Current Tracer - for findinding shorts in a PCB

[Jeff Liebermann]

On Thu, 8 Apr 2010 13:25:51 -0700, DaveC <invalid@invalid.net> wrote:

I use a sheet of liquid crystal material:
http://scientificsonline.com/product.asp_Q_pn_E_3072375
Make sure the board starts cold or at least at room temperature.

Great idea, Jeff.
Which temperature product would you recommend for this use?
Dave

Note that the above web page says "Locate Electrical Shorts on Circuit
Boards" near the top of the page.

The last remaining one I have hidden from the visiting kids is marked
25-30C. It's about right for testing PCB heating. If unavailable,
the next lower range might be better. Note that you're not going to
see much linear range in temperature indication. There is a rather
abrupt transition at some temperature in the range. Areas of the
board that are hot are bright blue. Most everything else is black. In
between is a smear of red and green. If there's an air gap between
the board and the liquid crystal sheet, the affected area grows
rapidly larger and is difficult to pinpoint. At that point, I would
suggest using a IR gun type thermometer.

<http://www.teachersource.com/Chemistry/LiquidCrystal/LiquidCrystalSheets4x4inch.aspx>
<http://www.teachersource.com/Chemistry/LiquidCrystal/LiquidCrystalSheets12x12inch.aspx>
There are other sources for less $$$.

--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558
# http://802.11junk.com jeffl@cruzio.com
# http://www.LearnByDestroying.com AE6KS

 

[N_Cook]

Scrim <nospam@nospam.nospam> wrote in message
news:I7vun.199336$ti6.7573@newsfe24.ams2...

stratus46@yahoo.com> wrote in message
news:442f6f03-dad3-49d1-9c5b-7a240a31226c@q23g2000yqd.googlegroups.com...
On Apr 5, 9:49 am, "Scrim" <nos...@nospam.nospam> wrote:
Is there a DIY circuit out there for a good current tracer? I have
a
valuable multilayer pcb with a short between a power supply line
and earth I
can't find. The basic idea is to inject a string of pulses through
shorted
circuit and use a small solenoid type sensor to follow the current
path
until the short is reached.

Thanks,

Alan

Has it ever worked or is this a failure? Are you thinking incomplete
etch or shorted bypass cap?

G˛

Thanks for the ideas. This is a high precision board that used to work.
Ideally I'd like to have a thermal camera, so I like the idea of trying a
digital camera in a dark room. Does it have to be a CCD? I'm not sure what
my web cam is (cmos?), but at least the infrared filter is easy to remove
and it doesn't have autofocus, so I can set it right with the lights on
first.
It's a multilayer board so tracing with an ohm meter seems like a bit of a
non-starter as I have no idea where the tracks go, and because of it's
precision I don't want to cover it in frost, although that's a great idea
for another time.
I've already repaired an area of board under an electrolytic that leaked.
The board was carbonised all the way through, but I think I've fixed that.
I've looked at all the tantalum capacitors I can see and they all seem ok.

Alan

So as it is a populated board, how do you know it is a in-board short and
not a shorted VTS , if SM can look very inniocent, or something similar?


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://diverse.4mg.com/index.htm

 

[Scrim]

"N_Cook" <diverse@tcp.co.uk> wrote in message
news:hpnj52$ego$1@news.eternal-september.org...

Scrim <nospam@nospam.nospam> wrote in message
news:I7vun.199336$ti6.7573@newsfe24.ams2...
stratus46@yahoo.com> wrote in message
news:442f6f03-dad3-49d1-9c5b-7a240a31226c@q23g2000yqd.googlegroups.com...
On Apr 5, 9:49 am, "Scrim" <nos...@nospam.nospam> wrote:
Is there a DIY circuit out there for a good current tracer? I have
a
valuable multilayer pcb with a short between a power supply line
and earth I
can't find. The basic idea is to inject a string of pulses through
shorted
circuit and use a small solenoid type sensor to follow the current
path
until the short is reached.

Thanks,

Alan

Has it ever worked or is this a failure? Are you thinking incomplete
etch or shorted bypass cap?

G˛

Thanks for the ideas. This is a high precision board that used to work.
Ideally I'd like to have a thermal camera, so I like the idea of trying a
digital camera in a dark room. Does it have to be a CCD? I'm not sure
what
my web cam is (cmos?), but at least the infrared filter is easy to remove
and it doesn't have autofocus, so I can set it right with the lights on
first.
It's a multilayer board so tracing with an ohm meter seems like a bit of
a
non-starter as I have no idea where the tracks go, and because of it's
precision I don't want to cover it in frost, although that's a great idea
for another time.
I've already repaired an area of board under an electrolytic that leaked.
The board was carbonised all the way through, but I think I've fixed
that.
I've looked at all the tantalum capacitors I can see and they all seem
ok.

Alan


So as it is a populated board, how do you know it is a in-board short and
not a shorted VTS , if SM can look very inniocent, or something similar?


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://diverse.4mg.com/index.htm

In fact I found the shorted component was a surface mount diode connected
directly across the capacitor on the reverse of the board, which I've
replaced. Now to connect it all up again and see if everything's working
again...

Alan

 

[DaveC]

In fact I found the shorted component was a surface mount diode connected
directly across the capacitor on the reverse of the board, which I've
replaced. Now to connect it all up again and see if everything's working
again...

Alan

And not one word about how you went about identifying the diode?

Dave

 

[N_Cook]

DaveC <invalid@invalid.net> wrote in message
news:0001HW.C7E5546000C3C3C3B01AD9AF@news.eternal-september.org...

In fact I found the shorted component was a surface mount diode
connected
directly across the capacitor on the reverse of the board, which I've
replaced. Now to connect it all up again and see if everything's working
again...

Alan

And not one word about how you went about identifying the diode?

Dave

So a TVS, but uni or bipolar ? and what caused the high A-t transient to
knock it out ? or it will likely reoccur


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://diverse.4mg.com/index.htm

 

[Scrim]

"DaveC" <invalid@invalid.net> wrote in message
news:0001HW.C7E5546000C3C3C3B01AD9AF@news.eternal-september.org...

In fact I found the shorted component was a surface mount diode connected
directly across the capacitor on the reverse of the board, which I've
replaced. Now to connect it all up again and see if everything's working
again...

Alan

And not one word about how you went about identifying the diode?

Dave

Good point, sorry! I used an old HP 546A Logic Pulser and an HP 547A Current
Tracer. Designed for use with logic circuits, but perfect for this job. You
can pick them up quite cheaply on eBay sometimes. Superb bit of kit, except
the labels - don't try to peel the plastic layer off. It looks like that
protective peel off plastic, but it's not!

Alan

 

[Scrim]

"N_Cook" <diverse@tcp.co.uk> wrote in message
news:hpp8et$fdq$1@news.eternal-september.org...

DaveC <invalid@invalid.net> wrote in message
news:0001HW.C7E5546000C3C3C3B01AD9AF@news.eternal-september.org...
In fact I found the shorted component was a surface mount diode
connected
directly across the capacitor on the reverse of the board, which I've
replaced. Now to connect it all up again and see if everything's
working
again...

Alan

And not one word about how you went about identifying the diode?

Dave


So a TVS, but uni or bipolar ? and what caused the high A-t transient to
knock it out ? or it will likely reoccur


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://diverse.4mg.com/index.htm



I can only try to fix what I can and hope for the best...

Alan