What is going on (impedance). - Page 1
Forum section: electronics / beginners

author: Oneminde
author_rank: Regular Contributor
timestamp: April 14, 2020, 05:27:26 pm
content: As the title imply, I have a question about impedance and to me, its an odd one.I was experimenting and made two foil conductors, sort of wanted to learn what happens (trend) when you go from a few to several conductors but maintain the same total gauge/mass. Since copper has a specific resistance ( 1.68x10^-8 ohm/m) one would expect that a certain gauge (AWG) should not change or be dependent upon how that mass is distributed, but in this case it looks like it is. And so I am here to ask why this is. Is it a false positive or something real.Each single conductor is separated by 5cm to maintain no to very little Lp. Test frequency 1kHz.Foil conductor #1.Single Sheet: 25x0.05mm = 1.25mm^2 = 17AWGTotal foil area: 3 x 1.25mm^2 = 12.5AWGSeries Inductance (Ls): 0.1 µHParallel Capacitance (Cp): 13.8 pFDC Resistance (DCR): 0.06 OhmSeries Resistance (Rs): 0.045 ohmFoil conductor #2.Single Sheet: 8.33 x 0.05mm = 0.416mm^2 = 31AWGTotal foil area: 9 x 0.416mm^2 = 12.5AWGSeries Inductance (Ls): 0.1 µH – (at 100kHz: 0.129 µH)Parallel Capacitance (Cp): 10.8 pFDC Resistance (DCR): 0.04 OhmSeries Resistance (Rs): 0.038 ohmSo what's going on with the resistance, why does the DCR and Rs go down when the foil area is spread out on more conductors ?

author: AVGresponding
author_rank: Super Contributor
timestamp: April 14, 2020, 06:19:57 pm
content: Two questions which might have a bearing on the answer:1. How confident are you about the uniformity of the gauge?2. How are you terminating them?

author: Oneminde
author_rank: Regular Contributor
timestamp: April 14, 2020, 06:47:49 pm
content: 1. They where cutout from a single strip and should be 99% identical. No foil was lost in the process.2. With the same copper strip.The length is 1 feet.

author: Dave
author_rank: Super Contributor
timestamp: April 14, 2020, 08:30:07 pm
content: Can you describe your test setup? What are the specs of your meter?I have a strong feeling that the measurement difference falls within the error bands of the measurement system.

author: DimitriP
author_rank: Super Contributor
timestamp: April 14, 2020, 08:56:41 pm
content: What you have going on is probably something like this:

author: Oneminde
author_rank: Regular Contributor
timestamp: April 14, 2020, 09:48:25 pm
content: Calculated reactance for 1kHz with the 0.1uH = 0.0006 ohmsThe DMM is the DE-5000. But I will upgrade to a bridge LCR meter, considering the APPLENT AT826.Going from 0.045 > 0.038 is a 15.5% difference, the LCR meter is suppose to be -/+3% or less

author: Oneminde
author_rank: Regular Contributor
timestamp: April 14, 2020, 09:53:05 pm
content: Its better than 3%https://www.deree.com.tw/de-5000-lcr-meter.html

author: Oneminde
author_rank: Regular Contributor
timestamp: April 14, 2020, 10:05:54 pm
content: There is too much of a difference to be caused by reading errors of the meter.The distance is as mentioned 5 cm between the conductors, for both foil #1 and #2. A side effect is also less C... so dunno what's going on.

author: Dave
author_rank: Super Contributor
timestamp: April 14, 2020, 10:34:44 pm
content: You're reading the spec sheet wrong.The tolerance is ± 1% + 3 counts.So your measurements of Rs have a tolerance of 3 mΩ and DCR a tolerance of 30 mΩ. While Rs error bands don't exactly overlap, they aren't that far apart.You also need to take into consideration, that the connections between the crossing foil strips are not perfect and gaining or losing a couple of milliohms can be very easy in practice.I'd suggest a better measurement method for the DCR.Do you have a stable, regulated power supply, that is capable of supplying constant current? Two decent multimeters?Put one of the multimeters in current measurement mode and wire it in series of the power supply. Connect it to the strips at the red markings (see attached image). Set the current to 1A. Use the other multimeter to measure the voltage between the green dots. Use Ohm's law to calculate the resistance. This will yield you a much more accurate result, with considerably better resolution.

author: Oneminde
author_rank: Regular Contributor
timestamp: April 14, 2020, 10:57:49 pm
content: Thanks Dave. I'm in dire need to get some proper equipment. This was a quick and dirty test, but as you point out, there are better ways to do proper resistance measurements

author: Oneminde
author_rank: Regular Contributor
timestamp: April 14, 2020, 11:23:04 pm
content: 2 dimensional conductor: Ohms per Squarehttps://en.wikipedia.org/wiki/Sheet_resistanceand here thehttps://en.wikipedia.org/wiki/Four-terminal_sensingcomes into play.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 15, 2020, 01:37:35 am
content: Likely dominated by contact resistance. Solder those lap joints.Tim

author: Cerebus
author_rank: Super Contributor
timestamp: April 15, 2020, 02:33:32 am
content: Go back and check the spec sheet for your meter. The lowest range for which it gives accuracy figures is the range 0.1 - 1. Measuring low resistance (40 min one case 60min the other) is really the territory of specific "low resistance" meters, also you're going to need to take a 4 wire measurement to have any hope of getting accuracy with that low a resistance. And yes, because of the shapes of your 'resistors' you're also into the territory of the weird and wonderful world of 'ohms per square'.

author: Oneminde
author_rank: Regular Contributor
timestamp: April 15, 2020, 12:33:16 pm
content: Thanks guy's. This was what I was after, so I am probably looking at a false positive and need to do a four probe sensing, which is great info in general. So thanks a lot.

author: Vovk_Z
author_rank: Super Contributor
timestamp: April 16, 2020, 08:19:07 am
content: DCR and Rs can differ because meter uses (can use) different metods to mesure them. And it is really hard to measure such low values so uncertainty of mesurement is large. I can't say which value is closer to real one.

author: Oneminde
author_rank: Regular Contributor
timestamp: April 16, 2020, 10:59:44 am
content: They are indeed very low values. I will revisit this when I have a pair of nice multimeters.

author: Oneminde
author_rank: Regular Contributor
timestamp: April 16, 2020, 10:49:00 pm
content: I think the Bryman BM869s (2 of them) will serve me well.