| Electronics > Repair |
| DIY magnetic probe to find a short? |
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| systemloc:
Trying to repair my 80's vintage function generator, an IEC F72. Found the 5V rail was low. Found it was actually shorted when I pulled the three terminal regulator and connected my lab supply and saw it current limit!! I have a schematic, and the board is a beautiful 80's through hole lab quality piece. The voltage rails are supplied through a few jumpers here and there, so I desoldered the jumpers to isolate the portion of the board with the short. Unfortunately, I isolated it to a still large section of the circuit, and nothing is getting hot!! Obviously, I checked the electrolytics, they are still fine. I tried using air in a can to put frost on the board and look for something warm, but I'm stumped. One difficulty is that the circuit branches to two subsections from the jumper I have isolated the problem to. I was thinking I could wind some wire around a piece of ferrite and use it with an oscilloscope to see an increase in potential and crudely determine which trace is carrying the several amps of current, and maybe follow it. Anyone have any experience or tips? Would that even work for detecting DC with sensitivity sufficient to track the current? Thanks for any advice! KI4TJ |
| xavier60:
I find shorts on laptop main boards by applying some current to the shorted power rail, then measuring voltage drops on the ground plane. This avoids having to know how the power rail track is routed. |
| helius:
See also this thread, this page, and this video. |
| duak:
hp used to make a current probe, the 428x, that used a ferrite core as a flux gate magnetometer. Applied Physics makes a newer version. Here's a link to one person's experience with the ilk: http://www.prc68.com/I/HP428.html The article discusses how it works and alternate solutions. Low voltage ceramic and tantalum caps can also short circuit. Since this is a 5 V circuit, could there be any transorbs or ICs that have failed? A good 4 1/2 digit DMM can easily show the mV voltage drops through typical traces with currents on the order of 100 mA. It may take a little while to puzzle it out though and a map with the various voltage readings can make it easier. Cheers, |
| Kleinstein:
A sensitive measurement of DC current by the magnetic field is difficult. It needs a reasonable higher current to make it work. If enough current is used, one might be able to use a good hall effect of GMR based sensor. A flux gate probe is tricky to get very small and might be influenced by other effects (like ferrites at larger distance, mybe eddy currents). Hall effect sensors tend to have quite some drift. The alternative way to trace current flow is looking at the low voltage drop, so using a high resolution voltmeter and the copper resistance. |
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