Fluke 8840A - is this normal? - Page 1
Forum section: electronics / repair

author: beowulfenator
author_rank: Contributor
timestamp: September 03, 2018, 05:03:07 pm
content: I got my hands on a used Fluke 8840A and I'm trying to figure out if it's working correctly.In DC voltage mode when leads are not connected the meter displays a constantly changing number in millivolt range, 5 millivolts or so. When leads are connected and shorted, it shows zero, or maybe 1-2 microvolts.A fully charged lithium ion battery shows 3.9379 volts, but what feels strange is that this number stays on the screen after I've disconnected the leads. And the last digit changes a bit, goes several microvolts back and forth.Is this how a meter should behave? I've compared this with an old HP3468, and that one just goes to zero after the leads are disconnected from the voltage source.In resistance mode (2-wire) shorted leads show 0, open leads indicate out of bounds value (OVER). Resistance is measured correctly, and as soon as I disconnect the probes from the resistor, the meter immediately displays OVER.Is it normal for this meter to work with voltage like that?

author: Kirkhaan
author_rank: Frequent Contributor
timestamp: September 03, 2018, 06:06:29 pm
content: Mine has similar behavior.

author: Gyro
author_rank: Super Contributor
timestamp: September 03, 2018, 06:08:34 pm
content: Yes, completely normal.The 8840 has a high impedance input (not 10M), this is to avoid circuit loading, it will float at the input voltage present across the probes (the only loading on the circuit is input leakage current- in the pA range). It will drop to 10Mat higher voltage ranges due to a resistive attenuator, check the manual to see which ranges.All meters (even handheld ones) will read over-range in some form when on the resistance setting with nothing connected. It effectively means 'infinite resistance'.

author: beowulfenator
author_rank: Contributor
timestamp: September 03, 2018, 07:17:04 pm
content: Thanks! I was mostly surprised by the fact that it "floats" the input voltage, unlike the HP meter (and any of my other cheapo handheld meters).

author: Gyro
author_rank: Super Contributor
timestamp: September 03, 2018, 08:09:30 pm
content: Yes, it does seem a bit strange the first time you come across it.

author: dacman
author_rank: Frequent Contributor
timestamp: September 04, 2018, 12:55:33 am
content: Which HP meter? The HP 34401A has a >10 G-Ohm mode, but at power-up is set to its 10 M-Ohm mode.

author: beowulfenator
author_rank: Contributor
timestamp: September 04, 2018, 07:56:53 am
content: Quote from: dacman on September 04, 2018, 12:55:33 amIn my case, HP 3468A.

author: mzacharias
author_rank: Frequent Contributor
timestamp: September 05, 2018, 06:07:59 pm
content: Quote from: Gyro on September 03, 2018, 06:08:34 pmYup. Above the 20 volt range it's a standard 10 MegOhm input impedance. This question comes around pretty often.My first 8840 fooled me; I thought it had some sort of non-switchable "touch-hold" feature. They do "hold" a voltage reading on that range.Pretty cool when you realize what's going on. 20V range at that impedance is pretty unusual I'm led to believe.Pretty sure my Keithley 2015 only goes to 10 volts at a high input impedance.