Lower voltage for resistance measurement for FLuke 289 - Page 1
Forum section: electronics / testgear

author: tonycstech
author_rank: Regular Contributor
timestamp: September 28, 2024, 06:18:35 pm
content: I have fluke 289.measuring 1.8v buck converter based circuit gives me incorrect resistance compared to other meters.It gives me ohms instead of kohms.This is because fluke 289 uses 2v to measure resistance and if you change the range, it will jump to 5v.This is a big problem for me. Is there a way to change it to lets say 1v like a normal multimeter uses ?Even 0.5v.How am i suppose to measure 1.8v low resistance circuit pushing 2v or even 5v through it ? It screws up the reading.Basically i need it to use about 1v in 500ohm range and 0.5v above, which i think is what every other meter uses more or less but clearly not 5vThanks !

author: TimFox
author_rank: Super Contributor
timestamp: September 28, 2024, 06:24:26 pm
content: Make your own circuit with a low-voltage cell (1.5 V or so) and a three-resistor "T" circuit that has an open-circuit voltage of <0.5 V.Measure the voltage with no load and with load connected, using your voltmeter.The rest is algebra.

author: tonycstech
author_rank: Regular Contributor
timestamp: September 28, 2024, 06:29:01 pm
content: yea simplest solution ever.very helpful

author: TimFox
author_rank: Super Contributor
timestamp: September 28, 2024, 06:33:21 pm
content: You want the third resistor (that connects to the device under test) to be roughly the same resistance or higher than the expected resistance of the device for a good result.

author: Fungus
author_rank: Super Contributor
timestamp: September 28, 2024, 07:02:50 pm
content: If the resistance changes with voltage then it's not really resistance.

author: TimFox
author_rank: Super Contributor
timestamp: September 28, 2024, 07:19:47 pm
content: Yes: that would be a non-ohmic device, many of which exist, especially in complex circuits.

author: tonycstech
author_rank: Regular Contributor
timestamp: September 28, 2024, 08:18:39 pm
content: Quote from: Fungus on September 28, 2024, 07:02:50 pmNo, it would be a charging capacitor.

author: tonycstech
author_rank: Regular Contributor
timestamp: September 28, 2024, 08:19:35 pm
content: Quote from: TimFox on September 28, 2024, 06:33:21 pmI cannot measure resistance under load.I mean i can but i do not want to power the device before knowing the resistance.This is madness.

author: TimFox
author_rank: Super Contributor
timestamp: September 28, 2024, 08:28:10 pm
content: Quote from: tonycstech on September 28, 2024, 08:19:35 pmWhat were you planning to do if you had a DMM whose output voltage was < 1 V?By "under load", I meant connecting the battery/resistors circuit to either nothing ("no-load") or your device under test, where "loaded" meant with respect to the external network.

author: J-R
author_rank: Super Contributor
timestamp: September 28, 2024, 08:31:10 pm
content: This thread is madness...Use a power supply set for the voltage you think you desire, then use your DMM to measure the voltage drop and calculate resistance. Ohm's law assumes ideal components, as does your DMM when in resistance mode.

author: TimFox
author_rank: Super Contributor
timestamp: September 28, 2024, 08:33:50 pm
content: It is not "madness", it is "ill-posed".

author: bdunham7
author_rank: Super Contributor
timestamp: September 28, 2024, 08:49:14 pm
content: Quote from: tonycstech on September 28, 2024, 06:18:35 pmThe F289 has a full-scale voltage of about 0.5V up to the 500k range. Its open-circuit voltage is higher, about 5V. There's nothing unusual about this, other meters may have more or less current or voltage in various resistance functions. The Lo-Ohms function is different and you wouldn't want to use it for your application.If you think the open-circuit voltage is causing some component to start (and stay) conducting when you initially connect the meter, then you could temporarily put a resistor across the test leads of the same value as the range you are using. So you'd manually set the range to 5k, put a 5k resistor across the test leads, connect the meter and then remove the extra resistor. As long as the resistance of your DUT is under 5k, it will not exposed to more than 0.5V this way.Another way would be to just use the next-higher range with a resistor equal to that range without disconnecting it. So use the 50k range and a 50k resistor and you'll have no more than a 10% error--and one you can calculate--when you measure your DUT. This would be handy if the first method doesn't work or you have no idea what your DUT resistance actually is.

author: tonycstech
author_rank: Regular Contributor
timestamp: September 28, 2024, 09:14:17 pm
content: so basically i need cheap junk multimeter to do precision work and the top leading brand is not good enough ?Thats all i am getting out of this.

author: BeBuLamar
author_rank: Super Contributor
timestamp: September 28, 2024, 09:14:33 pm
content: The Fluke is resistance measurement sends out a constant current not voltage. The curent changes with the ranges.

author: TimFox
author_rank: Super Contributor
timestamp: September 28, 2024, 09:28:47 pm
content: Constant current into load, and measuring voltage across the load, is a normal method to measure resistance, and it is meaningful for an actual resistor.Practical current sources have a maximum "compliance voltage", which is what you will find with no load across the source (just like how I told you to run my three-resistor circuit).I have a laboratory current source with accurate current and adjustable compliance, but I don't think you need something that complicated. Keithley 225https://accusrc.com/uploads/datasheets/4106_225.pdfWhat do you intend to measure on your device under test? A complex circuit with semiconductors is not a resistor, but you can certainly look for a short circuit (for example).

author: tonycstech
author_rank: Regular Contributor
timestamp: September 28, 2024, 09:52:56 pm
content: I dont have any problems with any cheap chinese multimeters to measure resistance on a complete semiconductors circuit, any of themBut there is one particular that throws the reading off significantly which makes be believe there is a problem.So if i cannot trust FLUKE for giving me expected reading while chinese junk does, i have no point to continue with this conversation because solutions are rather absurd.Anything from starting from adding resistors all the way to making my own multimeter.What is the point to pay premium for Fluke then ? Absolutely no point at this point.

author: J-R
author_rank: Super Contributor
timestamp: September 28, 2024, 09:57:08 pm
content: Spot on, that junk 289 is broken, better dump it:https://www.eevblog.com/forum/buysellwanted/

author: TimFox
author_rank: Super Contributor
timestamp: September 28, 2024, 10:11:10 pm
content: Different instruments connected to a givennon-linearcircuit will give different answers for the ratioV/I, ifIis different for the two meters. Why would your cheap meter give the better or more accurate value for an ill-defined parameter? Not every object between two terminals is an Ohmic resistor.

author: tonycstech
author_rank: Regular Contributor
timestamp: September 28, 2024, 10:38:11 pm
content: its not broken. I've seen bench meters do same thing, giving me weird readings.Only cheap chinese meters or smaller cheaper meters can measure it correctly.

author: tonycstech
author_rank: Regular Contributor
timestamp: September 28, 2024, 10:40:22 pm
content: Quote from: TimFox on September 28, 2024, 10:11:10 pmI dont know why. All i know is: there are hundreds if not thousands of people who use hundreds of different multimeters and they all report same values as my cheap junk.Yet Fluke reports completely different value, much lower.To me this sounds like a FLUKE problem than the circuit.

author: Helix70
author_rank: Unknown
timestamp: September 28, 2024, 10:44:38 pm
content: Quality meters do not "push voltage" to measure resistance, they apply a specific current, and the voltage (compliance voltage) adjusts according to the circuit it is connected to. Perhaps your understanding of how a multimeter works is the issue here. A quick google might help you.

author: TimFox
author_rank: Super Contributor
timestamp: September 28, 2024, 10:47:18 pm
content: Quote from: tonycstech on September 28, 2024, 10:40:22 pmPerhaps it is your misunderstanding?

author: Fungus
author_rank: Super Contributor
timestamp: September 28, 2024, 10:52:28 pm
content: Quote from: tonycstech on September 28, 2024, 10:38:11 pmSo use those instead...

author: tonycstech
author_rank: Regular Contributor
timestamp: September 28, 2024, 11:19:28 pm
content: Quote from: Fungus on September 28, 2024, 10:52:28 pmi do. I just wanted something professional with live graph view and true RMS.I guess FLUKE is a really a fluke/accident and not a multimeter.

author: Helix70
author_rank: Unknown
timestamp: September 28, 2024, 11:27:27 pm
content: When used incorrectly, any multimeter reading is a fluke.

author: TimFox
author_rank: Super Contributor
timestamp: September 28, 2024, 11:30:56 pm
content: If you connect your Fluke to a resistor, does it read correctly?Is that why they call it an ohmmeter?If not, it may be busted.

author: tonycstech
author_rank: Regular Contributor
timestamp: September 29, 2024, 12:16:18 am
content: So you are telling me that using two probes to measure resistance is not a correct way to do it ?its a 1.8v rail. Measuring it with 2v or even 5v at the probes will likely cause measurement issues so is there a way to lower the voltage in the firmware of the meter maybe ?

author: bdunham7
author_rank: Super Contributor
timestamp: September 29, 2024, 12:23:04 am
content: Quote from: tonycstech on September 28, 2024, 09:52:56 pmThe solution would be to understand the circuit in question and for that to happen you'd have to tell us exactly what circuit or DUT you are measuring. Your low-cost meters typically have lower test currents for the resistance ranges and if you have a circuit that is not ohmic, then you will likely see different results. This is why I suggested manually using a higher range on the Fluke as this will reduce the test current. For example, what do you get on this particular measurement if you manually select the 500k range? That's a 1µA test current.As far as giving you the "expected" reading, you can be pretty sure that the Fluke is giving you thecorrectreading, notwithstanding what a bevy of other cheap meters may say. Why that correct reading doesn't match your expectations or other meters is another story, one I'd encourage you to try to understand. You can rant and rave all you want and you won't change anyone's mind.

author: bdunham7
author_rank: Super Contributor
timestamp: September 29, 2024, 12:31:27 am
content: Quote from: tonycstech on September 29, 2024, 12:16:18 amNo, we're telling you that it isn't a resistor. When you say you are measuring a 1.8V rail, do you mean that you are measuring the part of the circuit that would be powered by a buck regulator with the regulator removed (or not)?I previously explained how to lower the maximum applied voltage to about 0.5V if you think that's the issue. A 50k resistor (47k is close enough) is all you need. You need to understand that the maximum (open circuit) voltage is only near 5V when the test leads are completely open. As soon as you have anything connected, the voltage drops. For the 500R to 500k ranges, if the resistance is within the selected range then the applied voltage will be less than 0.5V. However, the circuit you are testing is still not a resistor. Keeping the voltage below 0.5maycause it to behave ohmically--or not.

author: tonycstech
author_rank: Regular Contributor
timestamp: September 29, 2024, 12:39:53 am
content: yes its a buck converter circuit. nothing is removed.Removal of the buck converter is not an option. I need to know the circuit resistance before i do anything to it.Measuring at the coil.So if fluke gives me low ohm reading, it means there is a problem with the circuit.Meanwhile chinese piece trash says circuit is not low and the device is working normally after all.So yeah: "just use chinese junk" is the only option i have at this point and all the FLUKE hype is a joke UNLESS you work in some industrial setting.Otherwise its useless for modern electronics with multi phase controllers and processing units.Maybe some tractor wiring or construction site measurements its best for FLUKE use but i dont see any use for it in my case. Maybe not this model.

author: bdunham7
author_rank: Super Contributor
timestamp: September 29, 2024, 12:44:29 am
content: Quote from: tonycstech on September 29, 2024, 12:39:53 amI assure you people are using Fluke 289s on things other than tractors and construction. Have you tried or are you willing to try any of the things I've suggested?

author: J-R
author_rank: Super Contributor
timestamp: September 29, 2024, 02:03:34 am
content: https://youtu.be/YXeJ-BQnJtw?t=46

author: tonycstech
author_rank: Regular Contributor
timestamp: September 29, 2024, 02:12:43 am
content: Quote from: J-R on September 29, 2024, 02:03:34 amYep, there is a live demo for ya

author: J-R
author_rank: Super Contributor
timestamp: September 29, 2024, 02:25:28 am
content: Quote from: tonycstech on September 29, 2024, 02:12:43 amI guess there is no other way to say this, but even though you have mad repair skills, you probably need to hit the electronics books a bit...

author: tonycstech
author_rank: Regular Contributor
timestamp: September 29, 2024, 03:24:26 am
content: Quote from: J-R on September 29, 2024, 02:25:28 amAll i need to know is if there is any way to hack the device internals so it would obey my will. I think firmware driven device can be hacked by altering the firmware.

author: J-R
author_rank: Super Contributor
timestamp: September 29, 2024, 03:37:22 am
content: Quote from: tonycstech on September 29, 2024, 03:24:26 amOh, but adding a resistor to your test probe was too much to ask for? Come on now, you've run this train off the tracks...

author: bdunham7
author_rank: Super Contributor
timestamp: September 29, 2024, 03:43:04 am
content: Quote from: tonycstech on September 29, 2024, 03:24:26 amFWIW, in your video when you are showing the "wrong" reading of 218 ohms, the applied voltage is about 210mV so too much voltage isn't really the issue. Clearly this circuit is non-ohmic and is reacting to the different test currents being applied. You were correct when you said "I really don't understand how this meter works". Even If you could modify the firmware, what would you program it to do?QuoteEven though you may be good at what you are doing, you also clearly have no idea exactly what it is that you are doing and no inclination to learn. You never responded to my suggestions even though your video shows you already tried some of them. Why waste our time?

author: tonycstech
author_rank: Regular Contributor
timestamp: September 29, 2024, 04:22:12 am
content: Quote from: bdunham7 on September 29, 2024, 03:43:04 amI am not wasting anyone time here who isnt willing to waste it including my self.

author: BeBuLamar
author_rank: Super Contributor
timestamp: September 29, 2024, 09:15:15 am
content: Quote from: tonycstech on September 29, 2024, 03:24:26 amWhy do you want to hack it? The Fluke doesn't do what you want. Not measuring resistance correctly, not displaying correct AC voltage, not having live trend. Just sell it and buy something else. It's still worth good money.

author: csuhi17
author_rank: Frequent Contributor
timestamp: September 30, 2024, 04:29:54 pm
content: Quote from: J-R on September 29, 2024, 02:03:34 amWhy does Fluke start the resistance measurement from the Ohm measurement range? why not from highest MOhm to lowest Ohm?Got it.It was in continuity check mode. Should it have shown something other than 0.00 in the MOhm measurement range?