Measuring Large resistances
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From: John Larkin on 12 Nov 2009 17:06 On Thu, 12 Nov 2009 21:12:29 +0100, Kreyen <Ihatespam(a)nospam.com> 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 A quick+dirty way to do it is: Measure a power supply or 9-volt battery with your DVM. Then measure it again with the unknown resistor in series. Most DVMs have a 10M input resistance on their volts ranges. So Rx and the 10M DVM form a voltage divider, and you can do the math on that. You should be able to measure at least a Gohm or so to a couple of per cent accuracy. If your DVM has an infinite-impedance option (many bench meters do on their 10-volt ranges) you can make a divider from Rx and a 1G resistor, measure that with the infinite DVM, and do that math. Digikey has 1G resistors. A Keithley 610C measures up to 1e14 ohms. I got one on ebay for $150. I also built this: ftp://jjlarkin.lmi.net/99A260A1.JPG ftp://jjlarkin.lmi.net/99A260A3.JPG ftp://jjlarkin.lmi.net/99S260A.JPG John
From: Phil Hobbs on 12 Nov 2009 17:34 Fred Bartoli wrote: > Spehro Pefhany a �crit : >> On Thu, 12 Nov 2009 21:12:29 +0100, Kreyen <Ihatespam(a)nospam.com> >> 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 >> >> 100M isn't that high, well, depending on what kind of accuracy you're >> looking for. >> How much instability are you seeing? How much above 100M? >> LMC6042 has 2fA (typical) input bias current and costs a couple >> dollars one-off. >> A 100M 1% resistor with 50ppm/K tempco runs around five dollars. A 1G >> 1% resistor with similar tempco is maybe double that. >> Most inexpensive DMMs on the lower ranges have very high input >> impedance so the LMC buffer might not be necessary. >> >> > > 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... > > One cute method is to build a current-controlled oscillator, with a CMOS op amp connected as a Schmitt trigger, a voltage reference, a small MOSFET for the reset, and a good integrating capacitor. When you stick a current into the capacitor, the output frequency gives you the current. (No, this isn't the most accurate possible VFC, but it works pretty well for ~1% accuracy or a bit better.) Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
From: Didi on 12 Nov 2009 19:09 On Nov 13, 12:06 am, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > ... > > A Keithley 610C measures up to 1e14 ohms. I got one on ebay for $150. > ... My 17 years old Beckman DM27 xl multimeter has a 2G range on it. It can't do 0.1% accuracy at 2G, of course, but it does a good job for me (I use 1G resistors in HV biases, encounter them in HPGe detectors preamps etc.). Got it at about $100 back then. Dimiter ------------------------------------------------------ Dimiter Popoff Transgalactic Instruments http://www.tgi-sci.com ------------------------------------------------------ http://www.flickr.com/photos/didi_tgi/sets/72157600228621276/ Original message: http://groups.google.com/group/sci.electronics.design/msg/c51be927a616793e?dmode=source
From: Shaun on 12 Nov 2009 20:01 "Kreyen" <Ihatespam(a)nospam.com> wrote in message news: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 I just measured some Megaohm resistors with my Fluke 867b. accuracy 0.5% I measure two - 66 Megaohm resistors at 20% tolerance (not very accurate) measured 1 - 65.839 Megaohms measured 2 - 69.742 Megaohms Also I measured one 200 Megaohm resistor that came out of some medical equipment, unknown tolerance measured 196.46 Megaohms then readings were stable +/- .1 count in nanoSeimens and repeatable. You maybe working in an electrically noisy environment, try moving you meter to a different location where there is no extra cables around, maybe a noise filter would help too. Shaun
From: George Herold on 12 Nov 2009 22:03
On Nov 12, 5:06 pm, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Thu, 12 Nov 2009 21:12:29 +0100, Kreyen <Ihates...(a)nospam.com> > 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 > > A quick+dirty way to do it is: > > Measure a power supply or 9-volt battery with your DVM. Then measure > it again with the unknown resistor in series. > > Most DVMs have a 10M input resistance on their volts ranges. So Rx and > the 10M DVM form a voltage divider, and you can do the math on that. > You should be able to measure at least a Gohm or so to a couple of per > cent accuracy. > > If your DVM has an infinite-impedance option (many bench meters do on > their 10-volt ranges) you can make a divider from Rx and a 1G > resistor, measure that with the infinite DVM, and do that math. > Digikey has 1G resistors. > > A Keithley 610C measures up to 1e14 ohms. I got one on ebay for $150. > > I also built this: > > ftp://jjlarkin.lmi.net/99A260A1.JPG > > ftp://jjlarkin.lmi.net/99A260A3.JPG > > ftp://jjlarkin.lmi.net/99S260A.JPG > > John "A quick+dirty way to do it is: Measure a power supply or 9-volt battery with your DVM. Then measure it again with the unknown resistor in series. " Excellent, I knew there must be some way to use the 10Meg input impedance of the DMM. I'll try it tomorrow on some 1G R's I've got. George H. |