Yet Another Chinese DMM to Tear Down - Page 1
Forum section: electronics / projects

author: DW1961
author_rank: Frequent Contributor
timestamp: November 01, 2020, 01:55:45 am
content: I'll be receiving this soon. Two reasons I want to tear this one down. First, it's auto ranging. Second, it says it uses Ceramic Fuses! If any of you rember the last peice of garbage I reviewed after tearing down (And tossed into the trash can), we'll see if this one is any better.https://www.amazon.com/gp/product/B08DHHJPS1Here is the review I did on the POS:https://www.amazon.com/gp/customer-reviews/ROTO1DC8Z5O2E/ref=cm_cr_dp_d_rvw_ttl?ie=UTF8&ASIN=B07XCRJXJZ

author: sleemanj
author_rank: Super Contributor
timestamp: November 01, 2020, 02:40:37 am
content: If you're looking for cheap meters with ceramic (if 5x20) fuses from factory, then I currently like the ADM66 which I briefly reviewed here. The "ears" make it look a bit odd, much more so in photos than real life, but it's a capable meter for around the $20 mark.https://www.eevblog.com/forum/testgear/bside-adm66-6000-count-decent-small-sub-$20-usd-meter/The larger and a bit more expensive Peakmeter PM18A isn't bad either but the fuses are a longer size than 5x20 (I bodged in a 5x20 holder on mine because I don't care and have boxes of 5x20 ceramic fuses and no boxes of whatever size they are).https://www.eevblog.com/forum/testgear/pm18a-peak-meter-auto-ranging-6000-count/

author: MosherIV
author_rank: Super Contributor
timestamp: November 01, 2020, 10:30:54 am
content: Hi DW1961 and SleemanjThanks for the good work buying and tear downs.None of these show (visual) signs of all the input protection circuitry needed to protect the user or the dmm.A simple test is to select a function that is not Volts or Amps and measure mains to see if mains damages the dmm. Try Ohms, diode, (if they have the following) capacitance, temperature, etc.Some will survive and work ok but this still does not mean that the dmm is safe and can handle a mains surge in these functions

author: DW1961
author_rank: Frequent Contributor
timestamp: November 01, 2020, 05:49:11 pm
content: Quote from: sleemanj on November 01, 2020, 02:40:37 amI actually use the Amazon brand which is really a CES brand. They have actual CAT ratings and are on file. I just do this for fun.

author: DW1961
author_rank: Frequent Contributor
timestamp: November 01, 2020, 05:52:38 pm
content: Quote from: MosherIV on November 01, 2020, 10:30:54 amThe closest I can get to main is the circuit panel and it's only 240V (I think?).

author: DW1961
author_rank: Frequent Contributor
timestamp: November 02, 2020, 06:14:23 pm
content: Received a reply from the vendor after asking if the unit uses MOVs for surge protection:"This multimeter uses PTC (Thermistors). "Looking forward to seeing how large those PTCs are!Will we have a winner?

author: Alti
author_rank: Frequent Contributor
timestamp: November 03, 2020, 02:38:16 pm
content: The instrument clearly shows "---H" which probably means this thing is LIVE.This fat conductor dangling in front of his face is just hilarious, as he was looking for lost neutral.Quote from: DW1961 on November 01, 2020, 01:55:45 amA more important, is it going to reliably interrupt the short circuit current if you stick probes where you should not?It has "max 250V" molded on the case which limits measurements to 250V/sqrt2=177VAC so not spectacular. It is advertised "EN61010-1 CAT III 600V", there is a small asterisk there. With fuses 5x20 the highest rating one can get is 250VAC and 1k5A. There are no electrical cabinets with molded case circuit breakers that have 1k5A prospective short circuit currents. Lets hope he limits himself to poking with one lead only.

author: DW1961
author_rank: Frequent Contributor
timestamp: November 03, 2020, 05:58:13 pm
content: Quote from: Alti on November 03, 2020, 02:38:16 pmLOL, great catch! In the questions section the vendor says 600V. In the spec section it says 600V. And, on the case, as you said, 250V. I asked which one is it. I don't have an * on my page next to the entry "EN61010."I tested the flash light last night--I got it late last night. It's a good flashlight. Really projects a nice round beam, so there's that! I would rather of had a toaster so if I'm out working on 600V mains I can have some toast and coffee. I can't wait to dig into this thing.If it isn't a total POS, but completely fails protection status according to all of you, I'll use it for testing low voltage DC current and stuff like that--since it's auto ranging. It will be fine for that.

author: Alti
author_rank: Frequent Contributor
timestamp: November 03, 2020, 06:18:21 pm
content: Quote from: DW1961 on November 03, 2020, 05:58:13 pmWell maybe you have it near CAT rating printscreened on the case?This thing has three jacks, both inputs are limited to 250V, so?The only 600V situation with any chance of survival that can happen here is when only one lead is plugged, just what this dude is doing on the picture I linked.Why do you think he was asked to cover holes with his left palm to make a picture?

author: sleemanj
author_rank: Super Contributor
timestamp: November 03, 2020, 10:55:55 pm
content: Quote from: Alti on November 03, 2020, 06:18:21 pmPerhaps I don't follow what your intent in that was, but the meter will be quite happy to measure 600v in voltage mode without any issues whatsoever.You just should not use it on circuits with greater than 250v potential in the modes other than voltage. Ideally the meter manual will specify the maximum working voltage for the modes, although I'll admit for the cheap chinese meters they often have uncorrected errors in the manuals, it's under $20 whaddayouexpect.According to the manual of the ADM66 which of my collection was the nearest manual to reach, the other modes have "Overload protection: 250V DC or AC (RMS)", but I'm not going to try bunging the meter in resistance mode into a live wall socket to see if it survives, because I have no desire to potentially destroy a perfectly good meter by doing something that you shouldn't do in the first place. The meter doesn't have to survive such an error, it just has to not be harmful to you, and that's pretty unlikely in a rubber booted double walled case screwed shut meter in a Cat II environment.Quote from: Alti on November 03, 2020, 02:38:16 pmLittelfuse offer some with 500VAC (450VDC) ratinghttps://www.littelfuse.com/products/fuses/cartridge-fuses/5x20mm-fuses.aspxand Bel Fuse make some 600Vac (500 DC) rated oneshttps://nz.mouser.com/datasheet/2/643/ds-CP-0adkx-series-1218961.pdfThere are probably other manufacturers as well. These higher rated ones tend to be slow blow, but Littlefuse offers some quick blow which are higher than 250v as linked above.

author: Alti
author_rank: Frequent Contributor
timestamp: November 03, 2020, 11:57:41 pm
content: Quote from: sleemanj on November 03, 2020, 10:55:55 pmFrom your comment it seems that this device has a CATIII 600V rated voltage path and 2x250V current path and the warning applies to current measurement only, but not to voltage mode (where 600V limit applies)?QuoteIt seems we have different undersanding about CAT rating and life value here, I am glad this subject appeared.So you are saying that if I take this CATIII 600V rated multimeter in my left hand, I press in two leads (GND + Volts) and clip black lead to phase of some circuit breaker in that distribution board with prospective short circuit currents within the order of 25kA or thereabouts, situation as on the picture, now:- if I dial "Volts" and stick the probe with my right hand into another phase then you say this thing will show 413VAC,- but if I misdial "Volts" but select "mA" instead and stick the probe with my right hand same way and this 250V rated fuse that could technically interrupt maybe 1500A but won't interrupt 400VAC and 25kA that is going to flow (minus drop on vaporized leads) is going to show what on the display?How do you expect this story to end?QuoteThanks, indeed I did not realize such peculiarities existed.I just opened Littelfuse 487 series, these interrupt only up to 200A.This is not very useful for poking in electrical cabinet.

author: DW1961
author_rank: Frequent Contributor
timestamp: November 04, 2020, 01:59:13 am
content: Quote from: Alti on November 03, 2020, 06:18:21 pmI don't know. I wasn't there when the decided to do that.

author: DW1961
author_rank: Frequent Contributor
timestamp: November 04, 2020, 02:37:45 am
content: Soon, very soon, we're gig to tear this thing apart and see what lurks inside. I'm betting I find some hobos, a discarded tampon, and a used condom inside.Here is a good read. He talks about how MM manufacturers use diodes, ptcs, fuses, etc to protect and how cheap MMs do it, some bad and some ok. He said in one section, if I read correctly, that some remove the MOvs and use spark gaps instead.https://lygte-info.dk/info/DMMDesignProtection%20UK.htmlAnd his tear downs here:https://lygte-info.dk/info/DMMReviews.html

author: sleemanj
author_rank: Super Contributor
timestamp: November 04, 2020, 03:07:07 am
content: Quote from: Alti on November 03, 2020, 11:57:41 pmThe device inmycomment (BSide ADM66 also sold as Mustool MT66 and possibly other rebrands) is a Cat II rated meter. Not Cat III.Indeed looking at a sample of my cheapo chinese meters on my desk here...BSide ADM66 - Cat IIMestek DM90 - Cat IIRichmeters RM113D - Cat IINJTY T21D - Cat II (rebrand RM113D)BSide ADM02 - Cat IIVichy VC99 - Cat IIAneng A830L - Cat II (crappy manual meter)Digitech QM-1500 - Cat II (the ubiquitous crappy unfused manual meter)I have another couple somewhere in the garage but can't be bothered going to find them. Of the above the last two crappy manual meters Cat II is rather optimistic probably and I wouldn't use them on more than Cat I myself (ad they do get used on occasion, you can never have too many meters).The only Cat III meters I think I have arePeakmeter PM18A, this is a larger meter with larger fuses (but not HRC), and claims Cat III 1000 and Cat IV 600, but I would not trust to use it on more than Cat II.Richmeters 102 (Aneng AN8002) which claims to be 600v Cat III and 1000v Cat II, and, I would absolutely not trust that on more than Cat II, frankly Cat I is as far as I would use that little thing with the tiny 10mm fuses.Personally, if I was working in a Cat III (or IV) environment, I would buy a "name brand" meter, namely a Fluke. Because... that's work, significant danger, and a business expense.

author: Alti
author_rank: Frequent Contributor
timestamp: November 04, 2020, 09:38:50 am
content: Quote from: sleemanj on November 03, 2020, 10:55:55 pmQuote from: sleemanj on November 04, 2020, 03:07:07 amSo your advice about "a multimeter happily measuring 600V" does not apply to the AstroAI M4KOR that OP was asking for and which is depicted on the photo attached by me, but advice applies to some different multimeter?

author: DW1961
author_rank: Frequent Contributor
timestamp: November 04, 2020, 06:06:46 pm
content: I received a reply about the 250V vs 600V discrepancy."The "Max 250V" on the multimeter is the specification of the fuse. The fuse is mainly used to protect the current test. The multimeter can measure AC/DC voltage to 600V. The voltage measurement in the voltage range will not damage the meter, and the multimeter will only display “OL” if the range is over. "

author: DW1961
author_rank: Frequent Contributor
timestamp: November 08, 2020, 08:49:34 am
content: OK, so here it is. Looks like a desert on that PCB. One tiny PTC. CAT III 600Vhttps://www.amazon.com/gp/product/B08DHHJPS1Tear it up, boys!

author: DW1961
author_rank: Frequent Contributor
timestamp: November 09, 2020, 09:54:05 pm
content: Anyone care to comment on this thing?

author: floobydust
author_rank: Super Contributor
timestamp: November 10, 2020, 05:05:50 am
content: For USD$21 it's a step up from the usual. 5x20mm fuses with better PCB spacings, D5 is TVS for a bit better protection, J1 looks hackable as either the IR LED or Bluetooth add on?There are a few stupid pcb spacings mistakes, I'd swear there is only one company in china doing pcb layout for low end multimeters and they are still dumb. Follow the trace from the PTC to the rotary switch and ask why it runs so close to the ground pour - just over 1mm spacing? That's pushing it for a 600VAC rating, and the PTC would get fried if it did arc over.I wouldn't trash it beyond the Cat. III 600V spec (test is 6kV impulse) and I wonder how noisy the readings are. There is small capacitance on the rails and a boost-converter I think for decent LCD contrast so it might be wandering.Otherwise, I think these DMM's for review will soon be coated with ricin lol

author: DW1961
author_rank: Frequent Contributor
timestamp: November 10, 2020, 06:29:14 pm
content: Quote from: floobydust on November 10, 2020, 05:05:50 amHey Flooby, thanks. I didn't see that trace. I'll add it to the review. I didn't trash it. I just gave it an average rating of 3 stars due to puny protections, but otherwise said it was ok for household work. I think that's fair?Check out my other tear down. I think it is using MOVs this time:https://www.eevblog.com/forum/projects/yep-aaaaanother-chinese-dmm-to-tear-down!/

author: DW1961
author_rank: Frequent Contributor
timestamp: November 11, 2020, 04:03:56 am
content: Quote from: floobydust on November 10, 2020, 05:05:50 am"TVS" ?