Yep--AAAAANOTHER CHINESE DMM to tear down! - Page 1
Forum section: electronics / projects

author: DW1961
author_rank: Frequent Contributor
timestamp: November 09, 2020, 01:08:54 am
content: We tore into another one of this vendors DMMs a few weeks or months ago:https://www.amazon.com/dp/B08BYF5BJ5/ref=emc_b_5_tThe conclusion was that protection wasn't great, never use it on mains, but for other things, it's fine. CAT filings are nonexistent.And now we're going to tear another one down. This time, we're ding this one:https://www.amazon.com/gp/product/B08DKMKHYYThe price is the same, they seem to have the exact same functions, but the outward appearance has changed. I like it a lot better. Every time I see it the Beatles song "Come Together" enters my head: "Hear come old flat top. . . " lolThe two seem to have the exact same functions, and this one also has the weak "thermistor" (PTC) protection circuit without a mention of MOVs.I know I said I was never going to review another one of these things, but I just couldn't help myself. I have half a dozen of them now. Since I am 99% doing low voltage DC readings (and the occasional 120V reading), they will serve me just fine.However, I have my eyes on that Fluke 107!!! Tasty!Well, I'll try to tear it down tonight and post the images.

author: DW1961
author_rank: Frequent Contributor
timestamp: November 09, 2020, 05:51:13 am
content: Well, I have the images. I keep getting this message:"Your attachment has failed security checks and cannot be uploaded. Please consult the forum administrator."I'll try again tomorrow.It does look like this one might be using MOVs.

author: DW1961
author_rank: Frequent Contributor
timestamp: November 09, 2020, 09:51:40 pm
content: For some reason I cannot upload images anymore.I keep getting the :"Your attachment has failed security checks and cannot be uploaded. Please consult the forum administrator" error.So, here s a link to the files on my MegaSync server. You should be able to browse through them, and I have named them "name_##.jpg so we can refer to them easily.https://mega.nz/folder/Y6YXHaaA#mnWEZPtGNRNlqdV2AeSOWQOnce the images load, you can double click them to see the larger version.

author: DW1961
author_rank: Frequent Contributor
timestamp: November 10, 2020, 02:40:13 am
content: Guys. I really think this is using MOVs. The PCB says RV1 and RV2 and they looks like varistors. Really was hoping some would look and comment.It also has some PCB guard tracks too. It also has a real chip in it, not that blob chip stuff.Image link is working fine on my end.https://mega.nz/folder/Y6YXHaaA#mnWEZPtGNRNlqdV2AeSOWQ

author: xavier60
author_rank: Super Contributor
timestamp: November 10, 2020, 06:58:37 am
content: RV1 and RV2 look like MOVs to me also.I'm also curious about other aspects like the actual DC and resistance accuracy.Also the output voltage and current on the Diode test range and what voltage it measures up to.

author: DW1961
author_rank: Frequent Contributor
timestamp: November 10, 2020, 06:21:02 pm
content: Quote from: xavier60 on November 10, 2020, 06:58:37 amDid you see the small guard tracks near the 10A current side? Not really robust, but at least they are there. Also, the LED has it's own container isolating it further. Also, the large plastic dial and carriage help section off the bottom from the top, I think? I also see the bottom has a plastic guard protecting the the bottom of the unit? However, the fuses are small. And on one side they use a glass fuse. I'm not sure what that is except extra protection for the current side? It's an interestingly placed fuse. It has three fuses total.If so, it's looking pretty decent for 23 bucks. A lot better than their last effort we tore up.Here are the specs from the Amazon page:DC Voltage: 400mV, ±(0.5%+5) 4V/40V/400V, ±(0.5%+4) 1000V, ±(1.0%+6)AC Voltage: 400mV, ±(1.0%+6) 4V/40V/400V/750V, ±(0.8%+10)DC Current: 400μA/4000μA, ±(1.0%+10) 40mA/400mA, ±(1.2%+8) 10A, ±(1.2%+10)AC Current: 400μA/4000μA/40mA/400mA, ±(1.5%+10) 10A, ±(2.0%+5)Resistance: 400Ω, ±(0.8%+5) 4KΩ/40KΩ/400KΩ/4MΩ, ±(0.8%+4) 40MΩ, ±(1.2%+10)Capacitance: 10nF, ±(5.0%+20) 100nF/1μF/10μF/100μF, ±(3.5%+8) 1mF/10mF/100mF, ±(5.0%+10)Frequency: 100Hz/1000Hz/10kHz/100kHz/1MHz/30MHz, ±(0.5%+10)

author: DW1961
author_rank: Frequent Contributor
timestamp: November 11, 2020, 05:13:26 am
content: I added another picture so you can see the entire PCB up to the LCD. I can't take the LCD off because it is hard wired in.Sure would like to get a few more comments on this DMM. It seems like the protections are pretty decent, and the meter works really well. It looks like it is diode, PTC, and MOV bridge. Curiously, on the small current side, they use a glass fuse on the back side of the PCB (I think that's what they are doing), and then a they are finally using a real copper shunt other side. Can anyone confirm this?Also, I just noticed I only got one picture, but the jacks are solid, thick metal. Sorry about the image uploads. Multiple files are still not working correctly. Just use the link above. I put two new images in the folder. I'm thinking for the price, this MM is pretty damn nice, with some very decent protection and separation of circuits. It's only 25 dollars.

author: DW1961
author_rank: Frequent Contributor
timestamp: November 11, 2020, 06:14:27 pm
content: Anyone else want to get in on this? I'd appreciate it!

author: The Soulman
author_rank: Super Contributor
timestamp: November 11, 2020, 07:01:40 pm
content: Could have been worse..They did a poor job (hand?) soldering the current shunt tho.

author: schmitt trigger
author_rank: Super Contributor
timestamp: November 11, 2020, 07:41:39 pm
content: As you correctly mentioned, for basic low voltage work, these meters provide a tremendous features/price ratio.If you are a beginner, there is no excuse not to own a basic DMM.Many times I have found myself with the need to measure several parameters simultaneously.Using Flukes or Keithleys exclusively would quickly become an expensive proposition.And you don’t require 6 digits either.But the price point of these allows someone to have multiple DMMs without breaking the bank.Of course, one should own a good, trustworthy DMM as a sanity check.Thanks for taking the time and sharing your findings.

author: DW1961
author_rank: Frequent Contributor
timestamp: November 11, 2020, 09:11:04 pm
content: If you didn't see it, here is the link to all of the pictures.https://mega.nz/folder/Y6YXHaaA#mnWEZPtGNRNlqdV2AeSOWQI was hoping you guys might comment on protections. Mainly, these types of MM offer decent if not good accuracy, but their protections are always lacking, or even really poor. I saw a Fluke 107 tear down on Youtube, and this meter's jacks look hugely superior to the fluke's. The Fluke PCB looks better along with the huge Bussman fuse, but they both used PTCs and MOVs plus diodes for protection. As you can see, I'm mainly interested in their protections. However, this LCD is huge and all of the functions work really well, it's crisp,a nd the measurements come fast, along with instantaneous continuity.The glass fuse on the back is confusing, since they use a small but ceramic on the inside.But for sure, any comments on the IC etc.

author: rsjsouza
author_rank: Super Contributor
timestamp: November 12, 2020, 02:22:19 pm
content: Well, it surely looks that several elements are there (MOVs, PTCs, somewhat beefy diodes...) but it is very hard to simply look at the PCB and positively say the meter is safe... Or even safer than others.I have seen MOVs incorrectly connected, as well as fuses and PTCs. From what I learned from joeqsmith's videos is that frequently the small clamping transistors are the ones that tend to bite the dust - and these are frequently placed after the rotary switch, thus leaving it prone to arcing. Also, IIRC I have seen it also go through them straight to the main IC, thus rendering the meter unusable.I think a great textbook analysis is joeqsmith's series on improving the input protection of his UT61E. I have one of these meters and leave it unprotected anyways, given that I mostly use it in electronics and don't care for the extra protection circuitry reducing its frequency response.

author: DW1961
author_rank: Frequent Contributor
timestamp: November 12, 2020, 06:32:00 pm
content: Quote from: rsjsouza on November 12, 2020, 02:22:19 pmDid you see that huge carriage the rotary dial has? It's a really thick piece of plastic, and it screw on to the cover, where the dial slides in side. So the contacts have two thick pieces of plastic between them and the user. It's the first time I've seen that in the Chinese MMs since posting them. It also has it's own LED plastic sleeve around the LED part of it too, sectioning that aspect off from the entire board. First time I've seen that too.As far as the DMM not withstanding a maximum surge and destroying the electronics on it, I'm not going to hit them on that at all. I mean it's less than $27 bucks. As long as the protection circuitry is there visually, the aspect that protects one from getting harmed, that's the best we can do. That's the most important thing.I'm not understanding the glass fuse on the back of the PCB. Maybe it's there to help start the protection process in case of a surge? I say that because it is downstream of the ceramic fuse and the PTCs and MOVs.I wish more people would comment on this because I think it deserves it. I used it last night, and from a user POV, and it's pretty nice really. I'll do a video on it and switch all of the functions around so everyone can see how it works. One thing I do like about it is it leaves out all of the bullshit, such as, external flashlights and colored external LEDs that do nothing.Rotary dial for switch. There are three screws that attach the (for lack of other words) blast protection carriage to the PCB and the that covers the dial itself so there are two pieces of plastic between the switch contacts on the PCB and the user. It seems it would be really hard for any arc to pass through both of those plus the dial plastic itself:

author: DW1961
author_rank: Frequent Contributor
timestamp: November 12, 2020, 07:34:37 pm
content: One thing I DO NOT like is that the battery leads are soldered on instead of having them directly attached to the PCB.I received an email back from the vendor and he said they are using MOVs for protection but the ceramic fuse is only 250V. If I remember correctly, people were saying you need the DMM's CAT rated voltage for the fuse, or it's not CAT certifiable.

author: DW1961
author_rank: Frequent Contributor
timestamp: November 13, 2020, 06:42:26 pm
content: We had discussed teh CAT ratings and fuse requiremetn before, but this is what the vendor said:"Hello,The CAT voltage value is the pulse withstand voltage value, which is an insulation voltage value inside the shell protection, which is different from the voltage value of the fuse. The fuse of this multimeter can only reach 250V."So it sounds like what he is saying is that the fuse voltage does not need to be the same as the cat rating in order to have CAT certification.