Short circuit finder: what if ... - Page 1
Forum section: electronics / projects

author: saronno
author_rank: Regular Contributor
timestamp: February 16, 2023, 01:48:37 pm
content: Is a magnetic head from a old cassette recordersensible enough to pick up small magnetic field generated from a bad component?In alternative, what about a hall sensors?Probably there is already something like that, but I don't know ... so I ask you ...

author: hanakp
author_rank: Regular Contributor
timestamp: February 16, 2023, 04:07:20 pm
content: This has existed for decades, HP 547A, for example:You can even find its schematic online, but you will have trouble sourcing the parts. Tape heads are too bulky, but I suppose head from old HDD (pre-GMR) or floppy disk drive could serve as the sensor.

author: MarkL
author_rank: Unknown
timestamp: February 16, 2023, 05:54:52 pm
content: Here's a high-end, more modern version:https://www.aimtti.com/product-category/current-probes/aim-i-prober-520It uses a fluxgate magnetometer for current sensing. It's directional, so you can also determine which way current is flowing in a trace.

author: oz2cpu
author_rank: Frequent Contributor
timestamp: February 20, 2023, 10:14:02 am
content: what if :you know you have a short, but not where it is, right,so what i do is to first evaluate how much current and voltage the "problem" net can handle,normally a 1V source, current limited to 1A will be fine,apply that and measure mV DC voltage along the net, then you find it real fast.if you want fastest time to solve, take a thermal camera, and "see" where the heated component is,BUT sometimes the path to the shorted point, contain more resistance, and the short can be a super good short,in that case the thermal methode will fail.

author: ajb
author_rank: Super Contributor
timestamp: February 20, 2023, 03:58:37 pm
content: If you have a meter handy that can do 4-wire resistance measurements, that's another option. The tricky bit is the proper 4-wire probing. If you've got places to connect kelvin clips that works great, but on an all-SMT board things get a lot more challenging. A lot of "kelvin" probes that look like conventional meter probes but with two banana plugs won't work, because they make the kelvin connection inside the probe, which means they only compensate for the probe leads and not the contact resistance of the probe tip, which will swamp the resistance of the short you're trying to find. If you can use chopsticks in both hands than two pairs of conventional probes held the same way can work. There are kelvin probes with a pair of pogo pins at the tip, but they can be tricky to use on small SMT parts. Or you could make your own probes with something like bent sewing pins as the contact points so they can be placed close enough together for SMT pads.The gold standard would be something like Robin Renzetti's true kelvin probes, which he shows the making of here:They look like a conventional probe, but have a split carbide tip ground to an extremely fine pyramid point such that the two isolated halves make independent contact to the DUT. I wish I could buy a pair of those. (The whole series is worth watching, the amount of experience and thoughtfulness that goes into every decision and fabrication technique is really mind boggling. Robin used to be somewhat active here under the username robrenz, but now mostly shares his work on instagram.)

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 21, 2023, 01:56:06 am
content: Quote from: hanakp on February 16, 2023, 04:07:20 pmThe head out of a tape answering machine could be small enough, but it would have pretty limited bandwidth. Probably a VCR head would be the best commonly available choice, the connections on HDD heads are really tiny and hard to work with.