for Computing and Information Services Data Communications Group

1998\03\22@140038 by John Sanderson

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Hello PIC.ers,
..
>Date:    Sat, 21 Mar 1998 08:27:56 -0600
>From:    Martin McCormick <TakeThisOuTmartinEraseMEspamspam_OUTDC.CIS.OKSTATE.EDU>
>Subject: Re: OT -16F84 near brush with death!

>Nigel Goodwin writes:
>Never touch any metal work without first testing it, just rub it gently
>with the back of your finger, it will feel 'rough' if it's live.
..
if it *is* alive, and wants to kick you, then your muscles will retract
your fingers *away*  from the juice.
..
>        I have noticed this effect on equipment that was
>transformer-isolated, also.  I think it may be either due to capacitive
>coupling between the primary of the transformer and the chassis.  It is a
<snippo>
>on the housing of the motor and touching the body of the projector at the
>same time.  I have never since seen anything in public use as dangerously
>designed as that projector.  We basically condemned it and I cut the
>pigtail.
<more snip>
..
12+ years ago I did development work for a Co. that mostly was in the
machine shop turning-milling-welding-punch press production business.
One of their welding folk *wired up* the 3 lines going to a 380 volt
welding transformer. Of course, in this country's supply, that's 380
volt line-to-line, aye ....but the machine was a single phaser.
What do you think he did with the remaining yellow/green conductor?
Wired it into the 3rd phase, of course!
This (mobile, rubber wheeled) machine worked every day for +/- 3 months until
someone leant a piece of steel against both it...... and a building column.
<ear-plugs in>
    {earth-shatttering bang}
<ear-plugs out>
The local fault protection pulled out the power, I'm happy to say.
Ever since, I've wondered just how it was no-one was killed in all that
three-month period.
No amount of conventional RCD/  earth leakage at the industrial
limit of 250 mA...(saves eqpt., not people) , o/current, fault level,
or u/volt, shunt trip, phase loss protection etc. can save a body that
becomes part of this circuit.
..
Just to (only just) remain on-topic, can anyone venture a modern
(PIC naturally) way to guard against this kind of incompetence?

Best regards,    John
..
email from John Sanderson   /   JS Controls, Boksburg, RSA
Manufacturer & purveyor of laboratory force testing apparatus
and related products and services.
Tel/fax: Johannesburg 893 4154    Cellphone 082 453 4815

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