Sniff.  I killed another DMM :( - Page 1
Forum section: electronics / beginners

author: paulca
author_rank: Super Contributor
timestamp: April 03, 2023, 02:03:14 pm
content: Probing an active power source while the meter is in "Ohms" will do it.Lost my DC Volts and Ohms and everything else is now suspect.Thankfully it's been a few years since my last one, although it's the first time killed one this way, normally I overcurrent the Amp range and melt it with a short.I took it apart. The damage appears to have occured on the range select switch, looks like that was the high resistance point where it got hot, melted the contact copper into the knobs plastic holding for it. Trying to bend and/or reposition the contact failed. Looks like it got hot enough to have melted the solder mask when it was rotated as it was fouled with green solder mask. The main issue though is it's about 3mm shorter than it should be now.Multiple areas of plastic melting were found in addition, including the battery compartment, although there was previous damage in there from leaking AA cells.Time for a new meter.Not been a good week (again!) on Saturday I accidentally powered on my audio project, completely failing to notice it's power supply board was sitting on top of the MCU (not insulated). 2 minutes later when I moved the PSU board I got burnt. Then when I stupidly put my finger on the MCU to find out why it was drawing 1.6Amps I found out why. I have the white blister on my finger to prove the MCU is, in fact, dead. It not has a tattoo of my finger print on it too.

author: shapirus
author_rank: Super Contributor
timestamp: April 03, 2023, 02:05:50 pm
content: What meter was it? It shouldn't happen. A good meter must survive high voltage (meaning AC mains) input in any mode.

author: obsoletepower
author_rank: Contributor
timestamp: April 03, 2023, 02:20:16 pm
content: I feel for you Paul....but if you're doing things like probing AC mains you should not risk doing so using an inadequate multimeter for your own safety most importantly. You clearly enjoy high voltage applications and my advice is take this as a sign that you should be extremely cautious and check things 10 times if you have to for proper isolation and make sure the surfaces you are grabbing/touching are not live. I cannot stress enough how one mistake can send you to the hospital or worse.

author: bdunham7
author_rank: Super Contributor
timestamp: April 03, 2023, 02:31:19 pm
content: Perhaps you could try a Fluke 101 or 116 as they are completely impossible to kill with mains voltage or below regardless of the level of user stupidity.

author: paulca
author_rank: Super Contributor
timestamp: April 03, 2023, 03:03:28 pm
content: Quote from: obsoletepower on April 03, 2023, 02:20:16 pmI know. I spent ages making sure it was okay to probe the 240V. That was successful. It was the after thought to check the frequency. I rotated the MM dial, did not double check it, stuck the probes in. The device under test's relay toggled a few times and I pulled the probes out, knowing something was wrong, decided not to reattempt it. It was only later I discovered the meter was dead. Reads random AC voltages on any voltage setting and doesn't even switch into the ohms range anymore.It would seem that I did not rotate the dial to Hz but to the setting prior, Ohms. I believe if I'd paid attention, I could have just pressed the "Hz" button while in AC Volts and it would have worked.Yes. I have taken that onboard as to just how simple a mistake can be. Plan and then DO ONLY WHAT YOU PLANNED. No extensions, no "Just while Im here"The meter may not have survived, but at least it didn't make any drama during the affair either.Itiswas an Extech Ex350. £75 worth

author: paulca
author_rank: Super Contributor
timestamp: April 03, 2023, 03:12:33 pm
content: I was going to buy one of these anyway, for the current clamp.https://www.amazon.co.uk/UT210E-Current-Meters-Capacitance-Tester/dp/B00O1Q2HOQ/Any reasons this won't do me as a "general" meter anyway?I'd love a bench meter, but alas, other than an AliExpress job I don't have the budget and I don't have the time patience or risk apitite to shop second hand.

author: themadhippy
author_rank: Super Contributor
timestamp: April 03, 2023, 03:42:31 pm
content: QuoteIf you double up on the price of the amazon meter you can pick up an owen clone from cpc

author: shapirus
author_rank: Super Contributor
timestamp: April 03, 2023, 03:49:56 pm
content: Quote from: paulca on April 03, 2023, 03:12:33 pmJust maybe not as convenient as a regular DMM, and it has only a basic set of functions.Should be nice as a clamp though, and it has three ranges, the lowest of which is just 2A, which may be very useful. No min/max/peak functions though, that's a downside.

author: paulca
author_rank: Super Contributor
timestamp: April 03, 2023, 04:09:29 pm
content: I have 2 oh-so-nasty meters, even one of the obligatory yellow plastic ones! They will do for the LV DC stuff I need to do tonight, which is mostly "Is there 12V?" and that's about all I need.So I'll do a bit of shopping. Before I need to measure anything with something I need to trust.

author: shapirus
author_rank: Super Contributor
timestamp: April 03, 2023, 04:21:57 pm
content: Quote from: paulca on April 03, 2023, 04:09:29 pmOne reasonable approach is to get one good meter from a trusted brand (where I will of course recommend the BM869s as being the sweet spot in this category for its very reasonable price) to be used, among other, as a reference, and then, as you need them, additional specialized devices such as a current clamp or cheap meters such as Aneng to use when you need a second DMM.The "reference" meter can be a bench meter, but unfortunately trusted ones come at a much higher price than DMMs, unless you find a good second-hand offer.

author: obsoletepower
author_rank: Contributor
timestamp: April 03, 2023, 04:28:04 pm
content: Quote from: paulca on April 03, 2023, 03:12:33 pmA bench meter is great...if you have the budget for it and if you already have a good handheld one. My advice, get yourself a good, used handheld meter (previous post mentioned the Fluke 101 and 116 which are good but won't do for LV/LC applications). What is your budget so I can recommend something that would be appropriate?

author: paulca
author_rank: Super Contributor
timestamp: April 03, 2023, 04:44:35 pm
content: I don't think I am at the stage to afford a proper reference meter.What I have done previously is use more than one, cross reference new voltage meters against the others. If they all read "Good enough". It's fine. If one or more of them read differently, there is something to ponder before proceeding.Prime example. The LFP battery pack. When I hooked up the cell monitor to that I went through each cell and checked it with the Extech. They were within 10mV of each other across the range, "Grand. Not great, not terrible". 1/10th the cell voltage tolerance. I do similar with battery chargers. Annoyingly my what-would-be best battery charger removed the calibration from it's settings menu, so I have to accept the 100mV safety/nanny over reading.Probably the most precise measurements I would need to make would be around the feedback in Buck IC circuits. Call that ~1mV. I tend not to dabble too much in percision stuff. As soon as any greek symbols appear I go looking for pre-canned components.The attraction of the bench meter is the fact it can't move about. The fact it's always there. Always has test leads of various types connected. Always read to press a button and get a reading or just hook it up to a circuit and leave it connected while you work. Multi channel.Accurate? Precise? I'd take trustable enough and cut a 0 off the price.Second hand fluke is an option for the hand held. Thanks.For the bench meter. How bad, given my requirements, can a cheap AliExpress "big colour LCD" 2/3/4 channel bench DMM be? (Note I haven't looked for examples yet). EDIT: I always want one to stop the temptation of using the scope for that purpose!

author: BeBuLamar
author_rank: Super Contributor
timestamp: April 03, 2023, 04:55:18 pm
content: I have made all the mistakes that you did. Measuring Ohms on a power source, trying to measure voltage when the lead in the current socket. Nothing happens on the Ohms and just lost a fuse in the Amps.

author: paulca
author_rank: Super Contributor
timestamp: April 03, 2023, 05:31:13 pm
content: Quote from: BeBuLamar on April 03, 2023, 04:55:18 pmFrom looking inside the meter the copper contacts for the function rotary switch on the outer set has taken current, heated and mis-shaped the plastic such that those contacts no longer make contact ... destroying any of the functions which require those... and that's before we even get too look at what else is dead.It is likely due to this rotary contact switch being the highest resistance point in the circuit, it produced enough heat to disform the plastic in the 1 or 2 seconds inserted. The fuse may also be blown, but it's reading garbage and not selecting the proper function ranges. I could ask Extech for a replacement dial component and repair that. Then find the fuse is blown and fix that and find the problems go further.Maybe a future project, but like in relationships, when it's over, you know.

author: paulca
author_rank: Super Contributor
timestamp: April 03, 2023, 05:46:50 pm
content: Wow.I had 2 choices in the bottom of the odd tools bin.A Schieder Electric meter... or a cheap plastic unbranded "black" random meter.I went with the former, as it looked like a much more stubstansial product."They are all much the same, right?"Well, after a small "Oh shit" moment and it telling me my lead acid charge voltage was 16.33V and again directly across the PSU charging it's outlet it was 16.80V.... nope. Not even close.Got the other random black meter and it at least agrees with the battery charger.... 13.57V for a PSU indicated of 13.60V.... and it says the USB supply is 5.10V as opposed to 6.3V from the scheider electric branded one.Saved by a £3.99 multimeter eh.Should I microwave the schieder for fear someone might use it again or should I open it to find out if it has a calibration pot at least?

author: james_s
author_rank: Super Contributor
timestamp: April 03, 2023, 05:55:05 pm
content: Buy a Fluke and it will be the last meter you ever need. I've had mine for many years, haven't killed it yet.

author: JustMeHere
author_rank: Frequent Contributor
timestamp: April 03, 2023, 06:08:14 pm
content: According to your spec sheet, you have 600 v of overvolt protection.

author: paulca
author_rank: Super Contributor
timestamp: April 03, 2023, 06:13:55 pm
content: Quote from: james_s on April 03, 2023, 05:55:05 pmThe 101 or the 106 are in the ~£100 range. Simple, but built like a brick **** house and trustable.On the bench meter. There are "cheap" options for around £120. Typical brands like Owon.Is owon as bad an option for a bench DMM as it is for a scope?

author: shapirus
author_rank: Super Contributor
timestamp: April 03, 2023, 06:14:47 pm
content: Quote from: james_s on April 03, 2023, 05:55:05 pmOne can never have too many multimeters

author: paulca
author_rank: Super Contributor
timestamp: April 03, 2023, 06:15:35 pm
content: Quote from: JustMeHere on April 03, 2023, 06:08:14 pmIs that not protection for "me" rather than the meter?Also. I am not exactly sure what effect the "dugga dugga" of the inverter tripping rapidly had on the instantaneous output voltage. The meter didn't survive!I expect if the "fault" relay "dugga dugga'd" clicky clicky. Like the range relay going nuts in a PSU in current limit.... on a FET bridge->transformer, might produce some over voltage. I'd actually hope it doesn't, I'd like to hope the inverter is a bit better than that.Anyway, to me it doesn't look like over voltage, it looks like current flowed through the rotary contacts melting the plastic structure. No flash damage, no soot, no skid marks. Just melted plastic.It is available for post-mortem... only I don't have a reliable meter to diagnose with.

author: bdunham7
author_rank: Super Contributor
timestamp: April 03, 2023, 06:53:11 pm
content: Quote from: paulca on April 03, 2023, 06:15:35 pmA question with different answers depending on who you ask, apparently. According to Fluke, the meter should survive the rated voltage (not the transient tests) unharmed on any range.QuoteSo in contrast to your inverter which seems to have cut out due to overload, what if instead you had the connected the meter to a 3-ph 480V service panel with 10,000A fault current available? 💥

author: paulca
author_rank: Super Contributor
timestamp: April 03, 2023, 06:56:24 pm
content: Quote from: bdunham7 on April 03, 2023, 06:53:11 pmThen I would be hoping the CAT rating on the case etc would limit the amount of flash damage I might receive and most of the meter guts stay inside.

author: bdunham7
author_rank: Super Contributor
timestamp: April 03, 2023, 07:14:52 pm
content: Quote from: paulca on April 03, 2023, 06:56:24 pmThat's a lot of hoping. If the PTC they used (if they even have one?) can't even block the power from your inverter, there's nothing in that circuit that is going to be able to interrupt a high fault current. You'll be hoping that the circuit breaker on the mains circuit trips before you get third degree burns. That meter and its 🐱-rating isn't going to help you.Can you post photos of the board?

author: Eraldo
author_rank: Regular Contributor
timestamp: April 03, 2023, 08:13:44 pm
content: Plz post some pics of the places where the magic smoke escaped. There is something charming about it.Also as the others have already said it's strange the meter failed in the ohms range. Usually it has 250 - 300 v protection.Anyways do post some pics to analyze and see the built quality of the multimeter (it's interesting)

author: paulca
author_rank: Super Contributor
timestamp: April 03, 2023, 08:17:57 pm
content: Quote from: bdunham7 on April 03, 2023, 07:14:52 pmGive me time.-----I've been around a while. I have made far worse mistakes than this. (I do not see that as an excuse to take risk, but the opposite. Never rely on luck. The more lucky you feel or have been the more vulernible you are.)I openly accept my mistake. I got lucky. The meter is not at fault, I am. The meter did not make me pay badly, just financially, I would say it did fine. In my limited requirements.----Before I started back into the electronics hobby, actually one of the reasons I did. I had a voltage and current monitor circuit for a 13 Amp boiler running via a triac. All bought based on a forum post on a home brew forum. The trouble was, there were two different versions of the voltage current monitor. They had different instructions and different wiring diagrams. Without a clue which was which I decided to go with the instructions that came with MY unit. Nope. Bad idea. That put 240V straight across the current coil inputs. The circuitry in that part of the circuit instantly vaporised and plated the inside of the case with a lovely metallic copper finish. Unfortunately it also sent white hot jets of electronics out the air vents and melted the landlords carpet.In the end, other than a flash, a bang and the trip switch popping, nothing, including the 13A plug fuse was blown.Just a burnt carpet and an interesting copper plating of the plastic. (the electrolytic?)The difference there was, I was standing well back, expecting either "working" or "FLASH BANG". The carpet I didn't expect.A really dodgy meter... Would be just like holding that thing in my hand when that happens.I imagine a hot 240VAC voltage flowing through the wrong circuit in a DMM would result in similar result.

author: paulca
author_rank: Super Contributor
timestamp: April 03, 2023, 08:22:24 pm
content: Quote from: Eraldo on April 03, 2023, 08:13:44 pmThere was no smoke!However, the more screws I removed from the meter the more it smelt of "hospital", strong antiseptic. TCP?I will promise some photos.

author: Gyro
author_rank: Super Contributor
timestamp: April 03, 2023, 08:30:32 pm
content: This is where having an electrician's CAT ratedtwo polevoltage tester at hand comes in really handy. DMMs (or their operators) are too error prone for live stuff. By all means use a DMM, but use a tester to prove dead first.

author: Jwillis
author_rank: Super Contributor
timestamp: April 03, 2023, 08:45:26 pm
content: Quote from: shapirus on April 03, 2023, 02:05:50 pmI would like to know what brand it was to so i can avoid it. Never had a failure in a meter when voltage is applied in the Ohms setting.Destroyed one with over voltage once . But 3kV will do in just about any DMM.

author: AnalogTodd
author_rank: Regular Contributor
timestamp: April 03, 2023, 08:53:40 pm
content: Quote from: james_s on April 03, 2023, 05:55:05 pmThe labs at the semiconductor company I worked at for decades had Fluke 87 (various generations) sprinkled around like crazy. I bought one for my home use, and gifted one to my son as he got into his job (IT, both keyboard and hardware side). As said, these things are pretty much impossible to kill, hence the lab manager purchasing them for our use. Believe me, there are plenty of engineers and techs who can kill equipment without much trying.

author: theHWcave
author_rank: Regular Contributor
timestamp: April 04, 2023, 08:13:01 am
content: Flukes are expensive. I would go for a Brymen, built at least as good and generally more capable on the same price point. As a beginner meter, I always recommend the BM235. Built like a tank. I have used mine for years now. £71 here for example (no affiliation)https://telonic.co.uk/product/brymen-bm235-professional-multimeter-cat-ii-1kv-cat-iii-600v/. I bought my Rigol scope from them and a BM869s and their service was good.

author: mwb1100
author_rank: Frequent Contributor
timestamp: April 04, 2023, 08:18:34 pm
content: This teardown of an Extech 350 shows several PTCs and MOVs on the input as well as 900K (through three 300K SMT resistors) on the input:-https://www.eevblog.com/forum/testgear/extech_s-new-ex350-a-first-look/msg959909/#msg959909Should that have handled the situation better? Is there some defect in the input protection design on the 350? Maybe the SMT resistors aren't up to the task?

author: james_s
author_rank: Super Contributor
timestamp: April 04, 2023, 09:53:49 pm
content: Might be worth contacting the manufacture, they may be willing to replace it.

author: Swainster
author_rank: Frequent Contributor
timestamp: April 05, 2023, 01:35:55 am
content: FWIW, the only DMM I've personally owned from new which failed is a fluke. That said, I've seen quite a few unit-t DMMs fail on the job, though not while I was using them.Regarding the fluke (115), it failed while on the shelf - one day I turned it on and it was showing cal-err and was totally unusable. As such, I'd say go with the brymen. At least you won't be so annoyed if it decides to spontaneously fail on you. To fair to fluke, I also have a couple of decades old models which have never missed a beat.

author: paulca
author_rank: Super Contributor
timestamp: April 05, 2023, 09:08:29 am
content: I went for the Brymen BM235.

author: paulca
author_rank: Super Contributor
timestamp: April 05, 2023, 12:38:05 pm
content: Does the following link work? Post-mortem photos.https://photos.app.goo.gl/Jt4soTyt2fnhkUp1AI measured the fuses. 2 Ohm and 1.2 Ohm according to my £10 stand in.I measured the input resistor network, 890K Ohm.Given where the rotary knob melted, if it melted during the "incorrect use", it would have been over on the A / mA portion of the dial. There are visible marks and damage to the solder mask in that area.However. That melted spot is also directly under the battery compartment. (EDIT Just on the complete otherside of the board Paul!) The meter has already suffered cell leakage damage (which is when I oddly lost the continuity buzzer), but the meter was cleaned and worked for 3 years since.I did also replace a pair of Duracells in there that "felt" a bit waxy to the touch and a few flakes of white crust... about a month ago.Is it likely the "incorrect use" was just coindidentally on the slow progression of battery acid eating the rotatory dial structure?Why then the strong smell of anti-sceptic. I mean it "stank" on opening. Very obvious. Is that what battery acid eating rotary dial and solder mask smells like?On warranty, given the previous leakage damage and the meter being 5 years old. The best I'd foresee is just being nice and asking Extech if they want the meter back for fault mode diagnosis. I doubt I'd get a replacement.

author: paulca
author_rank: Super Contributor
timestamp: April 05, 2023, 01:02:19 pm
content: I did call ExTech to see if I could get a replacement rotary switch."We don't sell parts for that meter. You could return it for service and repair and that "might" be possible."That's a dead end. Best case I spend the money to return it. They tell me it's a goner and offer me 30% of a new one.So it's eWaste or make my own contacts.

author: joeqsmith
author_rank: Super Contributor
timestamp: April 05, 2023, 01:04:33 pm
content: Note how the traces connecting the pads of the rotary switch route such that the contact run over them. They have cut through the mask to copper. Also note how they placed the vias centered on the pads, right inline with the contacts. Turning the switch with the meter attached may have done it in.Enjoy your new meter.

author: Jwillis
author_rank: Super Contributor
timestamp: April 05, 2023, 08:25:47 pm
content: Quote from: paulca on April 05, 2023, 01:02:19 pmI am really surprised its an Extech . Considering the price one would expect a lot more. Think I'll stick to Mastech , and Brymen . Have one Fluke and its a nice meter but to much for my budget to get a second .

author: BeBuLamar
author_rank: Super Contributor
timestamp: April 05, 2023, 10:48:23 pm
content: Quote from: Swainster on April 05, 2023, 01:35:55 amI have encountered several Fluke meters that failed. Those include the 116, 87, 189 2 of them, 287 3 or 4 of them.

author: Swainster
author_rank: Frequent Contributor
timestamp: April 06, 2023, 12:30:21 am
content: Haha, the only other fluke I've bought new is a 189. Although it hasn't failed yet, it seems like I'm good at picking the 'exceptions that prove the rule'.BTW, running through the calibration procedure got the 115 running again, but improvising 600VAC required a bit of improvisation. Presumably some eeprom data had been corrupted... while sitting on the shelf (or more likely when first turned on with a low battery after sitting on the shelf for a few months, though there was no battery leakage).

author: alm
author_rank: Super Contributor
timestamp: April 06, 2023, 12:34:50 pm
content: Quote from: Swainster on April 06, 2023, 12:30:21 amThe plural of anecdote is not data. There have been a *lot* of Fluke meters produced over decades, so if you look in the right places I'm sure it's easy to find many broken meters. Brymen has been around for much shorter and produces in much smaller volumes, so naturally there will be fewer broken Brymen meters around.All brands have failures. To measure the failure rate you need access to data on a much larger, less biased, sample than any individual is likely have access to unless they work at a large distributor and manufacturer, and are probably not allowed to talk about it.

author: Swainster
author_rank: Frequent Contributor
timestamp: April 07, 2023, 06:32:15 am
content: Totally agree! Just to be clear, my remark was 'tongue in cheek'. In fact, I am more of a fluke enthusiast than a detractor, as the pride of my TE collection are both Flukes. That said, my personal opinion is that if you are buying new, Brymen DMMs offer better bang for buck for general hobby/DIY use.

author: dietert1
author_rank: Super Contributor
timestamp: April 07, 2023, 06:57:57 am
content: Note the position of the Hz range next to the V~ range on the BM869 recommended above. If we want to learn something from all this..Regards, Dieter

author: paulca
author_rank: Super Contributor
timestamp: April 07, 2023, 08:55:53 am
content: I want to point out. The meter was not connected to the test device while I rotated the dial.

author: David Hess
author_rank: Super Contributor
timestamp: April 07, 2023, 04:03:55 pm
content: The ohms mode on better multimeters will withstand hundreds of volts without damage.My Beckman RMS225 specifies overload protection of 500 volts DC or AC RMS for all ohms ranges. My Tektronix DMM916 specifies 850 volts peak for all ohms ranges. My very old Tektronix DM501 is protected by a 1/16 amp fuse, which I replaced with a pair of 28 volt grain of wheat bulbs, but the slightly later DM502 is protected to 130 volts DC or AC RMS continuously, or 250 volts DC or AC RMS for 1/2 hour, using a very small 120 volt filament lamp.

author: paulca
author_rank: Super Contributor
timestamp: April 10, 2023, 05:46:38 pm
content: Been using the new BM235 for a bit now.Found a few features I love. The fact the auto power off is "smart". So if you leave it monitoring a battery voltage, it stays monitoring it.Also, if you enable the "REC" function for MAX, MIN, AVG, it beeps every time the voltage changes. That can be very useful when charging, say, lithium cells and a voltage rise might be nice to "hear". Additionally it it starts beeping rapidly, the voltage is changing rapidly.The only downside with that feature is the audio beep times out after about a minute. It did give me an idea for adding sound to battery chargers and BMSes. So you CAN take your eyes off them when abusing them.