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Metal film resistors?
are metal film resistors wirewound or not? I've been trying to find this one
out. Someone who I know says they're not so are suitable for RF but Maplin catalog seems to say they are. someone please help! dr. x --- Outgoing mail is certified Virus Free. Checked by AVG anti-virus system (http://www.grisoft.com). Version: 6.0.799 / Virus Database: 543 - Release Date: 22/11/2004 |
They're not wirewound. They consist of a thin film of metal deposited on
a ceramic tube, which is helically cut to leave a conductor in a spiral pattern. A wirewound resistor has a lot of inductance because it's made of many turns of wire. A metal film resistor has a very small amount of inductance by comparison -- so little that it can be ignored for most purposes. Roy Lewallen, W7EL James Bond wrote: are metal film resistors wirewound or not? I've been trying to find this one out. Someone who I know says they're not so are suitable for RF but Maplin catalog seems to say they are. someone please help! dr. x |
James Bond wrote:
are metal film resistors wirewound or not? I've been trying to find this one out. Someone who I know says they're not so are suitable for RF but Maplin catalog seems to say they are. someone please help! dr. x Strictly speaking they are not "wire wound", but some are constructed with the film "wound" around a substrate, thus having a lot of inductance. If you dont't have the makers specs to tell you, the choices are to measure the inductance or to remove the coating and see what's underneath. -- Jim Pennino Remove -spam-sux to reply. |
are metal film resistors wirewound or not? I've been trying to find this one
out. Someone who I know says they're not so are suitable for RF but Maplin catalog seems to say they are. Strictly speaking they are not "wire wound", but some are constructed with the film "wound" around a substrate, thus having a lot of inductance. If you dont't have the makers specs to tell you, the choices are to measure the inductance or to remove the coating and see what's underneath. Several manufacturers (Caddock and some of the Vishay divisions) make metal-film resistors which are advertised as having about the same amount of inductance as a straight wire the same length as the resistor body. I believe that these have the film cut into a back-and-forth serpentine meander pattern, rather than a spiral. -- Dave Platt AE6EO Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
James Bond wrote:
are metal film resistors wirewound or not? I've been trying to find this one out. Someone who I know says they're not so are suitable for RF but Maplin catalog seems to say they are. They can be suitable for some RF applications. If you are thinking of the wire-ended ones up to 2-3W, they are a flat spiral of just a few turns of the metal film, on a ceramic core. Plugging the number of turns, diameter and length into the usual formula suggests inductances of less than 0.1uH, even for the larger ones. This means they have inductive reactances rising up to about 10 ohms at 30MHz. For example, if you wanted to use say a 47 ohm resistor as a dummy load, the SWR would be quite reasonable up to 30MHz. However, the inductance of the higher-value resistors goes up much less rapidly then the resistance does (they use a thinner film of a more resistive material, but not a lot more turns). The inductance of a 470R metal film resistor is not a lot higher than that of a 47R. So if you were to use ten 470R resistors in parallel to make your dummy load, the inductive reactance would be divided by 10 and the SWR would be much better. I have used multiple-paralleled 2W metal film resistors as high as 50MHz, and up to144MHz in situations where the inductance could be tuned out. The ones I tested can also be overloaded to red heat for several seconds without significantly changing resistance when cold - unlike the cheaper carbon film resistors which burn (as carbon does), or wire-wound resistors which generally develop a hot-spot and burn out. -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
On Wed, 24 Nov 2004 20:57:59 +0000, Ian White, G3SEK wrote:
Plugging the number of turns, diameter and length into the usual formula suggests inductances of less than 0.1uH, even for the larger ones. This means they have inductive reactances rising up to about 10 ohms at 30MHz. Ian, I've just measured a couple of resistors bought as spares for the FL-2100Z, 33 ohm 3W metal film used as a grid stopper, and 22 ohm 3W metal oxide used on the anode, again as a suppressor, with a coil wound round it (both of the originals were carbon). The metal film 33R measures 6.5uH and the oxide 22R measures 4.5uH on the aade.com meter. Both values (if the readings are correct) would represent a high ratio of X to R at HF frequencies... I have quite a surplus of them, so if you wish I can send one of each in the mail for you to check on your VNA. -- Duncan Munro http://www.duncanamps.com/ |
I've used "Flame Proof" resisters to build signal splitters, the
outputs were within .1 db of a commercial unit (made by Suhner) @ 850 Mhz. That ain't bad :). Howard. On Wed, 24 Nov 2004 18:59:16 +0000 (UTC), "James Bond" wrote: are metal film resistors wirewound or not? I've been trying to find this one out. Someone who I know says they're not so are suitable for RF but Maplin catalog seems to say they are. someone please help! dr. x --- Outgoing mail is certified Virus Free. Checked by AVG anti-virus system (http://www.grisoft.com). Version: 6.0.799 / Virus Database: 543 - Release Date: 22/11/2004 |
Duncan Munro wrote:
On Wed, 24 Nov 2004 20:57:59 +0000, Ian White, G3SEK wrote: Plugging the number of turns, diameter and length into the usual formula suggests inductances of less than 0.1uH, even for the larger ones. This means they have inductive reactances rising up to about 10 ohms at 30MHz. Ian, I've just measured a couple of resistors bought as spares for the FL-2100Z, 33 ohm 3W metal film used as a grid stopper, and 22 ohm 3W metal oxide used on the anode, again as a suppressor, with a coil wound round it (both of the originals were carbon). The metal film 33R measures 6.5uH and the oxide 22R measures 4.5uH on the aade.com meter. Both values (if the readings are correct) would represent a high ratio of X to R at HF frequencies... I have quite a surplus of them, so if you wish I can send one of each in the mail for you to check on your VNA. Those values seem surprisingly high, so yes, please do and I will measure them. Based on the standard inductance formula (and there seems no reason why that shouldn't apply in this case) it would seem to require very large numbers of turns to achieve 1uH. My posting was based on the ones I scraped the paint off in pre-VNA days, so it would be useful to collect more data on typical 'coil' dimensions and numbers of turns, together with the actual effective inductances. -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
On Thu, 25 Nov 2004 07:32:27 +0000, "Ian White, G3SEK"
wrote: Those values seem surprisingly high, so yes, please do and I will measure them. Please post the results here when you've done so, Ian. I for one would also be interested. -- "What is now proved was once only imagin'd." - William Blake, 1793. |
On Wed, 24 Nov 2004 18:59:16 +0000 (UTC) "James Bond"
wrote: are metal film resistors wirewound or not? I've been trying to find this one out. Someone who I know says they're not so are suitable for RF but Maplin catalog seems to say they are. They are not WIREwound, but the resistive element IS usually a helical film that is a layer on a ceramic body. As someone else mentioned, there are some types which are serpentine rather than helical, but these are always special and sold as such. Film resistors generally have fewer turns in their helix than wirewounds, so they will have more inductance than carbon or ceramic bulk resistors but less than wire wound resistors. - ----------------------------------------------- Jim Adney Madison, WI 53711 USA ----------------------------------------------- |
On Thu, 25 Nov 2004 07:32:27 +0000, Ian White, G3SEK wrote:
Duncan Munro wrote: I have quite a surplus of them, so if you wish I can send one of each in the mail for you to check on your VNA. Those values seem surprisingly high, so yes, please do and I will measure them. OK, will post them off tomorrow night. I'll include the manufacturers info (they were bought recently from Mouser in the US), and put two of each in so that you can crack one of each value open and see what they're made of ;-) -- Duncan Munro http://www.duncanamps.com/ |
"James Bond" wrote in message ... are metal film resistors wirewound or not? I've been trying to find this one out. Someone who I know says they're not so are suitable for RF but Maplin catalog seems to say they are. Metal film or metal oxide? There is a big difference. High ohmic values are almost always Oxide types and these are usually the continuous layer type with the occasional 'trim'. Really low value metal film are also a continuous tube or nearly so. It is the upper half of the metal film rage (100R-100k) that is spiral cut & consequently of appreciable inductance. Unfortunately, these are the values that are most needed. |
matt wilson wrote:
"James Bond" wrote in message ... are metal film resistors wirewound or not? I've been trying to find this one out. Someone who I know says they're not so are suitable for RF but Maplin catalog seems to say they are. Metal film or metal oxide? There is a big difference. High ohmic values are almost always Oxide types and these are usually the continuous layer type with the occasional 'trim'. Really low value metal film are also a continuous tube or nearly so. It is the upper half of the metal film rage (100R-100k) that is spiral cut & consequently of appreciable inductance. Unfortunately, these are the values that are most needed. I think metal film resistors are made by depositing metal vapor onto ceramic rods in a vacuum. Then they are attached to metal end caps with leads and either laser or abrasive engraved to set the final resistance. Some have only a turn or two, some have more. They are a lot less inductive than wire wound devices, and the best low inductance versions have a serpentine pattern engraved in the film, to keep the inductance very low. e.g. http://www.caddock.com/Online_catalog/power/power.html -- John Popelish |
On Sat, 27 Nov 2004 19:31:37 -0500, John Popelish
wrote: d the best low inductance versions have a serpentine pattern engraved in the film, to keep the inductance very low. Yes, minimal inductance, but doesn't that pattern give rise to more parasitic capacitance? -- "What is now proved was once only imagin'd." - William Blake, 1793. |
Paul Burridge wrote:
On Sat, 27 Nov 2004 19:31:37 -0500, John Popelish wrote: d the best low inductance versions have a serpentine pattern engraved in the film, to keep the inductance very low. Yes, minimal inductance, but doesn't that pattern give rise to more parasitic capacitance? Some. More than an inductive pattern? Not much, if any. -- John Popelish |
Just a comment.
Even such a thing as a small 1/2-watt resistor has distributed R, L and C. L and C can be calculated from physical dimensions. A resistor can be treated as a helically-loaded transmission line in exactly the same way as a helically-loaded antenna. If the frequency is high enough the radiation resistance can be taken into account. Just calculate the input resistance of the line with a short circuit at the other end and the job is done. The performance of dummy-load resistors can be determined in the same way. If you (in the plural) are unable to do this then you are unworthy to call yourselves engineers. Whatever happened to your education? ;o) ---- Reg |
On Sun, 28 Nov 2004 12:23:04 +0000 (UTC), "Regosaurus"
wrote: Just a comment. Even such a thing as a small 1/2-watt resistor has distributed R, L and C. Indeed. L and C can be calculated from physical dimensions. Only if you have x-ray vision. The package gets in the way. A resistor can be treated as a helically-loaded transmission line in exactly the same way as a helically-loaded antenna. If the frequency is high enough the radiation resistance can be taken into account. Just calculate the input resistance of the line with a short circuit at the other end and the job is done. The performance of dummy-load resistors can be determined in the same way. If you (in the plural) are unable to do this then you are unworthy to call yourselves engineers. Whatever happened to your education? ;o) I've never described myself as an engineer. I'm not one! But if I wanted to checkout how suitable any given resistor was for a dummy load, I'd use a network analyser. -- "What is now proved was once only imagin'd." - William Blake, 1793. |
But if I
wanted to checkout how suitable any given resistor was for a dummy load, I'd use a network analyser. -- ================================ If the network analyser is a computer program you'll still need X-ray eyes. |
Avery, I too have had a similar career to yours. I cannot but agree with all
that you say. A very apt description. Regarding newsgroups - "Abandon all rank ye who enter here." (Toc-H, on the Western Front, 1916.) To summarise - To find what a resistor does , measure it. If you are unable to measure it then model it with lumped components and then calculate. If lumps are not accurate enough then model it as a distributed transmission line, which it actually is, and calculate again. If you get similar answers for both procedures then you are laughing. If you don't know how to do these things then you are not qualified to call yourself an engineer which I'll admit is slightly off-topic. But if the cap fits then wear it! ---- Regards, Reg. |
I've used these as a dummy load and they work
fine even at 440mhz.. http://www.ohmite.com/catalog/pdf/tah_tch_series.pdf Mouser sells them.. http://www.mouser.com/catalog/620/415.pdf Hope this answers your question.. -Pete James Bond wrote: are metal film resistors wirewound or not? I've been trying to find this one out. Someone who I know says they're not so are suitable for RF but Maplin catalog seems to say they are. someone please help! dr. x |
On Mon, 29 Nov 2004 13:40:16 GMT, Pete D wrote:
I've used these as a dummy load and they work fine even at 440mhz.. http://www.ohmite.com/catalog/pdf/tah_tch_series.pdf I notice they say "very low inductance" but it would be nice to have a figure!!! -- "What is now proved was once only imagin'd." - William Blake, 1793. |
"Reg Edwards" wrote in message
... If lumps are not accurate enough then model it as a distributed transmission line, which it actually is, and calculate again. If you don't know how to do these things then you are not qualified to call yourself an engineer which I'll admit is slightly off-topic. Well, Reg, equivalent circuit modelling is not a common topic taught in colleges, _especially_ in an undergraduate curriculum, and there are one heck of a lot of EE's out there who work for companies that don't even _have- the facilities (a network analyzer) to properly measure what their resistor does... yet plenty of them are fine engineers. Electrical engineering is quite broad these days. There are guys who sit around designing communication systems who never touch soldering irons, and I'm sure plenty of them would claim you're not qualified to be an engineer because you can't derive some 'trivial' convolutional code off the top of your head. BTW, many SPICE simulators do a mediocre job of simulating lossy transmission lines. Most people who are going to be using components at frequencies where they care about distributed parasitics are probably (hopefully) using frequency domain simulators anyway, but that too is an area where today's undergraduate curriculum tends to be somewhere beween weak and non-existant. (Using simulators other than SPICE... e.g., harmonic balancers, periodic steady staters, linear frequency sims, etc.) ---Joel |
Duncan Munro wrote in message .. .
The metal film 33R measures 6.5uH and the oxide 22R measures 4.5uH on the aade.com meter. Both values (if the readings are correct) would represent a high ratio of X to R at HF frequencies... I wonder, though, if the AADE meter is not getting confused by the resistance of the resistor. The fact that the measured inductance is just about proportional to the resistance might be evidence for that. 73, Mike, KK6GM |
Mike Silva wrote:
Duncan Munro wrote in message . .. The metal film 33R measures 6.5uH and the oxide 22R measures 4.5uH on the aade.com meter. Both values (if the readings are correct) would represent a high ratio of X to R at HF frequencies... I wonder, though, if the AADE meter is not getting confused by the resistance of the resistor. The fact that the measured inductance is just about proportional to the resistance might be evidence for that. Duncan has kindly sent a couple of samples, with duplicates that have had the paint scraped off. I just arrived home from a few days away, so haven't had time to measure them yet. Each one is only an open spiral of about two turns along the whole length of the 3W resistor body, so you can see immediately that there's no way the inductance can actually be more than a few hundred nH. This actual, physical inductance is in *series* with the resistance. What seems to be happening is that the AADE meter displays the resistance and reactance in their equivalent parallel form, which is a function of the measurement frequency (which varies, but is understood to be in the order of a few MHz). Guessing a frequency and then doing the parallel - series transformation on 22 ohms in parallel with 4.5uH produces results in the right ballpark: R is still around 22 ohms but the *series* inductance is 100-200nH. I will try to measure the resistors tomorrow. -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
On 30 Nov 2004 12:37:57 -0800, Mike Silva wrote:
Duncan Munro wrote in message .. . The metal film 33R measures 6.5uH and the oxide 22R measures 4.5uH on the aade.com meter. Both values (if the readings are correct) would represent a high ratio of X to R at HF frequencies... I wonder, though, if the AADE meter is not getting confused by the resistance of the resistor. The fact that the measured inductance is just about proportional to the resistance might be evidence for that. Mike, I think you've hit the nail on the head. To be fair to AADE, they warn that the 'Q' of the inductor has to be reasonable to get a sensible measurement. The kind of measurement frequencies we are talking about are in the order of 700kHz. At that frequency, the inductance of the 'indicated' 4.5uH is 19.8 ohms, not a million miles from the 22 ohms of the resistor itself - this is not what I would call a reasonable 'Q' value. Fair play to AADE, it's designed to measure the inductance of inductors, not other components ;-) -- Duncan Munro http://www.duncanamps.com/ |
On Tue, 30 Nov 2004 21:04:28 +0000, Ian White, G3SEK wrote:
Guessing a frequency and then doing the parallel - series transformation on 22 ohms in parallel with 4.5uH produces results in the right ballpark: R is still around 22 ohms but the *series* inductance is 100-200nH. There is an additional complication in that there is another inductor in the box itself of 680uH, LX (or should I say RX) is in series with that. It's late now, but I will try and work out what's going on tomorrow night. I will try to measure the resistors tomorrow. If you get the opportunity, it would be much appreciated. -- Duncan Munro http://www.duncanamps.com/ |
Duncan Munro wrote:
The kind of measurement frequencies we are talking about are in the order of 700kHz. At that frequency, the inductance of the 'indicated' 4.5uH is 19.8 ohms, not a million miles from the 22 ohms of the resistor itself - this is not what I would call a reasonable 'Q' value. Fair play to AADE, it's designed to measure the inductance of inductors, not other components ;-) If the reactance is much lower than the resistance, it's generally inconsequential in a practical application. I think that's almost always the case for carbon film resistors, and I suspect it's nearly always the case for metal film resistors. Probably, if the Q is so low as to make measurement difficult, it's probably low enough that the X isn't important in a practical application. Roy Lewallen, W7EL |
On Wed, 01 Dec 2004 16:35:36 -0800, Roy Lewallen wrote:
Duncan Munro wrote: The kind of measurement frequencies we are talking about are in the order of 700kHz. At that frequency, the inductance of the 'indicated' 4.5uH is 19.8 ohms, not a million miles from the 22 ohms of the resistor itself - this is not what I would call a reasonable 'Q' value. Fair play to AADE, it's designed to measure the inductance of inductors, not other components ;-) If the reactance is much lower than the resistance, it's generally inconsequential in a practical application. I think that's almost always the case for carbon film resistors, and I suspect it's nearly always the case for metal film resistors. Probably, if the Q is so low as to make measurement difficult, it's probably low enough that the X isn't important in a practical application. (adds) ..... at the test frequency |
budgie wrote:
On Wed, 01 Dec 2004 16:35:36 -0800, Roy Lewallen wrote: Duncan Munro wrote: The kind of measurement frequencies we are talking about are in the order of 700kHz. At that frequency, the inductance of the 'indicated' 4.5uH is 19.8 ohms, not a million miles from the 22 ohms of the resistor itself - this is not what I would call a reasonable 'Q' value. Fair play to AADE, it's designed to measure the inductance of inductors, not other components ;-) If the reactance is much lower than the resistance, it's generally inconsequential in a practical application. I think that's almost always the case for carbon film resistors, and I suspect it's nearly always the case for metal film resistors. Probably, if the Q is so low as to make measurement difficult, it's probably low enough that the X isn't important in a practical application. (adds) ..... at the test frequency Sure. Any statement about a frequency-dependent property like X or Q applies only at the frequency at which the component has that particular X or Q. As you raise the frequency, the X of course increases while the R stays relatively constant. But other effects like shunt C and the physical length of the part eventually start coming into play, making the simplistic model of a series RL inadequate. My general experience has been that I can ignore the inductance of leaded carbon film resistors up to a frequency where the leads and component length become a problem, and I need to go to chip components. I've never seen significant reactance from the trim cuts on a thick film chip resistor -- the shunt C across the narrow cuts pretty much makes them invisible.(*) I suspect that carbon film resistors likewise have a narrow cut. But I don't have much experience with metal film resistors. I assume the base material has less resistivity, so is probably cut into thinner strips with more "turns" and more spacing between "turns". So there might be a combination of R and frequency where the reactance is objectionable, below the frequency where you need to abandon leaded parts. I'm watching this thread with interest for any good measurement results. I could try making some measurements up to 1.3 GHz with my 8505A network analyzer, but I wouldn't trust the results. I think the measurements probably would have to be done on a system with good, computer-directed calibration, a good set of calibration standards, a decent and well-characterized test fixture, and an operator who's very familiar with the many traps you can fall into when making subtle measurements like these -- and I have none of the above. (*) I've used thick film resistors at frequencies up to 20 GHz or so, in very sensitive time-domain applications. In those applications, I modeled nearly every component as a transmission line or a pi or tee approximation to a line, with the R in one or two lumps. Those models agreed quite well with actual results. Roy Lewallen, W7EL |
On Wed, 01 Dec 2004 19:57:48 -0800, Roy Lewallen wrote:
budgie wrote: On Wed, 01 Dec 2004 16:35:36 -0800, Roy Lewallen wrote: Duncan Munro wrote: The kind of measurement frequencies we are talking about are in the order of 700kHz. At that frequency, the inductance of the 'indicated' 4.5uH is 19.8 ohms, not a million miles from the 22 ohms of the resistor itself - this is not what I would call a reasonable 'Q' value. Fair play to AADE, it's designed to measure the inductance of inductors, not other components ;-) If the reactance is much lower than the resistance, it's generally inconsequential in a practical application. I think that's almost always the case for carbon film resistors, and I suspect it's nearly always the case for metal film resistors. Probably, if the Q is so low as to make measurement difficult, it's probably low enough that the X isn't important in a practical application. (adds) ..... at the test frequency Sure. Any statement about a frequency-dependent property like X or Q applies only at the frequency at which the component has that particular X or Q. I only added that because the test frequency was cited as ~700kHz and a casual reader may have taken the above to mean that the X was insignificant at the frequency of intended operation. |
budgie wrote:
I only added that because the test frequency was cited as ~700kHz and a casual reader may have taken the above to mean that the X was insignificant at the frequency of intended operation. Ah, thanks, I'd missed that. At 700 kHz, you'd never get enough reactance from a metal film resistor to be bothersome, and I doubt that you'd even be able to measure it. Roy Lewallen, W7EL |
Duncan Munro wrote:
On Tue, 30 Nov 2004 21:04:28 +0000, Ian White, G3SEK wrote: Guessing a frequency and then doing the parallel - series transformation on 22 ohms in parallel with 4.5uH produces results in the right ballpark: R is still around 22 ohms but the *series* inductance is 100-200nH. There is an additional complication in that there is another inductor in the box itself of 680uH, LX (or should I say RX) is in series with that. It's late now, but I will try and work out what's going on tomorrow night. I will try to measure the resistors tomorrow. If you get the opportunity, it would be much appreciated. Over a range from 50kHz to 50MHz, Duncan's two resistors measure about 22 ohms + 31nH, and 33 ohms + 23nH. To avoid having to construct a special test jig, I measured each resistor with about 30mm of bent wire leads, which would account for about 20nH of those measured inductance values. The very low inductance of the resistor body is completely consistent with the physical construction. On closer inspection, the metal film is an almost continuous tube, with a very narrow spiral gap of about 1.5 turns. The gap adjusts the resistance by slightly increasing the overall electrical path length, but it adds very little inductance. If you used very short leads instead of the longer lengths I had to use, these resistors would have a low SWR up to at least 144MHz. Thanks again to Duncan for supplying the resistors. -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
Ian, if it's not too much trouble, I'd be very interested in how the
measured inductance compares to that of the resistor body/leads only. I'd think that could be done by coating a similar-size resistor with conductive paint or foil and measuring with the same method. The difference between this measurement and the one you made would then show how much inductance is due to the spiral element, and would represent the minimum possible inductance for that resistor body type and lead length. Roy Lewallen, W7EL Ian White, G3SEK wrote: Over a range from 50kHz to 50MHz, Duncan's two resistors measure about 22 ohms + 31nH, and 33 ohms + 23nH. To avoid having to construct a special test jig, I measured each resistor with about 30mm of bent wire leads, which would account for about 20nH of those measured inductance values. The very low inductance of the resistor body is completely consistent with the physical construction. On closer inspection, the metal film is an almost continuous tube, with a very narrow spiral gap of about 1.5 turns. The gap adjusts the resistance by slightly increasing the overall electrical path length, but it adds very little inductance. If you used very short leads instead of the longer lengths I had to use, these resistors would have a low SWR up to at least 144MHz. Thanks again to Duncan for supplying the resistors. |
On Thu, 2 Dec 2004 12:01:03 +0000, Ian White, G3SEK wrote:
Thanks again to Duncan for supplying the resistors. No need, it's thanks to you for taking the time to measure them and post the results! At least I now know my linear is not going to fail because the carbon comps have been replaced with the metal oxide and metal film jobs. Thanks again Ian. -- Duncan Munro http://www.duncanamps.com/ |
Roy Lewallen wrote:
Ian, if it's not too much trouble, I'd be very interested in how the measured inductance compares to that of the resistor body/leads only. I'd think that could be done by coating a similar-size resistor with conductive paint or foil and measuring with the same method. The difference between this measurement and the one you made would then show how much inductance is due to the spiral element, and would represent the minimum possible inductance for that resistor body type and lead length. I could certainly do that, because Duncan has supplied pairs of resistors: one in original condition, and the other with the coating cleaned off, just ready for painting. However, the difference in inductance is going to be very small, and I'd need to build a test jig that can keep other stray inductances under control. Over the weekend, maybe... -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
The idea was to measure the coated resistor under exactly the same
conditions as the regular one, so couldn't you just use exactly the same setup as before? Roy Lewallen, W7EL Ian White, G3SEK wrote: Roy Lewallen wrote: Ian, if it's not too much trouble, I'd be very interested in how the measured inductance compares to that of the resistor body/leads only. I'd think that could be done by coating a similar-size resistor with conductive paint or foil and measuring with the same method. The difference between this measurement and the one you made would then show how much inductance is due to the spiral element, and would represent the minimum possible inductance for that resistor body type and lead length. I could certainly do that, because Duncan has supplied pairs of resistors: one in original condition, and the other with the coating cleaned off, just ready for painting. However, the difference in inductance is going to be very small, and I'd need to build a test jig that can keep other stray inductances under control. Over the weekend, maybe... |
Roy Lewallen wrote:
The idea was to measure the coated resistor under exactly the same conditions as the regular one, so couldn't you just use exactly the same setup as before? Afraid not... In the present setup, the main contribution to the total inductance comes from the long, floppy resistor wires, and I couldn't guarantee not to disturb their configuration while applying conductive paint to the resistor. To get a reliable answer, I'd need to reduce the lead length and make the whole thing mechanically more stable. Ian White, G3SEK wrote: Roy Lewallen wrote: Ian, if it's not too much trouble, I'd be very interested in how the measured inductance compares to that of the resistor body/leads only. I'd think that could be done by coating a similar-size resistor with conductive paint or foil and measuring with the same method. The difference between this measurement and the one you made would then show how much inductance is due to the spiral element, and would represent the minimum possible inductance for that resistor body type I could certainly do that, because Duncan has supplied pairs of resistors: one in original condition, and the other with the coating cleaned off, just ready for painting. However, the difference in inductance is going to be very small, and I'd need to build a test jig that can keep other stray inductances under control. Over the weekend, maybe... -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
I use them with no issues that I know of.
Perhaps someone else has the mathematical and physical explanation. I measured a few with my LC meter in the L mode and was unable to measure any L. "James Bond" wrote in message ... are metal film resistors wirewound or not? I've been trying to find this one out. Someone who I know says they're not so are suitable for RF but Maplin catalog seems to say they are. someone please help! dr. x --- Outgoing mail is certified Virus Free. Checked by AVG anti-virus system (http://www.grisoft.com). Version: 6.0.799 / Virus Database: 543 - Release Date: 22/11/2004 |
"Brian - KB9BVN" wrote in message nk.net... I use them with no issues that I know of. Perhaps someone else has the mathematical and physical explanation. I measured a few with my LC meter in the L mode and was unable to measure any L. "James Bond" wrote in message ... are metal film resistors wirewound or not? I've been trying to find this one out. Someone who I know says they're not so are suitable for RF but Maplin catalog seems to say they are. someone please help! dr. x Just a simple explanation. Not a comprehensive review. I do not know what meter, nor the circuit. Your depending on the circuit in your meter. Some Inductance measurement circuits need the resistive component to be canceled out (balanced by a bridge) to read the inductance. It doesn't mean it has no inductance, only that he resistance values is much higher than the reactance of the inductor. Therefore the meter sees the resistance only. To make it even worse, not all MF resistors are inductive. |
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