New Brymen BM2257 Multimeter

[bdunham7]

I also note Fluke ditched using clamp transistors for a while now in their newer models and instead is using diode arrays but they are not symmetrical.

Fluke has newer models?

The 87V does appear to be BAV199s.  IDK about the small meters, but on the 289 it appears they just can't make up their mind which to use.

[floobydust]

I consider the 87 an old dog, I love her dearly. Just a model number number Fortive keeps rehashing. That transistor clamp design goes back to at least 1989 in the Fluke 87, I don't consider it recent. Hand-selected parts and you don't know what their BV_EBO ends up, is it 5V or 7V or 10V etc. depending on batch and manufacturer.
A 7V spike into a DMM IC is not really a good idea, still too much. The days of 9V battery-powered multimeter (IC's) are gone as well, today the IC's are lower voltage. People are just copying the old design I think.

I was talking about for example, the Fluke 107 and 17B schematics. It's a diode party of BAV199, they must survive the few amp transient test impulse. The Fluke 107 Joeqsmith seemed to like its robustness, uses no transistor clamp scheme that I know of.  (The 17B transistor clamp circuit is a bit over my head unless I get some coffee).

BM2257 using SOT-23 diodes array? "N" marking code not sure what they are. Clamp transistors appear to be 2SC5866 "VLR" 6V but high leakage current.
Hopefully Dave will reveal the Brymen LowZ issue at hand. 121GW is just a switch to engage the PTC, no drama.

[Fungus]

Edit: Post removed - something mixed up in the system and that was about Uni-T.

[floobydust]

Very few are 600V or 1,000V rated, and even fewer have approvals. Hard to find them and see their datasheets.

[EEVblog]

  It would be great if we knew the two PTC part numbers, somebody cut off the heatshrink tubing over them to see that.

I doubt there's anything special about them.
Probably the most generic PTCs possible, whatever was cheap in Shenzhen that day.

The small one has no marking. I'll cut the heatshrink off the bigger one, but I suspect it's also unmarked.
Brymen don't substitute parts that I know of, so they would have a specific brand and model specced in.

[Fungus]

I doubt there's anything special about them.

Probably the most generic PTCs possible, whatever was cheap in Shenzhen that day.

There may not be anything special about them, but I doubt Brymen is sourcing gray market parts.

Post removed - something glitched in the system. Of course Brymen don't buy Shenzhen specials.

[EEVblog]

BM2257 using SOT-23 diodes array? "N" marking code not sure what they are. Clamp transistors appear to be 2SC5866 "VLR" 6V but high leakage current.
Hopefully Dave will reveal the Brymen LowZ issue at hand. 121GW is just a switch to engage the PTC, no drama.

Unless I missed something in my reverse engineering, the LowZ input shunts PTC2 (bigger one, also unmarked under the heatshrink) and R32 (1K) directly to COM, and then going through R33 (1k) and PTC1 (smaller one), clamped with VAR2 and VAR1 to COM. Then through the HV 10M directly to pin 26 of the BTC chip. No extra protection.
So anything that's happening here is happening inside the chip.

The "VLR" SOT-23 clamps are only used in Ohms mode. Q10+Q11 in parallel with Q7+Q8.

Edit: I missed the other side of the switch in Low-Z, but it wasn't in the input signal path, will trace it.

[bdunham7]

Can someone post a photo of their BM2257 in Low-Z mode with less than 0.928VDC on the input? 

[EEVblog]

Can someone post a photo of their BM2257 in Low-Z mode with less than 0.928VDC on the input?

It doens't display anything, just kinda flashed a bit IIRC, as if it was glitch reading or something.

[bdunham7]

Unless I missed something in my reverse engineering, the LowZ input shunts PTC2 (bigger one, also unmarked under the heatshrink) and R32 (1K) directly to COM, and then going through R33 (1k) and PTC1 (smaller one), clamped with VAR2 and VAR1 to COM. Then through the HV 10M directly to pin 26 of the BTC chip. No extra protection.
So anything that's happening here is happening inside the chip.

That sounds like what I'd expect there to be and the impedance should be ~2.2k all the way to zero.

I think maybe the answer is that the Auto-V switch logic that decides whether the display should be AC or DC may have a threshold or multiple thresholds.  When you supply it with less than the 0.928V and it shows all zeros, does it display the AC or DC symbol?  And where is the decimal point?

Edit:  our posts crossed in web continuum

It doens't display anything, just kinda flashed a bit IIRC, as if it was glitch reading or something.

So an Auto-V threshold makes sense then.  Next question:  How low will it go on ACV @ 50/60Hz?

[EEVblog]

So an Auto-V threshold makes sense then.  Next question:  How low will it go on ACV @ 50/60Hz?

Good point, I'll try it.

[EEVblog]

Edit: I missed the other side of the switch in Low-Z, but it wasn't in the input signal path, will trace it.

Nope, it's just physically switching the backlight in OFF mode. Seems odd, maybe reducing quiescent, or maybe backfeeding through the chip during power down or something.

[5U4GB]

Pop quiz: How many mA does it take to blow a 400mA fuse?

If it's a Chinese blade fuse you bought off Amazon, about 20 amps.

[Furna]

So an Auto-V threshold makes sense then.  Next question:  How low will it go on ACV @ 50/60Hz?

Good point, I'll try it.

I measured 0.9 ACV @50Hz

[kwass]

I get the same 0.900 V on 60Hz AC and a lower limit of 0.916 V on DC.

BTW the AUTO DC volts is useful for testing small coin cells as the load is typical of many devices they are used in.

[bdunham7]

Good point, I'll try it.

I think this "glitch", if it even can be called that, could likely be corrected improved in the firmware.  I was curious as to how my only Auto-Check Low-Z meter, the Fluke 116, handles mixed signals or AC with a DC offset as well as small voltages--so I checked.  It seems that the way the 116 handles it is to default to DCV (reads 0.0V DC with no input) and then measure and display the DC coupled RMS (aka TRMS AC + DC) value in all cases and either the AC or DC symbol depending on the mix.  Again, it appears to default to DC and then only displays the AC symbol instead if the AC component is signficantly higher than the DC component.  For example, 100mVAC and 200mVAC both display as 0.1 VDC and 0.2 VDC respectively, while 300mV displays as 0.3VAC.  I didn't test this extensively to see what the exact relationship was, but the AC component consistently needs to be significantly higher to get the AC displayed, at least at the lower levels.  Perhaps at higher voltages the thresholds will be relatively closer. 

IDK how the Brymen system works internally and we probably will never know, but very possibly if they just had it default to DC whenever the inputs are low enough that the circuitry can't decide otherwise then it would read smoothly to zero.  If these meters were really smart they would display both the AC and DC symbols when relatively significant components of either are present.  But maybe they just ran out of program memory space.....

[floobydust]

I drew a partial schematic of the BM2257 front-end and also agree, the (bigger) PTC2 get's grounded in LowZ mode (at S0B1) - not like some other BM235? I think that left the clamps in series which is what I was on about for the input impedance being two levels.

The only other DMM I know about with a deadzone is the AN8008 around +/-4uV it truncates to 0V to mask A/D noise.
I think here, for the BM2257 it might be for the superstitious multimeter users that freak out when it doesn't read zero?

Something else I noticed is some sloppy PCB layout again on the rotary switch rings. It's like the layout guy has 1 hour to pound this out lol.
The change to using four MOV's is also to save space.

[EEVblog]

I drew a partial schematic of the BM2257 front-end and also agree, the (bigger) PTC2 get's grounded in LowZ mode (at S0B1) - not like some other BM235? I think that left the clamps in series which is what I was on about for the input impedance being two levels.

Yes, the BM235 goes into a transistor zener clamp, hence will only conduct the PTC once it hits 7V or so. Interestingly, the BM235 does actually go lower than the BM2257 in LowZ, down to 0.5V

Something else I noticed is some sloppy PCB layout again on the rotary switch rings. It's like the layout guy has 1 hour to pound this out lol.

Yes, looks rushed! Didn't bother to reshuffle things when they they ran out of space, so added a bodge wire around the socket, a 10A jumper cable on the top, and a battery wire on the bottom.

[EEVblog]

My reverse engineered partial schematic, could have errors, stuff missing, but shows how it works.



Video to be released in the next few days:

[Kleinstein]

In normal operation there are 3 MOVs in series at the input. In low Z mode there are only 2 MOVs in series (VAR1+VAR2). So one may have a slightly earlier clamping, lower maximum voltage in low Z mode.

The loading part in low Z mode is just the resistor and PTC (looks like the same as for the ohm mode). There should be no good reason for a lower voltage limit in the low Z mode. There is just the point that the AC detection has a lower limit to detect AC.

[EEVblog]

In normal operation there are 3 MOVs in series at the input. In low Z mode there are only 2 MOVs in series (VAR1+VAR2). So one may have a slightly earlier clamping, lower maximum voltage in low Z mode.

It's still 1240V, above the max rating of the meter.

[bdunham7]

In normal operation there are 3 MOVs in series at the input. In low Z mode there are only 2 MOVs in series (VAR1+VAR2). So one may have a slightly earlier clamping, lower maximum voltage in low Z mode.

The loading part in low Z mode is just the resistor and PTC (looks like the same as for the ohm mode). There should be no good reason for a lower voltage limit in the low Z mode. There is just the point that the AC detection has a lower limit to detect AC.

Those MOVs are rated for 385VAC continuous, the lower "extra" MOV is 350VAC.  IDK for sure why they've added that lower MOV, but if the reason was to reduce leakage and keep the input resistance at 10M even at the maximum input voltage then that need would not apply to the Low-Z mode.   Another possibility is that the 4-MOV setup is a leftover design feature from a previous version that had the now prohibited 'CAT II/1000V' rating and now at CAT III/600V they don't need it but didn't bother to take it out. 

[floobydust]

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.

[bdunham7]

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.

With three in a row at 600V, that's 200V apiece and extrapolation puts that leakage current at a fraction of a µA.  At 600V a 10M input would have 60µA already, so not an issue.  With only two, the leakage current would be higher but in Low-Z mode your input impedance is probably less than 1M @ 600V and your current is >600µA and not very stable or certain anyway. 

[Furna]

[...] 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.