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Brymen BM789

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2N3055:

--- Quote from: bdunham7 on September 12, 2021, 08:11:41 pm ---
--- Quote from: 2N3055 on September 12, 2021, 06:42:39 pm ---It's not a bug. If you have DC signal with AC superimposed on top, you might get to the point where signal is DC pulsating between two levels, one of which goes over ADC range. But still might be too fast switching for meter to up range... Autoranging is not magical.. There are scenarios where you can confuse it.

--- End quote ---

The OP may be able to clear this up, but I didn't see that it was specifically an autoranging issue, more that the meter didn't read OVERLOAD when it was clearly overloaded.

Now the DC vs AC input is an interesting characteristic. There is the external impedance and then there is how things are coupled and measured internally.  For the Fluke 116, for example, VAC and mVAC are both DC-coupled inputs, but internally it appears that VAC is AC-coupled whereas mVAC is not.  So given a 0-1V square wave, the 600mVAC range displays 'OL' but the VAC range correctly displays ~0.500V.  And given a 0-500mV square wave, the 600mVAC range displays ~350mV but the VAC range displays ~0.250VAC.  So mVAC is TRMS AC+DC.  And a closer look at the front panel reveals that I should have known that, but I didn't.   :palm:



Now the OPs meter gives the option of DC, AC and DC+AC.  It functions properly on DC and DC+AC, but fails to uprange or indicate overrange on the AC scale, and then only in one direction.  That seems wrong to me.  Thinking about it, perhaps there is some clamping bit that is breaking down too soon and actually keeping the sensed voltage from reaching the overrange threshold.  If some other EEVBlogger has a 786 or 789, perhaps they could help the OP out by running this test and seeing if it is a bug/feature or if the OPs unit has a defect.

--- End quote ---

OP is pushing signal that is 1 V P-P, with offset such that signal dips only 200mV below zero. He is doing it on a 600mV range.
That range CANNOT range up. It is mV position on the switch. So meter being DC coupled it is overloaded and shows, pretty much crap...
 "......something above 1 V p-p (so above the specs for the mV scale) it reads around
650mv on the AC scale, and it stays so even if I raise the voltage to around 30 V .(Can't get further) ....."

He pushed 30V in 600mV input, that is NOT AC coupled.  So ADC reads full scale and that's it.   As I said there is NO ranging in that switch position. It is fixed 600mV position. I do agree that OVRLD indicator (together with backlight flashing) should be shown instead.

bdunham7:

--- Quote from: 2N3055 on September 12, 2021, 08:54:57 pm ---That range CANNOT range up... there is NO ranging in that switch position. It is fixed 600mV position. I do agree that OVRLD indicator (together with backlight flashing) should be shown instead.

--- End quote ---

OK, then it definitely isn't an autoranging issue!  :)

Yes, the lack of an overrange indicator in one direction seems to me to indicate a specific defect.

OP:  Send your meter back and if you get a replacement, tell us what it does.

Edit:  After thinking further, I'm still not a fan of the meter not reading a 1Vp-p signal in mVAC mode because that does seem to be explicitly within range--0.5VAC.  A true AC mode would imply blocking a reasonable DC component at some point in the signal chain.  That's what my 189 that has all three modes does.

Neutrion:
But do you have a meter which also has the same AC,  AC+DC, DC range? Because AC+DC is OK and it overranges properly.

I already contacted Brymen through the seller as mentioned, and their ansver was, that this is OK because the meter works well WITHIN the specs. So I suppose all the BM789 should work like this, and possibly the 786 as well. I hope some other guys with 789s will show up.

And the question is, will it be possible at all to change this in FW.

By the way 2N3055 did you check the 869s whether it does the same?

2N3055:
It doesn't behave the same but it is the same problem:

mV is same, only 600mV (500mV on BM689S) DC range.
For instance, if I feed 300mV RMS 400Hz signal with 700mV offset I get propper 300mV AC reading. If I go higher offset than that, it starts showing bad AC amplitude, less than it is. Until DC offset reaches 8V when it shows overload.
Any do you know why? Because you are (me in this case) severely overloading input amplifier with DC.

If you have mV level AC signal, you need to decouple it from DC by using capacitor as I said. Then you disconnect capacitor and measure DC volts separately.

There is  this misunderstanding that meter can simultaneously measure 10 mV AC  and 40V DC , or vice versa, because it has dual display.
It cannot.  Meter has ONE input attenuator and one ADC.  If you set it for 100V it cannot be in position for 10mV. And vice versa.

Meter that could measure 1 mV AC riding on top of 300V DC would need 2 separate  front end paths, with 2 separate attenuators and front end amplifiers. ADC could be multiplexed, but it would be nice to have two of those too.

Then you could simultaneously measure both AC and DC, completely independently (and at different scales)  at the same time and calculate AC+DC RMS and have that too. You could also measure frequency and duty cycle from AC path with good sensitivity. Also you could optimize AC and DC paths, AC for frequency response, and DC for best AC rejection.

I personally don't know of any meters that do that. I wish it existed, I would gladly buy one.
People that  work on tube amplifiers would love it.

It is up to the operator to know what to do or not.

Good practice requires that you start with meter in higher ranges anyways. Start with measuring AC  and DC in volts range that will also autorange where it needs to be. If there is ANY mention of volts then and there, you CANNOT measure in mV.
If you do have 10mV AC  riding on top of 10V DC, you can use capacitor. That will work well.

If you have 150V AC riding on top of 1 mV DC, you're out of luck. You are asking meter to suppress difference in amplitude of 150000 times...
Funny enough, meter might  have more suppression than that at 50Hz.. But input might start smoking too...
It probably won't, because Brymens have fantastic protection circuits.

But don't you ever confuse what meter can SURVIVE with what is expected for meter to measure.
You can only expect it to measure what is inside range of peak voltage of that range.
Everything else is abuse and survival of user error..

EEVblog:

--- Quote from: Neutrion on September 13, 2021, 11:35:09 am ---And the question is, will it be possible at all to change this in FW.

--- End quote ---

If it's related to the lack of a DC blocking cap in AC mV mode then there is likely nothing you can in firmware.
Dual display meters have a physical DC measurment path a separate AC coupled path, and they get measured separately.

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