Would you buy a precision µ/mOhm DMM adapter? - Page 1
Forum section: electronics / general

author: Shay
author_rank: Regular Contributor
timestamp: March 11, 2023, 07:02:37 pm
content: You're scrolling online, and you see a precision µOhm/mOhm DMM adapter.You click on it, and see the following:It has 3 ranges = 0.010A, 0.100A and 1.000A. All DC.It also has 4 wire Kelvin, with the DMM as the measuring device. Also A dip socket for the DUT, not just binding posts.The 1A range allows you to measure very small resistances, in the µOhm range, by applying 1A across the terminals, and then you connect your DMM into the output of the adapter, and measure the voltage. (R = V (dmm) / I (range))The other ranges are the same idea, just for higher resistances.Assume about 1% accuracy for the adapter itself, and good build quality, with good components (AZ OP), an over-resistance indicator, and single AA battery, used for powering the whole thing!The cost is about 40$-30$. it comes with silicone kelvin clips.Would you buy it?

author: shapirus
author_rank: Super Contributor
timestamp: March 11, 2023, 07:12:04 pm
content: Yes, if it measures in DC mode.If it's AC, then no, because I have a TR-1035.

author: richnormand
author_rank: Unknown
timestamp: March 11, 2023, 09:00:37 pm
content: Might be useful to locate shorts on a pcb?

author: Shay
author_rank: Regular Contributor
timestamp: March 11, 2023, 09:52:22 pm
content: Quote from: richnormand on March 11, 2023, 09:00:37 pmYes, that is indeed quite a common use. However contact resistance has to be low, else you'll get high measurements because of bad contacts.

author: shapirus
author_rank: Super Contributor
timestamp: March 11, 2023, 10:01:26 pm
content: Quote from: richnormand on March 11, 2023, 09:00:37 pmBut this is something that can be done with a regular DMM. What's the benefit of using an additional device for it?

author: SiliconWizard
author_rank: Super Contributor
timestamp: March 11, 2023, 10:11:37 pm
content: Problem with that is that 1A of test current is quite high. You better not mistakenly select this range and use it on a higher resistance value, which may overheat and blow up.If the adapter is well designed, this should not happen - it's basically a matter of having an appropriate compliance voltage for each range - but unless this compliance voltage was clearly stated in the documentation, I would not touch the device.One tricky thing design-wise is that a "safe" compliance voltage for the 1A test current, for instance, would be in the order of a few mV to a few tens of mV maybe. Designing a current source with this low a compliance voltage is not easy. That can be added as a voltage limiter though. Not rocket science, but I would be wary and would suspect this kind of adapters to not have much in terms of safety features, unless again clearly documented. Just a thought.

author: TizianoHV
author_rank: Regular Contributor
timestamp: March 11, 2023, 10:17:32 pm
content: Seems pricey, it depends on how much you need it.One simple alternative is to use a power supply with current limiting. If you don't have one and you need something small (is it small??) you could build something similar (maybe with only one current range) for much less.Silicon kelvin clips are fancy but can be inconvenient, to probe circuits etc.. it's better to have 4 separated wires with clips.Having a constant current source (I use a power supply) to measure contact resistances, relays, traces... is super convenient. (The advantage of a power supply is that you have full control of the voltage, Disadvantages: should be linear, heavy, the CV-CC transition could have current/voltage spikes killing stuff).

author: shapirus
author_rank: Super Contributor
timestamp: March 11, 2023, 10:40:09 pm
content: Quote from: TizianoHV on March 11, 2023, 10:17:32 pmCan even be replaced with just a regular CC source. Put (push? drive? not sure what verb to use here) known current through the element that you need to measure and read the voltage drop across it with a DMM. Calculate R = U / I.But it's much less convenient than a dedicated device. In comparison, for example, the TR-1035 meter is extremely convenient (and relatively cheap) for measuring low resistances, down to about 1 mOhm. One thing that it lacks, however, is measuring at DC current, which requires to take the effects of inductance and capacitance of the element being measured into account, if they can be noticeable at 1 kHz, the frequency that this device measures at.

author: Shay
author_rank: Regular Contributor
timestamp: March 12, 2023, 12:06:25 am
content: Quote from: shapirus on March 11, 2023, 10:01:26 pmNot 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.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.Quote from: SiliconWizard on March 11, 2023, 10:11:37 pmThank you for your note on over-resistance protection and the importance of documenting it; this is indeed very important in order not to burn resistors.Quote from: TizianoHV on March 11, 2023, 10:17:32 pm30–40 USD for a good unit is quite pricey, but not really if you consider any other professional (or just decent) alternative.A power supply with current limiting can probably get the job done, but it is not ideal; however, it has the advantage that you don't need to spend money, since most people who would use such things have a decent current limiting PSU. if you need to do a lot of low-value measurements or just use this to find shorts on a circuit board, it might be worth the $30-$40.noted on the Kelvin clips, which makes a lot of sense.

author: shapirus
author_rank: Super Contributor
timestamp: March 12, 2023, 10:18:02 am
content: Quote from: Shay on March 12, 2023, 12:06:25 amNot 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.

author: Shay
author_rank: Regular Contributor
timestamp: March 12, 2023, 12:43:37 pm
content: Quote from: shapirus on March 12, 2023, 10:18:02 amHave a look here:https://huntron.com/news/locate-short-circuits-milliohmmeter/

author: nctnico
author_rank: Super Contributor
timestamp: March 12, 2023, 12:49:29 pm
content: Quote from: Shay on March 12, 2023, 12:06:25 amYou 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.

author: shapirus
author_rank: Super Contributor
timestamp: March 12, 2023, 02:37:01 pm
content: Quote from: Shay on March 12, 2023, 12:43:37 pmGot it. So it's about actually finding the location of the short rather than simply detecting if one is present.

author: kripton2035
author_rank: Super Contributor
timestamp: March 12, 2023, 03:05:51 pm
content: this easy to make device finds shorts quickly.it injects only 30-40mA in the probes.http://kripton2035.free.fr/Projects/shorty-display.html

author: fcb
author_rank: Super Contributor
timestamp: March 12, 2023, 09:25:53 pm
content: 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.

author: thm_w
author_rank: Super Contributor
timestamp: March 13, 2023, 09:25:19 pm
content: Quote from: kripton2035 on March 12, 2023, 03:05:51 pmI would consider buying a dedicated device like this.If I want to measure low resistance with the DMM then I'll attach a power supply to the short. But that requires messing with two devices, or in your case DMM and adapter. Having it all in one might be nice. Also the probes may be specific to low ohm measurements.One or two 18650's would be capable of 1A easily. I'm not sure how long a single AA would last unless you are pulsing the current in short bursts.

author: MathWizard
author_rank: Super Contributor
timestamp: March 14, 2023, 05:21:38 am
content: I have kelvin probes for my cheap LCR meter, that would do better at DCR than my DMM's from my early days. Now I have 2 DMMs with 4wire mode.What about low voltage amplifiers probes for a scope ? Are there are ones under a few hundred $'s that are any good ? That would be a fun thing to try and copy old models of.

author: magic
author_rank: Super Contributor
timestamp: March 14, 2023, 07:56:45 am
content: I use a 100mA current source and ordinary DMM in mV range.Goes down to 1mΩ resolution with 3.5 digits or better with newer DMMs.Measurements are inherently 4-wire.Found it useful for testing things like connectors, cables or jumpers.

author: kripton2035
author_rank: Super Contributor
timestamp: March 14, 2023, 12:53:37 pm
content: you need 0.1mΩ resolution to precisely track down a short on a pcb.