Fluke 287 Inaccurate on 500Ohms Range - Page 1
Forum section: electronics / repair

author: vladsol
author_rank: Regular Contributor
timestamp: October 17, 2024, 03:31:29 pm
content: Hi all.According to the datasheet, the Fluke 28* has a short-circuit current of 1mA when measuring resistance at the 500-ohm range. Can someone check whether this value is exactly 1 mA or if it differs?I have a Fluke 287 with a significant deviation when measuring resistance below 500 ohms (in the 500-ohm range). Calibration of the multimeter does not fix the issue. On the 5K range and higher, this problem does not occur. I suspect the issue may be with the SL10327 chip, specifically in its current source.What's interesting: the deviation from the actual value decreases at the edges of the range. For example (resistor value, reading, difference):10Ω - 10.54Ω (+0.54Ω)20Ω - 21.06Ω (+1.06Ω)30Ω - 31.56Ω (+ 1.56Ω)40Ω - 42.03Ω (+ 2.03Ω)50Ω - 52.49Ω (+ 2.49Ω)60Ω - 62.91Ω (+ 2.91Ω)70Ω - 73.32Ω (+ 3.32Ω)80Ω - 83.70Ω (+ 3.70Ω)90Ω - 94.06Ω (+4.06Ω)100Ω - 104.14Ω (+4.1Ω)200Ω - 205.95Ω (+5.95Ω)300Ω - 305.43Ω (+5.43Ω)400Ω - 402.64Ω (+2.64Ω)500Ω - 497.61Ω (-2.39Ω)In the 5K range, the same resistors show:100Ω - 0.1000K200Ω - 0.2000K300Ω - 0.3000K400Ω - 0.4000K500Ω - 0.5000K

author: gamalot
author_rank: Super Contributor
timestamp: October 17, 2024, 06:56:07 pm
content: I don't think a precise value of current is necessary for resistance measurement, it just needs to be stable.

author: vladsol
author_rank: Regular Contributor
timestamp: October 17, 2024, 09:14:11 pm
content: Quote from: gamalot on October 17, 2024, 06:56:07 pmThank you for the photo! Unfortunately, in my case, this current is less than 1mA - around 0.94mA.Without an accurate current source (or without current feedback in the resistance measurement circuit), it won't be possible to measure the resistance precisely, since the resistance is measured based on the voltage drop across the terminals of the multimeter (or across the X resistance) with known current.

author: kblue
author_rank: Regular Contributor
timestamp: October 18, 2024, 01:39:38 am
content: 1mA for me too. Guess this should be expected? I wonder whats wrong with your unit. Hm.

author: beenai2018
author_rank: Contributor
timestamp: October 18, 2024, 08:35:03 pm
content: If the current was lower than 1mA wouldn't the resistance indicate be lower than normal ?In my mind, I's seeing somewhat of a linear line to 100ohm and then all over the place to 500.

author: vladsol
author_rank: Regular Contributor
timestamp: October 19, 2024, 07:36:41 am
content: Quote from: beenai2018 on October 18, 2024, 08:35:03 pmBefore calibration, the situation was a bit differentAfter calibration, it got better, but the issue still remains. Before calibration, the resistance was indeed lower (the resistance of a 100Ω (0.01%) resistor was measured as ~94Ω).I ordered a new SL10327 chip (checked the clamping diodes on the ISRC line, and they are OK), I will try to replace it... If the current value is 1mA, I will send it for calibration again.

author: armandine2
author_rank: Frequent Contributor
timestamp: October 19, 2024, 01:21:15 pm
content: Quote from: gamalot on October 17, 2024, 06:56:07 pmI wonder - may be?

author: floobydust
author_rank: Super Contributor
timestamp: October 19, 2024, 06:35:47 pm
content: Have you confirmed the Ohms-source protection circuit is not damaged - the PTC and 1k surge resistor, the clamp diodes etc. are OK?If you take the meter apart and heat up those parts, or the SL10327 - that can flush out the source of stray leakage current or high resistance.It could be a bad CMOS switch inside the SL10327 or the Ohms compliance voltage on the 500 ohm range is flushing out a problem.

author: vladsol
author_rank: Regular Contributor
timestamp: October 19, 2024, 07:41:29 pm
content: Quote from: floobydust on October 19, 2024, 06:35:47 pmYup, I have checked the current source protection and the sense line protection.The supply voltage of the SL10327 is ok, and the current-setting resistors R173, R71, R72 are fine as well. C14 and C17 seem alive too