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'[EE]: bad oscilloscope?'
2002\02\13@220312 by Rodrigo Valladares P.

picon face
Hi all,

After reading the benefit of an oscilloscope, i took one that is not used in
my work to debug (is this used in hardware too ?) a pic circuit. I have ZERO
experience with scopes, so, and i attach the probe to Ch1, the probe has a
clip, i grip it to gnd and the pin of the probe to what was i really want to
measure... BUT, i have a 50Hz superimposed over my signal, and showme not
really what i expect, then measuring the line ac make some spark in the
probe (the scope can accept 400 vac). Using a tester, and the scope off, i
discover that the clip is connected to the line ground (0 ohm), and i
suppose that the test probe was isolated from the main line. I'm wrong and i
must go to buy "measuring with a scope for newbies" or the scope is bad?, i
don't have none to ask in my work, because is not used and is there only by
a mistake

The scope is a tektronix tds-310, and the probes (both with the same result)
P6109B 10X

( BTW, 50 Hz is the line frequency here )

Thanks in advance

RVP.

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2002\02\13@224103 by David Duffy

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face
Rodrigo wrote:
>Hi all,
>
>After reading the benefit of an oscilloscope, i took one that is not used in
>my work to debug (is this used in hardware too ?) a pic circuit. I have ZERO
>experience with scopes, so, and i attach the probe to Ch1, the probe has a
>clip, i grip it to gnd and the pin of the probe to what was i really want to
>measure... BUT, i have a 50Hz superimposed over my signal, and showme not
>really what i expect, then measuring the line ac make some spark in the
>probe (the scope can accept 400 vac). Using a tester, and the scope off, i
>discover that the clip is connected to the line ground (0 ohm), and i
>suppose that the test probe was isolated from the main line. I'm wrong and i
>must go to buy "measuring with a scope for newbies" or the scope is bad?, i
>don't have none to ask in my work, because is not used and is there only by
>a mistake

Ouch !  Yes, it's normal for the CRO probe to be referenced to mains earth.
If you want to measure points at mains potential you will need to use a
isolated probe adapter or similar. The scope and/or the probes may have
been damaged now. I'm not familiar with your 'scope. Measuring mains
without the appropriate probes/adapters can be tricky & quite dangerous.
Regards...

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2002\02\13@224901 by Dale Botkin

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Common beginner mistake, you're seeing the difference in grounds between
teh scope and your board.  What you need to do is ground the scope probe
to your project ground.  There are usually ground clips that attach to the
probe and can clamp onto a ground on the board you're testing.  Need to be
CAREFUL especially if your project is not battery powered, though.  8)

Now I'm sure half a dozen people with more knowledge of how scope grounds
are isolated will jump in with more detail...

Dale
--
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curiosity killed the cat, I say only the cat died nobly."
         - Arnold Edinborough


On Wed, 13 Feb 2002, Rodrigo Valladares P. wrote:

{Quote hidden}

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2002\02\13@225322 by Dale Botkin

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Jeez, strike my last response, I replied before reading all the details.
You measured the AC Line voltage with the scope, and that made a spark?

On Wed, 13 Feb 2002, Rodrigo Valladares P. wrote:

{Quote hidden}

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2002\02\13@225733 by Alexandre Domingos F. Souza

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>Common beginner mistake, you're seeing the difference in grounds between
>teh scope and your board.  What you need to do is ground the scope probe
>to your project ground.  There are usually ground clips that attach to the
>probe and can clamp onto a ground on the board you're testing.  Need to be
>CAREFUL especially if your project is not battery powered, though.  8)
>Now I'm sure half a dozen people with more knowledge of how scope grounds
>are isolated will jump in with more detail...

       I just have a clip that goes from the scope, to the bench power supply ground ;o) Works every time. Never had any problems with this setup, since everything is powered thru the PSUs on the bench, and ALL the grounds of ALL the supplies are grounded togheter in a "star" way.


---8<---Corte aqui---8<----

Alexandre Souza
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2002\02\13@231156 by Rodrigo Valladares P.

picon face
> Jeez, strike my last response, I replied before reading all the details.
> You measured the AC Line voltage with the scope, and that made a spark?
>

Yes, i plug the scope in the main, switched on, self test passed ok, then
attach the probe to channel 1, the clip in the probe to one wire of ac line
(not shure if neutral or live), and the pin of the probe to the other wire
of ac..-> spark!.

i explain miself if the clip of the probe is connected with the earth of the
main, what i do is make a short between neutral o live to earth, across the
scope. what make me suspicius is i think that both terminal of the probe
(clip and pin) MUST be isolated from ANY lead of main (gnd, neutral and
live) -- but, David Duffy say "Yes, it's normal for the CRO probe to be
referenced to mains earth".

the project anyware is not battery powered, and is not isolated with a
transformer, but the above test was made without the project, only scope and
line.

Thanks.

RVP.

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2002\02\13@233108 by Giles Honeycutt

picon face
Rodrigo,
Just convert the 3 pin wall plug to a 2 pin wall plug.  (lift the ground
pin)
Believe it or not, you will be able to check almost any voltage then.  But
keep in mind that when you connect the ground of the probe to a circuit,
that the entire chassis of that scope will be at that potential.
Just a side note:
Be careful when using different wall plugs.  I have seen a situation that
one scope was plugged in on one wall, and we were measuring a circuit on
another wall.  The 2 walls were wired to different legs of the 220. (US wall
outlets are made up of 110 to ground of each leg of a 220 line)  Anyway,
with a simple wiring mistake in one outlet of neutral and ground getting
swapped (usually no problem)  when the probe touch the circuit, very big
sparks flew!!

Best regards,
Giles


{Quote hidden}

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2002\02\13@234505 by Jim

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  "> Just convert the 3 pin wall plug to a 2 pin
   >wall plug.  (lift the ground pin)"

We had a young tech who did this to measure
the grid drive to a small TWT (traveling wave
tube) one day (p/o the GM (Ground Mappping) RADAR
in a Panavia Tornado Nose RADAR) - the only problem
was the drive was WRT (with respect to) the cathode
of the TWT - and the cathode sat around -3000 VDC ...
the Tek 475 *did* stand off this voltage (the spec
states something like 500 V or so) ...

Normally a special isolation transformer and
insulated 'cage' for the scope was used - someone
had not informed this young man as part of his
training however.

Jim



{Original Message removed}

2002\02\14@005055 by Giles Honeycutt

picon face
Jim,
I would say he had many other problems also.  Perhaps he could get a voltage
doubler and start a jacob's ladder using his body and the scope as the
ladder??
By the way, a good example of high voltage stupidity:

www.drmegavolt.com/underpages/warning.asp
or got to http://www.drmegavolt.com for his main page.

What are you doing us so late?  I am going to bed now...

Best regards,
Giles


{Quote hidden}

>{Original Message removed}

2002\02\14@005717 by Rodrigo Valladares P.

picon face
> Just convert the 3 pin wall plug to a 2 pin wall plug.  (lift the ground
> pin)
> Believe it or not, you will be able to check almost any voltage then.

Thanks to all!

i don't know if this is the safest or the best way for doing what i want,
but it works for me, no sparks and no extraneous readings...

Thanks again

RVP.

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2002\02\14@005734 by David Duffy

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face
Giles wrote:
>Rodrigo,
>Just convert the 3 pin wall plug to a 2 pin wall plug.  (lift the ground
>pin)

NO !  NO  ! NO !   THIS IS NOT THE RIGHT WAY TO DO THIS !
Rodrigo has said he is a beginner - don't let him fry himself.
We don't need dead PicListers - they don't contribute to the list. :-)
Isolated probe adapters are the safe way to do these measurements.
Some of the TEK LCD 'scopes have isolated probes IIRC.

{Quote hidden}

This is exactly why you don't do this sort of thing.
Regards...

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2002\02\14@020527 by Kathy Quinlan

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----- Original Message -----
From: "Giles Honeycutt" <spamBeGoneprogrammer1spamBeGonespamHOTMAIL.COM>
Subject: Re: [EE]: bad oscilloscope?
> By the way, a good example of high voltage stupidity:
>
> http://www.drmegavolt.com/underpages/warning.asp


I wish the page mentioned all the details, Silicon Chip (Australia's only
decent Electronics Magazine now (RIP EA we loved)) featured the article, I
believe the event took place in Sydney, definitely in Australia, the guy in
the pic was under the influence of drugs at the time (GBH or something
IIRC). He was on the LV Stay bar which carried  ABC (Arial Bundled Cable),
which is basically Insulated cables bundled together, 440Vac phase to phase,
240Vac phase to neutral. The power co had isolated them, but were working to
isolate the section of HV cabling. They do not like blacking out whole
suburbs. The Guy had been up there for nearly 3 hours before he made
contact. which is why they had news crews around to capture the event. I
think he made a mention in that years Darwinian awards ;o)

Regards,

Kat.


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2002\02\14@054232 by Russell McMahon

picon face
> Giles wrote:
> >Rodrigo,
> >Just convert the 3 pin wall plug to a 2 pin wall plug.  (lift the ground
> >pin)
>
> NO !  NO  ! NO !   THIS IS NOT THE RIGHT WAY TO DO THIS !
> Rodrigo has said he is a beginner - don't let him fry himself.
> We don't need dead PicListers - they don't contribute to the list. :-)
> Isolated probe adapters are the safe way to do these measurements.
> Some of the TEK LCD 'scopes have isolated probes IIRC.


I agree that this is bad practice - especially for a beginner who has no
idea what they are doing.
No offence meant Rodrigo BUT you really are not aware of all the factors
involved and it is quite easy to kill yourself - or someone who comes by
while you are working - if you follow this advice.

That said, I have done this on occasion - but you have to be aware that the
scope is then at mains potential and be aware of other dangers and problems
it may cause. Rather than lifting the scope ground lead, which makes it
non-standard and potentially dangerous forever, I use a "tapon" or double
adaptor type plug (tghe type which allows several appliances to plug into
the same power outlet in a large stack of plugs).
Colour this tapon BRIGHT RED. Cut off the earth pin. Place it in the power
outlet and plug the scope power plug into it.  You now have a floating
ground scope that is just waiting to kill you, your fellow workers and your
visitors and to direct main voltage into other equipment if you common its
ground with other grounds under certain conditions. Only do it if you
understand all this and control it and even then don't do it if you can help
it.



       RM

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2002\02\14@054250 by Russell McMahon

picon face
>    "> Just convert the 3 pin wall plug to a 2 pin
>     >wall plug.  (lift the ground pin)"
>
> We had a young tech who did this to measure
> the grid drive to a small TWT (traveling wave
> tube) one day (p/o the GM (Ground Mappping) RADAR
> in a Panavia Tornado Nose RADAR) - the only problem
> was the drive was WRT (with respect to) the cathode
> of the TWT - and the cathode sat around -3000 VDC ...
> the Tek 475 *did* stand off this voltage (the spec
> states something like 500 V or so) ...
>
> Normally a special isolation transformer and
> insulated 'cage' for the scope was used - someone
> had not informed this young man as part of his
> training however.


Presumably this is an ISOxxxx certified facility. ??? :-)



       Russell McMahon

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2002\02\14@055101 by Edson Brusque

face
flavicon
face
Hello Picple,

> Yes, i plug the scope in the main, switched on, self test passed ok, then
> attach the probe to channel 1, the clip in the probe to one wire of ac
line
> (not shure if neutral or live), and the pin of the probe to the other wire
> of ac..-> spark!.

   hehehe, I have a very weird experience times ago. I was buildin
something on a prototype wireboard, with the mains AC going thru 4 diodes in
a bridge rectifier configuration and something (mostly resistors zener and
caps, I was building a no-trafo PSU). So I connected the scope probe clip on
circuit earch (that is constantly changing phase/neutral because of the
bridge rectifier) and ALL the phones on the house started ringing! It take
me some time to figure that one thing had to do with the other and
disconnect the clip. I waited some seconds with a bit '?' inside my head and
then curiosity made me connect the clip again...
RRRIIIIIIIIIIIINNNNNGGGGGGG! I disconnected and decided not to think about
it.

   Ohhh, I burned two modems when doing this.

   Best regards,

   Brusque

-----------------------------------
Edson Brusque
Research and Development
C.I.Tronics Lighting Designers Ltda
Blumenau  -  SC  -  Brazil
http://www.citronics.com.br
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2002\02\14@060728 by Kevin Blain

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First, I'll agree that this is potentially very dangerous. Think also of
the next person to use to scope...when it develops a fault.....

If you have a dual trace oscilloscope, remember that often you have an

'A-B'

Button. Use this to subtract one reading from another. You can then put
the A probe tip at the supply side input, and then the B probe some
voltage lower. The 'scope will then read the difference, I.e. -B volts
with respect to A. Often there is an invert feature too, so you can
switch this back to +B volts if you need to.


Aside from this, be extremely careful when working on mains voltage. If
possible use an isolating transformer between the kit you are measuring
and real mains. Use RCDs. Take extreme care. Remember that if you have
230V mains (as we do here in the UK) the peak voltage is 325V,
nominally, but the mains spec allows the RMS level to rise to 252V, so
the peak value will become 356V.

Regards, Kevin

> >
> > NO !  NO  ! NO !   THIS IS NOT THE RIGHT WAY TO DO THIS !

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2002\02\14@091546 by Eoin Ross

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Sounds more like the military, can't imagine ground mapping radar being in a civilian plane ... but maybe its used for ground proximity warning as well

Is it time to go OT? : )
>>> RemoveMEapptechspam_OUTspamKILLspamCLEAR.NET.NZ 02/14/02 05:38AM >>>
>    "> Just convert the 3 pin wall plug to a 2 pin
>     >wall plug.  (lift the ground pin)"
>
> We had a young tech who did this to measure
> the grid drive to a small TWT (traveling wave
> tube) one day (p/o the GM (Ground Mappping) RADAR
> in a Panavia Tornado Nose RADAR) - the only problem
> was the drive was WRT (with respect to) the cathode
> of the TWT - and the cathode sat around -3000 VDC ...
> the Tek 475 *did* stand off this voltage (the spec
> states something like 500 V or so) ...
>
> Normally a special isolation transformer and
> insulated 'cage' for the scope was used - someone
> had not informed this young man as part of his
> training however.


Presumably this is an ISOxxxx certified facility. ??? :-)



       Russell McMahon

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2002\02\14@092405 by Douglas Butler

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Technically cutting the ground pin is wrong, but at least here in the
USA it is how MOST small shops do it.  Assuming the "ground" of your
circuit is safe to touch there is no safety hazard.

A better way is to run your bench off of an isolation transformer.
Ground the test gear and the circuit ground at one point isolated from
the mains ground.  This reduces most measurement hum, and only costs
$100 or so, but still has a problem if the circuit "ground" is at a
dangerous potential.

Good isolated probe adapters are very expensive, and cheap ones have
lots of artifacts that corrupt your readings.  IMHO they are only for
special circumstances.  You have to work with them to learn what the
artifacts are and how to compensate for them.

On the other hand the description suggested the spark was not from the
ground clip, but from the probe TIP!  The only way you should get a
spark from the probe tip is by exceeding the probe voltage (400V?) so
that the probe breaks down internally.  Also if it was a 1X probe and
the scope input was set to 50 ohms you could get a spark, but I think
you said it was a 10X probe.

Sherpa Doug

> {Original Message removed}

2002\02\14@092716 by Jim

flavicon
face
Pre-dated ISOxxxx.

Jim

----- Original Message -----
From: "Russell McMahon" <RemoveMEapptechKILLspamspamCLEAR.NET.NZ>
To: <PICLISTSTOPspamspamspam_OUTMITVMA.MIT.EDU>
Sent: Thursday, February 14, 2002 4:38 AM
Subject: Re: [EE]: bad oscilloscope?


> > We had a young tech who did this to measure
> > the grid drive to a small TWT (traveling wave
> > tube) one day (p/o the GM (Ground Mappping) RADAR


<snip>

{Quote hidden}

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2002\02\14@093849 by Alan B. Pearce

face picon face
>BUT, i have a 50Hz superimposed over my signal,
>and showme not really what i expect

This sounds as though the power supply to the circuit you were looking at is
floating and not earthed. If you connected the scope probe ground lead to a
ground terminal on the power supply (which seems likely from the way you
write) then the power supply output probably is floating with respect to
ground. Join the black and green terminal on the power supply together, and
the hum signal should go away.

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2002\02\14@100802 by Dale Botkin

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On Thu, 14 Feb 2002, Alan B. Pearce wrote:

> >BUT, i have a 50Hz superimposed over my signal,
> >and showme not really what i expect
>
> This sounds as though the power supply to the circuit you were looking at is
> floating and not earthed. If you connected the scope probe ground lead to a
> ground terminal on the power supply (which seems likely from the way you
> write) then the power supply output probably is floating with respect to
> ground. Join the black and green terminal on the power supply together, and
> the hum signal should go away.

Along with all power on that circuit as the breaker pops with extreme
prejudice, if you're in the US where black is AC hot and green is ground.

Dale

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2002\02\14@104452 by Alan B. Pearce

face picon face
>Along with all power on that circuit as the breaker pops with extreme
>prejudice, if you're in the US where black is AC hot and green is ground.

hang on, he started off talking about running his PIC circuit from a power
supply, and all he could see on the scope was a massive hum waveform. I
don't recall seeing any PIC chips that run off mains voltage directly, he
definitely talked about a power supply.

I think you will find the power supply has red, black and green terminals on
the front, for positive, negative and ground, with the power supply being
totally floating from ground. Most bench power supplies I have seen are
wired this way.

The reference to mains seems to be a red herring (sic) that has developed
because he talked about trying to measure the mains with the probe. He also
says he has zero experience with scopes, so it would seem this was a
dangerous blunder of the inexperienced. I have not seen anyone else try and
get him back on the right track of debugging his pic circuit with a scope.

Original mail below with emphasis by me.

>After reading the benefit of an oscilloscope, i took one that is not used
in
>my work to debug (is this used in hardware too ?) a pic circuit. I have
ZERO
                                                   ^^^^^^
^^^^^^^
>experience with scopes, so, and i attach the probe to Ch1, the probe has a
 ^^^^^^^^^^^^^^^^^^^^^^^^^

>clip, i grip it to gnd and the pin of the probe to what was i really want
to
>measure... BUT, i have a 50Hz superimposed over my signal, and showme not
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

>really what i expect, then measuring the line ac make some spark in the

Now looking at the bits I have underlined, does this not make sense for an
ungrounded circuit behaving as a hum aerial ???? Lets get back to the
original problem.

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2002\02\14@114657 by Dale Botkin

flavicon
face
It does make sense to a certain degree.  All of my "bench" power supplies
are Power One and similar linear units liberated from other equipment.
None have separate ground connections on teh output; all have red & black,
and the black is tied to the input AC ground AFAIR.  The only place I have
a green wire is on the input side, where there' also a black AC hot wire.

Even if the PIC's DC gound was isolated from the AC ground, thogh,
attaching the scope ground clip should have tied PIC and scope grounds
together, eliminating th hum - no?  It always does for me.

As for what you were suggesting, if it was "tie the DC ground to the AC
ground", I'd agree.  That's not what I understood you to be saying fo just
reading the post though (but hey, I'm on my first cup of coffee, so I may
just be dense this morning).

Dale
--
"Curiosity is the very basis of education and if you tell me that
curiosity killed the cat, I say only the cat died nobly."
         - Arnold Edinborough


On Thu, 14 Feb 2002, Alan B. Pearce wrote:

{Quote hidden}

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2002\02\14@124057 by Peter L. Peres

picon face
>I just have a clip that goes from the scope, to the bench power supply
>ground ;o) Works every time. Never had any problems with this setup,
>since everything is powered thru the PSUs on the bench, and ALL the
>grounds of= ALL the supplies are grounded togheter in a "star" way.

Why do I infer from this that you do not work with anything small signal
and/or high frequency ? ;-)

Peter

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2002\02\14@230607 by Rodrigo Valladares P.

picon face
Why not? if the pic must be powered from the ac line without transformer (no
external psu) , and detect  voltage levels via interrupts and comparators,
send pulses to fire triacs and keep a multiplexed display via tmr0
interrupt, and so... i'm really more comfortable with software, but where
trying to figure why the interrupts don't work as i expect, and check if the
hysteresis feed via another port is enought to make the comparator work as i
expect, etc, etc, a scope can show what is not working.

i must confess that i make a special plug without earth, and check what i
want in ten minutes, and fix the problem. I don't know why if i can put the
probes of a ac powered multimeter in the line to check if i have 220V, i
can't put the probes of a 400V oscilloscope, this is a bit extraneous to me.

I understand that this must be done in few cases and taking all the
precautions.

Thanks to all.


RVP.


>
> The reference to mains seems to be a red herring (sic) that has developed
> because he talked about trying to measure the mains with the probe. He
also
> says he has zero experience with scopes, so it would seem this was a
> dangerous blunder of the inexperienced. I have not seen anyone else try
and
> get him back on the right track of debugging his pic circuit with a scope.
>

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2002\02\15@053604 by Alan B. Pearce

face picon face
>Why not? if the pic must be powered from the ac line without transformer
(no
>external psu) , and detect  voltage levels via interrupts and comparators,
>send pulses to fire triacs and keep a multiplexed display via tmr0
>interrupt, and so... i'm really more comfortable with software, but where
>trying to figure why the interrupts don't work as i expect, and check if
the
>hysteresis feed via another port is enought to make the comparator work as
i
>expect, etc, etc, a scope can show what is not working.
>
>i must confess that i make a special plug without earth, and check what i
>want in ten minutes, and fix the problem. I don't know why if i can put the
>probes of a ac powered multimeter in the line to check if i have 220V, i
>can't put the probes of a 400V oscilloscope, this is a bit extraneous to
me.

Well by all means have the PIC line powered - once you have the thing up and
running properly. For goodness sake use an isolating transformer between the
circuit under development and the incoming mains, otherwise you stand a very
good chance of looking the pictures Jinx posted in the [OT] list.

While working on the PIC itself, why not power it from a low voltage power
supply until you have it running properly? It will be a lot safer than
running it from mains level supply while working on it, even if you do have
an isolating transformer.

>I understand that this must be done in few cases and taking all the
>precautions.

Well if you ever want to get to a stage of drawing your pension, make this
ALL CASES, and always taking precautions. I would hate to try and count the
number of times I have had shocks off equipment despite taking all possible
safety precautions

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2002\02\17@223233 by ards, Justin P

flavicon
face
--
Good isolated probe adapters are very expensive, and cheap ones have
lots of artifacts that corrupt your readings.  IMHO they are only for
special circumstances.  You have to work with them to learn what the
artifacts are and how to compensate for them.

My only probe is now a electrical/magnetically decoupled probe as my lovely
3 year daughter chopped the lead into many pieces.

Justin

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