How to measure the inductance of a through-hole resistor? - Page 1
Forum section: electronics / projects

author: MisterHeadache
author_rank: Regular Contributor
timestamp: August 07, 2024, 01:22:42 pm
content: Here's my question: How can I measure the inductance (or inductive reactance) of through hole resistors of values from 1 ohm through 1 Megohm, fabricated from unknown construction, using instruments commonly available to the hobbyist?Background: I work on a lot of old RF electronics that use carbon composition through-hole (axial leaded) resistors. Carbon comps are good for those apps because they have low inductance, but they are hard to find nowadays (either expensive niche manufacturers or untrustworthy NOS). Ceramic non-inductives are avaible but pricey (~$2 apiece). Metal film I understand MIGHT be low enough inductance, but I have found only one (Vishay PR02-FS) which are spec'd as non-inductive, but they top out at 100 ohms and I need values upwards of 1 Megohm.There's plenty of useless blather online about the 'relatively low' inductance of metal film and some carbon film through-hole resistors, but what I want is DATA.So more specifically:Could I set up my nanoVNA to evaluate the inductance of my junk box of metal film/metal oxide/carbon film resistors?Could a relatively inexpensive LCR meter make this measurement? I understand most LCR meters cannot tolerate much resistance when making an inductive measurement.Could I make a resonant circuit with the resistor and a known capacitance and stimulate it with a step response and observe the response on a 'scope and determine the inductance?Lastly, I don't really need super accurate measurements. I'd settle with being able to do a side-by-side comparison to a carbon comp resitor of the same resistance value (which of course has lead length inductance anyway).

author: vk6zgo
author_rank: Super Contributor
timestamp: August 07, 2024, 01:58:51 pm
content: Quote from: MisterHeadache on August 07, 2024, 01:22:42 pmBack in the early 1960s, our Lecturer at Perth Tech school demonstrated the inductive coefficient of carbon composite & carbon film through the hole resistors of various values using a "Q meter".The carbon film resistors were trimmed to value at the factory by a spiral cut in the film, which added substantial inductance.I think modern metal film resistors are a bit more sophisticated, but perhaps not.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 07, 2024, 02:12:22 pm
content: Maybe something here will help you out as the same basics apply:https://www.eevblog.com/forum/rf-microwave/shunt-capacitance-of-1206-smd-resistors-jeroen-belleman-december-2010/

author: mag_therm
author_rank: Frequent Contributor
timestamp: August 07, 2024, 02:13:18 pm
content: For a nanoVNA test jig:Use a baseplate of about 75 by 50mm of un-etched 1 oz copper pcbSolder two of SMA jacks AMPHENOL RF 132289 at required spacing.That connector has easy access to solder the pins.Solder in your network or single resistor suspended at required height over copper and do S11 and S21. (screw a shorting plug in to Sma2 for S11)I also use the Vishay PR02 kit CCR-122 10R to 1 Meg.I recall testing and using a series of 47000 and 50 Ohm for 940:1 divider network which had low inductance at 14MHz etc but I did not record the values.

author: MisterHeadache
author_rank: Regular Contributor
timestamp: August 07, 2024, 03:04:30 pm
content: Thanks for the replies! I read Jeroen's article and I understand the VNA technique. Guess I was overthinking it! I'll build the jig and run some comparison tests and post my results. Question on the proper way to do a basic SOLT calibration with the jig, would it be:Short: solder the pin on port #1 temporarily to the ground plane on the PCBOpen: remove the short, have nothing between the pins on the portsLoad: solder an SMT 50 ohm resistor from the pin on port #1 to the ground plane on the PCBThrough: solder a length of bare wire from port #1 pin to port #2 pin

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 07, 2024, 03:20:03 pm
content: The basic shape you're looking for is a single-pole response.The basic setup will work in reflection (s11, resistor wired across RF jack from signal to GND), or transmission (s11, s12*) with a series (jack 1---resistor---jack 2) or shunt (jack 1---jack 2, resistor from trace to GND) arrangement. Which one is best, depends on value: small values (under Zo) are better in shunt, large values in series.*s22 = s11 and s21 = s12 by symmetry, but you can always check all four just in case, or, say, to reduce noise by combining curves.Then, fit a curve such that the component under test, in the jig arrangement given, is modeled as an R+L, or even (R+L || C) (or (R || C) + L, but you may not have data to disambiguate the two possibilities, for example, full transmission line effects might take over by the time series/parallel LC comes into question, and then a much more developed model is required; this will be in the GHz, corresponding to lead length).You'll find that carbon comp are more or less ideal, with some capacitance at high frequencies, and inductance given by lead+body length (major downside: excess noise, and value tends to drift up over time, and under voltage bias); carbon/metal film (normally spiral cut) aren't much worse (despite what you may think of the spiral), and, wirewound fall apart in the MHz. Wirewound are available into the megs, where C will also dominate at such frequencies; these will most strongly call into question the L/C placement in the model, but more than that, C is distributed along the winding so you may end up with a more detailed model anyway, even without transmission line effects.Tim

author: jonpaul
author_rank: Super Contributor
timestamp: August 07, 2024, 08:20:58 pm
content: Carbon Comp, MF, dep F about same as for a wire of that legnth.We use 1 nH/ftUp to GHz its insignificant,Measuer with HP4195A Impedance test fixture and Thru Hole binding post adapters.Jon

author: trobbins
author_rank: Frequent Contributor
timestamp: August 07, 2024, 11:58:09 pm
content: I use a Picoscope 4224A and FRA4PicoScope software to easily measure part impedance out to 1MHz. For an R-X series network, using an appropriate reference metal film R, I'm able to assess small value inductors like ferrite beads with <1uH, where the gain/phase plot nicely shows the impedance change of the inductor, with no indication of reactance from the resistor. What I haven't done is to measure the response for an R-R* network, where R* is say a WW or a vintage carbon comp to assess the response out to 1MHz.Are you not wanting to use normal PRO-2 resistors, with nominal impedance curves out to 100MHz, or is their impedance change too much ?

author: mawyatt
author_rank: Super Contributor
timestamp: August 08, 2024, 01:09:45 am
content: Quote from: jonpaul on August 07, 2024, 08:20:58 pmAgree a leaded component should have an inductance ~ of a straight wire of equal total length if the component doesn't have any geometry which allows field coupling or additional effective length.However, a for a 1mm dia wire the inductance is ~387nH/ftBest,

author: Smokey
author_rank: Super Contributor
timestamp: August 08, 2024, 02:52:40 am
content: Would a basic LCR meter like a DE-5000 not be able to measure inductance in this case?

author: vk6zgo
author_rank: Super Contributor
timestamp: August 08, 2024, 04:20:31 am
content: Quote from: T3sl4co1l on August 07, 2024, 03:20:03 pmThe old Philips spiral cut carbon film resistors were pretty terrible back in the 1960s.

author: mag_therm
author_rank: Frequent Contributor
timestamp: August 08, 2024, 12:48:12 pm
content: I tested some Vishay PR02 leaded resistors on the nanoVNA jig mentioned in #3 above.I did not cut the leads , and laid the resistor bodies on the copper baseplate between the SMA jacks to simulate how they might be used in old tube restoration.Up to about 4700 Ohm, the resistors were inductive, jX was between 10% (R=47 Ohm) and 80% (R=4700 Ohm) of R at 20 MHz.Up to about 100 Ohm, the resistors were inductive, jX was between 10% (of nominal R=47 Ohm) and 6% (of nominal R=100 Ohm) of R at 20 MHz.The nominal 4700 Ohm at 20 MHz measures 2660-j2360 OhmAbove 47000 Ohm, the resistor becomes a parallel network of R and -jX : (R*[-jXc])/(R+[-jXc])For example of 100000 Ohm resistor on the copper baseplate, at 20 MHz it becomes 307-j5590 Ohm.When the resistor was lifted about 15mm above the copper baseplate: 406-j6350 OhmI don't have any old carbon resistors here, it is likely that the higher values would have had have similar capacitive shunting.I also tested the only leaded RF Choke here, about size of a 3 Watt resistor, rated 294 uH at 0.2 AmpIt has a double pole at 4.97 MHz with a Q of about 16, and capacitive above resonance.If replacing RFC in old tube units best to check individually.scrots of sweeps. Marker 2 (Green) is at 20 MHzSweep S11 of Vishay_PR02 4700 Ohm:https://app.box.com/s/gj439ukiujaufjnhem655i8yq5qocrdzSweep S11 of Multicomp Size1206 4700 Ohmhttps://app.box.com/s/mautip8as1kgts73mw6ggfwqacusq206Sweep S11 of 50 Ohm Calhttps://app.box.com/s/1ejn2hc0zqn1baxrsgifoi9fhzfcvgj2

author: MisterHeadache
author_rank: Regular Contributor
timestamp: August 08, 2024, 02:09:38 pm
content: Quote from: mag_therm on August 07, 2024, 02:13:18 pmI built this fixture last night and did a SOLT calibration on it per my presumption above. I soldered in a 51 ohm resistor that I know is carbon comp and measured an almost flat S21 response from 1MHz to 100MHz. It was within a few tenths of dB to the theoretical value per Jeroen's formula (-3.6dB). But I ran out of time, will experiment with other resistors tonight.Quote from: mag_therm on August 08, 2024, 12:48:12 pmSo if I did my math right, that 4.7k resistor has an effective inductance of 30 microhenries at 20MHz? I'd think that would upset most 20MHz RF circuits!

author: mag_therm
author_rank: Frequent Contributor
timestamp: August 08, 2024, 02:59:05 pm
content: You are correct.At a frequency where the abs jXL = R, the impedance of the resistor is 1.41 times nominal R.Mostly not a problem.Looking at a HF receiver here (1963 Lafayette HE30) there are no circuits where the RF or IF signals go through a resistor except at the cathode of the 455 kHz Q multiplier which is a 10 k selectivity potentiometer.The plates have tuned coupling transformers, the low value cathode bias resistors are all capacitor bypassed.RF and IF gain and AGC are by variable grid bias.

author: mawyatt
author_rank: Super Contributor
timestamp: August 08, 2024, 04:02:08 pm
content: With a 4.7K resistor the effective series inductance will be swapped by the R, however the parallel equivalent capacitance will have an effect tho, as just ~1.7pF has an equal capacitive reactance @ 20MHz.For low resistance values the series inductance influences the overall impedance, for high resistances the parallel capacitance has more effect.Generally one can estimate the inductance using straight wire equivalents for the overall lead wires and resistor body as the effective length1, and the capacitance by the physical resistor body makeup.(1)https://chemandy.com/calculators/round-wire-inductance-calculator.htmBTW the physical resistor body distance of the PR02-FS (10mm) implies an equivalent inductance of ~5nH, add wire leads so the total length is ~40mm and the overall effective inductance becomes somewhere around 35nH.Here's a simple lumped element model for a leaded resistor, you should be able to "fit" measured values over reasonable frequency ranges to such.Best,

author: MisterHeadache
author_rank: Regular Contributor
timestamp: August 08, 2024, 08:27:27 pm
content: OK, thanks very much for the explanations. I'm now getting my head around how these components behave over frequency.Referring to mawyatt's lumped model, aren't L-sub-L (lead inductance) and C-Sub-R (body capacitance) largely the same regardless of ohm value within a particular manufacturer's series and body/wattage size? For laser trimmed or etched resistors which have helical groove patterns, doesn't L-sub-R vary, possibly substantially, depending on how much of a helical pattern they have cut into them to get the specific ohm value? That's the consistent caution that I've read about using metal film/metal oxide/carbon film resistors in RF circuits, and why I'm interested in quantifying their performance versus carbon comps of the same values.I understand that one has to look at the schematic and understand what function that specific resistor is doing. Let's say that a 4.7k carbon comp resistor was used in a feedback circuit or a filter circuit that would see significant signal magnitude at 20MHz. Wouldn't replacing it with that Vishay PRO2 with that much inductance likey upset the circuit? A resistor with 4.7k ohms of resistance and 80% of that in inductive reactance (3.76k ohms) would also introduce significant phase delay, right?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 08, 2024, 10:40:34 pm
content: I would wonder firstly whether your measurement is accurate. Very little phase error is necessary to read inductance that far out of whack. Repeat the measurement at other frequencies: does it follow consistently, or does it vary? Is the instrument properly calibrated? If there is an averaging option, might it prove useful?Tim

author: mawyatt
author_rank: Super Contributor
timestamp: August 09, 2024, 02:39:18 am
content: Don't know how much a spiral cut resistor body inductance increases, but with a 4.7K Resistor Real part, the body Inductive Imaginary part should be significantly smaller at even 20MHz and usually be ignored. 37uH to equal the 4.7K impedance @ 20MHz is not realistic in any practical resistor, about 1000 times too large!!However the body capacitive effects shouldn't be ignored, as a few 100s fF wouldn't be surprising!!Best,

author: mag_therm
author_rank: Frequent Contributor
timestamp: August 09, 2024, 12:41:07 pm
content: I am sorry I made a mistake interpreting my Vishay PR02 resistor sweep data in post #11The crossover from inductive to capacitive when mounted near the copper baseplate occurs at nominal value below470 Ohm(not 4700 Ohm)I have struck out and corrected post 11 and added scrots of the nanoVNAsaver S11 sweeps for the Vishay PR02 4700 Ohm and for a Multicomp 1206 4700 Ohm and for a 50 Ohm Cal

author: joeqsmith
author_rank: Super Contributor
timestamp: August 09, 2024, 02:25:57 pm
content: As pointed out already, a fair amount of the inductance is determined by lead length. Based on your question, I have no idea if you are trying to remove the effect of the leads.While not really what you are asking, this video attempts to demonstrate using a VNA to measure the impedance of a simple network using through hole parts and a breadboard. At 35 minutes, I show a simple RLC network sweeping from 10k to 50M. As I work through the problem, I eventually shorten the leads (37 min) and you can see what effect this has. Then I change to surface mount (38 min).Depending on what you are doing, construction can be every bit as important as the component selection.

author: MisterHeadache
author_rank: Regular Contributor
timestamp: August 09, 2024, 04:19:39 pm
content: Quote from: joeqsmith on August 09, 2024, 02:25:57 pmThe goal of my effort here is to determine using data if I can or cannot substitute modern construction through-hole resistors for original carbon composition resistors in vintage gear that operates at RF frequencies. Secondary constraints are defining the differences between constructions (metal film, carbon film, metal oxide), and upper limit of useable frequencies (my equipment would be primarly HF, so 30MHz and below, but up to 100MHz would be better. Certainly don't care about GHz operation).Since the applications must use leaded resistors, the natural inductance of the leads is expected, regardless of resistor construction. Knowing just how much 'worse' a modern resistor is as compared to a carbon comp would suffice.I am aware that ceramic resistors are available and have naturally low inductance, but paying $2 apiece gets painfully expensive in a hurry, and would be foolish to spend if there was a $0.10 alternative that would work fine. As would something like - "the series XYZ are OK to substitute but only in values above 1k because below that they are too inductive."Lastly, I emphasize chasing DATA over LORE - there's no shortage of 'OMG, don't ever replace your carbon comps with anything else!' LORE on the internet. If carbon comps were still readily available, there'd ne no problem. But AB stopped making them a long time ago, and Stackpole stopped a couple years ago, and only niche ($$$) manufacturers are left.(come to think of it, wasn't chasing DATA over LORE a STTNG episode??

author: mag_therm
author_rank: Frequent Contributor
timestamp: August 09, 2024, 06:50:06 pm
content: There is a range of carbon resistors athttps://www.surplussales.com/of NebraskaI buy tubes etc and metal film there, but I wouldn't buy carbon.Even if you don't use them for restoration, you could get a few of samples to compare on nanoVNA.About 10 years ago in a 1959 FM receiver I replaced nearly all of the carbon with smaller 1/4 Watt metal film, keeping the leads about the same.It re-aligned OK and is still running. L.O is 10.7 MHz above 88 ~108 MHz. At that time a 120V isolation transformer was added and the whole tube string was replaced with 6.3V equivalents.

author: bson
author_rank: Unknown
timestamp: August 09, 2024, 07:55:28 pm
content: I would imagine for any resistor other than wire wound, the inductance is going to be dominated by the loop area it forms with the return path underneath. If it has a circuitous return path or is part of a longer signal path the resistor itself probably makes little to no difference.If inductance really matters it's probably time to get acquainted with SMT...BTW, a wire wound resistor, like any coil, also has a capacitance in parallel with the inductance; this is the inter-winding capacitive coupling between adjacent turns. This should be measured for a ww resistor as much as the inductance.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 09, 2024, 08:23:44 pm
content: I've bought from surplussales as well and have had no problems with them.QuoteNot many 1Meg parts will be stable from DC to 100MHz but I understand what you are after.Looking at Digikey, they appear to have some stock of CC parts. You could stock up on the common parts while buying a range of parts for your experiment.If I had a stock pile of old CC and CF parts, I would attempt to run something. I'm in the same boat and would need to order a matrix of parts to run it. Looking forward to seeing what you come up with.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 10, 2024, 03:24:32 am
content: Using the LiteVNA64 to measure S21 for two sets of resistors. One CC, the other CF. Both sets are 1/2W. From 1-100MHz, outside of the CC's DCR being off, their AC performance is similar. Leads were left long and were measured at their ends. No attempt was made to compensate for the difference. CC measure roughly 87.3mm and CF 60.0mm, end to end. Note the CF's spiral pattern. The DCR for the 390 ohm CF measured 393 while the CC measured 400. For the 750, the CF measured 745 and the CC 785.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 10, 2024, 02:09:19 pm
content: Including S21 for the 24k and 220k CC & CF 1/2W parts. The unloaded fixture was also included to show it's contribution to the capacitive parasitics.I do not have any other type of 1/2W parts I can use for a comparison.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 10, 2024, 02:12:21 pm
content: Looking at various 1W & 2W CC. That 1M part is not very stable. Again remember the unloaded fixture is roughly 90dB.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 10, 2024, 02:16:04 pm
content: The 1W 56 ohm resistor behaved unexpectedly. It was highly inductive compared to the others. Shown are the 87ohm 1W and 47ohm 2W along with the 56 ohm. Inductance of the 47 & 87 appear similar but something is up with the 56 ohm.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 10, 2024, 02:23:47 pm
content: A rasp and sandpaper tells the story.The CF spiral has 3 turns or so compared with the WW. This is why we don't see much of a difference between the CF and CC. Now, if I snip the leads short the effects of the spiral pattern would be more pronounced.Good luck with your future repairs.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 10, 2024, 03:59:41 pm
content: Not a great comparison but showing two MF parts compared with the 1W CC. CC measure 77mm end to end where the 1/4W MF is 50mm.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 10, 2024, 04:16:56 pm
content: Ohmite Red Devil 82 ohm. I think these were CC.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 10, 2024, 07:20:24 pm
content: Wirewound in a phenolic case? Oooh, sneaky! I had no idea!Is the core ceramic? (Steatite? Alumina?) If you can check the Mohs hardness or something like that, that may be slightly identifying.Tim

author: joeqsmith
author_rank: Super Contributor
timestamp: August 10, 2024, 10:07:33 pm
content: Quote from: T3sl4co1l on August 10, 2024, 07:20:24 pmIt's a pretty old part and other materials may have been used. To expose it we start with a diamond stone to get a decent surface and finish it up with some 3000. Never grind anything that is unknown dry. Note the different layers of brass and copper.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 10, 2024, 10:09:53 pm
content: Dried and under the microscope, it has a woven pattern to it. I suspect the substrate is made from asbestos.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 10, 2024, 10:25:34 pm
content: Using a very sharp dental pick from my old dentist, I attempt to deform the substrate. It was very soft and spongy. You can see how I pushed a hole into it.Just a quick patent search for resistors and asbestos, first hit:QuoteSearching for lawsuits, Allen-Bradley Co. v. Erie Resistor Corporation 1939QuoteQuoteI would assume OP and anyone else working on old electronics and electrical devices is very much aware of the use of asbestos and follow some basic precautions.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 10, 2024, 11:47:43 pm
content: Comparing a 47 ohm RCD 5W with the 47 ohm 2W CC and what we now know is a 56 ohm WW. The 5W RCD part is roughly the same package size as the 2W CC.Datasheet may be found here:https://www.rcdcomponents.com/wp-content/uploads/2022/01/100-Series-Datasheet_RevI.pdfI doubt based on the comments about costs that this would interest OP but maybe someone else would find these useful:https://www.rcdcomponents.com/rcd-announces-release-of-cm-series-carbon-matrix-resistors/***Front to rearRCD 160 5W WW, Ohmite 2W CC, OhmiteRedLittle Devil

author: MisterHeadache
author_rank: Regular Contributor
timestamp: August 12, 2024, 03:06:22 pm
content: Guys, thanks for the very detailed test results, especially you joeqsmith! I've made a summary of the results so far:poster mfr/series construction value measurement reactance?mag_therm Vishay PR02 metal film 4.7k S11 strongly inductivemag_therm Multicomp 1206 unknown 4.7k S11 moderately inductivejoeqsmith unknown carbon comp 390/0.5W S21 not significantjoeqsmith unknown carbon film 390/0.5W S21 not significantjoeqsmith unknown carbon comp 750/0.5W S21 not significantjoeqsmith unknown carbon film 750/0.5W S21 not significantjoeqsmith unknown carbon comp 24k/0.5W S21 weakly capacitivejoeqsmith unknown carbon film 24k/0.5W S21 weakly capacitivejoeqsmith unknown carbon comp 220k/0.5W S21 moderately capacitivejoeqsmith unknown carbon film 220k/0.5W S21 moderately capacitivejoeqsmith unknown carbon comp 1k/1W S21 not significantjoeqsmith unknown carbon comp 1.5k/1W S21 not significantjoeqsmith unknown carbon comp 1Meg/1W S21 strongly capacitive/fixture effectsjoeqsmith unknown carbon comp 3.9k/2W S21 moderately capacitivejoeqsmith unknown carbon comp 47/2W S21 not significantjoeqsmith unknown wirewound in phenolic 56/1W S21 strongly inductivejoeqsmith unknown carbon comp 87/1W S21 not significantjoeqsmith unknown carbon comp 240/2W S21 not significantjoeqsmith unknown carbon comp 270/1W S21 not significantjoeqsmith unknown metal film 1k/0.25W S21 not significantjoeqsmith unknown metal film 82/0.25W S21 not significantjoeqsmith Ohmite red devil carbon comp(?) 82/0.25W S21 not significantjoeqsmith Ohmite red devil carbon comp(?) 82/2W S21 not significantjoeqsmith RCD 160 wirewound 47/5W S21 severely inductiveI see some key take-aways:Just because a resistor looks like a classic carbon composition resistor doesn't mean it's non-inductive! Those results on that 56 ohm / 1 watt resistor are alarming. I wonder how many folks over the years tried to use those instead of 51 ohm ones in a quick and dirty QRP dummy load and had frustrating results.The carbon film resistors so far look to have insigificant reactance.Any resistor over a couple hundredkiloohms looks difficult to quantify (fixture effects, capacitance starts to dominate)And this - ultimately one might have no choice but to measure every candidate resistor before using it in an RF circuit to know with certainty whether its reactance will be minimal or not.I've got my own measurements to do this week. And just for grins, I sent an email to tech support at Vishay, asking if they have lab data on the PR02 series for reactance. Won't hold my breath for a response...Quote from: mag_therm on August 09, 2024, 06:50:06 pmYep, I recently ordered some carbon comps from them and several had drifted more than 10%. I was not surprised and expected that because they are NOS and who knows how old they are. I will use some of them for my VNA comparisons.Quote from: bson on August 09, 2024, 07:55:28 pmMy desperation plan is exactly that - make a tiny bit of PCB to hold a 1206 SMT resistor (or possibly two, one on each side in parallel for higher wattage), then solder wire leads to it to make a cobbled-up low-inductance resistor replacement.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 12, 2024, 06:30:02 pm
content: See 7.20https://www.rcdcomponents.com/wp-content/uploads/2021/06/R-14-CF-MCF-Carbon-Film-Resistors.pdfSee 7.20https://www.rcdcomponents.com/wp-content/uploads/2021/06/R-08-GP-MGP-Metal-Film-Resistors-.pdfQuoteI had provided data for the fixture with no load attached just to answer that question. While it is 15 or more below, you would need to decide what your are really trying to measure. Parts with their stock leads unbent, bent, cut ..... In this case, the leads are straight and uncut. If they were all cut short we would greatly reduce the parasitics. As it is, they all have different lead lengths depending on the series.Keep in mind too that the voltage rating their ability to handle very high surge currents are a few reasons we may still use CC today. It all depend on the application and it's really up to the person doing the repairs to understand what parameters are important for each component they are replacing.

author: bson
author_rank: Unknown
timestamp: August 12, 2024, 09:21:16 pm
content: I'm surprised at that Vishay PR02! An inductive MF resistor...It also seems to have a significant parallel capacitance that becomes dominant at higher resistance values... But we're only looking at like 10-100MHz here. But then itisan axial THT part.From the datasheet:

author: MisterHeadache
author_rank: Regular Contributor
timestamp: August 13, 2024, 02:36:47 pm
content: Guys, thanks for this additional info! I have been looking at various data sheets and totally missed those impedance versus frequency plots on the Vishay PR01/02/03 data sheet. That's pretty much telling me exactly what I have been chasing, at least for that particular metal film resistor series. And a rep from Vishay did respond to me - and referred me to the data sheet.I did a quick search and found Allen Bradley's data sheet from the early 90's that also has impedance versus frequency curves for their carbon comp resistors. I'll spend some time studying them versus the Vishay curves and the RCD curves for carbon film. That will help immensely with my understanding the expected differences between classic carbon comps and modern carbon film and metal film.Also it is finally sinking in my aged brain what you guys have been trying to tell me:The series inductance typically dominates on low ohm resistors, causing the Z/R ratio to rise above 1.0 with increasing frequency.There is a 'sweet spot' of resistances where Z/R doesn't deviate much from 1.0. That is roughly between 100 ohms to several kiloohms, depending on a specific resistor series.Above the 'sweet spot', the parallel capacitance dominates, causing the Z/R ratio to decline below 1.0 with increasing frequency.And mawyatt's model is a useful approximation to simulate that performance.

author: MisterHeadache
author_rank: Regular Contributor
timestamp: August 13, 2024, 07:41:59 pm
content: I took the 0.1-100MHz impedance versus frequency plots from the Vishay PR01/02/03 [metal film], RCD CF [carbon film], and Allen Bradley [carbon composition] data sheets and re-plotted the curves on the same scale. I chose the Vishay and Allen Bradley 1W plots, but the RCD is 1/4 Watt, that's just the size they chose to show in the datasheet and they don't have anything else on their web site that I could find.There are three groupings: <10 ohms, medium (~10k), and high (100k). The values aren't exact for all three because all three didn't have curves for the exact same values. And I had to visually pick data points from them, so this is just an approximation. But it is more than adequate to show a few interesting observations:RCD CF25 [carbon film] shows the least impedance change over frequency and over the largest range of resistances. Maybe it's helped by being the 1/4W size instead of 1W size?Vishay PR01/02/03 [metal film] has significant inductive impedance at low resistance (1 Ohm), but has the second best performance at 100k.The beloved Allen Bradley [carbon comp] has virtually no change in impedance for values under about 100 ohms, but quickly becomes substantial with higher resistances. It has the worst performance of the three as frequency increases, but the Vishay catches up to it at 100MHz/100k Ohms.The biggest eye opener for me is just how poorly the Allen Bradley are shown to perform over frequency. Maybe because their data is with leads of only 1/4" in length, so there's minimal lead inductance to mix with the body capacitance at higher frequencies? Just a guess.There is this mythos about carbon composition resistors being so precious for proper function in RF circuits versus modern resistors. I can see that being true for low (a few hundred Ohms) applications, but that's it - the modern metal film seem to actually perform with less capacitive reactance above that. I wonder now if some older RF circuits that used CC's in the signal chain were actually tuned to work with the impedance characteristics of a CC, and substituting a non-CC (which ironically might have less reactance) could de-tune or upset the circuit performance.Here is a link to the AB datasheet that I found:https://www.hificollective.co.uk/sites/default/files/allen_bradley_data_sheet.pdf

author: MisterHeadache
author_rank: Regular Contributor
timestamp: August 20, 2024, 03:55:45 pm
content: I’ve been playing around with various resistors on my fixture. My nanoVNA is giving me results that jive with what’s been discussed here. Just for curiosity sake I’d like to be able to plot the results in |Z|/R0 format, like the datasheet curves. The nanoVNA software outputs .s2p files that have the data apparently in real and imaginary components normalized to 50 ohms impedance.How can I convert that data to |Z|/R0 format? I get that |Z| is the magnitude of the impedance (square root of sum of squares of real and imaginary magnitudes), and R0 is the DC resistance of the resistor. The values in the .s2p file are very small, quite a bit less than 1.0. I searched online but didn’t find an explanation that I could understand.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 20, 2024, 04:28:12 pm
content: I am not aware of a canned viewer that would do this. In the Allen Bradley data sheet that was previously linked, they also tested with a standard lead length of 1/4". They mention the fixture, mounting position and lead length having a significant effect on the high frequency characteristics and they are only looking at 10MHz. (pg 9)Do you have a LiteVNA?

author: joeqsmith
author_rank: Super Contributor
timestamp: August 23, 2024, 01:52:54 pm
content: Ohmite part was a Little Devil, not Red. They are indeed carbon comp as suspected. See OH series 2W in datasheet:https://app.mectronic.com/dsheet/OH3305_OHMITE.pdf

author: MisterHeadache
author_rank: Regular Contributor
timestamp: August 23, 2024, 03:46:25 pm
content: Quote from: joeqsmith on August 20, 2024, 04:28:12 pmI'm using a program called 'NanoVNA Saver' to control my Nano VNA-H. It captures a data sweep and then let's you move a frequency marker and interrogate the data at that marker frequency. Here's an example display for the marker. Notice that it calculates and displays the impedance in R+jX format, along with a bunch of other parameters. I am assuming I also can do this from the data in the .s1p and .s2p files, but I lack the understanding and the necessary formulas.Sample image is attached.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 23, 2024, 04:40:04 pm
content: Z is just the vector sum, so Z= SQRT(R^2 +X^2). DCR you have. For the Touchstone file format, the standard is free on-line. It's just a simple ASCII format that could be read into any spreadsheet and plotted there. Excel was limited to 65k samples or something. You may only be looking at 100-800 samples. So that shouldn't be a problem.I don't know anything about the Saver software but they should be following that same Touchstone standard. The header will tell you the format they are using. Could be MA, RI or DB. If it is just RI, you may just need to normalize the data to 50 ohms.As I understood, Saver is opened sourced so you could see what they are doing and also possibly modify it to suit your needs.

author: MisterHeadache
author_rank: Regular Contributor
timestamp: August 23, 2024, 10:11:05 pm
content: OK I think I have figured it out. The data in the .s2p file contains the real and imaginary components of the S parameters. What I'm looking to do then is convert those two numbers into the real and imaginary components of the transmission impedance. I found the formula here:https://docs.keysight.com/kkbopen/how-can-the-s21-to-transmission-impedance-zt-conversion-equation-on-the-4395a-operation-manual-be-derived-588282056.htmlThe math checks out correct in Excel when I process the .s2p data, just that working with complex numbers in Excel is a real pain.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 23, 2024, 11:29:32 pm
content: Another option may be to use METAS to import the file, display Z in the table format and select Z-Parameter. You can then cut and paste to Excel and plot?

author: Dave Wise
author_rank: Regular Contributor
timestamp: September 14, 2025, 07:49:17 pm
content: About that supposed-CC 56-ohm resistor that turned out to be WW. Color-banded resistors are WW if the first-digit band is wider than the others. I learned this in my teens, I don't remember where.

author: TimFox
author_rank: Super Contributor
timestamp: September 14, 2025, 09:45:40 pm
content: Back when Allen-Bradley carbon composition axial-lead resistors were the default, we needed 1 megohm gate-bias resistors for low-noise JFET amplifiers following a parallel-resonant sensor, where the ultimate SNR depended on the loadedQof the resonant circuit, including that parallel resistance. The frequency range of interest was between 3 and 10 MHz, for relatively low-field MRI.We found (first in the literature, then using an-hp-impedance meter and measuredQvalues) that the equivalentparallel resistanceof normal 1/4 W A-Bs fell off substantially above 2 MHz: the parasitic parallel capacitance was irrelevant since it was absorbed into the resonant circuit. The reason given in the literature was that the resistor mixed carbon powder and insulating (talc) powder to get high resistivity, and at high frequencies the talc tended to short out the carbon bits capacitively.At that time (pre surface mount), we found IRC "carbon ink" 1 megohm resistors, same physical size as the 1/4 W A-Bs, that still fell off a bit by 10 MHz, but were higher resistance than the A-Bs. I believe they had an unhelixed thin carbon coating on a ceramic core.

author: Traceless
author_rank: Frequent Contributor
timestamp: September 14, 2025, 10:04:58 pm
content: Quote from: MisterHeadache on August 07, 2024, 01:22:42 pmDepending on what you consider hobbyist range, and how high the required measurement frequency needs to be you might be able to use an LCR meter. I did usemine to measure resistance and inductance of resistors a while back. However those were quite the opposite of carbon composition resistors and had a lot of inductance. In case you wonder why the inductance on those resistors was so high - somebody posted an image of the inside of similar resistors herehere.