Simple Low Resistance Measurement
 

I'd like to be able measure from around 0.1R to let's say 0.5R. My various DMM's are 'marginal' down there.
Searching Google yields the very simple idea shown in the attached, which I found here http://diyaudioprojects.blogspot.com...asurement.html.

Wonder if anyone has any comments, good or bad, or other simple ideas to do this? I could pull out my Wayne Kerr bridge and set it up in Kelvin mode, but this project needs something a bit more "field" orientated.

Thanks
B

Re: Simple Low Resistance Measurement
 

Yes, that's the idea. But I think the LM317 needs some decoupling to avoid oscillation.

I have a professional meter that works like this, which I bought very cheaply.
The nice feature of a proper one if you can find it is that they have special dual connection croc clips. So each half of the "mouth" are separate. One does the current feed and the other is the voltage measuring contact - a very neat arrangement.

The 4-wire aspect is key to getting the right answer!

Re: Simple Low Resistance Measurement
 

Thanks for that info. I think I'll start by trying to get a good measurement of the resistance of the two leads, see how the corrected results come out, and think about 4 leads if that seems to be necessary.

B

Re: Simple Low Resistance Measurement
 

Here's one I knocked up in 'a local field' a few weeks ago ;D - I was going to tidy it up.... but you know how these things go. It looks pretty 'rough', but it's 5 - 10 minutes work, and it did the job perfectly - checking some low value emitter resistors.

Attachment 237131

Re: Simple Low Resistance Measurement
 

You really do need to 4 wire system if you want the reading to be meaningful.

The trouble is that contact resistance will be of the same order as the resistor unless you are very careful.

Re: Simple Low Resistance Measurement
 

I agree - I just double checked the 'field' board - running it to a low value resistor with a dmm in circuit (for mA), and then using a second dmm for mV drop across the resistor, I'm getting a steady 19.35mV drop at 88.0mA current = 0.21988 Ohms. The resistor is a brand new 0.22R, 1% metal film from Mouser. That's close enough for me.

Re: Simple Low Resistance Measurement
 

Best to avoid chrome plated crocodile clips as the plating can sometimes interfere with low resistance measurement.
Tinned clips are better. Screwed connection to brass connectors better still.

Re: Simple Low Resistance Measurement
 

It's worth appreciating how much test current can be safely handled by the "DUT". Not a problem for measuring the resistance of a bolted contact, but could be a concern for an smt resistor. I have a specialist milliohmeter bridge (Pontavi Th2) that applies 1.3A to the DUT, which is too much for some transformer windings and small parts.

Another comment is that 0.21988 is not the way to present a measured value when the value is a multiplication of two 3-4 digit values that are likely measuring down in the weeds already. Perhaps 0.22 absolute is a likely value, although doing comparative measurements can allow a better discrimination of the difference in resistance of samples.

Re: Simple Low Resistance Measurement
 

I stand corrected on the R value to 5 decimal places, that was sloppy, and I didn't intend to imply that level of precision. The DMM's I used specify resolutions of +/-0.1mA and +/-0.01mV at the values I cited, so technically I should have rounded the result to 0.22R. My point was more to illustrate that using the indirect method the OP had asked about, it is possible to get a much more meaningful measurement of a low resistance value than it is by using a direct resistance measurement. The same DMMs I used would have had a resolution of only +/- 0.1R at low resistance, so not too useful if you're trying to measure at the same level (and that's not even considering meter lead resistance).

We could also go into the 'accuracy' of each measurement - it's clearly a lot better using this type of indirect method, but frankly, if particularly high accuracy is needed, then we're back to using the right equipment etc., that the OP was looking to avoid in the first place.

Re: Simple Low Resistance Measurement
 

I rely on the calibration of a PSU to set a defined current through a suitable resistor, which is applied to the DUT, and measure the resultant mV with a DMM (4-wire method)

Re: Simple Low Resistance Measurement
 

I thought that the idea of four wire measurement was to remove the problem of contact resistance.

Re: Simple Low Resistance Measurement
 

It is, but as well as having four separate wires you also need four separate contact clips to the DUT. In that case it shouldn't matter if the clip contacts to the metering wires have a bit of resistance. But you may have to worry about thermocouple voltages at the contact points if the metals are different and the local environment is hot.

Cheers,

GJ

Re: Simple Low Resistance Measurement
 

Since thermocouples fundamentally measure temperature difference, providing everything is at the same temperature I don't think that the thermocouple effect matters - the law of intermediate junctions will be the saviour. The measurement may need to be done fairly quickly if somat is likely to get warm during the testing though

Re: Simple Low Resistance Measurement
 

Has anyone tried using an ESR Meter to measure low resistance? I don't see why it shouldn't work.

Re: Simple Low Resistance Measurement
 

ESR meters are usually 2 terminal and do not go down all that low.

Re: Simple Low Resistance Measurement
 

For the sake of comparison, I just measured the exact same 0.22R resistor on an ESR meter set up for low resistance measurements as well (a Russian 'ESR-Micro 4.0s'). With the fixed probes freshly zeroed, the measured resistance was exactly 0.22 Ohms with a cited tolerance of +/- 0.02 Ohms in the 0 to 1 Ohm range.

Re: Simple Low Resistance Measurement
 

Thanks for your various inputs. I don't have an ESR unit.
I think I need to measure somewhere in the range 0.08 to 0.12 ohm (expected) , ideally say within +-10% (or 20%) but I may find something nasty like, 0.2 ohm!

Waiting for the 317's to arrive.

B

Re: Simple Low Resistance Measurement
 

I found that an ESR based measurement technique is just as accurate and sensitive as a vintage Kelvin double-bridge that I have. I made up a diy ESR meter circuit from eevblog forum (below link) that gave me 0.1mohm sensitivity and 1 ohm full scale, and confirmed accurate at 47mohm - but yes that ESR meter needs 4-terminal connections as well. One main advantage of a modern ESR meter is the relatively low signal level applied to the DUT.

https://www.eevblog.com/forum/projec...-construction/

I have two ANENG 8009 cheap 4-digit meters for quick bench use. As an example, they provide a 1A FS with 0.1mA resolution, and accuracy hearsay is better than 0.5% at 1/3 FS from those with quality comparison meters. So with 100mA test current the accuracy is likely about 0.5%. A 200mohm resistor would read 20mV across it. The voltage accuracy on that 8009 meter is much better than for current by hearsay, but even at 0.5% accuracy the power measurement is about 1% accurate, so well within your aim even with cheap (but accurate) meters when using a 4-terminal setup with a steady dc test current.

Re: Simple Low Resistance Measurement
 

I stumbled upon this paper on the web by complete accident. https://www.homemade-circuits.com/si...ester-circuit/. I think it contains some interesting ideas which could be "bolted on" to a 317-based tester.

B

Re: Simple Low Resistance Measurement
 

The 317 method identified in the first post is so simple I'd suggest keeping a focus on it first, and letting any DMM do all the range switching. The main issue to confirm is that accuracy is acceptable and verifiable - otherwise you may mislead yourself right from the start.

Initially that means appreciating what meters you have, and what resolution they have when measuring low (MV) DC voltages, and whether any can measure low mA DC currents. Then checking their stated accuracy down at those low levels, and then cross comparing their measurements to see if they are all the same (given that none are new or in recent calibration).

If you have a sensitive and reasonably accurate mVdc meter, then setting the 317 for 10mA rather than 100mA may provide a better outcome for a few reasons, including the type of battery you need to use and any need for heatsinking the 317 (eg. if you just use a 12V batt or wall-wart or a 317L and say a 5V supply such as a USB supply wall-wart).