Tolerance for Precision Low-Resistance Resistor - Page 1
Forum section: electronics / beginners

author: Hobby73
author_rank: Regular Contributor
timestamp: May 21, 2016, 08:41:24 pm
content: I purchased a small lot of Dale (Vishay) resistors rated at 0.005 ohm 1%. In other words, 5 milliOhms or 5mR. I measured their resistance and they are all indicating lower resistance at around 4mR (after subtracting the 2mR internal resistance of my test leads).This is well outside of 1% tolerance, but that got me wondering if I have a correct understanding of the tolerance. Is it calculated differently for these low-resistance resistors? If it's calculated in the usual way, then 1% of 5 milliOhms is +/- 50 microOhms, which is a very tiny variance and below the precision of my measuring instrument. Or is it calculated differently, based on the range or some larger value that would make the measured reading of 4mR within spec?(Note: I am using "R" instead of the ohms symbol because the omega symbol is not supported in the forum font.)

author: doktor pyta
author_rank: Frequent Contributor
timestamp: May 21, 2016, 08:54:16 pm
content: You are writing about subtracting resistance of the leads, so I assume that You are using 2 wire ohmmeter probably from handheld DMM. This is not the way You should measure small resistance.Try to use lab power supply and pass 1 to 3 Amps through the resistor (if Your lab supply don't have ammeter use second DMM) and attach leads of Your DMM set for mV directly to the leads of the 5mR resistor.In other words only four wire (Kelvin) measurement makes sense in Your case.

author: Hobby73
author_rank: Regular Contributor
timestamp: May 21, 2016, 08:59:05 pm
content: It's a 4-wire kelvin measurement on my low-resistance meter. The resistance of the test leads is only 2mR (by comparison, the resistance of the test leads on my Fluke DMM is ~100mR).I just gave this some more thought. I had been focusing solely on the tolerance of the resistor. But on reflection, the accuracy of my test meter would encompass the rated value. So everything is probably OK. When you get down to these tiny values, it's not a spot-on measurement unless you have very expensive test equipment with ultra-high precision and accuracy, which I do not.

author: uncle_bob
author_rank: Unknown
timestamp: May 21, 2016, 09:16:37 pm
content: Quote from: Hobby73 on May 21, 2016, 08:59:05 pmHiWell it *can* be a darn good measurement, even with simple gear:Set up one meter to measure current. Use it, a load resistor and a variable DC supply to generate a current. For 0.005 ohms, something around 20A is a reasonable number (yes you need a big supply). At 20A, your resistor will have a drop of 100 mv. That's likely to be "in range" for a second meter. Yes, it's just a 4 wire resistance measurement. All you have done is moved up the test current "a bit". This of course *assumes* the resistor can handle 2W. If that's outside the limit, cut back the current a bit.Bob

author: danadak
author_rank: Super Contributor
timestamp: May 21, 2016, 10:42:59 pm
content: Here is an excellent resource for precision measuements -http://info.tek.com/KI-Low-Level-Measurements-Handbook-LP.htmlRegards, Dana.

author: Dr. Frank
author_rank: Super Contributor
timestamp: May 22, 2016, 08:18:44 am
content: Hobby73,you're right, such low Ohm measurements are rather delicate, in respect to the instrument, but also in respect to the resistor itself.You made several misconceptions already, for both.First, the resistor should be a 4 wire / Kelvin type.2 wires at 5mOhm is rather useless.If it's really a 2 wire type only, it's resistance very probably is defined as 5mOhm +/- 1% +/- x mOhm (for its leads).Also, the 5mOhm have to be defined at a certain length of its leads apart from the resistors body, let's say 10mm on each side.So you have apply your Kelvin test leads at exactly these points.Otherwise, you will have great errors on the order of several % per mm deviation.If you are using 4 Kelvin measurement, the instrument automatically subtracts, or better eliminates the test lead resistance.So you must not additionally subtract these 2mOhms for your test leads.Instead, the resistor measures as 6mOhms, obviously.So you have to take care about the mentioned correct resistor lead test points.Additionally, your instrument should also incorporate the Offset Compensation method, which is another big source of error on low Ohm, low test current / low test voltage measurements.Example: If your instrument measures these 5 mOhms with 100mA test current, it has to measure 500µV of test voltage.1% of these are 5µV, which is very well on the order of typical thermal offset voltages.Precision instruments therefore switch off the current, measure the offset, and subtract this from the subsequent measurements with test current applied.You easily see, that this effect depends also on the value of the test current; 1A mitigates this effect by a factor of 10.So, it would be helpful, if you would tell us the Dale type, and the model of your low ohm instrument, so that a real estimate of the possible errors is possible, by studying both specifications.FrankPS: I found Dale type WSK, which is an SMD, Kelvin type, 1%.How do you connect your test leads to the 4 reference points?That also could be a source of error, as these 1% apply only for a correct / defined SMD layout of the landing pads, as given in the specification on page 2!

author: Hobby73
author_rank: Regular Contributor
timestamp: May 22, 2016, 05:55:31 pm
content: Quote from: uncle_bob on May 21, 2016, 09:16:37 pmI understand how this test is an implementation of Ohm's Law, but to measure such a low value (5 mOhms) it seems like every component in the test setup would be a variable to contend with. For example, I would need to ensure the load is exactly 20.000A on the resistor, taking into account the effect of the wires (internal resistance) connecting to the load. And then there is the accuracy of the DMM in the mV range which introduces another source of variance. It seems like every item in the test setup would need to be calibrated just perfectly to get an accurate reading of the voltage drop.

author: Hobby73
author_rank: Regular Contributor
timestamp: May 22, 2016, 06:01:10 pm
content: Quote from: Dr. Frank on May 22, 2016, 08:18:44 amThe resistor is described here:http://www.mouser.com/search/ProductDetail.aspx?R=0virtualkey0virtualkeyWSR25L000FEBI'm also attaching a photo of the resistor. It doesn't have wire leads but metal contacts on its surface.When I touch the resistance meter probe tips against each other, that's when I see 2 mOhms displayed so I assumed I needed to subtract that from the reading. Are you saying the meter will make the necessary adjustment automatically?

author: Cerebus
author_rank: Super Contributor
timestamp: May 22, 2016, 07:07:02 pm
content: Quote from: uncle_bob on May 21, 2016, 09:16:37 pmDouble check your device's datasheet before taking this advice too literally. I have some Vishay VCS201 resistors, also 5mR, that are only rated for 10A. I'm sure they'd tolerate more as an overload (they are rated for 1.5W - 3 times what 10A would produce) but they might not be in spec at that much current.

author: Dr. Frank
author_rank: Super Contributor
timestamp: May 22, 2016, 08:23:04 pm
content: Quote from: Hobby73 on May 22, 2016, 06:01:10 pmYes, the 4W mode is intended to cancel any lead or tip resistances.If you really use 4W method, I can't understand, which TWO tips you are touching against each other , out of the four?For an SMD 2W component , the same is valid, what I stated about the leaded one.. so your measurement is depending on the 4 points, i.e. where exactly you apply these.I really doubt, btw, that this 2W 0.005 Ohm resistor is correctly specified with 1% .. its resistance cannot be defined that precisely w/o 4W connections.Vishay sometimes makes bad specs!Frank

author: uncle_bob
author_rank: Unknown
timestamp: May 22, 2016, 09:33:26 pm
content: Quote from: Hobby73 on May 22, 2016, 05:55:31 pmHiIf the power supply, the current meter, the leads, and the resistor are all in series, you can be pretty darn sure that what ever the current meter reads is going through your resistor.The resistor you show in the picture is a 4 wire part. It's designed to be surface mounted on a pc board. Two of the contacts are used for "load" two contacts are used for "sense". To test it with high current, you will need to solder it on a board. That's the only way to get the dissipation rating and move current into the contacts properly.Bob

author: Dr. Frank
author_rank: Super Contributor
timestamp: May 22, 2016, 09:49:02 pm
content: Quote from: uncle_bob on May 22, 2016, 09:33:26 pmSorry Bob, please have a look inside its spec, especially the proposed pads, it's a 2 wire resistor only.So you may artificially create a 4W sensing by PCB layout, maybe.The rest of your statementsThe rest of your statements are darn rightFrank

author: uncle_bob
author_rank: Unknown
timestamp: May 22, 2016, 11:39:06 pm
content: Quote from: Dr. Frank on May 22, 2016, 09:49:02 pmHiI took a quick look at the picture as it posted and *assumed* it was the part we normally use. It's not a useful part in that package. There is a "same size" package with 4 pads from the same guys that is useful.Bob

author: Hobby73
author_rank: Regular Contributor
timestamp: May 22, 2016, 11:46:06 pm
content: Quote from: Dr. Frank on May 22, 2016, 08:23:04 pmHere is the 4-wire Kelvin meter I am using (photo attached). It has three resistance ranges and I'm using the lowest one (200 mOhms) to measure this resistor.Remember the old days ... analog resistance meter with dial ... when I touched the probe tips together to "zero" the ohm meter. On modern digital meters, that's not an option so I thought I needed to subtract any resistance indicated when I touch the two probe tips together. This meter has only two probes and each probe has two wires connected to it from the meter.When the meter is on the highest range (20 Ohms), I touch the two probe tips together and it indicates 0 ohms.When the meter is on the middle range (2 Ohms), I touch the two probe tips together and it indicates 0 ohms.When the meter is on the lowest range (0.2 Ohms or 200 mOhms), I touch the two probe tips together and it indicates between 1.5 - 2.0 mOhms. I assume this reading is a result of the high sensitivity in this low range and there is some tiny resistance in the probe tips which occurs beyond the Kelvin adjustment.

author: uncle_bob
author_rank: Unknown
timestamp: May 22, 2016, 11:50:02 pm
content: Quote from: Hobby73 on May 22, 2016, 11:46:06 pmHiThat's not a 4 wire setup. What you have is a funky two wire. You need proper two contacts per probe stuff to do 4 wire at low resistances.Bob

author: Hobby73
author_rank: Regular Contributor
timestamp: May 22, 2016, 11:54:22 pm
content: Quote from: uncle_bob on May 22, 2016, 11:50:02 pmI have tested this meter with two-contact precision resistors in the hundreds of mOhms range and it is spot-on. But this is the first resistor I acquired that is in the single-digit mOhm range so I was a little disappointed. On the other hand, I am off by only 1 - 2 mOhms which isn't too bad.

author: uncle_bob
author_rank: Unknown
timestamp: May 22, 2016, 11:57:41 pm
content: Quote from: Hobby73 on May 22, 2016, 11:54:22 pmHiThe contact resistance on a part like you are looking at is death when using a single point probe.Bob

author: Hobby73
author_rank: Regular Contributor
timestamp: May 23, 2016, 01:04:10 am
content: Quote from: uncle_bob on May 22, 2016, 11:57:41 pmThat's an excellent point. For a 5 mOhm resistor with two contacts, I wonder if the contact resistance is included in its rating.I just checked the data sheet and it doesn't specify on this point.http://www.mouser.com/ds/2/427/wsr-239755.pdf

author: uncle_bob
author_rank: Unknown
timestamp: May 23, 2016, 01:15:01 am
content: Quote from: Hobby73 on May 23, 2016, 01:04:10 amHiThat's just the point. The resistor is rated with the entire pad soldered to a pc board. You are probing it with a tiny little point. The area of your contact compared to the area of the "real" contact is probably > 1000:1.Bob

author: Hobby73
author_rank: Regular Contributor
timestamp: May 23, 2016, 03:00:08 am
content: Quote from: uncle_bob on May 23, 2016, 01:15:01 amAh, I see. To expand the contact area, it would be ideal if I had some conductive putty to apply where the probe touches the resistor pad.

author: Dr. Frank
author_rank: Super Contributor
timestamp: May 23, 2016, 07:15:20 am
content: Now, that explains everything.. the meter may have 4W sensing, but the probe tips have not.Also, the resolution is not high enough; it will display 5.0 mOhm, so the resolution of 100µOhm alone already creates 2% error.. so absolutely no chance to check 1% accuracy!Unsuitable tool for this purpose.Frank

author: uncle_bob
author_rank: Unknown
timestamp: May 24, 2016, 12:44:48 am
content: Quote from: Hobby73 on May 23, 2016, 03:00:08 amHiTo do the contact properly, you need to solder the part to a copper pad. Anything short of that will have to much resistance. Even a copper pad and solder are a bit to high resistance in this case. The real answer is a properly done 4 wire probe setup.Bob