BM235's strange V and mV value - Page 1
Forum section: electronics / testgear

author: lionel
author_rank: Newbie
timestamp: April 25, 2023, 07:40:55 am
content: Hello everyone,I recently bought a BM235 (from Welectron) and noticed a strange behaviour on it. When selecting the V or mV fonctions without electrical contact, the reading value isn't at zero (especially in mV) and keeps changing. This happens with no probes connected when i move the multimeter, but especially when i move the probes in the air. In A or mA mode, on the contrary, the zero is dead on.Nevertheless, measuring an AA mignon, tension shows 1.610V without any fluctuations.Is this an expected behaviour or should I consider my multimeter faulkty ?PS: I can try posting a video showing what happens if needed.

author: EEVblog
author_rank: Unknown
timestamp: April 25, 2023, 07:54:12 am
content: All multimeters have an upper tolerance to electric fields. Perhaps you are in a particually high electric field environment?

author: Fungus
author_rank: Super Contributor
timestamp: April 25, 2023, 08:41:45 am
content: Quote from: lionel on April 25, 2023, 07:40:55 amYes. Any high-impedance circuit will pick up radio waves. That's what your meter is showing.Try putting some leads in and shorting them together. It will zero.

author: lionel
author_rank: Newbie
timestamp: April 25, 2023, 08:52:12 am
content: Quote from: EEVblog on April 25, 2023, 07:54:12 amNot to my knowledge, but who knows ? And no high tensions lines (that, I am sure of).Quote from: Fungus on April 25, 2023, 08:41:45 amI can confirm that shorting the probes together put the Brymen back to zero. What kind of high-impedance circuit could that be ? I mean, what kind of equipment in an individual house could cause such radio waves ?Anyway, these are good news, I won't have to send it back for RMA.

author: EEVblog
author_rank: Unknown
timestamp: April 25, 2023, 09:26:30 am
content: Quote from: lionel on April 25, 2023, 08:52:12 amHe's talking about the multimeter, it has a high impedanc input. Every meter will pick up stray electric fields on the mV range.Some high end meter even have near infinite imput impedance input on the mV range, and these meter will often display overload when the leads are just open.This is perfectly normal, once you measure a circuit that circuit will have vastly lower impedance and not be inpacted by any surrounding stray fields.

author: Fungus
author_rank: Super Contributor
timestamp: April 25, 2023, 09:39:44 am
content: Quote from: lionel on April 25, 2023, 08:52:12 amThe multimeter input jack.Quote from: lionel on April 25, 2023, 08:52:12 amThe wires in the walls.

author: lionel
author_rank: Newbie
timestamp: April 25, 2023, 10:07:08 am
content: Thank you very much for your answers, both of you !Just for the record, two videos to show the phenomenon:<iframe width="560" height="315" src="https://www.youtube.com/embed/pWZlHLWSPiQ" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" allowfullscreen></iframe><iframe width="560" height="315" src="https://www.youtube.com/embed/vcx2WKG7QPg" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" allowfullscreen></iframe>

author: lionel
author_rank: Newbie
timestamp: April 25, 2023, 01:04:03 pm
content: Now I really feel ashamed because, whereas I searched the Internet before asking here, I now figure out that this question has been answered almost everywhere...

author: Pfriemler
author_rank: Regular Contributor
timestamp: April 25, 2023, 01:15:35 pm
content: A coupile of months ago I had a chinese DMM with a strange behavior: in 400 mV DC range (no autorange, only dial) there was a stable mV reading without leads plugged and it went up in minutes up to overload. I sent it back for other reasons - bad choice: It had a very high Z input (I guessed somewhat ~800 MOhms), useful for some very special measurements in MOhm resistor voltage dividers. It was not a bug, it was a feature.With usual 10 MOhms impedance meters, the behavior shown in the YT shorts are - as Dave said - "perfectly normal". To avoid such effects, some Brymens (and other meters, too) have those low-Z ranges (I just grabbed a BM789 with it). Impressive, as it was a new experience to me.

author: Fungus
author_rank: Super Contributor
timestamp: April 25, 2023, 01:35:39 pm
content: Quote from: Pfriemler on April 25, 2023, 01:15:35 pmWhat meter was it? I wouldn't mind a very high Z meter...If you look at the listings for Chinese meters on Aliexpress they've started putting mV range disclaimers on them, they must be tired of people claiming refunds for this "problem".

author: Pfriemler
author_rank: Regular Contributor
timestamp: April 25, 2023, 03:13:12 pm
content: Quote from: Fungus on April 25, 2023, 01:35:39 pmedit: damned, link does not work ...?UEZNIRN PM8225:https://www.amazon.de/Multimeter-Spannungsmessr-Strommessger%C3%A4t-LCD-Anzeige-Hintergrundlicht/dp/B0BFLDDSVCOnly for the 400 mV. I wish, I have a 20V or so ... btw: this was my reason to look for an hi-z meter:https://www.eevblog.com/forum/beginners/how-to-measure-in-meg-ohm-voltage-dividers/msg4612876/#msg4612876

author: Neutrion
author_rank: Frequent Contributor
timestamp: April 25, 2023, 03:54:43 pm
content: Fungus: It never hurts to have yet another meter. We are waiting for your review!

author: Pfriemler
author_rank: Regular Contributor
timestamp: April 25, 2023, 04:12:22 pm
content: Quote from: Neutrion on April 25, 2023, 03:54:43 pmwell, in fact I consider to re-buy it just for the hi-z feature. 16 Euros incl. shipping. Did a hires pic from the inside. But this is offtopic here ...

author: Kleinstein
author_rank: Super Contributor
timestamp: April 25, 2023, 06:25:34 pm
content: With moving the probes around, it is quite easy to pic up electrostatic charge and possibly create extra charge from triboelectricity or piezo electric effects in the probe cables. So that behavious is normal, even with 10 M input impedance.A few handheld DMMs offer very high impedance in a mV range (e.g. up to some 200 mV or 600 mV).With higher grade bench meters high impedance is common, but often only to some 12 V. 20 V high impedance is less common (e.g. Sigilent 3065 or Rigol 3068 or Keithley 2001+2002).This usually needs a supply up to this voltage - so nothing to expect in a handheld meter.

author: EEVblog
author_rank: Unknown
timestamp: April 26, 2023, 04:49:29 am
content: BTW, these "ghost voltages" are so much of problem for electricans on mains wiring (due to capacitive coupling), that any decent electrican multimeter has a Low Z (low impedance) mode which instead of the usual 10M input impedance, it has only a few k ohms impedance. Your BM235 has this mode.So when you are working on live wiring you don't read a "phantom voltage" like that 100V, thinking the line is live when it's now. The low impedance will ensure that the meter reads zero.

author: Fungus
author_rank: Super Contributor
timestamp: April 26, 2023, 06:01:25 am
content: Included for comedic value:Ref: The Aneng AN870 page on Aliexpresshttps://www.aliexpress.com/item/33004463675.html