Fluke 8842A voltage left on display (and probes) after a voltage reading! - Page 1
Forum section: electronics / testgear

author: corrado33
author_rank: Frequent Contributor
timestamp: May 28, 2014, 01:49:42 am
content: Hi all,I recently acquired a cheap 8842A.The problem it has is that after a DC voltage measurement, the display will still contain the measurement of what you just measured (almost like a "hold" function). However, over time the display reading will drop and eventually return to 0ish. If you short the probes this happens quite quickly. At first I thought this may be normal operation, but then I measured the voltage OF the probes with another multimeter. If the display had 8.7V on it, the probes measured 8.7V! (Dropping quickly as I was making the measurement, but still, that's not good!)Also, if you switch to other modes, then back to the DC voltage mode, it'll display 2V or so on the display!I (quickly) looked at the manual, but I can't find a troubleshooting table. Just paragraphs and paragraphs of "If you think there is a problem with this area or measurement."Any ideas?EDIT: Apparently this is normal... I have never seen a DMM do this before, not even someone's precious HP 3478A. What would this do to a circuit if say, I had measured 24V then went and measured some dinky digital pins on a micro or something?I did quickly skim the manual for the "How to do DC Voltage readings" section, and some other sections as well, no mention of this. Maybe it's in the theory of operation section.I found one site on yahoo answers of all places that says it's simply displaying the stray capacitance of the A/D input circuit.Why is this meter so poorly documented online as compared to the 3478A? When I did a bit of research I never found any huge problems with this meter...

author: quint
author_rank: Contributor
timestamp: May 28, 2014, 02:51:12 am
content: This is a common question regarding many bench DMMs. These meters typically have very high (several GigaOhm) input impedance and thus they don't return to zero on voltage ranges when they are left open circuit. If you short the probes together, the reading should return to (nearly) zero. If not, there is something wrong.

author: corrado33
author_rank: Frequent Contributor
timestamp: May 28, 2014, 03:33:58 am
content: Thanks for the info quint. The display does read nearly 0 when I short the probes together. However, that was not the case when the unit was warming up, it read 120ish mV, but after a few minutes of warm up it seems to be all fine!

author: mzacharias
author_rank: Frequent Contributor
timestamp: May 28, 2014, 10:51:55 am
content: The 8842A has an input impedance of 10 gig-ohms or so up through the 20 volt range. When you select the 200 volt range, or when it auto-ranges above 20, the impedance drops to a standard 10 meg-ohms (you can hear a relay click in at this point). On the manual 200 volt range you will notice it does not hold the voltage this way.The residual voltage you see remaining on the 20 volt range is held by circuit capacitance (or sample and hold cap?), with the circuit impedance too high to bleed off even this in any short time frame.Bothered me too with my first 8840A until I understood a little more about the way these things work. Now I look at it as a feature rather than a fault.