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Sci.Electronics.Basics -> troubleshooting method for micro-controller board ?
There are 103 messages in this thread.
You are currently looking at messages 80 to 100.
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Author: EeyoreDate: 21:59 09-11-07
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tonym924@gmail.com wrote:
> Eeyore wriote:
> > > "robb" <s...@where.on.net> wrote :
>
> > > > so i tested by connecting 5V/1A DC source between Vin and
> > > > GND-Pin3 and i checked the voltage with DVM between Vout and
> > > > GND-Pin3 and i was getting 3.47 V
> >
> > Pointless. It won't operate correctly with only 5V in.
>
> Agreed.
>
> > A linear voltage regulator has to have several volts more on the
> > input than the output to function correctly.
>
> Some need 1-2V more, others around 1/2V (I think I've seen
> LDOs down to 0.2V dropout [i.e. Vout-Vin]).
Sure, I was generalising somewhat for brevity.
As you say the value may vavy from several hundred millivolts for true LDO types
to as much as 3V in rare cases. 2V is typically a safe value to ensure correct
operation.
Graham
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Author: robbDate: 21:52 15-11-07
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"robb" <some@where.on.net> wrote in message
news:13iv5djrkhk1u04@corp.supernews.com...
> i have a micro-controller board with 3 big 40DIP ICs, 10 or so
16
> DIP support ICs. with 32v/8v/5v transformer supply
>
> i believe the 32v shorted with 5V line
>
> So....where does one start to diagnose such a *big* problem ?
>
I can only work on this in short segments of time and am usually
tired ... so i am moving backwards.
Anyway the revisit.
The last diagnostic consensus was to remove voltage regulators
and inject a good 5v power into the 5v lines and look for
warming components to get a fix on problems to repair.
Since my 5.2V *aged* lantern battery power supply did not impress
many i cobbled a simple 5v power supply from a 18 VAC/2.5A wall
wart, a 100V/1.5A bridge and 7805 5V regulator plus some
resitors and a 6V bulb to test.
I connected the 5v (through 450 Ohm) to the PCB 5v and 0v lines.
i measured ~60 mA curent nothing warmed ...i idecremented the
resistance by 100 for each iteration of testing the measured
currernt increased slightly until i made it to (100 Ohms) and
then the current spiked to ~+ 1.5A and the 7805 fried ?
but still no heat on the board anywhere, no trace no component no
IC ?
do i need to do something different with power supply test ?
thanks for any help,
robb
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Author: Arfa DailyDate: 05:03 16-11-07
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"robb" <some@where.on.net> wrote in message
news:13jq1ck2o3e8n21@corp.supernews.com...
> "robb" <some@where.on.net> wrote in message
> news:13iv5djrkhk1u04@corp.supernews.com...
>> i have a micro-controller board with 3 big 40DIP ICs, 10 or so
> 16
>> DIP support ICs. with 32v/8v/5v transformer supply
>>
>> i believe the 32v shorted with 5V line
>>
>> So....where does one start to diagnose such a *big* problem ?
>>
>
> I can only work on this in short segments of time and am usually
> tired ... so i am moving backwards.
>
> Anyway the revisit.
>
> The last diagnostic consensus was to remove voltage regulators
> and inject a good 5v power into the 5v lines and look for
> warming components to get a fix on problems to repair.
>
> Since my 5.2V *aged* lantern battery power supply did not impress
> many i cobbled a simple 5v power supply from a 18 VAC/2.5A wall
> wart, a 100V/1.5A bridge and 7805 5V regulator plus some
> resitors and a 6V bulb to test.
>
> I connected the 5v (through 450 Ohm) to the PCB 5v and 0v lines.
> i measured ~60 mA curent nothing warmed ...i idecremented the
> resistance by 100 for each iteration of testing the measured
> currernt increased slightly until i made it to (100 Ohms) and
> then the current spiked to ~+ 1.5A and the 7805 fried ?
>
> but still no heat on the board anywhere, no trace no component no
> IC ?
>
> do i need to do something different with power supply test ?
> thanks for any help,
> robb
>
Do you mean that the 7805 got hot and shut down ? One of those should never
"fry", because they are SOA protected. I suspect that something else
happened there, because if you do the math, it is impossible for 1.5 amps to
flow through a 100 ohm resistor, with only 5v driving it ... Apart from
that, most variations of the 7805 are only rated to 1 amp, although there
are exceptions.
When you cobbled together your test supply, did you mount the regulator on a
heatsink, and most importantly, did you place a 4u7 cap, paralleled with a
0u1 cap directly across the output and ground pins of the regulator, as
close to the device as you could get them? This is *very* important, to
prevent the regulator bursting into vicious ultrasonic or higher
oscillation. It may be just that you reached a current level where this
happened, and the meter didn't know what to do with what it was measuring,
so just displayed some meaningless nonsense. These regs do get very hot when
they oscillate. Although not strictly necessary, it's considered good 'belt
and braces' to put a 0u1 cap between input and ground pins as well.
The more I think about the 'problem' that this board has, the more non
'real-world' it seems. I can't really see any reason why three unrelated
fuses, on three unrelated rails, should have blown, unless there was a
serious and easily visible problem. The only thing that did occur to me was
input voltage. You're not by any chance firing the thing up on UK 240v
power, and its transformer is actually rated for 100 or 110v ? That would do
it ...
Arfa
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Author: robbDate: 07:32 16-11-07
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"Arfa Daily" <arfa.daily@ntlworld.com> wrote in message
news:SVd%i.24443$6v.23228@newsfe2-gui.ntli.net...
>
> "robb" <some@where.on.net> wrote in message
> news:13jq1ck2o3e8n21@corp.supernews.com...
> > "robb" <some@where.on.net> wrote in message
> > news:13iv5djrkhk1u04@corp.supernews.com...
> >> i have a micro-controller
> >> i believe the 32v shorted with 5V line
> >> So....where does one start to diagnose such a *big* problem
?
> >>
> >
> > i cobbled a simple 5v power supply from a 18 VAC/2.5A wall
> > wart, a 100V/1.5A bridge and 7805 5V regulator plus some
> > resitors and a 6V bulb to test.
> >
> > I connected the 5v (through 450 Ohm) to the PCB 5v and 0v
lines.
> > i measured ~60 mA curent nothing warmed ...i idecremented the
> > resistance by 100 for each iteration of testing the measured
> > currernt increased slightly until i made it to (100 Ohms) and
> > then the current spiked to ~+ 1.5A and the 7805 fried ?
> >
> > but still no heat on the board anywhere, no trace no
component no
> > IC ?
> >
> > do i need to do something different with power supply test ?
> > thanks for any help,
> > robb
> >
> Do you mean that the 7805 got hot and shut down ? One of those
should never
> "fry", because they are SOA protected. I suspect that something
else
> happened there, because if you do the math, it is impossible
for 1.5 amps to
> flow through a 100 ohm resistor, with only 5v driving it ...
Apart from
> that, most variations of the 7805 are only rated to 1 amp,
although there
> are exceptions.
>
Thanks for help Arfa,
I am describing from memory, the last thing i recall was
decrement too the 100 Ohm resistor and the amp meter's numbers
were jumping around and 1.5 is the largest number i caught with
my eye.
as for fried 7805 ... the 100 ohm resistor started to smoke and i
think i saw the 7805 let out a puff of smoke and now the 7805
does not produce output ? so i assumed it was fried
this all happened very quickly
>
> When you cobbled together your test supply, did you mount the
regulator on a
> heatsink,
>
the pcb i took it from had a small flat square of aluminum bolted
to it 1.5cm x 1.5cm x 1 mm
> and most importantly, did you place a 4u7 cap, paralleled with
a
> 0u1 cap directly across the output and ground pins of the
regulator, as
> close to the device as you could get them?
>
no
>
> This is *very* important, to prevent the regulator bursting
> into vicious ultrasonic or higher oscillation.
>
uh-oh,
>It may be just that you reached a current level where this
> happened, and the meter didn't know what to do with what it was
measuring,
> so just displayed some meaningless nonsense. These regs do get
very hot when
> they oscillate. Although not strictly necessary, it's
considered good 'belt
> and braces' to put a 0u1 cap between input and ground pins as
well.
>
i'll try the test again with those mods
>
> The more I think about the 'problem' that this board has, the
more non
> 'real-world' it seems. I can't really see any reason why three
unrelated
> fuses, on three unrelated rails, should have blown, unless
there was a
> serious and easily visible problem. The only thing that did
occur to me was
> input voltage. You're not by any chance firing the thing up on
UK 240v
> power, and its transformer is actually rated for 100 or 110v ?
That would do
> it ...
>
it is 115v plugged into 115v supply
i suppose if i could address the issue of the seemingly unrelated
supply rails i could probably figure it out
if this info helps ....
the machine *was working* when i replaced a VFD display driver
chip. That was easy problem to fix as it was very specific. I
started with closest components, checked pin signals with oscope
and i had a datasheet for guidance. Everything was working i just
needed to bolt it together.
*BUT* i found a yucky ring in peaks of the clock signal and
thought i should do something to repair that (find the source of
ring) again more very easy specific tasks, just follow traces and
check for expected behavior what goes in and what comes out
well i forgot to put the oscope probe tip guard on (to prevent
shorting pins) and i bridged the 32v VFD supply line to the 5V
rail (a little snap) and all sorts fo jitters came out of the
connected devices, i disconnected all those devices and i found
one lone fuse between a bridge and the transformer was blown i
think the 8V or 16V line that supplied the voltage regulators and
servo controler chips {L387,L298}
i replaced fuse , powered on and all three fuses blew
now i am here with very nebulous problem with no specific ideas
about how to repair
looking for help
thanks again for your help arfa,
robb
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Author: robbDate: 09:12 16-11-07
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"Arfa Daily" <arfa.daily@ntlworld.com> wrote in message
news:SVd%i.24443$6v.23228@newsfe2-gui.ntli.net...
>
> "robb" <some@where.on.net> wrote in message
> news:13jq1ck2o3e8n21@corp.supernews.com...
> > "robb" <some@where.on.net> wrote in message
> > news:13iv5djrkhk1u04@corp.supernews.com...
> >> i have a micro-controller
> >> i believe the 32v shorted with 5V line
> >> So....where does one start to diagnose such a *big* problem
?
> >>
> >
> > i cobbled a simple 5v power supply from a 18 VAC/2.5A wall
> > wart, a 100V/1.5A bridge and 7805 5V regulator plus some
> > resitors and a 6V bulb to test.
> >
> > I connected the 5v (through 450 Ohm) to the PCB 5v and 0v
lines.
> > i measured ~60 mA curent nothing warmed ...i idecremented the
> > resistance by 100 for each iteration of testing the measured
> > currernt increased slightly until i made it to (100 Ohms) and
> > then the current spiked to ~+ 1.5A and the 7805 fried ?
> >
> > but still no heat on the board anywhere, no trace no
component no
> > IC ?
> >
> > do i need to do something different with power supply test ?
> > thanks for any help,
> > robb
> >
> Do you mean that the 7805 got hot and shut down ? One of those
should never
> "fry", because they are SOA protected. I suspect that something
else
> happened there, because if you do the math, it is impossible
for 1.5 amps to
> flow through a 100 ohm resistor, with only 5v driving it ...
Apart from
> that, most variations of the 7805 are only rated to 1 amp,
although there
> are exceptions.
>
Thanks for help Arfa,
I am describing from memory, the last thing i recall was
decrement to the 100 Ohm resistor and the amp meter's numbers
were jumping around and 1.5 is the largest number i caught with
my eye.
as for fried 7805 ... the 100 ohm resistor started to smoke and i
think i saw the 7805 let out a puff of smoke and now the 7805
does not produce output ? so i assumed it was fried
this all happened very quickly
>
> When you cobbled together your test supply, did you mount
> the regulator on a heatsink,
>
well the regulator had a small flat square of aluminum bolted
to it 1.5cm x 1.5cm x 1 mm when i pulled it from some other pcb
> and most importantly, did you place a 4u7 cap, paralleled
> with 0u1 cap directly across the output and ground pins of
> the regulator, as
> close to the device as you could get them?
>
no
>
> This is *very* important, to prevent the regulator bursting
> into vicious ultrasonic or higher oscillation.
>
ohhhh
> It may be just that you reached a current level where this
> happened, and the meter didn't know what to do with what it
> was measuring, so just displayed some meaningless
> nonsense. These regs do get very hot when
> they oscillate. Although not strictly necessary,
> it's considered good 'belt and braces' to put a 0u1
> cap between input and ground pins as well.
>
i'll try the test again with those cap mods
>
> The more I think about the 'problem' that this board has,
> the more non 'real-world' it seems. I can't really see
> any reason why three unrelated
> fuses, on three unrelated rails, should have blown,
> unless there was a serious and easily visible
> problem. The only thing
> that did occur to me was input voltage.
> You're not by any chance firing the thing up
> on UK 240v power, and its transformer is actually
> rated for 100 or 110v ?That would do
> it ...
>
it is 115v plugged into 115v supply
i wish i knew enough to speculate the connection , for now i
can only report what i see and do. i suppose if i could address
the issue of the seemingly unrelated supply rails i could
probably figure it out
FWIW, if this info helps ....
the machine *was working* when i replaced a VFD display driver
chip. That was an easy problem to fix as it was very specific. I
started with components that were related to the VFD (ie. driver
chip) , i checked pin signals with oscope and i had a datasheet
for guidance. Driver chip had 6 bad lines that would not allow
user button presses to be seen by main board. I replace the DIP40
with a PLCC chip and Everything was working i just needed to
re-assemble and bolt the pieces together.
***BUT***
i stumbled across a "yucky" ring in the peaks of the clock signal
to the VFD driver chip and i thought i should do something to
repair that (ie find the source of ring) again more very easy
specific tasks, just follow traces and check for expected
behavior (in vs outs) what goes in and what comes out
well i forgot to put the oscope probe tip guard on (to prevent
shorting pins) and i bridged the 32v VFD supply line to the 5V
rail (a little arc snap) and all sorts of jitters came out of the
connected devices, i disconnected all those devices and i found
one lone fuse between a bridge and the transformer was blown. i
think the 8V or 16V line that supplied the voltage regulators and
servo controler chips {L387,L298, etc}
i replaced the one fuse, powered on and all three fuses blew
since then i have checked transformer output (ok), desoldered the
L298/L387 ics, continuity tests on various parts of PCB (73 Ohms
between 5v and 0V all around the board), tried to feed 5v into
the 5v line as suggested on the group to feel for warming
components,
now i am here with a very nebulous problem with no specific ideas
about how to repair
and looking for help
thanks again for your help arfa,
robb
BTW there are pictures of all this stuff on
(alt.binaries.schematic.electronics)
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Author: robbDate: 09:12 16-11-07
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"Arfa Daily" <arfa.daily@ntlworld.com> wrote in message
news:SVd%i.24443$6v.23228@newsfe2-gui.ntli.net...
>
> "robb" <some@where.on.net> wrote in message
> news:13jq1ck2o3e8n21@corp.supernews.com...
> > "robb" <some@where.on.net> wrote in message
> > news:13iv5djrkhk1u04@corp.supernews.com...
> >> i have a micro-controller
> >> i believe the 32v shorted with 5V line
> >> So....where does one start to diagnose such a *big* problem
?
> >>
> >
> > i cobbled a simple 5v power supply from a 18 VAC/2.5A wall
> > wart, a 100V/1.5A bridge and 7805 5V regulator plus some
> > resitors and a 6V bulb to test.
> >
> > I connected the 5v (through 450 Ohm) to the PCB 5v and 0v
lines.
> > i measured ~60 mA curent nothing warmed ...i idecremented the
> > resistance by 100 for each iteration of testing the measured
> > currernt increased slightly until i made it to (100 Ohms) and
> > then the current spiked to ~+ 1.5A and the 7805 fried ?
> >
> > but still no heat on the board anywhere, no trace no
component no
> > IC ?
> >
> > do i need to do something different with power supply test ?
> > thanks for any help,
> > robb
> >
> Do you mean that the 7805 got hot and shut down ? One of those
should never
> "fry", because they are SOA protected. I suspect that something
else
> happened there, because if you do the math, it is impossible
for 1.5 amps to
> flow through a 100 ohm resistor, with only 5v driving it ...
Apart from
> that, most variations of the 7805 are only rated to 1 amp,
although there
> are exceptions.
>
Thanks for help Arfa,
I am describing from memory, the last thing i recall was
decrement to the 100 Ohm resistor and the amp meter's numbers
were jumping around and 1.5 is the largest number i caught with
my eye.
as for fried 7805 ... the 100 ohm resistor started to smoke and i
think i saw the 7805 let out a puff of smoke and now the 7805
does not produce output ? so i assumed it was fried
this all happened very quickly
>
> When you cobbled together your test supply, did you mount
> the regulator on a heatsink,
>
well the regulator had a small flat square of aluminum bolted
to it 1.5cm x 1.5cm x 1 mm when i pulled it from some other pcb
> and most importantly, did you place a 4u7 cap, paralleled
> with 0u1 cap directly across the output and ground pins of
> the regulator, as
> close to the device as you could get them?
>
no
>
> This is *very* important, to prevent the regulator bursting
> into vicious ultrasonic or higher oscillation.
>
ohhhh
> It may be just that you reached a current level where this
> happened, and the meter didn't know what to do with what it
> was measuring, so just displayed some meaningless
> nonsense. These regs do get very hot when
> they oscillate. Although not strictly necessary,
> it's considered good 'belt and braces' to put a 0u1
> cap between input and ground pins as well.
>
i'll try the test again with those cap mods
>
> The more I think about the 'problem' that this board has,
> the more non 'real-world' it seems. I can't really see
> any reason why three unrelated
> fuses, on three unrelated rails, should have blown,
> unless there was a serious and easily visible
> problem. The only thing
> that did occur to me was input voltage.
> You're not by any chance firing the thing up
> on UK 240v power, and its transformer is actually
> rated for 100 or 110v ?That would do
> it ...
>
it is 115v plugged into 115v supply
i wish i knew enough to speculate the connection , for now i
can only report what i see and do. i suppose if i could address
the issue of the seemingly unrelated supply rails i could
probably figure it out
FWIW, if this info helps ....
the machine *was working* when i replaced a VFD display driver
chip. That was an easy problem to fix as it was very specific. I
started with components that were related to the VFD (ie. driver
chip) , i checked pin signals with oscope and i had a datasheet
for guidance. Driver chip had 6 bad lines that would not allow
user button presses to be seen by main board. I replace the DIP40
with a PLCC chip and Everything was working i just needed to
re-assemble and bolt the pieces together.
***BUT***
i stumbled across a "yucky" ring in the peaks of the clock signal
to the VFD driver chip and i thought i should do something to
repair that (ie find the source of ring) again more very easy
specific tasks, just follow traces and check for expected
behavior (in vs outs) what goes in and what comes out
well i forgot to put the oscope probe tip guard on (to prevent
shorting pins) and i bridged the 32v VFD supply line to the 5V
rail (a little arc snap) and all sorts of jitters came out of the
connected devices, i disconnected all those devices and i found
one lone fuse between a bridge and the transformer was blown. i
think the 8V or 16V line that supplied the voltage regulators and
servo controler chips {L387,L298, etc}
i replaced the one fuse, powered on and all three fuses blew
since then i have checked transformer output (ok), desoldered the
L298/L387 ics, continuity tests on various parts of PCB (73 Ohms
between 5v and 0V all around the board), tried to feed 5v into
the 5v line as suggested on the group to feel for warming
components,
now i am here with a very nebulous problem with no specific ideas
about how to repair
and looking for help
thanks again for your help arfa,
robb
BTW there are pictures of all this stuff on
(alt.binaries.schematic.electronics)
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Author: cliftoDate: 11:07 16-11-07
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robb wrote:
> I connected the 5v (through 450 Ohm) to the PCB 5v and 0v lines.
> i measured ~60 mA curent nothing warmed ...
That's pretty good. If you then shorted the PCB 5V and 0V lines, you would
have 450 ohms across 5V and should read 11.1 ma.
> i idecremented the
> resistance by 100 for each iteration of testing the measured
> currernt increased slightly until i made it to (100 Ohms) and
> then the current spiked to ~+ 1.5A and the 7805 fried ?
To get 1.5A through 100 ohms you need to apply at least 150V to it.
--
Angry American flags attack Hillary Clinton!
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Author: robbDate: 13:11 16-11-07
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"clifto" <clifto@gmail.com> wrote in message
news:mqr015-1b8.ln1@remote.clifto.com...
> robb wrote:
> > I connected the 5v (through 450 Ohm) to the PCB 5v and 0v
lines.
> > i measured ~60 mA curent nothing warmed ...
>
> That's pretty good. If you then shorted the PCB 5V and 0V
lines, you would
> have 450 ohms across 5V and should read 11.1 ma.
>
> > i idecremented the
> > resistance by 100 for each iteration of testing the measured
> > currernt increased slightly until i made it to (100 Ohms) and
> > then the current spiked to ~+ 1.5A and the 7805 fried ?
>
> To get 1.5A through 100 ohms you need to apply at least 150V to
it.
>
thanks for reply clifto,
i will have to believe you on the numbers
and although the numbers may seem incorrect from a electronic
formulaic analysis i can only report what i see on the equipment
i use and with the setup i use.
The DMM amp reading numbers were fluctuating wildly on my last
test and 1.57 ???was something i thought flashed by as highest #
maybe it was .57 but i do know that whatever the number the 100
ohm resistor had a stream of smoke going up and the 7805 appeared
to have a puff of smoke released before i could dis-connect now
i am hopping somebody can help me decipher what went wrong with
my testing
even operator errors would be useful as long as it helps nme
omove toward a repair/
robb
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I'm reading this thread with some interest, but I'm more puzzled as it
goes on, there should be something we are (and the OP is) missing.
On 16 Nov, 19:11, "robb" <s...@where.on.net> wrote:
> "clifto" <cli...@gmail.com> wrote in message
>
...
...
> it.
>
> thanks for reply clifto,
>
> i will have to believe you on the numbers
>
> and although the numbers may seem incorrect from a electronic
> formulaic analysis i can only report what i see on the equipment
> i use and with the setup i use.
>
> The DMM amp reading numbers were fluctuating wildly on my last
> test and 1.57 ???was something i thought flashed by as highest #
> maybe it was .57 but i do know that whatever the number the 100
> ohm resistor had a stream of smoke going up and the 7805 appeared
> to have a puff of smoke released before i could dis-connect now
> i am hopping somebody can help me decipher what went wrong with
> my testing
>
hmm 78xx don't smoke easily, maybe if you apply a too high input
voltage (> 30V I believe) or if it goes into self-oscillation if an
output capacitor is omitted. A 100 ohm resistor, assuming we are
talking about a 1/4 W one or bigger, shouldn't smoke easily with only
5V even on a short to ground, it should become quite hot but not smoke
instantly, if it smoked I believe the input voltage was higher.
Power supply problems shouldn't be that difficult to troubleshoot,
even if once I had almost the same problem with a simple three tubes
amplifier, but that was another story :-)
Regards
Francesco
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Author: ian fieldDate: 13:41 16-11-07
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<francesco.messineo@gmail.com> wrote in message
news:93c88014-5860-4492-8dd3-19290c5d3a0c@n20g2000hsh.googlegroups.com...
> I'm reading this thread with some interest, but I'm more puzzled as it
> goes on, there should be something we are (and the OP is) missing.
AIUI the OP is trying to find which/how many 5V rail devices on a heavily
populated logic board are S/C after a short between 36V & 5V feeds.
A slightly risky strategy would be to use the 5V O/P from an old (pre-3.3V)
AT PSU, which typically can supply 200A or more via progressively lower
current limiting resistors, at some point the current will be high enough to
distinctly heat any S/C components enabling rapid identification, obvious
pitfalls are S/C Unobtainable Logic Arrays, firmware ROMs the supplier
refuse to sell as spares, the risk of blowing circuit traces (unrepairable
in multi-layer boards) and the risk to eyesight from exploding S/C
capacitors!
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Author: cliftoDate: 14:20 16-11-07
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robb wrote:
> "clifto" <clifto@gmail.com> wrote...
>> robb wrote:
>> > I connected the 5v (through 450 Ohm) to the PCB 5v and 0v lines.
>> > i measured ~60 mA curent nothing warmed ...
>>
>> That's pretty good. If you then shorted the PCB 5V and 0V lines, you would
>> have 450 ohms across 5V and should read 11.1 ma.
>>
>> > i idecremented the
>> > resistance by 100 for each iteration of testing the measured
>> > currernt increased slightly until i made it to (100 Ohms) and
>> > then the current spiked to ~+ 1.5A and the 7805 fried ?
>>
>> To get 1.5A through 100 ohms you need to apply at least 150V to it.
>
> thanks for reply clifto,
>
> i will have to believe you on the numbers
>
> and although the numbers may seem incorrect from a electronic
> formulaic analysis i can only report what i see on the equipment
> i use and with the setup i use.
What I was trying to get you to realize is that there is probably a
problem with your equipment, or with your understanding of its use.
When the PCB supposedly draws over five times as much current as a
dead short, it should tell you that what you think you're seeing isn't
what is actually happening.
> The DMM amp reading numbers were fluctuating wildly on my last
> test and 1.57 ???was something i thought flashed by as highest #
> maybe it was .57 but i do know that whatever the number the 100
> ohm resistor had a stream of smoke going up and the 7805 appeared
> to have a puff of smoke released before i could dis-connect now
> i am hopping somebody can help me decipher what went wrong with
> my testing
If you used a 1/10 watt 100 ohm resistor, putting 5 volts across it
should dissipate 2-1/2 times its capability, or 0.25 W. That might
make it release smoke.
--
Angry American flags attack Hillary Clinton!
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Author: ian fieldDate: 14:34 16-11-07
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Author: robbDate: 15:16 16-11-07
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"clifto" <clifto@gmail.com> wrote in message
news:s47115-ak8.ln1@remote.clifto.com...
> robb wrote:
> > "clifto" <clifto@gmail.com> wrote...
> >> robb wrote:
> >> > I connected the 5v (through 450 Ohm) to the PCB 5v and 0v
lines.
> >> > i measured ~60 mA curent nothing warmed ...
> >>
> >> That's pretty good. If you then shorted the PCB 5V and 0V
lines, you would
> >> have 450 ohms across 5V and should read 11.1 ma.
> >>
> >> > i idecremented the
> >> > resistance by 100 for each iteration of testing the
measured
> >> > currernt increased slightly until i made it to (100 Ohms)
and
> >> > then the current spiked to ~+ 1.5A and the 7805 fried ?
> >>
> >> To get 1.5A through 100 ohms you need to apply at least 150V
to it.
> >
> > thanks for reply clifto,
> >
> > i will have to believe you on the numbers
> >
> > and although the numbers may seem incorrect from a
electronic
> > formulaic analysis i can only report what i see on the
equipment
> > i use and with the setup i use.
>
> What I was trying to get you to realize is that there is
probably a
> problem with your equipment, or with your understanding of its
use.
> When the PCB supposedly draws over five times as much current
as a
> dead short, it should tell you that what you think you're
seeing isn't
> what is actually happening.
>
[trim]
>
> If you used a 1/10 watt 100 ohm resistor, putting 5 volts
across it
> should dissipate 2-1/2 times its capability, or 0.25 W. That
might
> make it release smoke.
>
Thanks for help clifto,
i do realize what you are saying which is why i said, "i
consider operator error to be a possible problem".
this amateur horse brain needs some help getting itself in front
of the cart.
i've been told that the 73 Ohm resistance between the 5V and 0v
is not so bad and is somewhat hopeful that i did not incinerate
all the ICs.
if i can diagnose without removing the ICs then that would be
great, otherwise i may need to fall back to brute force
diagnostics and remove ICs one at atime and check things and
iterate .
i was hoping to learn something a little more sophisticated.
i was uing 1/4 watt resistors.
i plan to try the capacitors to prevent oscillation,
thanks for help and for any more ideas you may have,
robb
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Author: robbDate: 15:23 16-11-07
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<francesco.messineo@gmail.com> wrote in message
news:93c88014-5860-4492-8dd3-19290c5d3a0c@n20g2000hsh.googlegroups.com...
> I'm reading this thread with some interest, but I'm more
puzzled as it
> goes on, there should be something we are (and the OP is)
missing.
>
> On 16 Nov, 19:11, "robb" <s...@where.on.net> wrote:
> > "clifto" <cli...@gmail.com> wrote in message
> >
> > and although the numbers may seem incorrect from a
electronic
> > formulaic analysis i can only report what i see on the
equipment
> > i use and with the setup i use.
> >
> > The DMM amp reading numbers were fluctuating wildly on my
last
> > test and 1.57 ???was something i thought flashed by as
highest #
> > maybe it was .57 but i do know that whatever the number the
100
> > ohm resistor had a stream of smoke going up and the 7805
appeared
> > to have a puff of smoke released before i could dis-connect
now
> > i am hopping somebody can help me decipher what went wrong
with
> > my testing
> >
>
> hmm 78xx don't smoke easily, maybe if you apply a too high
input
> voltage (> 30V I believe) or if it goes into self-oscillation
if an
> output capacitor is omitted. A 100 ohm resistor, assuming we
are
> talking about a 1/4 W one or bigger, shouldn't smoke easily
with only
> 5V even on a short to ground, it should become quite hot but
not smoke
> instantly, if it smoked I believe the input voltage was higher.
>
thanks for help Francesco,
i do not under-estimate my ability to mis-understand.
so i consider myself missing something as distinct possibility.
but what ?
source is a 18-24v ac 1000mA wall wart that feeds the bridge and
then 7805.
>
> Power supply problems shouldn't be that difficult to
troubleshoot,
> even if once I had almost the same problem with a simple three
tubes
> amplifier, but that was another story :-)
>
someoner suggested that i add caps to prevent oscillation i will
try and i hope to give more accurate report and numbers after i
make the change.
thanks for help francesco and any other ideas you may have,
robb
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Author: robbDate: 15:29 16-11-07
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"ian field" <dai.ode@ntlworld.com> wrote in message
news:svl%i.37809$uH.577@newsfe4-gui.ntli.net...
>
> <francesco.messineo@gmail.com> wrote in message
>
news:93c88014-5860-4492-8dd3-19290c5d3a0c@n20g2000hsh.googlegroups.com...
> > I'm reading this thread with some interest, but I'm more
puzzled as it
> > goes on, there should be something we are (and the OP is)
missing.
>
> AIUI the OP is trying to find which/how many 5V rail devices on
a heavily
> populated logic board are S/C after a short between 36V & 5V
feeds.
>
> A slightly risky strategy would be to use the 5V O/P from an
old (pre-3.3V)
> AT PSU, which typically can supply 200A or more via
progressively lower
> current limiting resistors, at some point the current will be
high enough to
> distinctly heat any S/C components enabling rapid
identification, obvious
> pitfalls are S/C Unobtainable Logic Arrays, firmware ROMs the
supplier
> refuse to sell as spares, the risk of blowing circuit traces
(unrepairable
> in multi-layer boards) and the risk to eyesight from exploding
S/C
> capacitors!
>
Ian has the problem packaged precisely in a nutshell.
but i don't remember the explosion risks better get my safety
glasses.
having said that ian i think i will desolder the rom, ram, and
8031 just in case they are ok and are not replaceable as you
point out.
the board was mfg mid 80's so i am pretty certain it is only two
layer board.
thanks for the nutshell version,
robb
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Author: ian fieldDate: 16:31 16-11-07
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"robb" <some@where.on.net> wrote in message
news:13jrvakamk9ii1b@corp.supernews.com...
>
> "ian field" <dai.ode@ntlworld.com> wrote in message
> news:svl%i.37809$uH.577@newsfe4-gui.ntli.net...
>>
>> <francesco.messineo@gmail.com> wrote in message
>>
> news:93c88014-5860-4492-8dd3-19290c5d3a0c@n20g2000hsh.googlegroups.com...
>> > I'm reading this thread with some interest, but I'm more
> puzzled as it
>> > goes on, there should be something we are (and the OP is)
> missing.
>>
>> AIUI the OP is trying to find which/how many 5V rail devices on
> a heavily
>> populated logic board are S/C after a short between 36V & 5V
> feeds.
>>
>> A slightly risky strategy would be to use the 5V O/P from an
> old (pre-3.3V)
>> AT PSU, which typically can supply 200A or more via
> progressively lower
>> current limiting resistors, at some point the current will be
> high enough to
>> distinctly heat any S/C components enabling rapid
> identification, obvious
>> pitfalls are S/C Unobtainable Logic Arrays, firmware ROMs the
> supplier
>> refuse to sell as spares, the risk of blowing circuit traces
> (unrepairable
>> in multi-layer boards) and the risk to eyesight from exploding
> S/C
>> capacitors!
>>
>
> Ian has the problem packaged precisely in a nutshell.
>
> but i don't remember the explosion risks better get my safety
> glasses.
>
> having said that ian i think i will desolder the rom, ram, and
> 8031 just in case they are ok and are not replaceable as you
> point out.
>
> the board was mfg mid 80's so i am pretty certain it is only two
> layer board.
>
> thanks for the nutshell version,
> robb
In my experience of situations like this (such as regulation failure in a
SMPSU) its quite common for a small number of devices - sometimes as few as
1, to go S/C and "crowbar" the rail effectively protecting the remainder of
devices. Occasionally a few devices will blow O/C instantaneously after
going S/C, which can lead to further problems after the S/C devices are
replaced - if the Vcc bond wire to the S/C die blows, the I/P & O/P pins cn
the S/C die can load the O/Ps of other chips connected to it, this can be
tedious to trace on bussed logic, its possible to snip the tops of DIL chip
pins to isolate the chip while leaving enough pin on the chip body to solder
the pin back on to (you can always replace the chip later if you want to)
with the pin isolated you can use a pulse injector and scope to find if the
snipped chip or something else on the bus line is still holding it down.
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Author: Arfa DailyDate: 17:50 16-11-07
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<snip much>
> i plan to try the capacitors to prevent oscillation,
>
That is the most important thing that you must do before going any further.
If not properly bypassed, 78 series regulators will *readily* oscillate
violently, particularly if you have long leads attached to them. Connect 0u1
between "in" and ground, and 4u7 in parallel with 0u1 between "out"
and
ground. It is *essential* that these are soldered *directly* to the pins of
the device, not some distance away down the connecting wires.
Arfa
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Author: cliftoDate: 18:41 16-11-07
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Arfa Daily wrote:
> That is the most important thing that you must do before going any further.
> If not properly bypassed, 78 series regulators will *readily* oscillate
> violently, particularly if you have long leads attached to them. Connect 0u1
> between "in" and ground, and 4u7 in parallel with 0u1 between
"out" and
> ground. It is *essential* that these are soldered *directly* to the pins of
> the device, not some distance away down the connecting wires.
Might as well add the back-bias protection diode with anode at "out" and
cathode at "in" while he's at it.
--
Angry American flags attack Hillary Clinton!
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Author: Arfa DailyDate: 20:57 16-11-07
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"clifto" <clifto@gmail.com> wrote in message
news:nem115-ne9.ln1@remote.clifto.com...
> Arfa Daily wrote:
>> That is the most important thing that you must do before going any
>> further.
>> If not properly bypassed, 78 series regulators will *readily* oscillate
>> violently, particularly if you have long leads attached to them. Connect
>> 0u1
>> between "in" and ground, and 4u7 in parallel with 0u1 between
"out" and
>> ground. It is *essential* that these are soldered *directly* to the pins
>> of
>> the device, not some distance away down the connecting wires.
>
> Might as well add the back-bias protection diode with anode at "out" and
> cathode at "in" while he's at it.
>
Yeah, if you like, though probably an unnecessary complication for this test
setup. Going back to the story of how this unfortunate situation arose, I
think that there is a valuable lesson for Robb to add to what he's learning
here, and that is "If it ain't broke, don't fix it ..." The fact that it was
working after replacing the VFD driver chip, should have been an end to the
job. It obviously didn't matter that the clock was a bit scruffy, as
observed on the 'scope, as the item was managing to use it satisfactorily.
Depending on how fast the clock is, and how good a probe was being used on
the 'scope, the signal may not even have been 'wrong' in the first place. It
might just have *looked* as though it was. It's pretty likely that any clock
inputs on the chips are internally schmitt'd anyway, to make sure that a
clean signal is being passed on into the chip's circuitry.
Arfa
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Author: EeyoreDate: 22:51 16-11-07
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Arfa Daily wrote:
> Apart from that, most variations of the 7805 are only rated to 1 amp
Not any more.
1.5A is the norm. Look at any up-to-date data sheet.
Graham
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