Measuring Large resistances
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From: Robert Baer on 12 Nov 2009 23:24 Jan Panteltje wrote: > On a sunny day (Thu, 12 Nov 2009 21:12:29 +0100) it happened Kreyen > <Ihatespam(a)nospam.com> wrote in <biqof5tmr1nlhjrj5ogpm02cg126it0j7n(a)4ax.com>: > >> I am having problems trying to measure resistance values above >> 100MegaOhms. My Laboratory Ohmeter gives unstable values. >> >> Is ther anyway apart from the usual dc bridges of getting accurate >> resistance measurements. >> >> Thanks guys. >> >> Kreyen > > Sure, 100 volt in 100 MOhm = 1 uA. > How accurate can you measure current? Most hand-held DVMs are 10 Meg input and have a 200mV full scale setting; equivalent to 20nA full scale - allowing one to use reasonably low voltages for deriving large resistance values.
From: Robert Baer on 12 Nov 2009 23:26 Kreyen wrote: > On Fri, 13 Nov 2009 01:59:20 +0530, "pimpom" <pimpom(a)invalid.com> > wrote: > >> Kreyen wrote: >>> I am having problems trying to measure resistance values above >>> 100MegaOhms. My Laboratory Ohmeter gives unstable values. >>> >>> Is ther anyway apart from the usual dc bridges of getting >>> accurate >>> resistance measurements. >>> >> Are you trying to measure a resistor or something else? What >> range variations are you getting? Did you consider surface >> leakage and other interfering factors? What level of accuracy do >> you need? >> > > I'm trying to measure 3 resistances to be used for calibration > purposes.. I'm getting variations of several percent while I'm > looking into a t least a 0.1 percent accuracy level. > > Kreyen *That* is a color of a different horse..the DVM used would have to have that accuracy, leaves out handhelds!
From: Jim Yanik on 13 Nov 2009 00:01 Fred Bartoli <" "> wrote in news:4afc7e1b$0$21967$426a74cc(a)news.free.fr: > Spehro Pefhany a �crit : >> Most inexpensive DMMs on the lower ranges have very high input >> impedance so the LMC buffer might not be necessary. Nonsense;they have the same impedance on their lowest range as the others,unless they disconnect the input divider.They don't. (I did that on a TEK DM501 to make cal measurements for a 577/177 curve tracer.) also,I checked a Harbor Freight DMM and it was only 1 Megohm input R. (and the first one was way out of calibration,read a 1.5v alkaline cell at 1.9v) >> >> >> > > High value resistors are (depends on the model) somewhat unstable. > I currently have a batch of Dale 1G/1%. > They measure fine with 100V bias which is the datasheet measuring > conditions. When measuring them at low voltage, they're all over the > place, from +3ish% to +7ish%. Yep, not even grouped... > On the contrary I've some Caddok rated <0.02ppm/V! > > A friend of mine worked at, IIRC, Vishay/sfernice on that specific high > value resistors 'feature' and went auditing some of the 'production' > lines (the quotes are his). He said the voltage dependency was mostly a > prod issue. High value Rs is a very small niche market and as such, > production inherited some highest tech tools, like hand lapping and the > likes. Being labor intensive it was relocated in low labor cost > countries, with poor buildings (he said, almost backyard :-) and less > than ideal handling cleanliness. > > That could explain a lot some of the strange behaviors... > > seems like they could just use carbon film deposition on glass substrates and laser-trim them,all machine handled. -- Jim Yanik jyanik at localnet dot com
From: ChrisQ on 13 Nov 2009 06:42 Kreyen wrote: > I am having problems trying to measure resistance values above > 100MegaOhms. My Laboratory Ohmeter gives unstable values. > > Is ther anyway apart from the usual dc bridges of getting accurate > resistance measurements. > > Thanks guys. > > Kreyen You could build a simple bridge circuit that depends only on the *ratio* between two resistors to determine the unknown. Put a known precision resistor in series with the unknown value as the lower resistance of one leg of the bridge. Then, the other leg using a second precision resistor and decade box and helipot / trim pot of suitable value. Put a null voltmeter between the two leg midpoints and adjust pot to get a null. Then, calculate the ratio with a few sums to determine the unknown. Bridge excitation voltage doesn't need to be precision or stable either. You should be able to get very high accuracy with this route, depending on the bridge supply and dvm resolution. Null methods resolve to zero load on the unknown leg divider at null... Regards, Chris
From: Spehro Pefhany on 13 Nov 2009 10:59
On Thu, 12 Nov 2009 20:26:57 -0800, Robert Baer <robertbaer(a)localnet.com> wrote: >Kreyen wrote: >> On Fri, 13 Nov 2009 01:59:20 +0530, "pimpom" <pimpom(a)invalid.com> >> wrote: >> >>> Kreyen wrote: >>>> I am having problems trying to measure resistance values above >>>> 100MegaOhms. My Laboratory Ohmeter gives unstable values. >>>> >>>> Is ther anyway apart from the usual dc bridges of getting >>>> accurate >>>> resistance measurements. >>>> >>> Are you trying to measure a resistor or something else? What >>> range variations are you getting? Did you consider surface >>> leakage and other interfering factors? What level of accuracy do >>> you need? >>> >> >> I'm trying to measure 3 resistances to be used for calibration >> purposes.. I'm getting variations of several percent while I'm >> looking into a t least a 0.1 percent accuracy level. >> >> Kreyen > *That* is a color of a different horse..the DVM used would have to >have that accuracy, leaves out handhelds! Maybe *your* handhelds. |