SO.... What Capacitance are your Resistors...? - Page 1
Forum section: electronics / projects

author: jaunty
author_rank: Regular Contributor
timestamp: February 10, 2016, 01:34:01 am
content: So - I've been working on this project I've been trying to troubleshoot and it's a real head scratcher. Just on a whim I decided I would check to see if there was any significant capacitance to the resistors from a new source I was using... 0.4µF!! - So i went back and measured a bunch of resistors in my 'collection' - both carbon and metal film etc - and get around a maximum of 4 nF... that's a HUNDRED times more capacitance. I tried searching on the topic on the forum here - but I'm wondering if the resistors I'm using - despite having passed with FLYING COLOURS for tolerance and 'tempco' etc - are messing up my circuit...! (the circuit in question is a controller board with lots of op amps, some 4011BEs and an 8048 processor...)any opinions? observations?

author: Ian.M
author_rank: Super Contributor
timestamp: February 10, 2016, 01:38:28 am
content: Your capmeter is broken or unsuitable. A typical resistor will have a capacitance in the pF range.

author: jaunty
author_rank: Regular Contributor
timestamp: February 10, 2016, 01:42:34 am
content: measure some - you'd be surprised. My 'cap meter' is a humble DMM (tek TX1 aka Fluke 183) but I trust it entirely in the nF range. It does a FINE job reading down to about 2-3 pF.... 400nF is DEFINITELY something.... unless you can debunk why this is happening... but seriously - try it yourselves....

author: jaunty
author_rank: Regular Contributor
timestamp: February 10, 2016, 01:54:27 am
content: Quote from: blueskull on February 10, 2016, 01:47:09 amThanks Blueskill - a valid point - though I have tried all up and down the series of values and get a consistent 4nF for the 'usual types' whether it's 1 megohm or a 43 ohm... - why are these new ones measuring out as having 100 TIMES as much capacitance (400nF) as all my other resistors (and yes - also consistently - independent of value).. and should i question this as the source of a potential problem for troubleshooting. I DO get that this runs contrary to 'conventional wisdom' but even thought these chinese resistors measure out with flying colours - I'm wondering if there's something in their construction that make them actively capacitative...

author: jaunty
author_rank: Regular Contributor
timestamp: February 10, 2016, 02:12:35 am
content: Quote from: blueskull on February 10, 2016, 02:07:09 amits really the 400 nF ones that I'm worried about - the '4 nF' ones were more my known 'good' controls.... I'll grant that the 4nF figure is likely distorted in some way by the measurement... but I guess what I'm trying to figure out is - if there's some way the ones measuring 100 times higher are messing up my circuit (apart from desoldering and replacing 100 odd resistors)

author: c4757p
author_rank: Super Contributor
timestamp: February 10, 2016, 02:25:06 am
content: Yeaaaah, none of your resistors have 400nF of capacitance. Nope. No chance. Nor 4nF. I don't knowwhyyour 1M resistors are measuring 4nF on that meter, but they shouldn't be. Try an LCR meter.

author: retrolefty
author_rank: Super Contributor
timestamp: February 10, 2016, 02:31:26 am
content: Quote from: c4757p on February 10, 2016, 02:25:06 amThat's my gut feeling also.

author: jaunty
author_rank: Regular Contributor
timestamp: February 10, 2016, 02:36:41 am
content: Quote from: retrolefty on February 10, 2016, 02:31:26 am... and what sort of 'evidence' for it do you want? can i mail you a sample resistor?? I have a cheapo ebay ESR meter around but I'd have to dig it out.But that still is a moot argument when it comes to the 100x higher reading with the chinese ones - what then, do you think could account for THAT?

author: c4757p
author_rank: Super Contributor
timestamp: February 10, 2016, 02:40:33 am
content: Nobody's looking for evidence, we believe you that your meter is reading that number. However, having... erm, used resistors before, I'd bet money that it's readingwrong. I don't know the exact method that meter uses, so I can't really say why.The ESR meter is even worse. It's specifically designed for one type of part - electrolytic capacitors. You're doing a pretty specialized measurement (extremely lossy low-value capacitors, essentially), and really should be using a proper LCR meter.

author: BravoV
author_rank: Super Contributor
timestamp: February 10, 2016, 02:47:28 am
content: Claiming an ordinary carbon or metal resistor with 400 nF parasitic capacitance, its like claiming a fist sized transformer can power the electricity need for the whole housing block.

author: jaunty
author_rank: Regular Contributor
timestamp: February 10, 2016, 02:49:36 am
content: Quote from: c4757p on February 10, 2016, 02:40:33 amSure- I get the skepticism... i'm just trying to get a few people to try a simple experiment with theirs - and ideally I'd like to get a second source to confirm or deny my findings. It's kind of a big deal i think (?) if true. I have extremely high faith in the TX1 I'm using to get these measurements... at the very least - it can do a FINE job of distinguishing between a 10pf and 22pf cap in the same mode... so I'm trying to figure out not so much what accounts for the lower 4nF reading but WHY this other batch or resistors are so much higher ... (as mentioned there is ZERO correlelation to resistive value - so it's an objective phenomenon)

author: retrolefty
author_rank: Super Contributor
timestamp: February 10, 2016, 02:50:22 am
content: Quote from: jaunty on February 10, 2016, 02:36:41 amMy vote is to have you mail a few to Dave for a mailbag day evaluation. He should be able to measure/test on a larger variety of test equipment then most of us have and the whole forum gets to see/learn the results.

author: jaunty
author_rank: Regular Contributor
timestamp: February 10, 2016, 02:51:42 am
content: Quote from: BravoV on February 10, 2016, 02:47:28 amI said no such thing. Please be careful what you paraphrase! I said the batch that i currently have - measured that much. So - I went to measure out all my existing 'known good' resistors - and got a value 100x lower (however you want to interpret that). So i'm trying to figure out 'why'...... and - as for your comparison... no - not really... one could, for example - take an SMT capacitor and dip it into a resistor shaped mold and call it a resistor. That's one solution - even though a completely facetious one.

author: jaunty
author_rank: Regular Contributor
timestamp: February 10, 2016, 02:55:26 am
content: Quote from: jaunty on February 10, 2016, 02:51:42 am

author: jaunty
author_rank: Regular Contributor
timestamp: February 10, 2016, 02:56:35 am
content: Quote from: retrolefty on February 10, 2016, 02:50:22 amthat's sort of tantalizing - though I'm sure i'll have moved well on by then - but perhaps it will be interesting.

author: onlooker
author_rank: Frequent Contributor
timestamp: February 10, 2016, 03:42:17 am
content: Just to echo my reply to OP's another thread. Did you use the parallel mode of your LCR meter?

author: jaunty
author_rank: Regular Contributor
timestamp: February 10, 2016, 03:56:52 am
content: Quote from: onlooker on February 10, 2016, 03:42:17 amon the Tek meter it's a 'delta' button - like 'rel' mode on some other meters for measuring small capacitances. But yes if so.

author: jaunty
author_rank: Regular Contributor
timestamp: February 10, 2016, 04:04:21 am
content: for reference:

author: jaunty
author_rank: Regular Contributor
timestamp: February 10, 2016, 04:05:24 am
content: hopefully people can read upside down - guess that one's for dave...

author: onlooker
author_rank: Frequent Contributor
timestamp: February 10, 2016, 04:06:35 am
content: I do not know Tek meters. The capacitance of a resistor should be modeled as in parallel to the resistance. That is, it should be measured as such. A series model make no sense since it will give infinity DC resistance. The user manual should tell how to set the parallel mode on your Tek meter.Edit: I see. It is a DMM, not a LCR. I do not think it is capable of measuring parallel capacitance. Most likely, it can only measure capacitance with minor series resistance component. You need a proper LCR.

author: Ian.M
author_rank: Super Contributor
timestamp: February 10, 2016, 04:16:20 am
content: Its *TOUGH* if you don't trust your test equipment *OR* any of your resistors. However if you put one of your 1M supposedly 400nF resistors in series with an ordinary NE-2 style neon bulb, and power it from between 150V & 200V DC (which doesn't need to be well regulated), with 47uF decoupling across the supply, *IF* your measurements are correct, the flash rate can be expected to be between 1 and 10 Hz. (it depends on the RC time constant, the supply voltage and the breakdown and holding voltages of the NE-2 bulb) If, as we expect, you see a steady glow, the resistor's not got much capacitance. As a final check, try adding a 470nF 200V cap across the 1M resistor and watch it flash.

author: jaunty
author_rank: Regular Contributor
timestamp: February 10, 2016, 04:24:07 am
content: Quote from: onlooker on February 10, 2016, 04:06:35 amhaha - well what's a 'proper' LCR to you?

author: onlooker
author_rank: Frequent Contributor
timestamp: February 10, 2016, 04:28:35 am
content: This forum has a lot dicussions on de5000 for around $100. There are several other similar ones with a little higher prices.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: February 10, 2016, 04:29:14 am
content: Garbage in, garbage out. I just "measured" a bunch of resistors using my cheap ProsKit MT-5210 LCR meter, which is reliable when measuring _capacitors_ on the Capacitance range. Depending on the resistor type (carbon comp, metal film, wirewound, etc.) and the Range setting on the meter, I too can obtain "capacitance" readings in the 0.4 uF or even greater range. What does this prove? Garbage in, garbage out. It proves that test equipment, when not used properly, will give misleading readings.I haven't tried doing the same thing with my Fluke 83 or 87 multimeters using their Capacitance settings .... what would be the point?Sure, if you want to crosscheck and determine if the "400nF" reading for a _resistor_ is accurate, just build some simple oscillator circuit with other components of known values and measure the frequency. I think you'll find that the "400nF" reading is an error, and the lesson to be learned is .... don't use a DMM on capacitance range to measure resistors!

author: edavid
author_rank: Super Contributor
timestamp: February 10, 2016, 04:31:26 am
content: Quote from: jaunty on February 10, 2016, 04:24:07 amAn LCR meter uses a phase sensitive detector, so it can separate resistance from reactance.LCR meter: DER-EE DE-5000, GR Digibridge, HP 4260/4270/4280 seriesnot an LCR meter: any DMM, AADE LC meter, Chinese LC-200, AVR based "component tester"

author: jaunty
author_rank: Regular Contributor
timestamp: February 10, 2016, 04:31:33 am
content: Quote from: Ian.M on February 10, 2016, 04:16:20 ami'm EXTREMELY confident with my test equipment. it's you who isn't confident. Also they aren't 1M resistors - ALL of them do that - all the chinese ones. The european and american ones DO NOT do that. As I already said. it's the difference between these results that interests me. And something nobody seems to be willing to address...

author: edavid
author_rank: Super Contributor
timestamp: February 10, 2016, 04:34:51 am
content: Quote from: jaunty on February 10, 2016, 04:31:33 amPerhaps a difference in inductance. Why don't you make a voltage divider with your "400nF resistor" and a 400nF capacitor? If you plot the voltage ratio over a range of frequencies, you'll get a good idea what's going on.

author: jaunty
author_rank: Regular Contributor
timestamp: February 10, 2016, 04:37:57 am
content: Quote from: alsetalokin4017 on February 10, 2016, 04:29:14 amSure- but that's not the issue. Perhaps you misread. Perhaps i'm not being articulate enough (?)The issue WAS - WHY does this particular brand read as having a 'capacitance' (however you want to interpret that - it's immaterial) that is 100x higher than the reference resistors. And what does that mean for a circuit?So - well what if you DID use your fluke 87 and get the same results? what then? (The TX1 is at LEAST as good - probably better than a Fluke 87)

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: February 10, 2016, 05:11:59 am
content: Hi,I just tried a couple of my meters.First a Fluke 87I took at 1 Meg 1/4 metal film resistor and a 0.47uF film capacitor.I measured them individually, in series and in parallel.If I measure just the capacitor I get a reading of 450nF which is close enough.If I measure the 1 Meg resistor I get a reading of 140nFIn parallel I get 0.6uF and in series I get around 0.1uFAll of which seems about right for series and parallel capacitors.However,the Fluke 87 Manual says:Fluke 289I measured the capacitor first. I got a reading of 452nF.I tried to measure the resistor on the capacitance range, I got an unstable reading.I tried the series combination. The Fluke 289 reads the correct value of capacitance, even with a 1 Meg resistor in series.This just shows, that some meters don't work with a parallel resistor when measuring capacitance.I will try the same components on an LCR meter, HP4274A, but it is too late....Regards,Jay_Diddy_B

author: Ian.M
author_rank: Super Contributor
timestamp: February 10, 2016, 06:03:07 am
content: @Jaunty: Why the hostility? I provided an extremely simple method of proving whether or not your resistors have significant parallel capacitance. If they don't, your capmeter (you said its a Tek TX1 multimeter on nF range) is quite simply lying to you because its measurement method cannot tolerate parallel resistance (i.e leakage current).Why a 100:1 difference from your other resistors? Well that's going to be down to your capmeter's autoranging being fooled slightly differently,Is it going to make a difference in your circuit? Maybe. If its very high impedance, high gain OPAMP circuit, or is a HF or higher RF circuit, the inductance of spiral cut film resistors may become an issue,However you haven't told us what trouble you are having with your circuit., In fact all we know is: "(the circuit in question is a controller board with lots of op amps, some 4011BEs and an 8048 processor...)"

author: jaunty
author_rank: Regular Contributor
timestamp: February 10, 2016, 06:32:58 am
content: Quote from: Ian.M on February 10, 2016, 06:03:07 amno hostility meant at ALL Ian - you were the one suggesting that I didn't trust my equipment - it's not at all true.i most certainly do. I don't think there is any reason NOT to. It's the best and most consistent meter i could find ... understanding or interpreting results is another thing altogether though. as you know....the big question for me is "are these chinese resistors actually creating a problem for me?" and creating some kind of cumulative issue in the circuit itself.... and so this spurious issue with this 'capacitance' or whatever it translates to in real world terms seemed to me something worth investigating - because I'd never seen or considered it before.As for the issues in particular with a circuit - well I won't trouble anyone here with that - since I have another forum I go to for help - since its' a replica of a board from a synthesizer from the early 80s... so it's a good place to source such problems - i was just at my wit's end and thought maybe this weird behaviour of the resistors might bear exploration (hence me being here)... that's about it...

author: Ian.M
author_rank: Super Contributor
timestamp: February 10, 2016, 07:36:15 am
content: A typical 1/4W axial leaded film resistor has a body length L of 6.5mm, diameter D of 2.3mm, with metal end caps each of up to 25% of the total body length.This can be approximately modelled as a parallel plate capacitor consisting of two plates of area (Pi/4)(D^2) = 4.15 mm^2 or 4.15E-6 m^2, separated by a dielectric of thickness D/2 = 3.25mm or 3.25E-3 m.Alumina ceramic, which is commonly used as the substrate for film resistor bodies has a dielectric constant of 9 - 10.Z5U ceramic (as used in high value ceramic caps) has a dielectric constant (relative permittivity) of up to 18000. Its the highest dielectric constant ceramic that's commonly available.You'll find the formulae for calculating the capacitance here:http://www.rfcafe.com/references/electrical/capacitance.htmHere's an online calculator for it that will accept mm and mm^2 and output in pF:http://www.calctool.org/CALC/eng/electronics/parallel_platePlug in the numbers and you will see that a 1/4W thin film on alumina resistor should have a stray capacitance of no more than 0.11pF. If the manufacturer FUBARed it and used the much more expensive Z5U ceramic for the substrate, it *MIGHT* have as much as 200pF of capacitance. In practice, the ends aren't parallel plates, but little cups, so most of their surface is further apart, and the substrate diameter is significantly less than the body diameter so the capacitance will be significantly less.If your resistors are different dimensions, plug in the numbers for them. You wont get more than a few pF for any normal body ceramic and are unlikely to get into the nF range even if you used the dielectric constant for Z5U ceramicIt is therefore obvious that your meter cannot be trusted to measure the stray (shunt) capacitance of a resistor, no matter how much faith you have in its accuracy when used for other purposes.

author: bson
author_rank: Unknown
timestamp: February 10, 2016, 08:25:40 am
content: Keysight 1733C handheld LCR meter, 1.2k resistor (Vishay-Dale 1W):R=1200.1 ohm @ 1kHzZ=1200.1 ohm 0.00deg @ 1kHzPar C = 3-5pF @ 10kHzThe parallel C is so small it's likely just noise and meaningless for an axial component. The meter would need to be carefully calibrated with a short consisting of a section of the lead to factor out these kind of error sources. Or it's really just measurement voltage noise. 10kHz is the lowest frequency it would even give me a reading on!4nF? Almost certainly a test setup error.

author: Seekonk
author_rank: Super Contributor
timestamp: February 10, 2016, 03:05:51 pm
content: This is the political season so it only follows that the hard questions are not answered.Instead the the question they wished was asked is answered.The voltage divider suggestion is a good idea. I would use the two types of resistors in a non inverting R1/R2 op amp circuit and then test with R2/R1 comparing frequency response. This would make it independent of probe effects. It certainly appears that some quality of this resistor is able to fool a simple capacitance meter. If this is a real real it should show up in the frequency response of an op amp. If it is capacitive I would think the model would be a spiral capacitor. I would heat both resistors with a torch to burn off the enamel and see how many spirals it has. It might give some clue. The ceramic may have some unknown quality. Wouldn't be surprised if it was filled with some waste from another process, like they did with sheetrock.

author: jaunty
author_rank: Regular Contributor
timestamp: February 10, 2016, 08:17:01 pm
content: you would make a great engineer... but a lousy scientist.yes if you refer to a common design model for such resistors and limit your understanding to such published data tables, then you can refute anything you want. So the problem must be my crappy meter...even if i weren't using a 'high end' meter and it was just a cheap $100 chinese affair and not a $400 meter- it wouldn't make any difference. The result would still be completely objective... (as mentioned previously) i am simply curious what gives these resistors such a different property to the others...Quote from: Ian.M on February 10, 2016, 07:36:15 am

author: PedroDaGr8
author_rank: Super Contributor
timestamp: February 10, 2016, 08:33:07 pm
content: I think some people are getting hung up on the number being wrong. What I gathered from this thread is thatJaunty does NOT think the number is real. He doesn't think that his resistor actually has 0.4uF of capacitance.What he is asking is:1) WHY does this one read 0.4uF, not 4nF like the other resistors he has. Not that the number is real, but why is it different from the rest (including ones with similar resistance values).2) If he can figure out why it is different, is there a possibility that this difference is the reason for his circuit messing up.

author: jaunty
author_rank: Regular Contributor
timestamp: February 10, 2016, 08:39:00 pm
content: Ahhh yes! Political Season! You're talking about the Superbowl right ?? (haha - just a little joke - as it seems more and more - we seem to treat 'politics' like a team sport.. without consulting actual 'issues' - my team for the win!)At any rate - TOUCHÉ! THANK YOU for that acknowledgement!! Yes - I thought perhaps a little surgery might be in order... i also wonder if there's a 'cleaner' way to possibly intervene - like using liquid nitrogen to break it open.. (?) - I am worried the heat might affect (or destroy) some interesting aspect of the construction.Quote from: Seekonk on February 10, 2016, 03:05:51 pm

author: Seekonk
author_rank: Super Contributor
timestamp: February 10, 2016, 08:44:14 pm
content: I have operated 1/4W CF resistors at currents that made them glow red. After an hour the enamel was burned off, but resistance had changed less then 1%. Don't think a torch would do any damage.

author: The Doktor
author_rank: Regular Contributor
timestamp: February 10, 2016, 09:01:09 pm
content: Jaunty, don't know where you're located, but I'm in NJ, USA. I've got a few month old DER-EE DE-5000 and lots of free time. I'd be happy to measure a few of each type if you wanted to send them to me. If you're not in USA, you can probably get a similar offer from somebody closer, just post a request in this thread.Ed

author: jaunty
author_rank: Regular Contributor
timestamp: February 10, 2016, 09:30:39 pm
content: yes - sure- i'm in california... shoot me your address (there's PM on this thing right??) - would be fun - who knows... i'm hoping to uncover the big chinese resistor conspiracy (haha)...Quote from: The Doktor on February 10, 2016, 09:01:09 pm

author: edavid
author_rank: Super Contributor
timestamp: February 10, 2016, 09:53:45 pm
content: Quote from: jaunty on February 10, 2016, 08:17:01 pmThis is a misconception you have... that meters having a C function, can be measured on a single scale from "crappy" to "high end". For the measurement you are talking about, the best DMM is not as good as the crappiest LCR meter. A simple RC bridge would also be much better than your DMM. The TX1 is just incapable of making an accurate measurement in this situation.

author: wraper
author_rank: Unknown
timestamp: February 10, 2016, 10:05:52 pm
content: Quote from: jaunty on February 10, 2016, 08:17:01 pmThe problem is not in the meter indeed. But between the chair and the meter. There is no problem in the multimeter because it is incapable of measuring anything other than capacitors on the capacitance range. How it will fail to do this depends. A bit different resistance or a bit of inductance in the resistor can make it fail to measure with very different results. When this "something" is on the border in which auto-range starts acting, you might not get any stable results at all but just display flashing some different nonsense all of the time.

author: jaunty
author_rank: Regular Contributor
timestamp: February 10, 2016, 11:03:29 pm
content: Quote from: edavid on February 10, 2016, 09:53:45 pmgosh-i'm not certain why it would matter... you're saying it's measuring a difference that doesn't actually exist...? that if a meter sees a difference then the circuit will not?

author: jaunty
author_rank: Regular Contributor
timestamp: February 10, 2016, 11:08:32 pm
content: Quote from: wraper on February 10, 2016, 10:05:52 pmand yet... that's not true. if it WERE true then it would not be repeatable, would it? - and one would be getting spurious DIFFERENT results from the SAME samples - and not give a consistent reading depending on the resistor. It blows my MIND the lengths people will go to in order to support their own beliefs... rather than have to accept something that runs counter to their training and experience.

author: wraper
author_rank: Unknown
timestamp: February 10, 2016, 11:17:05 pm
content: Quote from: jaunty on February 10, 2016, 11:08:32 pmTry them with another (even crappiest meter). Then talk something about repeatability. Same resistors give the same result when failing to measure them properly. Unstable results will be seen only if auorange will start acting.

author: wraper
author_rank: Unknown
timestamp: February 10, 2016, 11:22:08 pm
content: Also, measure the DC resistance of each resistor (writing the value until the last digit the meter shows) you are measuring. This could tell something why they are measured differently under the C range.

author: jaunty
author_rank: Regular Contributor
timestamp: February 10, 2016, 11:28:12 pm
content: Wraper - I think you're getting a bit too obsessed with the 'capacitance' thing a bit too much. Perhaps it's my fault for titling it so precociously lke i did.

author: The Doktor
author_rank: Regular Contributor
timestamp: February 10, 2016, 11:30:20 pm
content: Jaunty, just sent you my info.Ed

author: The Electrician
author_rank: Frequent Contributor
timestamp: February 10, 2016, 11:36:17 pm
content: DMMs measure capacitance by applying a (relatively) constant current and measuring the time taken to charge to a certain voltage. When this method is working properly, one can see a ramp of voltage across the capacitor as the DMM is carrying out the measurement. This ramp is repetitive and looking at it with a scope can tell us whether it's working in this case.Here's what I see across the DMM leads in capacitance measuring mode with nothing other than the 10 megohm scope probe connected to them:With a 1 meghom resistor connected, I get this waveform across the DUT and the DMM leads:There's no ramp. The DMM is struggling to make a measurement and just can't do it. The result of the measurement is meaningless.Jaunty, what do you see of you look at the DMM's waveform with the two classes of resistor you're measuring?

author: edavid
author_rank: Super Contributor
timestamp: February 10, 2016, 11:40:01 pm
content: Quote from: jaunty on February 10, 2016, 11:03:29 pmIt matters because you are trying to measure a small capacitance in parallel with a resistance.QuoteI'm saying there's no evidence that it's measuring a difference incapacitance.QuoteYou haven't shown your circuit (why not?), but yes, it's quite likely.

author: The Electrician
author_rank: Frequent Contributor
timestamp: February 10, 2016, 11:44:48 pm
content: If I measure the capacitance of several resistors with an impedance analyzer, I get results like this. There is an effect known as the "Boella effect" that causes the parasitic capacitance of resistors to vary with frequency. See:http://www.edn.com/electronics-blogs/anablog/4309676/the-Boella-effectandhttps://www.bing.com/search?q=boella+effect&qs=n&form=QBLH&pq=boella+effect&sc=2-13&sp=-1&sk=&ghc=1&cvid=C4F6F6882C064753A2EF5296043B04A9Here's the measured capacitance of a 1 watt precision molded 1 megohm resistor vs. frequency. The bottom of the scale is 1/10 pF:Here is the measurement on a 1 watt carbon comp resistor:And here is the measurement on a 1/2 watt metal film resistor:

author: The Electrician
author_rank: Frequent Contributor
timestamp: February 10, 2016, 11:59:14 pm
content: In the previous post, I showed that the parasitic capacitance of a small metal film resistor was much less than 1 pF.When I measure the capacitance of the carbon comp resistor with its larger parasitic capacitance, I get a value of 7 nF on a Fluke 189.The measured value for the metal film resistor is 9 nF. This is interesting; the DMM's measured value of the resistor with smaller parasitic capacitance is larger than the resistor with larger parasitic capacitance.The waveforms from the DMM when making these measurements vary a lot with different resistors. Some of this variation is probably due to the differences in parasitic capacitance, and some is due to other factors.But one thing I do see is large variation in capacitance as measured by the DMM method with respect to apparent small physical variations in the resistors.These values gotten from the DMM measurement are mostly meaningless. They certainly don't correlate with the actual parasitic capacitance as measured with an impedance analyzer.An impedance analyzer can correctly measure the parasitic capacitance of resistors and inductors, but a DMM's capacitance range can't; it's strictly intended to measure the capacitance of devices whose dominant property is capacitance.

author: vk6zgo
author_rank: Super Contributor
timestamp: February 11, 2016, 01:57:13 am
content: Quote from: jaunty on February 10, 2016, 11:28:12 pmYes,if you had said something like this at the outset:-"I am getting funny readings if I try to read the parasitic capacitance of some resistors using the "C" range on my TX1 DMM.I know it isn't really capacitance,as the values are much higher than expected.My new Chinese metal film resistors read "400nF,whereas my older resistors from other sources read 4nF.What kind of resistor characteristic would cause :-(a) An obviously false capacitance reading(b)One which varies so widely."We could then have answered "We don't know!" & gone home.Back in the day,the device of choice to look for strange effects on resistors,would probably have been a "Q" meter,with an LCR bridge as second choice.Today it may be a VNA,with an LCR meter as second choice.The last actual LCR meter I used was an '80s version,but it was very capable.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: February 11, 2016, 04:29:44 am
content: Hi Group,Here are some pictures from my experiments:First I have a Fluke 87 connected to a 0.47uF Film capacitor. The more experienced readers will recognize the Phillips (Mullard) capacitor. The meter reads the right value.Then I connected a 1M Ohm metal film resistor. The meter is reading 0.147 uFSo I tried the 1M resistor in series with the capacitor and I got 0.098uF:If I check (C1x C2) / (C1+ C2)I get (450n x 147n) / (450n + 147n) = 110nFThis is close to the 0.098uF measured.I then tried the parallel combination:I got 0.59uFI then tried the same experiments with a Fluke 289:First the 470nF capacitor:I tried the 1M resistor but I did not get a stable reading, so there is no photograph.I then tried the 1M and 470nF in series:I got a reasonable reading for the capacitor.HP4274A LCR MeterThe HP 4274A is a very accurate LCR Meter that was probably introduced in the late 1970's but a very capable instrument.Here is the open circuit reading after zero calibration:That is zero fF. a fF is a femto Farad 1-15F or 1/1000 of a pFHere is the same meter with the 1M Resistor in the test fixture.The value is 160 fF or 0.16pF. This is very similar to the 0.11pF predicted by Ian.M in an earlier post. There are reasons why the value is higher. One is the metal end caps on the resistor construction reduce the distance.Comparing the measurements from the various meters, I would conclude that even some very good DMMs are fooled by a high value parallel resistor into giving erroneous readings. The more specialized LCR meter give the correct answer. The correct answer matches the calculated value of the capacitance from the physical dimensions and the material properties.I learned from this thread to be careful measuring capacitors with a DMM.Regards,Jay_Diddy_B

author: The Electrician
author_rank: Frequent Contributor
timestamp: February 11, 2016, 05:10:32 am
content: Jay_Diddy_B, compare what I got as shown in the third image of reply #52. Around .19 to .16 pF over the swept frequency range for the metal film.It's clear that DMM capacitance ranges are only suitable for measuring actual capacitors. I posted elsewhere about DMM capacitance measurements. Let me find the post and I'll edit in a link.Link at:https://www.eevblog.com/forum/testgear/about-capacitance-measurement-with-dvms/msg489690/#msg489690I asked for other people to try some of the measurements I described, but everybody's too busy with other stuff!

author: jaunty
author_rank: Regular Contributor
timestamp: September 16, 2018, 03:03:59 am
content: interesting - all the replies i've gotten since first posting this - sorry I've been distracted with other work stuff since first posting about this topic ... i've looked into a few more results with 'other' (vishay, welwyn wirewound) resistors since then and found i get a null capacitance result ... though of course that's all the result from a DMM - but that fact is not uninteresting at any rateHOWEVERi find it interesting the replies i'm getting on the topic ... from the perspective that capacitance is generated by a set of relationships between materials that have a capacitor like structure (think of an RCA jack maybe - or perhaps even a silicon junction on a transistor etc) i don't find it SOOOO surprising in principle at least that some electronic products CAN have capacitance or capacitance like properties ... i just wanted to ask to see if this was a 'known' issue or simply the result of the measuring or measurement method etc ... seems the topic is still open ... might make a great topic for a research paper.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 16, 2018, 04:41:41 am
content: Yeah a DMM isn't going to tell you that, they require low leakage to register anything as a capacitance. That's a consequence of how they measure it, an RC oscillator at low frequency and low current, basically, usually.More generally, any impedance that varies against frequency is inductive or capacitive, to some extent. Even if you only know |Z|, you can recover the RLC equivalent because the network must be causal (no negative inductance, resistance or capacitance), and (to be perfectly technical), |Z| is the magnitude of a continuous analytic complex function Z(f) for which the Kramers-Kronig relations apply.The only real way to test a component, is to sample that function at enough points -- measure the impedance or resistance or reactance at enough frequencies -- to derive the network to the desired accuracy.If you know already that a given component should be simple -- a carbon film resistor, or a ceramic capacitor, for example -- it's probably okay to sample a few points, and extrapolate the 2nd order (series/parallel RLC) model from that.Also given that the component is simple, like these examples, you can guess that ESL should be on the order of the physical length, and EPC on the order of the physical width. If the resistance (ESR/EPR) is large with respect to the aspect ratio times the impedance of free space (377 ohms), we expect C to dominate. If smaller, then L.Tim

author: mzzj
author_rank: Super Contributor
timestamp: September 16, 2018, 08:44:53 am
content: If I stick multimeter probes to wet soil and try to measure the circumference of earth based on that sometimes I get up to 1 kilogram difference in the measured distance between earth and sun.In other words: Like about 100 people have tried to explain, You have wrong tool for the job. GIGO

author: MK14
author_rank: Super Contributor
timestamp: September 16, 2018, 09:07:08 am
content: Quote from: jaunty on September 16, 2018, 03:03:59 amAround 2.5 years worth of distractions!

author: MK14
author_rank: Super Contributor
timestamp: September 16, 2018, 09:48:12 am
content: The following article on the parasitic capacitance of resistors.http://www.resistorguide.com/resistor-capacitance/Seems to be saying thatQuoteSo, if you tried to test that resistor type, you would be trying to detect 0.05pF, probably using meter probes with lots of pF, and possibly with your hands holding it, adding even more pF. (Not to mention, the resistors resistance, will completely mess up the reading, as well, typically).Analogy:You are putting a 0.001g postage stamp (ok, it probably is more like 1g, really) onto your Stones and Pounds, weighing scale. Standing on the scales, hoping to see the 0.001g postage stamp weight, on top of your, 5 to 50+ stone weight.The combination of the resistors resistance, and trying to accurately measure just <0.05pF, is a difficult thing to measure accurately.As a side note, will it be 2020, or 2021, when you reply ?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 16, 2018, 12:03:08 pm
content: A very basic introduction, but falls well short of a complete description.Curiously, the diagram shows ESL, which is promptly forgotten in the following equations, and the ratio of R to L or C is not at all noted in the text (scanning it, anyway).Such generalizations are not helpful.Again, capacitance dominates at high values. It is hard to find a wirewound resistor that's bad as low as 50kHz, but perhaps a high value (~100k?) precision part would be. Those aren't very common nowadays.Capacitance absolutely does not dominate at low values. At least, I will be impressed -- and very interested -- if you can present to me a 0.01 ohm resistor that is capacitive-dominant at a few MHz and costs the same as any other.Tim

author: MK14
author_rank: Super Contributor
timestamp: September 16, 2018, 01:42:14 pm
content: I wonder how the resistor manufacturer's, measure the tiny capacitance, of a tiny surface mount resistor, of a type which has very low stray/parasitic capacitance ?Especially over a very wide range of resistor values, such as 0.1 Ohms up to 10 Megaohms.It's tricky to make contact with it, without (typically adding) to the stray capacitance.Especially if it is a smaller resistor manufacturer, who can't afford to buy expensive ($12,000), test equipment. E.g. A small plant in India, making resistors. But maybe they can or should afford the right equipment.Another possible snag, is ignoring the stray inductance, which will also attempt to hinder accurate measurement of the resistors stray capacitance.But I can believe there are tried and tested methods and/or old journals with the answers.

author: ArthurDent
author_rank: Super Contributor
timestamp: September 16, 2018, 03:47:04 pm
content: Here is a simple way to check this ‘resistor is really a capacitor theory’. You can use a resistive divider on D.C. to get a ratio voltage and this is common in metrology. You could also use a resistor divider on A.C. like in a scope probe but you might have to adjust compensation depending on the use and frequency. You can use a capacitive divider only on A.C. and this is pretty common as well.If you use 2 equal resistors as in figure 1, the voltage, V1, at the output will be V/2. If you use a signal generator and use A.C. with equal capacitors as in figure 2 then the output voltage, V1, will also be V/2.If you ‘think’ you have a resistor that actually has a large capacitance then if you choose a real capacitor of the same value for the lower capacitor in the divider circuit in figure 3, then if the resistor that you think is a capacitor is placed in the top half of the divider, the output should still be, V1, of V/2.Try it with a generator at various frequencies from 1Khz to 1Mhz and let me know what you get for V1.

author: MK14
author_rank: Super Contributor
timestamp: September 16, 2018, 04:26:41 pm
content: One issue that comes to mind. Is that the resistors stray capacitance, is probably rather sensitive to frequency (and probably other factors, such as voltage).So the method you described, may limit the choice of frequencies.E.g. low frequencies, may be unsuitable.I guess the resistors datasheets could do with a typical stray capacitance value vs frequency (and voltage if it gives much change, which it is known to with some capacitor types, anyway).

author: ArthurDent
author_rank: Super Contributor
timestamp: September 16, 2018, 05:21:40 pm
content: MY14 - "Is that the resistors stray capacitance, is probably rather sensitive to frequency ..."I realize that and just mentioned using 1Khz to 1Mhz for checking to eliminate both high and low frequencies. This was kind of a wild ass guess and I thought this range might work for testing. However, I think that the output voltage from figure 3 will probably be too low to measure because the ratio of capacitance will be so large.

author: MK14
author_rank: Super Contributor
timestamp: September 16, 2018, 05:38:12 pm
content: Quote from: ArthurDent on September 16, 2018, 05:21:40 pmI bet Marvin the Robot, brain the size of a planet, would know how to measure it.0<.05pF expected stray capacitance from some resistor types, mentioned above. Would present a significant challenge.Actually, no. This thread seems to have the answers.See a post, many posts above this one.It doesn't seem to like me trying to link to it directly.Please goto post "Reply #55"It shows a HP LCR meter, being used to measure down to 1/1000ths of a pF.

author: ArthurDent
author_rank: Super Contributor
timestamp: September 16, 2018, 05:44:29 pm
content: "I bet Marvin the Robot, brain the size of a planet, would know how to measure it."He's too depressed to solve so menial a problem.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 17, 2018, 06:18:41 am
content: Quote from: MK14 on September 16, 2018, 04:26:41 pmIf you use a series equivalent (R + C), you'll have a f^2 dependency.The parallel equivalent is the more accurate first order model.The impedance is measured on a fixture with a VNA. Pad capacitance, connecting traces and all that are de-embedded, leaving only the contributions due to the component itself.A 49.9 ohm 1206 chip resistor will fall over at >3GHz. Yes, a high bandwidth is needed to test some components.Tim

author: MK14
author_rank: Super Contributor
timestamp: September 17, 2018, 02:54:11 pm
content: Quote from: T3sl4co1l on September 17, 2018, 06:18:41 amThanks.Yes, needing >3GHz, is a very surprising fact. Intuitively, one would have expected, more like 10KHz (as in simple capacitance meters, such as some DMMs), to do the job of component testing and measurement. But that is because intuition, can get it badly wrong at times.In this case intuition is likely to be getting it wrong, because of NOT taking into account the tiny signals the very low stray capacitance gives (stray sub pF capacitance, does not do much at 10KHz, in most circuits). Compared to the big dominance of other factors, until they are removed/minimised using the techniques described in your post.