Current Tracer - for findinding shorts in a PCB

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

Reply to
Scrim
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Sometimes, a low ohms meter such as the Bob Parker ESR meter, can be a valuable aid to finding PCB shorts. Also, allowing a limited non-damaging current to flow round the PCB and through the short, can cause a rise in track temperatures, that can be seen in the dark, with a reasonably sensitive CCD camera.

Arfa

Reply to
Arfa Daily

I purchased a Cen-Tec non contact laser thermometer that would work well to measure track temperatures. I use it to find hot running semiconductors and capacitors and just about everything else. They are about $30 USD and have helped me track down lots of problems.

Reply to
Meat Plow

Stick the board in the freezer. When you remove it and apply current limited power to the suspected circuit, the frost will melt showing just where the short is.

Reply to
tm

On Apr 5, 9:49=A0am, "Scrim" 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. >

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

G=B2

Reply to
stratus46

"tm" wrote in news:hpd96c$71g$ snipped-for-privacy@speranza.aioe.org:

Spray freeze is faster, or if you have a can of freon "duster" around, turn it upside down (it's the same stuff). Hose down the suspected area, turn on a current limited supply across the short, and Voila!

Doug White

Reply to
Doug White

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

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

Reply to
Scrim

A normal CCD infrared camera will not see the heat you are looking for. Only a thermal imager will do that and they are $$$$$$$ not cheap. Maybe you could rent one?

It would be nice if you had the board layout. A good DVM on the mV range could find the short. Or the ESR meter suggested by someone else would work.

Reply to
tm

I dunno about DIY circuits, but a circuit tracer and milli-ohm meter should be very helpful in trying to find a low resistance path on a board without an schematic.

If you're lucky, and/or patient, you might find one cheaply on eBay or elsewhere.

Toneohm is one brand name for shorts locators, and I bought one of their older models years ago for about $20 on eBay.

The circuit-tracing types of shorts locators generally have a digital display, but the helpful feature is the tone ouput, which allows the operator to test circuit locations without watching the display, as the tone changes as the test points get closer to the short.

The models I've seen use 4-wire Kelvin probe sets, which allows the circuit tester to ignore circuit component resistance values.

-- Cheers, WB .............

Reply to
Wild_Bill

t
a
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I've replaced lots of physically leaking 'lytics and where there is one, there are usually more. If you don't initially see the 'puddle', look for brown 'mist' around 'lytics. You might try powering it up and see if you can read the Voltage variations around the board. You might find a single cap that decided to be a wire. Tantalums are known to do this and I replaced one (32 years old) just last week. Monolythic ceramics fail this way sometimes too. Once in 40 years of doing this I had a 'tytic that shorted out.

G=B2

Reply to
stratus46

If it were me, I'd want to fix it via any means possible and worry about calibration later.

Reply to
Fester Bestertester

h I

d

To find such a short magnetically, you should use a "point" sensor [see below], not a coil. You really need to get in close to the path. Don't go too high, else the GND planes will start to attenuate. The

1kHz to 2 kHz range is fine. Pulsing at a high enough rate will move you out of the low 1/f noise range, and these sensors are much smaller than any coil you can put in there, short of using a SQUID.

These people make such a sensor and are knowledgeable: NVE Analog Sensor Catalog, NVE Corporation, 11409 Valley View Rd, Eden Prairie, MN 55344

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For the pusling supply, you might even get away with using something as simple as a function generator. When shorted, like clip lead to clip lead that constructs a 1 inch diameter they can supply around 100 mA, and I've measured that at distances of over 3 feet away.

Depending on how skilled you are [and determined to make this cheaply on your own] you could do this using your computer's soundcard and coils.

Reply to
Robert Macy

Fester Bestertester wrote in news: snipped-for-privacy@news.eternal-september.org:

hey,you can frost it,then bake it dry in an oven.

--
Jim Yanik
jyanik
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Reply to
Jim Yanik

Robert Macy Inscribed thus:

A scrap HDD RW head works well.

--
Best Regards:
                     Baron.
Reply to
Baron
[about finding a short in a printed wiring board]

ted

t

Great idea! Any old AC wallwart is a good current-limited power source, I've got a bin of those ... around here ... somewhere...

Reply to
whit3rd

I use a sheet of liquid crystal material:

Make sure the board starts cold or at least at room temperature. The reason I only have one sheet is that all my friends (and their kids) have pillaged the rest of my collection. It doesn't work on the component side of the board, but works quite well at locating hot spots on the circuit side. Otherwise, an IR thermometer works well. Try to get the 12:1 optics, not the 8:1.

--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558
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Reply to
Jeff Liebermann

HP made a current pulser and magnetic current sensor that can work in these situations. Another way is to stuff current in the power leads and use a high-resolution voltmeter to find the point of lowest voltage. A tek 576 curve tracer makes a 4-terminal measurement and you can see it on the scope screen. And when you get close you can get lots of amps at low voltage to burn out the short. High-resolution time-domain-reflectometer can be used, but you have to probe between caps...and be extremely careful you don't blow the TDR on residual board volts.

I've used a thermal imager for this. That's the best way if you can get access to one. Shorts between ground plane areas are very difficult to pinpoint without thermal sensing.

Reply to
mike

h I

d

A lot of good suggestions and I have one other. In our factory the most useful tool we had for power/ground shorts was the current limited power supply and a sheet of temperature sensitive liquid crystal, same stuff used in the mood ring craze of the 70's I believe. At the time Edmund Scientific was our source for the stuff.

Set the current limit to a "safe" level and lay the LC sheet on top of the component side of the board and watch for a color change in the LC sheet.

We had several methods for tracing shorts, including the Tone Ohm tracer but the LC sheets were easy to use and worked best for power/ ground shorts.

Good Luck, Rush

Reply to
rush14

A great selection of ideas. many thanks to all who contributed. The board is a 7 1/2 digit multimeter so I'd prefer not to do anything that might upset it if I can avoid it, but if all else fails it will go in the freezer! I look forward to thermal cameras becoming cheap enough to own, but in the mean time I'm going to try a combination of the other ideas presented here until solve it. Thanks for the help,

Alan

Reply to
Scrim

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

Dave

Reply to
DaveC

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