Current Tracer - for findinding shorts in a PCB
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From: mike on 7 Apr 2010 03:05 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. > > Thanks, > > Alan 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.
From: rush14 on 7 Apr 2010 12:15 On Apr 5, 12:49 pm, "Scrim" <nos...(a)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 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
From: Scrim on 7 Apr 2010 12:30 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
From: DaveC on 8 Apr 2010 16:25 > 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
From: Jeff Liebermann on 8 Apr 2010 18:19
On Thu, 8 Apr 2010 13:25:51 -0700, DaveC <invalid(a)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(a)cruzio.com # http://www.LearnByDestroying.com AE6KS |