I was an idiot and may have destroyed my multimeter, any way I can test it? - Page 1
Forum section: electronics / beginners

author: Cyber Akuma
author_rank: Regular Contributor
timestamp: September 20, 2023, 07:01:11 pm
content: I am sure most of you probably can tell from the title can already tell what happened but let me get into it. I have a Klein MM400.My father asked me to look into a charger for his wireless drill, he had already opened up the AC adapter for some unsafe reason and told me he thought the transformer itself was damaged. It has been a while since I last used a multimeter and had forgotten hot to properly use it, and stupidly plugged the lead into the 10A port. I tried checking if any DC voltage was coming out of the transformer but of course the display said nothing, so then I wanted to make sure AC voltage was even coming into the transformer.Yes, I am fine, the pop was thankfully at a distance from me on the transformer itself.The multimeter still turns on, I plugged the red lead properly into the other port and it still seemed to measure AC voltage ok. But since this multimeter was the most advanced electronics testing equipment I had in the house, and it's now possibly damaged, I don't know any other way I can test it's other functions to see if it's even still working properly. I don't even have any other cheaper multimeter to check it's fuses to see if they blew. I will need to replace the leads as the incident seems to have taken a small chunk out of the tip of the red one, but I have no idea if this is still working, if it's fuses blew but it's otherwise fine, or if it's permanently damaged now. Yes it's no $100-300+ Fluke, but it's no $15 Radioshack special either and I would like to try to see if it's still working properly before replacing it.Here's pictures of the inside:(Damn was it annoying to screw the PCB back into it after the photos, mainly the part where the selector dial is held in place by two tiny spring-loaded ball bearings that were very very easy to lose or mis-align, still not 100% sure I put that part back together right as the selector switch feels a bit stiffer than before)

author: WimWalther
author_rank: Regular Contributor
timestamp: September 20, 2023, 07:10:02 pm
content: If the 10A circuit is fused, you very likely took out the fuse. If no fuse, then it's extremely likely that SOMETHING blew out, either in the meter or the device under test.Is that xfmr still outputting voltage?Anyway, that piece of heavy, bare copper wire that looks like a W is probably the 10A current shunt. If it's not all blowed up & crap then the meter is probably just fine. But like I said, something very likely blew.. somewhere. So find & fix it.ETA: Only hi-end meters tend to have fuses for the high current range. Inexpensive units run bare and unprotected. A slip like you made typically blows traces associated with the high-current shunt off of the board, leaving ugly black burn marks.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 20, 2023, 07:11:40 pm
content: If the mishap happend at a relatively low voltage ( like < 200 V), chances are high that only the 10 A fuse is blown. The diode test of the meter should likely still work and could be used to check the fuse (out of the holder).A proper fuse may be expensive and one may use a lower rated fuse for a few initial tests - not to oder a $10 fuse for a broken meter.

author: wasedadoc
author_rank: Super Contributor
timestamp: September 20, 2023, 07:18:14 pm
content: Most likely the 10A fuse blew. The resistance ranges do not use that fuse. See if the resistance ranges are alive by switching to them and shorting the probe tips together. Should read zero or close to it. If so, remove the 10A fuse and put it between the probe tips.

author: WimWalther
author_rank: Regular Contributor
timestamp: September 20, 2023, 07:23:22 pm
content: Oh look, there's a big ol 10A fuse in there. Dang thing was covered by some notify on my screen.. =/It's likely shot.

author: Cyber Akuma
author_rank: Regular Contributor
timestamp: September 20, 2023, 07:25:08 pm
content: Quote from: WimWalther on September 20, 2023, 07:10:02 pmThre are two fuses in it (the white ones in the photo) one is listed as 600V 500mA and the other as 600V 10A, so it looks like it does have a fuse over the 10A. No visible signs of it having blown though, but it's not transparent.QuoteDidn't check, not sure if it would be a good idea to plug that thing back in after this. The reason it blew was because I was using the 10A port right? I can't imagine why it would not be causing a short or anything while it was plugged in but caused a bang like that as soon as I put the test leds on the AC side of the input wires. They were directly coming in from the AC jack itself so there shouldn't have been anything that got crossed.QuoteThe M-shaped one labelled R34 you mean? I notice there is a glass component connected to it, I think it might be a diode?Quote from: Kleinstein on September 20, 2023, 07:11:40 pmIt was US AC current, so yeah, 120V.So that diode is for the continuity test function?QuoteWell, is there a way I can test if it's working before ordering a replacement fuse without needing another multimeter?Quote from: wasedadoc on September 20, 2023, 07:18:14 pmSo basically use the resistance measurement as a continuity test to check the fuse? How can I be certain that function is still working though?

author: bdunham7
author_rank: Super Contributor
timestamp: September 20, 2023, 07:38:36 pm
content: The Klein MM400 is a perfectly OK meter and has reasonable 600V ceramic fuses. I'm 99% sure you just blew the 10A fuse, no need to disassemble it so far. Even the tip of the test lead can be touched up with a file, sandpaper or bench grinder.https://www.kleintools.com/catalog/test-measurement-accessories/fuse-6x32-10a-600v-mm300400

author: wasedadoc
author_rank: Super Contributor
timestamp: September 20, 2023, 07:44:30 pm
content: Quote from: Cyber Akuma on September 20, 2023, 07:25:08 pmIf the restance ranges show zero or near zero when the probe tips are shorted and OL or dashes or something similar when the tips are not shorted, they are working well enough to test the 10A fuse. If the fuse is OK it will give the same reading as when the probe tips are shorted together. (One probe in the Common jack and the other in the Volts/Resistance/Continuity/Diode jack.)

author: Gyro
author_rank: Super Contributor
timestamp: September 20, 2023, 07:50:55 pm
content: Quote from: Cyber Akuma on September 20, 2023, 07:25:08 pmVisible signs of a 10A 600V fuse having blown would be abadthing!

author: TimFox
author_rank: Super Contributor
timestamp: September 20, 2023, 08:03:32 pm
content: You should sit down before looking up the price of a proper 10 A replacement fuse, rated for DMM service.

author: Ian.M
author_rank: Super Contributor
timestamp: September 20, 2023, 08:31:08 pm
content: Do *NOT* attempt to test the 10A fuse in circuit - it must be removed from its meter, if you are using the same meter to test it.They are ****ing expensive because they are specially made to high quality standards with a reinforced tube and arc suppressing filler to break a high energy circuit without exploding (up to the meter's CAT rating), or at least to contain the explosion enough that it doesn't breach the housing and try to turn you into a crispy critter. Then of course there's the usual retail ripoff markup for anything sold in small unit quantities with safety certification!

author: WimWalther
author_rank: Regular Contributor
timestamp: September 20, 2023, 08:31:56 pm
content: Quote from: TimFox on September 20, 2023, 08:03:32 pmThe large HV fuses which protect meters like the Fluke 8050A are maybe even more expensive.. like $24 a pop, last I looked.Fortunately, the 8050A high current range is covered by a standard AGC 2A, common & cheap.

author: Zero999
author_rank: Super Contributor
timestamp: September 20, 2023, 08:32:45 pm
content: Remove the fuse and check it with the multimeter's continuity setting, which will work without the fuse in it.It's easily done. It's why I use a meter with an idiot alarm, which beeps if the lead is inserted into the current socket and the meter is set for voltage.

author: Cyber Akuma
author_rank: Regular Contributor
timestamp: September 20, 2023, 08:44:56 pm
content: Quote from: bdunham7 on September 20, 2023, 07:38:36 pmGood to hear it would be just the fuse that blew. Not sure how well the tip can be touched up though since it kind of took a small chunk out of it. But a new set of leads cost about half what the multimeter itself cost, so I can hopefully go that route instead of buying a new set.Quote from: wasedadoc on September 20, 2023, 07:44:30 pmYeah, the fuse is definitely dead. Resistance shows no difference as if I was not connecting the leds to anything, and the continuity test shows no continuity.Quote from: TimFox on September 20, 2023, 08:03:32 pmQuote from: Ian.M on September 20, 2023, 08:31:08 pmI checked, they're $10-13, ouch.And yes, I tested it outside the multimeter.Quote from: Zero999 on September 20, 2023, 08:32:45 pmWould have been a useful feature in this case. I checked and there is apparently a MM420 model that is basically the MM400 but with a warning if the leads are in the wrong sockets. Wonder why I didn't just get that since the price difference is small, either it didn't exist when I bought this or I might have read of some issues with the 420.So the 10A is just to measure current over 500mA and nothing else? I used the 10A because I was worried that the adapter was definitely going to be outputting more than 500mA. Does the current only matter when you are measuring current and not something else like voltage even if the current would be above 500mA?

author: Zero999
author_rank: Super Contributor
timestamp: September 20, 2023, 08:54:19 pm
content: Quote from: Cyber Akuma on September 20, 2023, 08:44:56 pmThe actual current flowing depends on Ohm's law:I = V/RThe meter will have a high input resistance, typically around 10M Ohms, so the current will be tiny. If the voltage source is 100V and can supply 10A, only 10µA will flow because of the high resistance.The current setting should never be connected to a voltage source. It will have a very low resistance, which is why it drew a huge current, causing a loud bang and the fuse to blow.

author: BeBuLamar
author_rank: Super Contributor
timestamp: September 20, 2023, 08:57:07 pm
content: Or you can simply use the meter without the fuse. Don't measure current with it and it would be just fine.

author: TimFox
author_rank: Super Contributor
timestamp: September 20, 2023, 09:01:18 pm
content: You can measure low values of current through the 10 A switch setting and 10 A fuse, but with lower resolution.However, as stated above,neveruse the 10 A connection to measure any voltage: the two panel connectors go directly to a very low resistance "shunt" to obtain probably 100 mV into the voltmeter circuit at 10 A (0.010\$\Omega\$), which is why there is a 10 A fuse there.On a typical DMM, the input resistance in voltage mode (using COM and VIN terminals) is 10 M\$\Omega\$, although some cheap ones are maybe 1 M\$\Omega\$ and expensive meters often have very high input resistance on the low-voltage ranges.

author: TimFox
author_rank: Super Contributor
timestamp: September 20, 2023, 09:03:12 pm
content: Quote from: BeBuLamar on September 20, 2023, 08:57:07 pmThis is why one often sees such meters at one's employer that don't work on the 10 A range (where a technician forgot to inform his supervisor what he did), along with ones sold through eBay at low prices.

author: bdunham7
author_rank: Super Contributor
timestamp: September 20, 2023, 09:16:04 pm
content: Quote from: Ian.M on September 20, 2023, 08:31:08 pmI'll agree that you should never test the innards of a meter with itself if any covers are off, but in the case of this and most meters with current fuses, simply running a single lead from the normal V+ input to the current input and then selecting continuity is a safe and reliable way of checking the fuse.

author: BeBuLamar
author_rank: Super Contributor
timestamp: September 20, 2023, 10:04:28 pm
content: Quote from: TimFox on September 20, 2023, 09:03:12 pmActually I blew the fuse in my work Fluke 87V often and I always replaced the fuse because I don't have to pay for it.

author: Veteran68
author_rank: Frequent Contributor
timestamp: September 20, 2023, 10:53:19 pm
content: Quote from: Cyber Akuma on September 20, 2023, 08:44:56 pmSince I don't think I saw this question addressed directly: the "A" settings on your meter are there to measure current (i.e. Amps), never voltage. Current measurements require your meter probes to be IN SERIES with the circuit under test, meaning you break the circuit and place the meter probes in line to complete the circuit through the meter. The big 10A fuse is on the 10A probe jack; since you have a 500mA fuse you should have another port labeled for mA or mA/uA which protects that jack when measuring small currents. Voltage measurements do not use the fuses.Placing probes from live to neutral when measuring current, as you would if measuring voltage, is directly shorting them together with only the very low impedance current shunt as a load. Pretty much the same as if you stuck a fork in an electrical outlet! Fortunately Klein are not bottom barrel meters so its damage is likely limited to the blown 10A fuse. Cheaper meters could potentially break down even more catastrophically (though unlikely at only 120V mains, as the energy isn't all that high compared to 230V or 480V or higher systems). If it weren't fused at all, the meter would be damaged probably beyond (practical) repair.No offense intended, but I strongly suggest you look up how a multimeter works, because this is basic DMM operational knowledge and without it, you might not only destroy equipment, but (at least with an unsafe meter) potentially hurt yourself, burn down your house, etc.

author: ledtester
author_rank: Super Contributor
timestamp: September 20, 2023, 11:48:47 pm
content: Recently I've seem some meters from aliexpress being shipped with a warning sticker over the current inputs. And after my dad blew out a cheap meter by making the same mistake I put a piece a tape over the 10A jack - at the very least it will serve as a reminder that extra care needs to be taken when using that input.

author: alligatorblues
author_rank: Regular Contributor
timestamp: September 20, 2023, 11:57:07 pm
content: A cheap and dirty continuity tester is a flashlight with the battery cover unscrewed and the circuit bridged by the questionable fuse.

author: JustMeHere
author_rank: Frequent Contributor
timestamp: September 21, 2023, 12:44:06 am
content: Pull out the 10A fuse that you probably blew. And since that thing doesn't seem to have a port for mA, also pull out that fuse. Wait awhile, possibly overnight. Give the thing the smell test. The chances you blew one or both fuses is a certainty. The chances you broke anything else is slim. There is no visible damage on the switch. The other functions are isolated from the amp measurement side.

author: Stray Electron
author_rank: Super Contributor
timestamp: September 21, 2023, 02:02:18 am
content: Quote from: alligatorblues on September 20, 2023, 11:57:07 pmI was going to suggest that to the OP. When I was young and started tinkering with electronics I made simple continuity testers by taking a bare piece of solid conductor copper wire (from a piece of Romex) and wrapped it around a D sized flashlight battery and bent a loop in one end to hold a flashlight bulb pressed against one of the battery terminals and then the other end of the battery and the 2nd end of the copper wire as my test points. It's crude but something like that would work fine for testing fuses and other low resistance items. Don't use something like this for testing anything with power applied to it! Other than perhaps a small dry cell battery.Not to be dick about it, but buying a few of those stupidly expensive fuses will teach you to double check your meter settings before taking any reading with it. Just be glad your meter had a fuse or you would be buying a new meter!I'll also suggest that you go down to Harbor Freight and buy one of their cheap $5 meters and use it for things that don't require precision or that involve safety related items. I got several of them when they used to give them away and they're surprisingly good meters. Mine has lasted me for over ten years so far. I keep one in my tool box and use it more than all of my GOOD meters combined.

author: Cyber Akuma
author_rank: Regular Contributor
timestamp: September 21, 2023, 05:27:19 am
content: Quote from: Veteran68 on September 20, 2023, 10:53:19 pmYeah that I know, the problem was I had it in voltage mode and tried to directly measure AC voltage through that port.QuoteWait, they don't? So the fuse was bypassed when I was using the 10A port to measure voltage?QuoteSo it did go through the fuse then?QuoteI used to, it's just that it has been a while.Quote from: alligatorblues on September 20, 2023, 11:57:07 pmQuote from: Stray Electron on September 21, 2023, 02:02:18 amYeah, I was thinking of trying that, but I am not sure where something like that even is. I used to have so many bits and bobs like that when I was younger. Come to think of it, are incandescent flashlight bulbs still being manufactured or were those banned alongside house bulbs? They're much easier to wire up than a LED for something like this.Quote from: JustMeHere on September 21, 2023, 12:44:06 amIf both fuses were blown wouldn't it not be able to measure anything at all? I was still able to measure 120V AC through the other port as well as test continuity and resistance.By the way, do you think my leads need to be replaced? The blast took a chunk out of the red one mostly, here are the best images I could get of them:(Imgur likes to resize these, would be best to go to the web page and then load the image file directly instead of try to view the tiny version it will try to wrap their HTML around)

author: Shonky
author_rank: Frequent Contributor
timestamp: September 21, 2023, 05:42:39 am
content: The 10A fuse isonlythere to protect the 10A range and only applies to the 10A socket.The 10A socket isonlyused to measure current, never voltage.If you try and measure a voltage with the 10A current range, you are short circuiting the voltage. Effectively you are putting a piece of wire across the voltage source. This causes a high current to flow and the fuse blows to protect the meter and to some extent it also protects the voltage source. It is only there to protect the meter though.With it blown only current measurement on the 10A range will no longer work asusming nothing else was damaged when you short circuited the power supply which is likely. The voltage, resistance, diode ranges etc will all continue to work.There are no fuses on the other ranges like voltage as they are a high impedance and therefore a large current can't flow.I also suggest looking for primer on using a multimeter and in particular the difference between measuring voltage (in parallel to the source) and current (in series).

author: Ian.M
author_rank: Super Contributor
timestamp: September 21, 2023, 10:21:31 am
content: The red lead is definately salvageable. Regrind the tip to a conical point without going back too far, preserving the groove accessories latch onto. Rotating the tip at an appropriate angle against a flat stone in a Dremel on low speed will do a decent job of it. In fact you just inspired me to do exactly that for the red probe from my previously owned Fluke 21 that I recently got cheap because the previous owner had made the same dumb move, and while checking it out, I was able to show the pawn shop that the 300mA range was dead. Fortunately for me, the Fluke 21 in question takes a regular 20mm 630mA HRC fuse . . .I'm tempted to replace my Fluke leads because the red one has a couple of divots out of the side of its tip, but aren't going to as once the edges were smoothed slightly, they don't affect function, and are a perpetual reminder to me to check range and connections before probing.If you work with big motors or other equipment with high inrush current, you may want to treat yourself to a cheapish CAT III rated AC/DC clampmeter, to save on the cost of 10A range fuses! Clampmeters aren't the best for accuracy as conductor positioning within the jaws and jaw core end surface condition can affect the reading a bit, but are far more than good enough to determine if its safe to wire in a more accurate (but delicate) meter. Don't buy bargain basement imports (because fake ratings) - you at least want one from a recognisable manufacturer that is invested in maintaining the credibility of their brand.

author: Veteran68
author_rank: Frequent Contributor
timestamp: September 21, 2023, 12:35:02 pm
content: Quote from: Cyber Akuma on September 21, 2023, 05:27:19 amYes, but you cannot measure voltage (or the other non-current modes) through the current jack. Regardless of whether you INTENDED to measure voltage, your meter was configured to measure current despite what position the dial was in. Some meters attempt to protect you from a mismatch between the measurement range selected and the jacks being used, by either physical means (shutters that close the amp jacks in voltage modes, and vice-versa, and physically block the dial from selecting an incompatible range) or electronic means (such as a loud beep, flashing lights, or a warning message on the display). Unfortunately, the majority of meters, especially the low-cost ones, will not prevent you from making this mistake. The only safety system you can truly rely on is the one between your ears.Quote from: Cyber Akuma on September 21, 2023, 05:27:19 amNo, the 10A fuse is ONLY used for the 10A jack. You cannot measure voltage through the 10A jack, it is for measuring current. The voltage jack bypasses all fuses.As we're explaining, the current jack places only a low impedance shunt (essentially, a short piece of wire) and a fuse between the probes. That's it. So using the probes in parallel with a circuit, as you would when measuring voltage, is shorting the circuit and allowing maximum current to flow. Which blows the fuse (or minus a fuse, blows up something in the meter if not the meter itself). Again, to measure current you must break the circuit and put the probes in series with the circuit, not in parallel like for a voltage measurement. Or else use the aforementioned current clamp, which is by far easier if accuracy isn't critical.The voltage jack has a high impedance and does not need a fuse to limit current.Quote from: Cyber Akuma on September 21, 2023, 05:27:19 amYes. That's why the fuse is blown.Quote from: Cyber Akuma on September 21, 2023, 05:27:19 amI understand. But this is fundamental knowledge and based on the conversation here, you're still figuring it out. I think all of this will become clear to you if you spend some time on some basic education in DMM use. There are countless articles and YouTube videos on the subject. Some basic meters (like the Fluke 101, still a Fluke but inexpensive and made for simple use) don't even have a current jack, as honestly for most homeowner use the need to measure current is an uncommon one compared to voltage. They are the safest to use because you can't make this mistake with them.And to be clear, I'd say virtually ALL of us, even seasoned, professional engineers and electricians, have made this mistake at least once in our lives. However it isn't typically due to a lack of understanding of how the meter works, but rather due to carelessness in not paying attention to where the probes are plugged in as you're switching between different measuring modes. You switch the dial but forget to change the probe jacks, or vice-versa. So we're not judging you for it happening, we're just admonishing you to learn a bit more about your tool and what you're trying to do with it, as it's potentially very risky to your person and property to not have a fundamental understanding of how DMMs should be used.As to the damage to the probe, it looks minimal and entirely salvageable. Even if it weren't, probes are not expensive to replace. And you don't HAVE to buy replacement Klein probes. You can buy entirely serviceable probes for less than $10.