General > General Technical Chat
Would you buy a precision µ/mOhm DMM adapter?
Shay:
--- Quote from: shapirus on March 12, 2023, 10:18:02 am ---
--- Quote from: Shay on March 12, 2023, 12:06:25 am ---Imagine you're trying to find a short circuit on a PCB. assume that a normal trance ranges from 0.01 to 0.05ohms, yet a short circuit is 0.001 ohm, you wont be able to measure the differences.
--- End quote ---
Not sure I get the point. 0.05 Ohm across two points on a PCB is essentially a short circuit for any practical purpose, save for certain cases like current shunts.
--- End quote ---
Have a look here:
https://huntron.com/news/locate-short-circuits-milliohmmeter/
nctnico:
--- Quote from: Shay on March 12, 2023, 12:06:25 am ---
--- Quote from: shapirus on March 11, 2023, 10:01:26 pm ---
--- Quote from: richnormand on March 11, 2023, 09:00:37 pm ---Might be useful to locate shorts on a pcb?
--- End quote ---
But this is something that can be done with a regular DMM. What's the benefit of using an additional device for it?
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Not quite. Most DMMs can't really measure well below 0.1 ohms, and even at <1Ohm the resolution is limited to something like 1.5 digits (0.X). and of course, the resistance of the leads adds up, and most don't have a relative function.
Imagine you're trying to find a short circuit on a PCB. If you assume that a normal trace resistance ranges from 0.01 to 0.05 ohms, but a short circuit is 0.001 ohm, you won't be able to measure the differences.
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You can. But you'll need to take relative measurements and put the probes in the exact same spots relative to the component you want to measure. I have used the VC8145 DMM (cheap 4.5 digit bench DMM) for this purpose to find which MOSFET is defective from a few parallelled MOSFETS uing simple probe leads. The probe leads themselves already represent about 1.4 Ohm but you can still see the relative difference in resistance.
Then again, using the Ohm setting on a DMM or a milli-Ohm meter to find a short somewhere on a PCB is not the best way to go. That is a solution looking for a problem. Using a current limited power supply (set to 0.5V to 1V / 100mA to 1A of current) and a DMM in the mV range generally works much better. The short is where the voltage is the lowest. This even works on PCBs with large power planes using a super cheap 3.5 digit DMM. The probe connection is not critical because no current flows through the probes. Using a thermal camera to find the short is even faster IF the short can be heated up.
@Shay: I'm not sure whether such an adapter is usefull. If you have a DMM + power supply, you can measure both voltage and current with better than 1% accuracy using equipment most people already have. Besides that, I wouldn't design anything to run from a battery but use USB power instead.
shapirus:
--- Quote from: Shay on March 12, 2023, 12:43:37 pm ---Have a look here:
https://huntron.com/news/locate-short-circuits-milliohmmeter/
--- End quote ---
Got it. So it's about actually finding the location of the short rather than simply detecting if one is present.
kripton2035:
this easy to make device finds shorts quickly.
it injects only 30-40mA in the probes.
http://kripton2035.free.fr/Projects/shorty-display.html
fcb:
Are you familar with Polar Instruments "ToneOhm" products? They only make the '950' now and they are quite expensive (if you don't use them much).
If you are making a device (primarily) as a fault locator, then you should spend some time with a ToneOhm.
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