Did something really stupid; is my DMM a goner? - Page 1
Forum section: electronics / repair

author: cigmas
author_rank: Regular Contributor
timestamp: April 18, 2021, 10:23:19 pm
content: I was measuring something on one of those racket-style bug zappers... while it was live. After a spark at one of the leads, the DMM's screen went blank and no longer turns on. It was in DC volt mode.Is there any chance of repair or is the chip likely fried? Even though I was not using one of the fused ports, I checked the two fuses for continuity with a bulb. I don't see anything visibly burnt or damaged.RIP Thsinde 19B+ ? At least it wasn't very expensive, but I really like this DMM and I'd likely get another if it can't be saved.

author: Jeff L
author_rank: Contributor
timestamp: April 18, 2021, 10:30:04 pm
content: It probably took a hit of 1500volts--- I'd be very surprised if it survived.

author: wraper
author_rank: Unknown
timestamp: April 18, 2021, 10:32:20 pm
content: You will need to replace main IC. That will require removing black blob and soldering new IC on the pads around that blob. The main problem is to figure out what it is

author: IanB
author_rank: Super Contributor
timestamp: April 18, 2021, 10:39:11 pm
content: If the screen doesn't turn on the meter is probably toast. You would have to recycle it and get another.Check out Joe's meter robustness tests to find a meter that is less likely to be damaged by such a mishap.https://www.eevblog.com/forum/testgear/hear-kitty-kitty-kitty-nope-not-that-kind-of-cat/

author: wraper
author_rank: Unknown
timestamp: April 18, 2021, 10:40:44 pm
content: Found a picture of PCB with IC in soldered package. Though it's blurry and I cannot figure out what's written on it.

author: wraper
author_rank: Unknown
timestamp: April 18, 2021, 10:49:21 pm
content: Apparently it's DM1106EN. It seems to be a clone of HY12P66 and DTM0660L, but check yourself if they are actually compatible.https://thearduinoshed.wordpress.com/2020/04/13/uni-t-ut210e-current-clamp-hacking/EDIT: DTM0660L can be purchasedhttps://www.aliexpress.com/item/1005002016637380.htmlHowever the issue is that your PCB unlike newer one has only pads for larger COB on PCB (blob IC mounted on a small adaptor circuit board which is then soldered on main PCB). Therefore to replace IC you will also need to make adaptor board or make a dead bug.

author: rsjsouza
author_rank: Super Contributor
timestamp: April 18, 2021, 11:03:56 pm
content: Quote from: IanB on April 18, 2021, 10:39:11 pmThat is the most probable cause, unless a voltage regulator was shot.In your first photo, I think I see a small hole on the top left of the black blob - it could be dust, though.

author: Gyro
author_rank: Super Contributor
timestamp: April 19, 2021, 08:26:18 am
content: The material used to cover those blob top ICs is very hard. Make sure that you have a chance of completely removing it before considering buying a replacement IC.

author: mikerj
author_rank: Super Contributor
timestamp: April 19, 2021, 08:37:40 am
content: Strange that they went to the expense of fitting proper ceramic fuses but not a single MOV in sight.

author: SeanB
author_rank: Super Contributor
timestamp: April 19, 2021, 10:08:30 am
content: Quote from: mikerj on April 19, 2021, 08:37:40 amCeramic fuses are very cheap to make, just a clay that is blended, then fired after extrusion, and ultra cheap to make. Add the crimped ends and the thin wire inside. MOV on the other hand needs precise blending of the exotic material oxide powders, then forming, drying, then sintering, then plating on the silver electrodes, soldering the wire leads on and then dipping in epoxy and heat curing, then classifying the voltage range. A lot higher cost.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 19, 2021, 10:32:24 am
content: MOVs are not absolutely needed. There are alternative ways to clamp a voltage. The MOVs may give better performance for ESD protection to the switch - which may very well have been the failure path in this case.The ceramic fuses (especially the high current one) are usually more expensive than MOVs. The MOVs are made in large quantities with not that exotic materials needed.When bought in larger numbers the ceramic fuses may also be more afforable.

author: MadTux
author_rank: Frequent Contributor
timestamp: April 19, 2021, 10:41:58 am
content: Tinywiny protection diodes inside the chip are certainly not enough to clamp an overvoltage event, much less capable in sinking enough current to blow a fuse. So MOVs or at least big external protection diodes are needed.Anyway, RIP, chinese piece of junk

author: Kleinstein
author_rank: Super Contributor
timestamp: April 19, 2021, 11:51:14 am
content: The MOVs don't stand the current to blow the fuses. The fuses are only for the current ranges and that current will flow through the wire type shunt for the large fuse and the 5 diodes (likely similar to 1N4007) close to the lower range fuse.MOVs would mainly clamp the current from ESD or similar higher overvoltage. Normal voltage significantly exceeding the ranges are normally clamped by diodes at or inside the chip. The current is there limited by a large resistor (here it looke like the 4 x 2.5 M) and thus not that large (e.g. 100 µA for 1000 V at the input).

author: Renate
author_rank: Super Contributor
timestamp: April 19, 2021, 12:01:28 pm
content: Just out of curiousity, did you look at the other side of the board with the selector switch to see if you could see any arcing?

author: MadTux
author_rank: Frequent Contributor
timestamp: April 19, 2021, 02:39:41 pm
content: A nice ESD spark is quite a bit more than 1kV, more in the 10kV range, so already 1mA for the diode to clamp.Then there is parallel capacitance on the resistors, arc-over on resistors, diode turn on time....Everything which a MOV would easily swallow, but a tiny diode inside the chip clearly can't.Otherwise, that thing wouldn't have failed.I've also killed a cheapie in a similar way years ago. Thought I could simply measure the voltage on chainsaw ignition coil instead of hooking it to a spark plug. I then turned the chainsaw flywheel rather slowly and the DMM was gone and the coil confirmed as good.... Ooops.

author: cigmas
author_rank: Regular Contributor
timestamp: April 19, 2021, 05:00:32 pm
content: Thanks so much for all the comments! For some reason I'm not getting email notifications of replies even though I'm subscribed for notifications to my thread. (Yes, nothing in spam either.)Would replacing the chip affect calibration validity? That's assuming I could even source the correct chip (@wraper thanks for the reasearch!) and manage to replace it at all, as that black stuff is really hard and I have no idea how to remove it without potentially destroying the board.Are there other components I should measure first in case the failure is not the chip? I would have to revive an even cheaper multimeter to measure anything.It might be a cheapo device for most of you, but it was the nicest DMM in this household and will be missed... all because of a stupid mistake. Poor thing.On a side note, is there a safe way to measure the high voltage of the zapper to know what the DMM got hit with? You know, without killing anything. This zapper makes a fatter spark than I've seen from other similar devices. Though maybe it's more trouble than it's worth.

author: cigmas
author_rank: Regular Contributor
timestamp: April 19, 2021, 05:27:33 pm
content: And yes, the speck on the chip in the image above is just dust.I got a little bold with the disassembly on the other side. Not much to arc, nothing under the backlight, plus the display's contacts are nothing I've seen before. Wonder if they still work now...

author: IanB
author_rank: Super Contributor
timestamp: April 19, 2021, 06:17:49 pm
content: Quote from: cigmas on April 19, 2021, 05:00:32 pmIn general the question is less about the voltage and more about the energy of the discharge. To measure that energy you would generally have to use an oscilloscope--you might for example discharge through a resistor, plot the voltage against time across the resistor, and measure the area under the curve. But of course you would have to really know what you are doing, or you could damage the oscilloscope the same way you damaged the meterThe lesson to learn here is that choosing a meter with robust input protection can save you from this kind of mishap. There are meters out there that will survive trying to measure a bug zapper without being damaged.

author: Renate
author_rank: Super Contributor
timestamp: April 19, 2021, 06:18:19 pm
content: Quote from: cigmas on April 19, 2021, 05:00:32 pmI used to have a high voltage probe for working with the HV on color TVs which ran to 30 kV or so.Most of them were 1000:1 resistor dividers, but you had to make sure that you DVM had the correct input resistance for that model probe.I'm not even sure now if there was a "low" resistor in the divider or if they relied entirely on the DVM for the "low" load.That would make the "top" resistor 10G for a 10M DVM.https://www.fluke.com/en-us/product/accessories/probes/fluke-80k-40Kind of pricey for my taste. I don't do many CRTs nowadays.

author: fordem
author_rank: Regular Contributor
timestamp: April 19, 2021, 06:38:12 pm
content: Quote from: cigmas on April 19, 2021, 05:27:33 pmGoogle elastomeric zebra connector - they are fairly common with LCD displays - and yes, they're quite sturdy, and survive repeated disassembly & reassembly, and even the occasional cleaning with an alcohol wipe when the display has missing or faded segments.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 19, 2021, 08:55:17 pm
content: There are not many more parts to fail - so chances are near 100% it is the main chip that failed.Replacing the chip would ruin the calibration - there may be some ratios still valid, but not much.

author: cigmas
author_rank: Regular Contributor
timestamp: April 19, 2021, 10:45:41 pm
content: Quote from: IanB on April 19, 2021, 06:17:49 pmThat would be a nice, but also risky, analysis! I will carefully step back now.Quote from: IanB on April 19, 2021, 06:17:49 pmYes but at what cost? Looking through the meter testing google doc, I could have killed meters worth an order of magnitude more.Are there better options at a similar price point ~$35 CAD?https://www.amazon.ca/multimeter-MeasuresVoltage-Capacitance-Transistors-Temperature/dp/B075WRQYX5/The nice thing about getting another Thsinde 19B+ is that it's a "known quantity" that I was happy with. On the other hand, there are newer "cheapo" DMMs that seem about as well reviewed (paid? ..) with updated features. eg:https://www.amazon.ca/KAIWEETS-Multimeter-Auto-Ranging-Capacitance-Temperature/dp/B07SHLS639/This "HT118A" (one of many such clones) adds NCT, duty cycle, and some other stuff, while retaining the temperature probe (but loses the REL feature??), though I don't care for that aesthetic. Plus I haven't seen others use as beefy a 10A fuse as the Thsinde other than Fluke, though I doubt I'll be using it for high current applications.Any preference between the above or something else priced similarly?Quote from: Renate on April 19, 2021, 06:18:19 pmAha, my first thought was "voltage divider" too but thought it still risky to DIY if I don't know roughly what the voltage is. Yes quite pricey, but I guess that's relative to what it's attached to.Quote from: fordem on April 19, 2021, 06:38:12 pmThanks!! That is such a cool connector! At least the silver lining of this mishap is that I learn something interesting.Quote from: Kleinstein on April 19, 2021, 08:55:17 pmI was afraid of this. In that case it's not worth attempting a repair. Maybe I'll just get a second one of the same so that I can use the dead unit for spare parts, though hopefully that will not be necessary.

author: james_s
author_rank: Super Contributor
timestamp: April 19, 2021, 10:49:27 pm
content: It's dead, Jim.Those bug zapper things produce quite substantial voltage, >1kV is typical. Always be aware of what you are trying to measure, even a good quality multimeter can be damaged by trying to probe something well beyond what it is rated to tolerate.

author: IanB
author_rank: Super Contributor
timestamp: April 19, 2021, 11:35:39 pm
content: Quote from: cigmas on April 19, 2021, 10:45:41 pmUnfortunately, $35 is in the "disposable" category of gadgets. At that price point you have to treat it as fragile and be really careful what you do with it. It will be fine for measuring batteries and low voltage electronics, but don't take it close to anything you wouldn't want to touch with your fingers.

author: rsjsouza
author_rank: Super Contributor
timestamp: April 20, 2021, 12:13:38 pm
content: The Thsinde is a good meter, provided you respect its limitations, which unfortunately you found in a not-so-ideal way. If you like it, by all means get another one.Safer meters in more or less the price bracket of up to US$50 may come in the shape of a Klein MM400 or a Fluke 101. But they are less featured.

author: aqibi2000
author_rank: Regular Contributor
timestamp: April 20, 2021, 02:20:26 pm
content: List it on eBay as “untested”

author: cigmas
author_rank: Regular Contributor
timestamp: April 22, 2021, 12:24:04 am
content: Quote from: rsjsouza on April 20, 2021, 12:13:38 pmThanks for the suggestions. I looked up the Klein MM400 (https://data.kleintools.com/sites/all/product_assets/documents/instructions/klein/MM400_1390110ART_WEB.pdf) and it seems nice, but most of the range and accuracy specifications are less than the Thsinde, cost is about double, and it doesn't mention added input protection or safety. The capacitance max range of 200 uF is a bit of a bummer, as I've needed to measure 12 mF capacitors before which worked with the Thsinde's max 6000 uF range.How useful is the NCV feature on multimeters? In particular, I foresee needing to drill into a wall and wondered if this would help avoid areas with wiring. That's mainly the reason I also looked at the HT118Ahttps://www.manualslib.com/manual/1860345/Kaiweets-Ht118a.html?page=23#manualThe measurement specifications are pretty similar to the Thsinde. I just wonder if its rather extensive feature set and appearance makes it more gimmicky, or if it adds decent value.Quote from: IanB on April 19, 2021, 11:35:39 pmSpeak of the devil. Here's the only other DMM around here, taken out in its prime by a battery leak. Jealous of those abominable "leads"?The tips came off long ago, but not before having a chance to enjoy intermittent connections.Can it work again? Maybe, sort of. Or not really..Quote from: james_s on April 19, 2021, 10:49:27 pmIn hindsight it was such a stupid user error, but I still don't know what the voltage is on the zapper. Here's a video of its rather large and loud spark, which is even more so in person, just for fun:https://i.imgur.com/MvIFIot.mp4

author: IanB
author_rank: Super Contributor
timestamp: April 22, 2021, 12:48:16 am
content: Quote from: cigmas on April 22, 2021, 12:24:04 amCoolBut the size of the bang does not tell you much about the voltage. I made enormous bangs as a child by shorting out an 18,000 µF capacitor charged up to only 12 V. And an accidental discharge of a 300 V photoflash capacitor nearly made me jump out of my seat.

author: SilverSolder
author_rank: Super Contributor
timestamp: April 22, 2021, 12:49:00 am
content: To measure such a high voltage, you need to divide it down by putting a whole bunch of resistors in series, measure what happens on the last one, and multiply the number back up, if you see what I mean.You need to use enough resistors that the spark can't just jump over all of them and blow the meter again!

author: cigmas
author_rank: Regular Contributor
timestamp: April 22, 2021, 02:01:32 am
content: Quote from: IanB on April 22, 2021, 12:48:16 amTrue. I've shocked myself with ~18 kV by absentmindedly putting my thumb over the shielding and center pin of a BNC cable.That made no spark, but left my forearm tingly for a good 20 minutes. I wish I could say it happened only once...As for capacitors, a lab tech once showed me a discharge of a nearly coke-can-sized capacitor whose capacity I don't remember. It was a frightful sound, and apparently could be quite dangerous.Quote from: SilverSolder on April 22, 2021, 12:49:00 amMakes perfect sense, but I don't feel like risking it! haha

author: rsjsouza
author_rank: Super Contributor
timestamp: April 22, 2021, 03:22:42 pm
content: Quote from: cigmas on April 22, 2021, 12:24:04 amYes, that is exactly what you are trading off: features and specs for safety. Klein is a brand used by people on the field and has great accountability with regards to safety - in other words, if it is shown their products claim safety but do not deliver, they will be sued pretty harshly. A completely different scenario when compared to a low cost Chinese brand.Higher up in price and features are the Klein MM600 and MM700 with a bit higher capacitance and other features, but they will set you back somewhere closer to US$100, which takes them pretty close to EEVBlog's BM235 and Brymen's BM257.Amprobe and Greenlee also have some interesting models as well (AM-560, DM210A), but the equation is the same: features versus safety.Overall, you need to keep in mind that, when measuring higher voltages and spikes, any destructive accident can happen even if the meter or the voltage divider are sized for the voltage - inductive loads can cause several kV spikes.Quote from: cigmas on April 22, 2021, 12:24:04 amThe NCV is a useful feature but it is highly dependent on the use case. For example, the only meters I have used to "look through walls" are the U1282A and the Amprobe AM530, which can set their sensitivity. Others such as the Bside ZT-Y (Aneng SZ-18), the Richmeters 113D, the Richmeters RM219 (Aneng AN870) are less sensitive and can "see through" thin walls.Also, keep in mind that some meters are designed for the Chinese market and can reliably detect only 240V, not the regular 120V here in North America.

author: rsjsouza
author_rank: Super Contributor
timestamp: April 22, 2021, 03:31:52 pm
content: Quote from: aqibi2000 on April 20, 2021, 02:20:26 pmOr: "Pulled from a working environment..."(the environment was working, not the meter)

author: cigmas
author_rank: Regular Contributor
timestamp: April 24, 2021, 12:38:17 am
content: @rsjsouza Really appreciate the mention of so many models and how they compare. When you say safety do you mean the CAT II/II/IV ratings? Since I don't foresee use in high energy applications, I'm less concerned about those for now. I don't see any mentions of input protection of the type which could have prevented my DMMs untimely demise.Maybe the best teardown review of the HT118A I've seen so far, by Kerry Wong:TL;DR: Seems good as far as cheap meters go, but nothing to write home about, and no input protection to speak of. Biggest downsides IMO are the omissions of the REL button and the big HRC fuse for the 10A jack when compared to the Thsinde.Also from his general review, the NCV appears sensitive but it's doubtful if it would work through drywall distances:https://youtu.be/CxoAun4GRl8?t=771Maybe it's worth trying out.

author: rsjsouza
author_rank: Super Contributor
timestamp: April 24, 2021, 03:48:11 am
content: Quote from: cigmas on April 24, 2021, 12:38:17 amSafety is usually tied to the CAT ratings; the meters that were tested for these ratings are usually sized to withstand voltage transients higher than their nominal rating while maintaining their integrity without harm to the operator. The fine print is that the meter does not need to survive - in other words, these ratings don't evaluate robustness but safety.What does that tell you? Simple: if you still poke your meter -any meter- to a voltage source higher than its nominal rating (that is what bug zappers do), any outcome is fair game, as long as the meter does not explode in your hands (a bug zapper wouldn't have such energy).With this in mind, the answer is: don't zap your meter. If you do, the chances (but not guarantees) of a properly rated meter to survive are much higher due to the better design and quality of its components.As a side note, our regular meter zapper "joeqsmith" has an excellent channel and a very long thread in this forum about meter robustness. But even still, his tests are very controlled and limited to a short burst lasting only a cycle or two applied to a meter input - a zapper continuously generate these pilses and therefore with an increased chance of actually destroying something.Quote from: cigmas on April 24, 2021, 12:38:17 amThe Rel is a useful feature but, for your use in the lab, the HRC fuses are 100% optional. Worse, they cost more... Much more.Again this is related to the safety: in case you are working on a circuit capable of delivering ten thousand Ampères, the HRC fuses are absolutely mandatory, as they are rated to quench the arc flash generated at those levels. In a lab or even around the outlets of the house you would be hard pressed to find such power - unless you do experiments à la Photoinduction.Overall: get the Thsinde as it seems to work well for you and you like it. Just don't poke it on these zappersGood luck in your quest!

author: cigmas
author_rank: Regular Contributor
timestamp: April 24, 2021, 07:51:45 pm
content: It makes sense that safety is priority, but it does seem strange to me to use that rating as an implicit correlatation to input protection instead of more concrete specifications of the limitations. Maybe it is difficult to guarantee.Quote from: rsjsouza on April 24, 2021, 03:48:11 amLesson learned.I'm curious, what is your reason to suggest the Thsinde over the HT118A?

author: rsjsouza
author_rank: Super Contributor
timestamp: April 25, 2021, 01:13:12 am
content: Quote from: cigmas on April 24, 2021, 07:51:45 pmYou have a point but, whenever a specification is made in an industry consortium, many manufacturers chime in with their inputs. In this case, I suspect that someone like Fluke and their very generous price point would simply go over spec and keep their meters robust enough to withstand mistakes. Other companies could do "just enough" to cater to lower price points. The justification? You can't fully protect against stupidity.Quote from: cigmas on April 24, 2021, 07:51:45 pmWell, nothing in particular apart from the fact you really expressed your satisfaction with it in previous posts.In my own opinion, the HT118A lacks the REL button. Also, IIRC its bargraph is not well implemented (too slow).(Edit) part of a sentence disappeared

author: cigmas
author_rank: Regular Contributor
timestamp: April 27, 2021, 03:47:22 am
content: I see, thanks! I might just flip a coin to choose..PS. Found this extensive filterable list of reviews and disassembled pictures of many DMMs:https://lygte-info.dk/info/DMMReviews.html