New Brymen BM2257 Multimeter

[EEVblog]

Quite high voltage MOV's are not available in the small 5mm size. Fluke went with their custom Epcos S05K575 (575VRMS) 910V@1mA.
I see Brymen simply used two in series to get the same net clamping voltage in 5mm size, using off the shelf parts.
(625VRMS) 1,000V@1mA is a 10mm part, one used in predecessors BM235, BM786 etc. with a smaller 7mm in series.

You want the smallest size part for cost, low capacitance but big enough to pass the 61010 transient test which is a few amp spike.
Leakage current looks high I'm not sure if anyone has actually measured it at max. input voltage say 1,000VDC or 600VAC to see if is an unknown issue.
Clamping voltage I estimate is max. ~3,000V total in a few amp transient. joeqsmith has the data.

I forgot the datasheet link:
https://www.cnr.com.tw/cloudSpace/CNR_D.pdf

Will test the leakage @1000V

[tautech]

[...] joeqsmith has the data.

While reading the last few posts, I thought it is time someone send a BM2257 to joeqsmith.
After Dave's video, we can get one more interesting video.

Not just yet.....there might be a revision like there was with the GW.......

[EEVblog]

Tested.
TLDR; Not a problem at 1200V DC and 1100V AC

[Furna]

Tested.
TLDR; Not a problem at 100V DC and 1100V AC

Watch the typo ... 1000V DC

[bdunham7]

Whoa...wait!  It has an undocumented 6000V range?   

[EEVblog]

Whoa...wait!  It has an undocumented 6000V range?   

In theory yes, try it   

[geralds]

[/quote]
Not just yet.....there might be a revision like there was with the GW.......
[/quote]

Would that be a FW update? If so, is it now possible to update the FW oneself (i.e. without sending it in for service)? I understand this was (generally) not possible with other Brymens?

Also (and again, apologies for such a Newbie query - I am a complete beginner and trying to learn...): How relevant is the LoZ "issue" for LV/electronics? In practice, would you use LoZ mode for electronics / hifi repair works?

Thanks!

[EEVblog]

Would that be a FW update? If so, is it now possible to update the FW oneself (i.e. without sending it in for service)? I understand this was (generally) not possible with other Brymens?

Only the BM780 series is designed for firmware updating. But even then the user can't do it, only a Brymen dealer who has access to the programming tool.

Also (and again, apologies for such a Newbie query - I am a complete beginner and trying to learn...): How relevant is the LoZ "issue" for LV/electronics? In practice, would you use LoZ mode for electronics / hifi repair works?

It's mainly for mains ghost voltage elimination, but there are niche uses for lower voltage work. For repair it's useful for safely discharging capacitors. But I have a video on that:

[Kleinstein]

The current specs for the MOVs are maximum values. As typical for leakage currents the actual value is often much better than the specs.
For those 200 Mohms range effective resistance already surface contaminations could be an issue.

The slightly higher current in the AC test can come from parallel capactance. The input impedance is not just the 10 M ohms, but also has a few pF of parallel capacitance inside the meter and also the cables outside. So I don't think it is leakage at the MOVs, more like the capacitance of the MOVs.

[dkarst]

I've been following this thread and don't think I've seen anything mentioned on the BM789.  It has been on the market with Low-Z capability (one if its key differences from EEVblog BM786).  I believe the spec for it is Low Z functionality with input < 1 volt.  I don't know if anyone here has looked at the input circuit topology.   Possibly, since the new BM2257 has this behavior at 0.9X volts, Brymen noted the behavior and made the spec <1V.  Maybe for the BM2257 they wanted to have a bit more headroom from spec and changed it to 1.5V.  It may also have a totally different input.

I guess just speculating since I don't have a BM789.  Just hadn't seen it discussed and thought interesting.   

[EEVblog]

I've been following this thread and don't think I've seen anything mentioned on the BM789.  It has been on the market with Low-Z capability (one if its key differences from EEVblog BM786).  I believe the spec for it is Low Z functionality with input < 1 volt.  I don't know if anyone here has looked at the input circuit topology.   Possibly, since the new BM2257 has this behavior at 0.9X volts, Brymen noted the behavior and made the spec <1V.  Maybe for the BM2257 they wanted to have a bit more headroom from spec and changed it to 1.5V.  It may also have a totally different input.

Welcome to the forum. I'd bet it's the same arrangement as the BM2257.

[J-R]

My BM789 bottoms out at exactly +/- 0.9000V DC in Auto V/LoZ mode.

[5U4GB]

It's mainly for mains ghost voltage elimination, but there are niche uses for lower voltage work.

I was recently doing some alarm re-wiring and there were ghost voltages everywhere, I wanted to use an NCV to check which sensor was wired to which input and it was impossible, almost every sensor registered a voltage present when disconnected.  As did the mains input to the alarm even though it was disconnected at the switchboard.

Really tempted to get a BM2257 now, I'd always considered Low-Z a bit of an extravagance (given the cost of the meters it came with), but after the recent episode...

[floobydust]

LowZ is a must have. Digital multimeters are too sensitive out in the real world - you'll get ghost readings due to capacitive AC coupling of mains, as well as DC leakage currents due to moisture and corrosion. Unless you have an analog VOM lol. 10MEG input impedance or higher leads to goose chases, I've been burned a few times troubleshooting.

[xKertx]

Does it have an option in it's settings that let's you choose whether you use normal alkaline batteries or Ni-Mh rechargeable ones?
Some quality meters have it (my Keysight multimeter and Vaisala thermo-hygrometer both have it) and i think it's a really nice feature. The low battery warning would be more accurate and you will not overdischarge/destroy your rechargeables.

[Furna]

Does it have a option in its settings whether you use normal alkaline batteries or Ni-Mh rechargeable ones?
Some quality meters have it and i think it's a really nice feature. The low battery warning would be more accurate and you will not overdischarge/destroy your rechargeables.

Short answer: no, there is no such setting.

Anyway in the general specifications at pag. 17 of the manual you can find the "Low Battery" voltage below approx 2.5V (so 1.25V each cell).
AFAIK lowest safe voltage for Ni-Mh is 0.9V.
IMHO perfectly safe to use Ni-Mh but (very) short duration since max for Ni-Mh is 1.42V and nominal 1.2V.

[Kleinstein]

Showing low Bat at 1.25 V is a bit on the high side for NiMH or NiCd. That is arealy above the 1.2 V nominal voltage.

The problem is not the maximum voltage - that can be at some 1.6-1.7 V, similar to very new alkaline cells.
The point is that the meter could show low battery when NiMH is still 95% full.

[xKertx]

Well the nominal voltage of the Ni-Mh is 1,2V when there is about 60% charge left. So that 1,25v threshold means you get the low battery warning all the time and eventually ignore it and have to guess when the batteries need to be charged.
It's little dissapointing because other than that it seem like a really nice DMM. I like to use ni-mh in my better equipment to avoid leakeage problems. 
I wish Dave could give them a hint and maybe they could add it to future firmware versions.

[Furna]

I guess there is also a cut-off min voltage ... so it is not that you get the low battery and ignore it ... the meter will shutdown.
What is the cutoff voltage? I have no idea.

[bdunham7]

Showing low Bat at 1.25 V is a bit on the high side for NiMH or NiCd. That is arealy above the 1.2 V nominal voltage.

The problem is not the maximum voltage - that can be at some 1.6-1.7 V, similar to very new alkaline cells.
The point is that the meter could show low battery when NiMH is still 95% full.

That depends on load.  The open-circuit voltage for a NiMH battery is almost always above 1.2 and usually even 1.25V after use at light loads.  Even at 50mA load on an AA cell, a 1.25V cutoff would yield more than 2/3 of the total capacity of the battery.  Most reasonable handheld multimeters will have a battery load much less than 50mA.  The ones that use 9V batteries can be below 1mA.

https://citeseerx.ist.psu.edu/document?repid=rep1&type=pdf&doi=5edecfd325c3787e2dd76a7460005bf5abe1758f

[Furna]

Test leads shorted and selector on Continuity/Beepr
still working at 1.77V (drowing 25mA)
shutdown at 1.76V
so 0.88 each cell
I would say safer for Ni-Mh ...
Anyway there must be a reason why Brymen choose 2.25V for low batt ... usually to guarantee reliable results.

Measurement done with a PSU

[Fungus]

Anybody got any Batterizers? 

[mwb1100]

NiMH has a flatter voltage/discharge curve than alkaline, so the low battery behavior might be OK.

Has anyone experience with using NiMH in the BM257s which is specified to show low battery when below 2.3V (no surprise that it's basically the same as the BM2257)?

[xKertx]

NiMH has a flatter voltage/discharge curve than alkaline, so the low battery behavior might be OK.

Has anyone experience with using NiMH in the BM257s which is specified to show low battery when below 2.3V (no surprise that it's basically the same as the BM2257)?

Yes, at low discharge currents the NiMh voltage is 1,2V most of the time, until it is almost depleted and the voltage drops.
The 2,5V threshold for the low battery warning is too high because the batteries will be half of the time below that.
2,3v (like on the bm257) would be much better because it would be below the NiMh's nominal voltage.

[mwb1100]

Yes, NiMh will be 1,2V most of the time until it is almost depleted and the voltage drops.
That's why the 2,5V is too high.
2,3v would be better

Yes - I got lost about what the low battery threshold for BM2257 is...  too many numbers between 1.2 and 2.5V being thrown around here for my feeble mind to keep track.