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| Brymen BM789 |
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| Caliaxy:
--- Quote from: bdunham7 on September 15, 2021, 03:20:37 am --- --- Quote from: Caliaxy on September 15, 2021, 01:07:25 am ---most of the meters I tried have the same issue: Fluke 101, 107, 17B+, Uni-T UT61e, Agillent U1252A, BM235, BM689S. --- End quote --- Do you mean they all displayed wrong readings or did they display an overload indicator? --- End quote --- They all display meaningless numbers instead of OL, exactly like BM789. Forgot to (re)mention GW121. |
| joeqsmith:
I would hate to think I left a few of you with the idea the having a meter show zero volts while in it's ACmV mode is unique to Brymen's new BM789, here is another one for the UNI-T fan boys. Of course, switch it over to tri-display mode and we get a feel for what's really going on. Lucky for me, the battery was charged this time when I went to use it. :-DD Again, this meter has been damaged, repaired and then the PCB was modified but I doubt any of that would have an effect on these measurements. |
| AndrewBCN:
--- Quote from: bdunham7 on September 15, 2021, 12:35:03 am --- --- Quote from: AndrewBCN on September 15, 2021, 12:00:23 am ---No, 2N3055 got it right, the AC V position on the Fluke 87V has 6000 count resolution, whereas the separate BM789 ACmV position has 60,000 count resolution. If you use the Auto position on the BM789 it will autorange exactly the same as the Fluke 87V. --- End quote --- That's preposterous. The bottom 10% of a 60,000 count range is not equivalent to a full 6000 count range, especially when you are talking about TRMS. And the 87V has a 'hi-res' mode, but I don't have one so I can't tell you the details of that. But it's OK to be silly if you like--this is EEVBlog after all. --- End quote --- I don't have a 87V either but I can read (the online user manual): the Hi-Res mode does not work with the ACV ranges. So, as I wrote, if you set the BM789 to Autorange ACV position, it will autorange exactly the same as the 87V. The main difference with the 87V in that respect (ACV measurements) is that the BM789 provides an extra, separate, high-resolution, dc-coupled ACmV manual range, which as 2N3055 noted, requires a little bit of thinking to be used. As for being "silly", I am sorry to say but that's one way to describe this entire thread. So I guess a little bit of humor is not entirely out of place. :-DD |
| 2N3055:
I go to bed, and children in other time zones get in a fight right away.. :-DD Look, people. This topic is chock full of emotions and many hurried responses without doing homework. So let's sumarize: - Meters that do AC+DC type of measurements (combined) do that by keeping front end DC coupled. - Meters SHOULD AC couple in AC only measurements so you could measure 20mV AC riding on top of 100V - For some reason Brymen doesn't do that on AC mV range. That is NOT good way to do it. - Because of that, if you plan to measure 20mV AC riding on top of 100V using mV range, on Brymen you have to use external capacitor. Like Joe said, just throw it in a box, if you do it on a regular basis. - There are other meters that do the same thing as Brymen and also those that have AC mV range properly AC coupled. Make note that there is no instrument available that can measure in single AC+DC measurement (single or dual display) that can do mixed scales for AC and DC. - Would I like that Brymen did it differently, so it also AC couples when AC only mV? Yes, of course I would. That is pretty much only thing they didn't do perfectly. On my BM869 I miss that and fact that it doesn't remember preferences between powerups. - Is it a problem in practice? It might be annoying but not a problem if you know it. Would it be better if they did AC coupling? Yes. - Overload indicator gets confused by DC offset overdrive only. AC overdrive without offset works perfectly. It will show OVL. - Whole problem is caused by DC offset overloading input stages. All of this applies only to dedicated mV AC measurements. Volt ranges properly AC couple and work perfectly. At all times meter is safe and won't be damaged. Measurements won't be valid if operated outside operating envelope. That is summary. |
| AndrewBCN:
Let's summarize (again): 1. The OP found a way to "fool" his shiny new Brymen BM789 when set to the DC-coupled high-resolution ACmV range. This basically involves saturating the input circuitry such that the DMM cannot measure the (small) AC component because of a (large) DC bias. Under these conditions the DMM neither detects an overload condition nor provides a correct measurement of the (small) AC component. 2. joeqsmith has demonstrated that a number of different DMMs from different brands can be "fooled" using a similar method. I think the lesson here, if there is one, is that using a DMM (any model from any brand) to get correct measurements always involves a minimum of thinking - as 2N3055 reminded us. As well as spending a few seconds to read and understand the User Manual. Edit: Sorry 2N3055, we seem to have posted our summaries almost simultaneously. I defer to yours which is much better than mine. Edit2: I can think of a number of reasons why Brymen did not include a series capacitor in the ACmV range front end, and similarly for other DMMs from other brands. But that would be a matter for (reasoned) discussion in another thread, if you ever want to. I for one would be very interested in your opinion. |
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