Simple Low Resistance Measurement

[Trevor]

Lm 317 and the steel version are not super stable you will find the output will vary according to the temperature of the regulator I designed a power supply that used these and found that I needed a thermistor to compensate for device temp
Trev

[trobbins]

I just set up a 317L for 10mA to about -0.015% based on two 6 digit DMMs I have. Very simple finger heating of just the 317L lowered the output level, but similarly the same heating of the setting resistors raised the output level by about the same amount (about 0.0005mA either way). That's pretty reassuring given the room is about 15C and the datasheet shows a minor but consistent fall in reference voltage above about 15C.

I'll set up a 317T for 100mA tomorrow, as well as check for operation in to an inductive load (eg. when measuring the DCR of a transformer winding), as that scenario could well require a flyback diode for protection.

[Oldcodger]

I had a similar problem circa 1983 in Zimbabwe. One of my sites was a highly politically sensitive fuel yard. It was electrically insulated/isolated from the main rail network, but my handbook stated that the resistance from rail to rail and across rail joints must be less than 0,1 ohm. All I could do was to drill the rails and bond over the joints and ( with a large soldering iron heated in the fields outside the yard) connect between rails. No one at rail HQ could suggest a way of verifying that I'd met the standards.

[G0HZU_JMR]

The LM317T method looks quite neat. I'm tempted to build one just to make reasonably accurate 100mA current source.

If you are OK with large and chunky SMD parts and you want to explore below 0.1 ohm I can recommend these Ohmite FCSL SMD parts to use as a reference. I've used these a few times. They are about 7.5mm x 14mm in size with a chunky SMD contact strip running down the 14mm length.

https://www.farnell.com/datasheets/1850916.pdf

I've just tested a couple here on a 4 wire meter and they are marked as 0.01 ohm (1%) and they reliably measure out OK on my meter at 0.01 ohm +/- 0.001R. The Ohmite part number is FCSL150R010FE.

To measure a value this low with the LM317T would be a challenge but I'd expect it to do a fairly good job if you select a higher value such as 0.1 ohm.

[trobbins]

Measurement accuracy will always come down to whatever meters are available for setup, and in addition whether there is a meter with an accurate current measurement, or the alternative of a meter with an accurate voltage measurement and known precision resistor.

Almost nobody would have a calibrated meter unless they just purchased a meter, or had meters at work that were annually calibrated - so the outcome for most of us is what confidence we have in the meters we have.

That's one advantage of buying a cheap new 4-digit handheld meter like the ANENG 8009 or similar, as they can provide reasonable mV measurement accuracy - so for example a sample 0.33 ohm 5% 3W emitter ballast resistor shows a 3.186mVdc meter reading with my 10mAdc 317L CCS using 4-point connection scheme. That reading varies 0.04 mVdc when the 4-point clips move the voltage points from next to the resistor body out towards the ends of the resistor terminal wires (where the current clips are).

[G0HZU_JMR]

Fortunately, for general stuff like this you can just buy a few reference resistors that cover the range you want to measure. This can be used as a means to cross check any results if there is any concern about calibration.

For example I just checked some of these tiny SMD 0.499R 1% resistors and the indicated result was within 0.002R on two meters so that was good enough for me.

https://uk.farnell.com/vishay/rcwe06...1-w/dp/2518233

Buying a pack of precision resistors would cost very little and this should offer high confidence when used as an in house check/calibration.
I would expect to have much lower confidence in the indicated result if all I had to go on was a cal certificate from a recent calibration of the 4 wire meter. Even if it was formally calibrated every day I'd still favour the cross check with a precision 0.1 ohm resistor.

[AmadeusMozart]

I've got a Simpson 260 meter that can measure relatively low but around 2 ohms or less it is not much use.

I picked up from ePray a 10 ohm 1% 30 Watt thick film resistor and put that in series with the unknown resistance. Connect this to the Simpson 260 and then with a digital multimeter I measure the DC voltage that the Simpson 260 puts out (across the series 10 Ohm and unknown resistor ) and the DC voltage across the unknown resistance. A simple arithmetic then works out the unknown resistance.

Ofcourse one can use a power supply but then you'll run the risk of overloading the unknown resistor.

[Oldcodger]

Another way ( avoiding the temperature varying PSU) is to use a 7106/7. it's got its own internal regulator . Only problem is getting hold of the precision resistors. But perhaps looking online for variations to set in series/series parallel might sort that problem out. It might be a project to be tried. The data sheets are out there.

[trobbins]

I set up a 317T for 100mA on a small protoboard, using a 5.2Vdc regulated smps wallwart (USB power supply). The 317T tab is thermally conductive with the PCB vias underneath the tab and to one side, but no additional heatsinking to ambient has been applied as yet. The 317T package increases in temp by about 7-8degC using an IR gun, with the 100mA load being the current measurement paths of 2 meters (ie. a low voltage <<1V). The constant current changes with 317T temp by about 0.24% after 5-10 mins (and for my setup it reaches 100.00mA).

For many, that kind of current variation is a non issue, so the simplicity of using a 317 is very apparent.

At 100mA, the 317 temperature change can be alleviated in a few ways - including using a heatsink; an input dropping resistor; and a thermistor as Trev has tried.

Restricting the range of resistances to be measured to say below 10 ohm, means the 317 only has to regulate the output up to about 1V, so for a 1.7V min differential across the 317 and a 1.25V sense, the input voltage to the 317 could be as low as about 1+1.25+1.7 = 4V, and so a dropping resistor could reduce 317 power dissipation by about 2/3rds in my situation by using the 5.2Vdc supply with a 1 ohm dropping resistor.

I also confirmed that adding a 1N4004 flyback diode across the CCS output terminals does not affect the current level, and could be worthwhile for protection when measuring transformer windings.

[Bazz4CQJ]

Thanks for those inputs. Sorry to say, delivery of my 317's has been delayed , so I'm still on the starting line!

Doing a little experiment with my Aneng AN8008 and my Wayne Kerr LCR bridge, I'm struck by the issue of making contacts that will give dependable and stable results. Maybe some gold-plated croc clips would be good?

B

[Chrispy57]

Hi Baz,
if you are still having supply-chain problems I will happily pop one from my stock in the post for you. I have the 100mA and 1.5A versions sitting idly on a shelf.

Cheers
Chris

[Bazz4CQJ]

Hi Chris,

many thanks for the offer, but the 317's arrived Ok, and I'm now doing some experiments.

B

[Wendymott]

Not exactly part of the thread, but the principle is sound.....many years ago I helped develop the Ground Resistivity meters, used by Archaeologists to measure earth resistivity to help locate buried walls and back filled trenches, where the background resistance and the resistance change could be similar. My late friend Keith Pickard devised the "back off" method , where the moving coil meter had a voltage fed to one side, proportional to the resistivity between two probes, and the other side of the meter was fed with a switched DC "backoff" voltage, using an accurate DC source. The nominal range of the meter was 0 - 1Kohm, in 10 X segments of 100 R FSD .. we also had a X 10 range to measure 0 - 100R in 10R steps.
This could be easily scaled to very low resistances. Our instrument used a 4 wire method with a Constant current fed via 2 wires and the sensor was the other two. Since my involvement, over 40 years ago, electronic components have developed to make it very simple to scale in your application.
We used a constant current of 10m/A AC..... to overcome polarisation of the probes in the ground, and to give us a fighting chance of measuring the return AC voltage, which was synchronously rectified. Just an Idea

[trobbins]

Synchronous detection of a single frequency that is not associated with the mains or other local signals was an excellent method for extracting a low level signal from a very noisy environment. We used an AD630 for continuous large battery ESR measurement in on-line battery systems.