eevblog BM786 multimeter bug? - Page 1
Forum section: electronics / testgear

author: geonak
author_rank: Newbie
timestamp: August 28, 2021, 04:50:25 pm
content: Hello everyone, long time follower first time poster.A while back I had bought an eevblog bm786 multimeter, to replace my cheap chinese one.I was working on an atx psu yesterday, and was testing the rectifier diodes, along the way I had some doubts about the results of the 786 and compared it with my cheap chinese one. To my surprise the 786 was saying the diodes were shorted while the chinese one measured the voltage drop correctly. (video:https://youtube.com/shorts/qROTB7-rbQY?feature=share)The component I was testing is an 30l60ct, and I tested 3 of them with the same results.Mind you I am a beginner so it might be a user error, or maybe it is a bug/problem with the multimeter?So I am posting the question here to hear your thoughts and understand what is going on.- is it a user error?- if not, is it a problem with my specific unit?- is it a prpblem with all 786s?Thank you for your help!Cheers,G

author: AVGresponding
author_rank: Super Contributor
timestamp: August 28, 2021, 05:32:52 pm
content: Is the reading on that yellow meter 0.56V or 56mV? Can't see the dp in the video.If it's 56mV, that's way below what the Vfshould be, and close enough to the reading from the 786.

author: bdunham7
author_rank: Super Contributor
timestamp: August 28, 2021, 05:36:55 pm
content: Both meters are actually telling you about the same thing. The device is a power Schottky, which means leaky. The DT830 and BM786 probably have different diode test currents (I believe the DT830 is not constant) and therefore one reads 56mV and the other 46mV--that's not a discrepancy at all if the test currents are not matched. As for the reverse voltage, again the Schottkys are leaky and in this case the test current of the BM786 is passed by approximately 2.3 volts--which seems low but read the datasheet of the component and the BM786 to see if they match what you are seeing. The DT830 has a maximum diode test range of 2 volts, so anything larger than 2 volts shows as an open circuit.Nothing appears broken here--both meters and all 3 components are probably fine.

author: AVGresponding
author_rank: Super Contributor
timestamp: August 28, 2021, 05:54:40 pm
content: https://www.onsemi.com/pdf/datasheet/mbr30l60ct-d.pdf

author: bdunham7
author_rank: Super Contributor
timestamp: August 28, 2021, 06:02:38 pm
content: Those seem less leaky then these:https://pdf1.alldatasheet.com/datasheet-pdf/view/966812/CITC/CF30L60CT.htmlAnd we don't know his ambient temperature.

author: floobydust
author_rank: Super Contributor
timestamp: August 28, 2021, 06:19:31 pm
content: Someone would have to measure the BM786 diode-test current. I hope it's not really low.In the field I use DMM's with higher 2-5mA test current for diode-test. Even AN8008 applies 1.7mABut getting Vf 0.0459V or 0.057V seems wrong regardless of test-current. That Schottky rectifier is rated maybe 10uA leakage at a few reverse volts. Even 100uA leakage should not scuttle the readings this much.

author: bdunham7
author_rank: Super Contributor
timestamp: August 28, 2021, 06:52:15 pm
content: EEVBlog BM786 manual says diode test current 'typical' is 0.35mA. I don't know whether that is a proper precision current source or what. I know the DT830 isn't. If there is a 'standard' for this, it should be 1mA because that is the most common value you see in a datasheet both for Vfand zener voltage. Of course there are plenty of exceptions to that, but if you only have one value to choose, 1mA would be it IMO.The datasheet I linked with the same part number is markedly different from the ONSemi one regarding reverse leakage. I think the reading he is getting are plausible.

author: floobydust
author_rank: Super Contributor
timestamp: August 28, 2021, 07:25:58 pm
content: You have to know your test equipment very well, in the field or a repair situation.Rough math gives OP's diode leakage around 93uA which high but it's a cheap semi. Spec is 100uA max from the datasheet at 60VRM 25C, not 2.2V at test here.The wonky low forward-voltage 47-56mV I've never seen unless the part is damaged or made in the gutters of guangdong.

author: geonak
author_rank: Newbie
timestamp: August 28, 2021, 08:17:53 pm
content: Thank you all for the awesome clarifications.I did some more testing based on your suggestions, and I confirm that I was reading the value on the chinese meter wrong, it is indeed in mv.In my mind I was expecting a value in the half volt range so my mind jumped to that conclusion, and the beep on the 786 with the lack of it on the yellow one threw me off also, which was obviously wrong.As to the part, I had pulled it out of a good brand atx psu that had suddenly died, and I was experimenting with it mainly to learn, I'd hope they are using good branded non cheap chinese ones.Based on all the answers above, I am thinking those rectifiers have gone bad? But what are the odds that all three were bad? ( the psu had 6 I had only pulled 3 out), is it one of those instances where a chain reaction breaks multiple things, or is there something I am missing?Thank you all again!

author: bdunham7
author_rank: Super Contributor
timestamp: August 28, 2021, 09:18:24 pm
content: Quote from: floobydust on August 28, 2021, 07:25:58 pmIf you look at the graphs you see that the reverse leakage is rather flat with voltage but varies radically with temperature. By these graphs, these just look like they are performing as if they were bit hotter than room temperature--but we don't have the datasheet for whatever the particular manufacturer is here nor do we know how much variance from typical would be acceptable. They're 30A trench Schottky's, their Vfat 350uA is going to be pretty low. I would test them with more current before concluding anything.

author: bdunham7
author_rank: Super Contributor
timestamp: August 28, 2021, 09:21:47 pm
content: Quote from: geonak on August 28, 2021, 08:17:53 pmI think they are probably not bad, but I could be wrong and I would test them with a higher current meter to be sure. In my experience it is not all that likely that they would all fail in that specific way. Perhaps slow-roasting these has a specific effect that I don't know about.

author: geonak
author_rank: Newbie
timestamp: August 29, 2021, 11:36:28 pm
content: Just to make sure I am understanding correctly, you are saying that for these diodes with their very high current capability, the constant current sent by the multimeter is not enough to generate the full voltage drop? And to test that properly, I should pass through them a higher current and measure the voltage drop manually?If that is the case, is there a criteria to indicate for which diodes the meter in diode mode would be enough? Is there a property of the diode like a minimum current or something that I should be looking out for in cases like these?Cheers,G

author: bdunham7
author_rank: Super Contributor
timestamp: August 30, 2021, 12:02:40 am
content: Quote from: geonak on August 29, 2021, 11:36:28 pmYes, and you can look at the datasheet for clues. The forward voltage is listed for 3A and 15A, but be aware that the junctions will quickly heat up and the forward voltage will be low. Otherwise you can look at the first two charts 'Forward Characteristics" and "Reverse Characteristics" and assume there will be considerable variance due to the tempco of these devices. You'll see that the chart doesn't even provide any data for forward currents less than 10mA and while the reverse current might be high if you were testing at 25C, these aren't necessarily the same devices in the datasheet. You could reasonably test these with 1 ampere. You really would want to know what the reverse current is at a higher voltage as well.QuoteA meter like that will be fine for small signal diodes, silicon rectifiers up to a certain size (almost anything that plugs into a normal wall outlet), etc. I would rather see a 1mA or better, a selectable precision test current simply because many datasheets list that--and it lights up LEDs better. Schottkys, large power rectifiers and so forth you need to do more work. I don't think you want a normal DMM to be using huge test currents because of the potential for damaging stuff.

author: geonak
author_rank: Newbie
timestamp: August 30, 2021, 12:06:21 am
content: Amazing explanation, thank you very much for all the information.things are much clearer now!Cheers,G

author: CDaniel
author_rank: Frequent Contributor
timestamp: August 30, 2021, 09:38:00 am
content: Every good multimeter would differentiate easily a bad / good diodeSo that diode is bad ... in the video you also seem to test it in reverse with BM786 and is conducting also ... more clear than this it cauldn't be

author: Kartika
author_rank: Regular Contributor
timestamp: January 06, 2022, 10:06:47 am
content: diode test with multimeter can easily deceive you ,,, use light bulb (with approximate leakage current) in series to see the real state of affairs.