Bad resistors
in [Design]
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From: DrParnassus on 23 Jun 2010 14:01 On Wed, 23 Jun 2010 10:12:47 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Wed, 23 Jun 2010 09:28:26 -0700, DrParnassus ><DrParnassus(a)hereforlongtime.org> wrote: > >>On Wed, 23 Jun 2010 08:50:37 -0700, John Larkin >><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >> >>>Dale says they're expensive because hardly anybody buys them. >> >> >> If they have to do a lot run simply because one order triggered it, >>they lose money if it isn't a pretty big order, and SMD has expiration >>dates due to termination oxidation issues so they cannot simply store the >>over-production stock either. Though I will take old parts because they >>are usable for proto builds. > >As I said, they are axial RC07 types, not SMD. > >They do stock 22M, available from distributors at a more sensible >price, and 33M, also distributor stock maybe. The 50M ones are >apparently not much in demand. > >All *are* 1% parts. And we can measure them accurately. > >I need a small constant current, and it's a battle between Johnson >noise current and shot/excess noise. It's not easy to generate a nA >level quiet constant current. The alternative was ten 10M >surface-mount thinfilms in series, which we don't have room for. > >John > You left out one over f.
From: DrParnassus on 23 Jun 2010 14:02 On Thu, 24 Jun 2010 03:48:22 +1000, "Phil Allison" <phil_a(a)tpg.com.au> wrote: >The bands can change color, especially with age or heat. > Bwuahahahahahaaha! Not on any of the brands real men buy!
From: George Herold on 23 Jun 2010 14:29 On Jun 23, 12:35 pm, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Wed, 23 Jun 2010 09:10:07 -0700 (PDT), George Herold > > > > > > <gher...(a)teachspin.com> wrote: > >On Jun 23, 11:50 am, John Larkin > ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > >> On Wed, 23 Jun 2010 08:24:02 -0700, DrParnassus > > >> <DrParnas...(a)hereforlongtime.org> wrote: > >> >On Wed, 23 Jun 2010 19:28:15 +1000, Sylvia Else <syl...(a)not.here.invalid> > >> >wrote: > > >> >>On 23/06/2010 9:54 AM, DrParnassus wrote: > >> >>> On Tue, 22 Jun 2010 21:51:47 GMT, Jan Panteltje > >> >>> <pNaonStpealm...(a)yahoo.com> wrote: > > >> >>>> Got 10 resistors on tape marked 2M7. > >> >>>> Used one, circuit did not work right. > >> >>>> Took some testing ... before I measured the resistors. > >> >>>> Of the 10 several are 620 k and not 2M7. > >> >>>> Bought fromwww.voti.nl, so beware. > >> >>>> All are marked with the right color code for 2M7. > > >> >>>> New to me :-) > > >> >>> A normal handheld DVM barely measures 1MOhm correctly so anything up > >> >>> there and over will not likely be easily measured with cheap handhelds. > > >> >>Still, you'd expect it to give the same wrong reading for the same > >> >>resistances. > > >> >>Sylvia. > > >> > That should be where their other clue comes from, because the readings > >> >are typically not repeatable. > > >> But they are, to a fraction of a per cent. In the case of the carbon > >> film resistor, I can hold the resistor body and see the resistor TC > >> effect, also repeatable. High-ohm resistors do tend to have bad TCs. > > >> There is nothing different between using and measuring a 10K resistor > >> or a 10M resistor. Or, assuming a little basic cleanliness, a 10G > >> resistor. > > >> We recently needed a 100M metal-film resistor to use in a photodiode > >> TIA thing. We tested a bunch of thickfilms and all had too much excess > >> noise; the testing itself is a nuisance. We settled on some axial RC07 > > >Was this 1/f noise? Noise that only showed up when you drove some > >current through them? > > Right. I don't know if it was 1/f or wideband shot noise, or maybe > both, but they were a lot noisier than the metal films, which seemed > to be at the Johnson noise level, independent of bias. Got a guesstimate on what "a lot noiser" was. Twice the johnson noise over the frequency band? ten times. This obviously depends on the bias level too. > > Capacitance limited our home-made test rig, so the noise spectrum was > hard to decode. The bottom like is that the thickfilms were noisy and > the Dales weren't. Yeah a few picofarads of stray C can sink a lot of 100 Meg ohm voltage noise. > > I was just thinking, in the shower, about developing a cool high-ohms > resistance and resistor noise measurement box, USB interfaced. That > would be fun. I probably won't do it, because it would be a fair > amount of work with an unknown market. Putting all the gain you need in one little area can lead to some problems. 10^4 was doable, but at 10^5 I got gain peaking, which screws up the measurment. George H. > > John- Hide quoted text - > > - Show quoted text -
From: John Larkin on 23 Jun 2010 15:19 On Wed, 23 Jun 2010 10:51:39 -0700, DrParnassus <DrParnassus(a)hereforlongtime.org> wrote: >On Wed, 23 Jun 2010 09:26:14 -0700, John Larkin ><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >> >>Read the Agilent and Keithley specs and then you can start weaseling >>about the dictionary definition of "accurately." > > > You do not understand. I am not talking about the equipment. Then what are you talking about? Are the Agilent and Keithley instruments unusable? You are on record here as saying that high-meg resistor measurements are "impossible to make accurately at the bench or in the field." So why do Agilent and Keithley sell these instruments? Explain this: ftp://jjlarkin.lmi.net/Keithley_1gig.JPG That's a 1G thickfilm resistor, from Digikey as I recall. If you paw the Pomona plug thing or the resistor, trying to add fingerprints, nothing changes, after the brief electrostatic transient. In the 10G and up turf, things have to be shielded, as any movement nearby can pin the meter. On the 1e-11 amps range, with a tiny Pomona "antenna", I can kick the needle by rubbing my foot on the carpet, standing 10 feet away. And it goes to 1e-14. It goes negative. John
From: DrParnassus on 23 Jun 2010 16:10
On Wed, 23 Jun 2010 12:19:33 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >> You do not understand. I am not talking about the equipment. > >Then what are you talking about? Are the Agilent and Keithley >instruments unusable? Zero common sense. If it ain't the gear it the hand that is on the gear, dippity doo da. |