20K ohm meter - Page 1
Forum section: electronics / testgear

author: cowasaki
author_rank: Frequent Contributor
timestamp: December 30, 2019, 11:03:00 pm
content: I’ve been messing around with valve equipment recently and a lot of the older equipment gives voltages using a 20K ohm meter. Is there any way I can simulate that, work out the correct reading using a modern DMM or does it not make much difference.If it matters I have. Keithley 2100, 2 PM2525s and a few hand held meters.

author: Yansi
author_rank: Super Contributor
timestamp: December 30, 2019, 11:07:30 pm
content: 20k meter? Don't you mean actually a 20kohm/V (per volt) meter (i.e. analog meter with 50uA full scale movement)?Well, modelling that is especially easy: Just use appropriate parallel resistance with the DVM you already have.Note that this scales with the range, so for example a 100V range will be 2Mohm.

author: cowasaki
author_rank: Frequent Contributor
timestamp: December 30, 2019, 11:33:06 pm
content: Quote from: Yansi on December 30, 2019, 11:07:30 pmYes I did mean 20Kohm/V

author: andy3055
author_rank: Super Contributor
timestamp: December 31, 2019, 05:03:05 am
content: Quote from: cowasaki on December 30, 2019, 11:03:00 pmIt should not matter. You can use any modern day DMM. Make sure you use the appropriate scale if it is not auto-ranging.

author: floobydust
author_rank: Super Contributor
timestamp: December 31, 2019, 06:04:43 am
content: I use extra caution, poking around say 500VDC with expensive multimeters I manual range up, so the DMM is not on the mV range and then hit with 500V. I don't know how tough the Keithley's are, many threads on the forum with blown mux or front end seems odd.Use decent test leads and don't slip with the probes...

author: Cubdriver
author_rank: Unknown
timestamp: December 31, 2019, 08:07:11 am
content: Quote from: andy3055 on December 31, 2019, 05:03:05 amYou can use a modern DMM, but be aware that unless you add the shunt resistance mentioned by Yansi (20k ohms/volt for the scale used) that your readings will be higher than they would if taken with the 'proper' meter listed in the schematic as the DMM's 10M input resistance will not load the circuit as much as the 20k/V meter would. For quick-and-dirty troubleshooting this is probably ok as long as you're aware of it, but if you need accurate readings for some reason then the shunt should be added.-Pat

author: Yansi
author_rank: Super Contributor
timestamp: December 31, 2019, 09:37:21 am
content: Quote from: floobydust on December 31, 2019, 06:04:43 amWhat?! If an autoranging multimeter can't handle 500V, it is not correctly designed then.

author: TimFox
author_rank: Super Contributor
timestamp: December 31, 2019, 03:43:17 pm
content: Tube equipment service information usually specified node voltages measured with specifically either a 20,000 ohm/V VOM (50 uA movement) or a VTVM with 11 megohm input resistance. In the latter case, there was 1 megohm inside the probe which isolated the meter cable when measuring a node with AC or RF on the DC level. For the VOM case, if the service information specified it, then there should be no problem with high frequencies. VOMs were made from 1000 ohm/V (1 mA) el-cheapos up to 100,000 ohm/V (10 uA) premium models, but 20,000 ohm/V (e.g., Simpson 260) were the most common.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 31, 2019, 05:13:36 pm
content: Modern bench meters should survive 500 V in the low voltage ranges. One nV meters may be an exception - at least I would avoid such stress to such a delicate instrument. The more stressing case is usually ESD - spark gabs may sometimes be to slow, and ESD can sometimes be stronger than expected.Also the resistance mode could have some trouble - this is often less well protected and overload specs may be to a lower level.In most cases the higher impedance DMM is OK. One may get a slightly higher reading, but test point voltages and the old analog meters are usually not that accurate anyway.

author: floobydust
author_rank: Super Contributor
timestamp: January 01, 2020, 04:58:42 am
content: Quote from: Yansi on December 31, 2019, 09:37:21 amIt's not static overload i.e measuring 500VDC with a DMM autoranged down to mV, that I am mentioning.When connecting multimeter test leads to a HV low impedance source, I have seen a tiny spark sometimes which I attribute to the fast dV/dt hitting the input capacitance of the DMM, causing a spike.There is DMM (differential) input capacitance due to the MOV's or divider AC compensation capacitors. It does depend on a DMM's design.There is DMM (common-mode) input capacitance. Mains-powered DMM's have higher input capacitance to earth ground (some even add a Y-cap) mostly due to the power transformer's windings and electrostatic shield.Handheld DMM's, there may be a foil shield and human body capacitance due to being held in your hand or steel workbench.The very long ago Fluke multimeter recall was because they could crash or reboot from that tiny spark, the EMI burst affected the MCU. Their fix mostly involved adding a series ferrite bead to COM which implies it was a common-mode sensitivity; the (+) input is not as susceptible because of the 1k and PTC lower dI/dt.I see the Keithley 2001 repair threads on EEVblog are a train wreck of blown mux, JFETs, op-amps on the front end. So I consider them delicate bench multimeters - their front-end designs - they are not industrial by any means because precision is their goal. Thermal EMF's, leakage current from extra protection components are unwanted. You can't have it all, toughness and uV/ppmprecision.

author: fcb
author_rank: Super Contributor
timestamp: January 01, 2020, 09:37:15 pm
content: I wouldn't go near valve gear with a 2100, as others have said, they aren't the most robust. Also, meters like the 34401A tend to float the whole unit against the COM input, so they tend to have more capacitance to ground than a floating handheld, a bench DMM might help tip your circuit into oscillation.Also the most basic handheld DMM will exceed the accuracy and linearity of an Avo8 or Simpson 260.

author: Yansi
author_rank: Super Contributor
timestamp: January 01, 2020, 11:06:03 pm
content: Quote from: floobydust on January 01, 2020, 04:58:42 amOkay got it. But the meter having insignificant capacitance to ground shall be an obvious clue for the designers of such unit, especially for guys from Keysight/Agilent/HP or Keithley.Sure the lab units are only mostly CAT I and not CAT III or IV as handhelds are, but that does not mean it should get blown just from a measurement of a stiff power supply and if it does, I mean the design of such unit was done incorrectly.I wouldn't have ever imagined it is possible to blow a Keithley like this O_o

author: fcb
author_rank: Super Contributor
timestamp: January 01, 2020, 11:47:17 pm
content: Quote from: Yansi on January 01, 2020, 11:06:03 pmYou'd think so. But I guess it's down to what your idea of 'insignificant' is. Something like the 34401A's capacitance to ground comes through coupling via the mains transformer, and certainly if you study isolated supplies in a Keithley SMU they are designed for very low coupling. Remember that they are probably only bothered about line frequencies and no-one really calibrates beyond 850Hz. We test the leakage current on our power analyser at 5kV AC and you can measure uA level leakages attributable to various couplings (data-isolator, transformer, general PCB design) - it's quite surprising just how capacitive the data-isolators are.

author: Fungus
author_rank: Super Contributor
timestamp: January 02, 2020, 05:34:24 pm
content: Get a Fluke 101. They're really cheap and you'll have real difficulty blowing one up.https://www.eevblog.com/forum/testgear/fluke-101-getting-cheaper/