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Brymen BM789

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Neutrion:
Hi everyone, finally I had to register myself to clarify something, but was reading the forum for a
while, so not a huge loss...
I am starting a new one for the BM789 as it does not seems to have an own topic yet.

So I got the bm789 with FW. version 08. (The Brymen multimeter, not the other bm789 equipment one can find on youtube,of which I was hoping to see an in depth review, done by the lady presenting it, after accidentally clicking on it, and watching the whole presentation.)

So I found two problems:

1.If I use the mV Scale and feed in a squarewave with 50% duty cycle without(or very minimal like 100-200mV) zero crossing (though it behaves almost the same way with less or more than 50%) 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)
 DC or AC+DC overranges properly.
If I change the duty % it will be a bit less than 650mV, around 500mV AC. So it peaks at 50%

The strange thing is, that if I chane the polarity of the leads, it behaves correctly and overranges around 650 mV AC which it should. So with one way the leads connected it behaves as expected, but not the other way around. I assume it depends on which polarity the squarevawe has.

Do you thing this is normal or acceptable? Before anyone mentions it, I know that I should use the higher ranges for signals like this, but when someone measures a lower level signal, and it raises up
above the limit, there is no way of knowing it, you just stop seeing anything above this level, while the voltagemight be skyhigh. I can not test it with higher voltages unfortunately, but would be nice if someone could do that.

So I am not vorrying about accuracy above the specified limit, but about the fact that it does not show overrange on the AC scale on the mV scale.

How do other meters behaves in this scenario?


2. Thats a minor issue, but still annoying: When measuring milli or micro amps, if on auto range, the measured currentis above the displayed range, so the meter has to switch range, on AC+DC mode first it displays "OL" for a second or more and only if you keep the leads connected it displays the proper values.
I thought, it is just a question of a timeout value in the FW, but Brymen says it is not possible to change it.
Should I keep it or not?


Ps:
Thanks to Dave for the inspiring Videos!

And also thanks to Joe Smith for your extensive work, wish we could see this level of testing with some more consumer goods as well, it would mean much less garbage be produced!

I would be especially interested in your oppinion on this, and whether it is a firmware bug or maybe hardware related.
I contacted Brymen through the seller, but they say it is all good and fine with the meter.



 

2N3055:
mV range on these meters is NOT AC coupled. It is DC coupled.
So what you see is because of that.
You can check it with another meter, just measure resistance of input on BM789.

On my BM869S (that is same in that regard) I usually keep it in DC mV mode, and put AC and DC dual display most of the time.
That enables me to see more at the same time. But BM869S has dual display.

You could also take a 1uF block capacitor and put it in series with meter probe. That would AC couple the meter on mV range.
V AC, OTOH, is AC coupled...


Neutrion:
But with the dual display you see at least that it overranged. But with the 789 if I don't check it all the time in dc or ac+dc it could mean the voltage is sometimes all over the place and I don't even see it.

How does this capacitor trick affect the accuracy in dc or ac+dc modes? Did you check the whole bandwith with it?

How do other meters cope with this?

2N3055:

--- Quote from: Neutrion on September 12, 2021, 04:45:53 pm ---But with the dual display you see at least that it overranged. But with the 789 if I don't check it all the time in dc or ac+dc it could mean the voltage is sometimes all over the place and I don't even see it.

How does this capacitor trick affect the accuracy in dc or ac+dc modes? Did you check the whole bandwith with it?

How do other meters cope with this?

--- End quote ---

Dual display is the point. That is why I insist that single display meters are less capable than dual display.
Simple is not simpler all the time.

To me simplest meter would have big screen with simultaneous DC component, AC component (RMS), AC+DC (True RMS combined), frequency, duty cycle and accuracy limits. All at the same time. That would be simplest meter to use. You would have position for V, A, Ohms, Diode test. In Ohms, it would show resistance, voltage over resistor, current used and also accuracy limits.

Connecting through the capacitor is not a trick. This is what you do when you want to AC couple signal (strip it from DC component). DC component will be affected, of course, i.e. it won't be measured at all. Only AC will be passed, and that one will be pretty much perfect in a frequency range of the meter. It will have high pass effect. 1 uF will have impedance of 3180 Ohm at 50 Hz. That will make a divider with input impedance of meter (10MOhm + 600 pF for mV range, that is 3,466 MOhm combined at 50 Hz.). That makes it 1/1000 of influence, 0.1 %. That is one and half order of magnitude less then meter specs in that range. So simply ignore it. At 100KHz, parasitics will be negligible, so high end won't have any influence.

Draw a circuit in Ltspice and play with it. That is a good way to a get feeling for how it works.

Neutrion:
Well yes dual display is alway better so I was also thinking a lot about the other Brymens. But than finally this became the choosen one. Was waiting on Joes channel for it to give me a final go, but he also not tested this thing.


--- Quote from: 2N3055 on September 12, 2021, 05:16:12 pm ---
Connecting through the capacitor is not a trick. This is what you do when you want to AC couple signal (strip it from DC component). DC component will be affected, of course, i.e. it won't be measured at all. .

--- End quote ---
Oh yes, I read parallel first, but that might wouldn't make any sense :) But thanks for the calculation, though this way I ended up having an ac only range.

But anyway, there are obviously ways around, but do you have exact the same issue with the 869s?
Also the second issue with the ac+dc mA measurement? Cause that one would be easier to solve in FW I suppose.



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