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#1
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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 |
#2
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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/ |
#3
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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/ |
#4
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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 |
#5
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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 |
#6
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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 ----------------------------------------------- |
#7
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![]() "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. |
#8
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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 |
#9
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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. |
#10
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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 |
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