Multimeter exploded during ACV measurment - What did I do wrong?

Question

^{simulate this circuit – Schematic created using CircuitLab}

A DT-830B meter.

I bought a new transformer, and I was trying to measure the output voltage. I plugged the probes on 'VΩmA' (not 10 A) and COM. and set it to 750(not 100% sure if i put it on 750 or 200) ACV.

Then I put my probe on the right side of the picture, which is the transformer output, and I got no numbers reading.

Then I wanted to check if the outlet was working right, and put the probe like the picture. My outlet has only two outputs, with no neutral/hot labeling, just two of them and they are 60 Hz 220 VAC.

Anyway, as I put the probes like the picture, the multimeter made zipping noises and it didn't display numbers. Maybe it was shorted inside? I put it like the picture again, and the fuse exploded.

Did I do anything wrong? I didn't think that I needed to swap to 10A because (I thought) it's only used when measuring currents. I just wanted to measure voltages.

Can you tell me what i did wrong?

• oh, and I actually did put the probe on voltage measurement terminal.. It says VΩmA. I put my probe on VΩmA and COM.

Also I dont remember where I bought it, but the multimeter says DT-830B, and no make printed.

Plus, I think it was under 10 dollars.

Well, some of you wanted the inside of that meter. So I'm uploading some of the pictures. The inside looked simpler than I expected...

Answer 1

Ok, let's first get some things out of the way that may have to do with misapplication of a multimeter...

Depending on the exact type of multimeter you use, your mileage may vary, but here's my guess about what happened, assuming your multimeter has separate inputs for current and voltage measurements, often labeled "[mA] [A] [COM] [V,Ω]" or something along that line...

No matter how you set the dial, if you don't connect the leads to the "Volts" input (and to any of the "Amps" inputs instead), you connect your multimeter's internal current sense resistor (shunt) across your transformer's output. This means, in basic words, you are creating a near-short across your transformer, and any (usually large!) current your transformer is able to deliver will rush through your poor multimeter.

Hmmm... considering your edits/clarifications... The multimeter should not become damaged if you connect the probes to "COM" and "V-Ohm-mA" and put the dial to any of the "Volts" positions. With any other setting (Ohm, Amps), you put the multimeter's current sense resistor (shunt) across your transformer's output (bad!), or the current source that your multimeter uses to test resistors will try to drive against the transformer's output (and it will find out that there's no way to win in this fatally hopeless situation).

Since you mention (in a later edit) that you can pretty much rule out any of these issues, there is of course a (somewhat rare & remote) possibility of a fault within the multimeter, and we're looking at this now...

The layout of the traces, and of any wires and components inside the multimeter must of course be designed to withstand the voltages they are exposed to during normal operation and allow for some safety margin. The pictures you edited into your question look like your multimeter may actually have contained a small spark gap because of horrible manufacturing skills - one gets what one pays for...

Here's a picture of a spark gap you can buy if you need controlled breakdown properties:

(Source: Wikipedia)

Here's a picture of a possible spark gap that no one actually wants ;-)

It appears that the three wires used to connect the main board and the banana socket board are (i) soldered with horrible quality and, more importantly, (ii) should have been clipped before the assembly was put into the enclosure. I guess the two top wires may have become bent while the instrument was put together and were really close to each other. Once you applied your transformer's voltage to the terminals, you probably ended up causing sparks between the wires. Note how the [10A] jack is connected to the [COM] jack by the shunt resistor (the big thing that looks like a U-shaped wire), so the middle wire can cause arcing to any of the two outer wires. By the looks of it, you had sparks between the top and middle wires, because there are little balls left from the arc's heat (sorry, i can't find an English word for Schmelzperle, maybe someone can edit).

So, yes, there is a possible piece of evidence that you used your multimeter the right way and you actually observed a fault caused by bad manufacturing.

What to do now?

Given you are a trained electrician (disclaimer, disclaimer ;-), you could clip the wires, fix the bad soldering, re-assemble the multimeter and chances are it will still work, maybe even better than ever before ;-)

It might be a very good idea, though, to limit the use of your repaired multimeter (or any similar model) to safe, low-voltage measurements, because it's worth considering...

Some notes about safety

Just like there's a direct, low-resistance path between [COM] and [10A], there is also a connection between the transistor socket and the three inputs on the bottom right. You can download a report with impressive pictures and a short video from the website of a German authority. The text is German, but the pictures tell the story pretty well. Since it's a publicly available report issued by a government agency, I have taken the freedom to copy two pictures.

One shows a very bad idea - do not attempt to try this at any time, neither at home nor somewhere else:

Another one shows an explosion probably caused by a cheap fuse not capable of breaking large currents. Note the giant transformer in the background, such impressive "boom" can usually not be achieved on a domestic outlet. However, if you subject a multimeter to DC (as when testing, say, a computer's switching power supply), arcs will sustain (because the current has no zero crossing like in AC). Note how your multimeter developed an internal spark even though you used it correctly, because it lacked the proper clearance and creepage distances. With DC, the spark might turn into an arc and indeed cause a fire, maybe even right in your hand holding the meter.

Again, pictures taken from Hessisches Ministerium für Soziales und Integration

Answer 2

When measuring voltage your probes must be connected to the jack labeled "ACV" and to COM. The Multimeter must be set to "ACV" as well. When you connected one of the probes to the 250mA jack and measured across 220V you put 220V directly across the shunt resistor used for measuring current. Here's a simple diagram of how the multimeter measures current:

The probes are represented by the "dots" at the top and bottom of the image.

A multimeter actually can't measure current exactly. Instead it measures the voltage across a known resistance (the shunt resistor) and calculates the current using Ohm's Law. Generally the 250mA measurement has a shunt resistor value of approximately 1 ohm (though it will vary depending on which meter you have). Let's assume it is 1 ohm though. You connected 220V directly across it, which means that based on Ohm's Law, 220V/1R = 220A tried to flow through it. At that current the shunt would need to dissipate 48.4kW (\$I^2 \times R\$, or \$V \times R\$). That's not happening, and it would have melted long before that. When measuring voltage, ALWAYS make sure your probes are connected to the voltage measurement jack. Also, when you measure current, make sure to put your meter IN SERIES with the load, not across it.

Answer 3

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1 shows what you did. You connected a milli-ammeter (250 mA) across the mains. It probably has a resistance of about 1 Ω which, by Ohm's Law, would cause a current of \$ \frac {V}{R} = \frac {220}{1} = 220 A\$ to flow very briefly before the fuse blew. Meanwhile the digital electronics which was expecting to see about 250 mV across the 1 Ω resistance (the shunt) would have seen almost mains voltage. This almost certainly damaged the electronics unless it is a high quality meter with excellent protection.

Figure 2 shows what you should have done. i.e., Switch the meter to volts and use the V and COM sockets.

^{simulate this circuit}

Figures 3 and 4 show a hypothetical multimeter circuit. The meter is full scale when 250 mV is placed across its terminals.

• To use it as a 250 mA ammeter we use a 1 Ω shunt resistor, measure the voltage drop across the resistor when current is flowing through it and read that off as mA. Placing 1 Ω across the mains causes a very high current to flow.
• To use it as a 250 V meter we need to divide the voltage by 1000:1. This we can do with a pair of resistors. In this case I have chosen the pair to give 1 MΩ total resistance - similar to many digital meters. At 250 V across the probes the voltage is divided down to 250 mV across the meter.

Figure 3 circuit will blow the meter. Figure 4 circuit will work and survive.

Answer 4

If your meter is of the type where 250 mA and Volts/Ohms are the same jack (you hadn't said that, but you had implied that in your description, and I, at least, am familiar with that sort of set up), it's just a junky meter that couldn't take 220 Volts, assuming you did, in fact, have it already set to measure AC Volts on a range suitable for 220VAC when you connected it.

Some "inexpensive" meters are also "cheap" in the low-quality, not actually suitable for the task, sense. You may want to shop a bit more carefully for your next meter.

Editing: Now that you have said that, and the model number and lack of make of the meter (which amazon sells for something like $6.30, with ebay probably going lower) would appear to confirm that you did have it correctly connected to measure voltage, and it was, indeed, just cheap (which is not actually inexpensive, if you have to buy one that works after the cheap thing dies.)

Answer 5

The existing answers do a good job of explaining why you shouldn't try to measure voltage with the test leads plugged into the current (amps) jacks on meters with dedicated jacks for current measurement -- the current shunt is a nigh-short, hence the presence of a fuse to protect it from blithering idiot moments (they're relatively common).

However, they do not explain the other half of what happened, which is the noises and damage. Cheap meters (anything under 50USD retail, basically, but especially the sub-25USD category) use ordinary glass 5x20mm or 6.3x32mm (3AG) fuses. These fuses are only rated to break surge currents up to a few dozen or perhaps a hundred amperes at 250VAC, and a mains outlet can supply several hundred amperes or more until the house fuse blows or the breaker trips. The result is that the under-rated fuse's element explodes violently instead of melting quietly, destroying the fuse, and perhaps allowing other parts of the meter to be destroyed as well.

Better meters (north of 75 USD typically, with an authentic listing from UL, CSA, TUV, or Intertek ETL) will have ceramic-bodied fuses capable of breaking kiloamperes at well upwards of 250VAC. These fuses often use a sand filler that is turned into an insulating glass around where the element initially breaks and arcs, snuffing out the arc before it can consume the entire element violently. They also have other design features, such as internal plastic shields and slots in the circuit board, that keep arcs from bypassing the fuse, or any failures of the fuse from damaging other parts of the meter.

BTW: considering your meter is one of those cheapies that multiplexes voltage and current probing onto the same jack, using the range switch to select between the multipliers and the mA shunts -- any number of things could have happened, not just an exploding fuse. ('Tis why you don't see that design on a Fluke.) Cheap meters not only cheap out on the fuses, they omit other input protection components used to keep overvoltages from damaging sensitive meter bits (there are high-voltage resistors, surge-clamping varistors and diodes, and PTCs that heat up to shut off excess current flow to protect the voltage and resistance functions on a proper meter), and do not provide sufficient clearance and width for tracks that carry high voltages and/or currents, leading to internal arcing.

Answer 6

One problem with buying a meter like this is that ... there's no brand behind it, no proper datasheet, etc.. That's really bad because you have zero guarantees towards its safety and ability to meet its "specifications". In fact, this "model" seems to appear under half a dozen different brand names and occasionally no brand name at all. You want to know who designed and manufactured your meter, or at least who's getting it certified.

There's a manual available for what seems to be a very similar model. You'll note that it states:

CAT I-Measurement Category I is for measurements performed on circuits not directly connected to mains. ( Examples are measurements on circuits not derived from mains, and specially protected (internal) MAINS-derived circuits. In the latter case, the transient stresses are variable; for that reason, its necessary that the transient-withstand -capability of equipment is made known to the user.). Don’t use the equipment for measurement within Measurement Categories II,III and IV.

This multimeter (which may or may not be the same as yours despite the same "model" number) is only rated for non-mains use and is not designed to cope with high transients (brief voltage spikes), nor with low-impedance supplies! Depending on your supply quality and your local environment, transients in the kilovolt range can happen dozens of times a year (link found to be broken Nov 2023).

Now, at this point it's pure speculation whether your multimeter was blown up by a coincidentally well-timed transient or due to another fault, but the fact remains that you should not use a multimeter outside of what they are rated for anyway.

Others have mentioned getting a better multimeter. It'll probably do you good to read up on meter category ratings and safety (Link found to be broken Nov 2023) first - it's not a particularly long document, and it's fairly easy to read.

Do note that rating markings do not necessarily mean anything - anyone can print a couple bits of text. Also, CE markings are similarly self-tested (ha...). If it's been externally tested by a reputable group, e.g. UL listed, then you can usually find the certification from the certifier (registration required, but appears to be free) (don't just trust stickers/printing on the meter) so you know it's properly tested.

Answer 7

I've got also a DT-830B, but mine is different. It is also for CAT II measurements and it has a lead fuse inside:

I can't see a lead fuse in your DT-830b and share Bobs opinion/answer.

Answer 8

The problem appears to be with your DMM.

On a multimeter with separate terminals for ammeter and other functions, the ammeter terminal is connected to the common terminal via a low-impedance path, while the other terminal is connected to a higher-impedance path. From your question, you did plug into the high-impedance terminal, which suggests that the meter was unable to cope with the 220VAC input.

If the probe was connected to the low-impedance ammeter terminal, there probably would have been a massive spark at the probes when you connected them and the whole meter (not just the fuse!) would likely have caught fire or exploded, because there would be only milliohms of impedance and you'd have thousands or tens of thousands of amps going through the meter. There's no way a small fuse of the sort used in DMMs could absorb and cut off that kind of current. Given that this was not the case, the meter was probably faulty. The ammeter is intended to be connected in series with the device under test, not in parallel as you would a voltmeter. Never use the ammeter on a low-impedance power source!

In the future, it would be a good idea to invest in a high-quality multimeter from the likes of Extech, Fluke, or Keysight (formerly Agilent)—you may need to spend upwards of $100 for a good one, although Extech has some solid ones for a bit less. Cheap DMMs can fail under high voltages in dangerous ways. I managed to blow the micro/milliammeter function on my Craftsman DMM by hooking it up to a 330VDC source (photoflash capacitor), even though the meter was specified for up to 500V above earth ground! (Thankfully, there was no explosion or fire or even any noise when that happened.)

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I just noticed your edit and it definitely looks like there's no fuse holder where one should be. There are large copper pads on the PCB that have nothing on them and are simply shorted. This is quite clearly a safety hazard, and I would not use this sort of meter for anything higher than 24V, if even that.

Even my cheap Craftsman DMM has two fuses: one for the low-amperage volt/ohm/capacitance/milli/microammeter measurements, another for 10A ammeter. That meter has seen several high-voltage measurements and, save for the milli/microammeter failure described above, has been working safely for the last five years. Even a store brand like Craftsman has a major company behind it (Sears) and that provides a minimum level of assurance that the meter meets certain safety standards. I wouldn't trust it for professional work, but at least I know it won't blow up on me in day-to-day use (mainly as a battery tester). I also have a cheap pocket meter from Craftsman, and while that unit is noticeably less accurate (it seems to read a bit high), it too has a fuse. Both meters have safety certifications on the packaging: UL for the larger meter and ETL for the smaller meter; both are rated to CAT II, at 600V for the former and 300V for the latter.

Do yourself a favor and get a decent meter from a well-known brand. Extech would be a good place to start—they have some good meters that aren't too expensive. Make double-sure it has safety certification marks (and be sure to check if they're real), that it is fused, and if it has a 10A or similarly rated ammeter, that it has a separate high-amperage fuse for it.

Answer 9

I believe the explanation is quite simple. Although most of the answers here cover (very well) all the aspects involving this kind of measurement, a point was missed.

That kind of multimeter does NOT have a 250 ACV scale.

Actually, if you take a close look at the picture, there is no 250 scale in any unit. These multimeters are based on the ICL 7101, which has a 3 & 1/2 digit display driver, so the highest number it can display is 199.9 .

If you had set it to 750 ACV, there would be a chance for survival. Always use a scale higher than what you expect to read.

Answer 10

Cheap meters should never be used anywhere near mains voltage for the reasons discussed.

Even my relatively expensive meter (IDM65) had issues with voltages approaching the maximum possibly because it also specifies a battery type as well. If you check other items like smoke detectors they say "use the specified battery only" for a good reason. Put a different battery in and it worked fine up to +599V DC.

Incidentally that meter suffered from calibration issues but I determined that it was caused by water exposure years earlier corroding all the SMD devices.

Wonder why

Answer 11

That type of meter and small variations of it are sold all over the place where the lowest price is all that matters. I've seen it where the fuse is only connected to a single function, such as the 200mA current measurement, and it is bypassed in other modes. The resister dividers used to set the Voltage ranges are shared with the current measuring function, and there are no bypass diodes, so overloading the current measuring function will fry a resistor and then Voltage measurements will be too high after that. The resistance measurement has a 1kohm resistor rather than a 1kohm protection PTC.

That meter will have a 1Mohm resistor near to the IC that connects to the Voltage measuring input of the IC. The other end of that resistor is connected directly to the Voltage probe when the meter is set to the 200mV range. That protects the meter up to 300 Volts, assuming the proper resistor was used. In your case, the meter probably developed a short in it due to a faulty function selection switch and the AC Voltage got connected one of the low Voltage power supply sections of the meter and destroyed the IC.

A proper meter will be protected to what is printed on the holes where you put the probes in regardless of what function the meter is set to. The Voltage and resistance hole is typically protected up to 300 Volts. The exception is that the Voltage can typically go up to 1000V if the meter is set to the 2 Volt or higher range. There is also a spark gap to protect from static discharge and other low current overloads. The current measuring function is protected by the interrupting current of the fuse that protects that.

Typical features:

• Spark gap on the input.
• 1Mohm resistor protecting the IC up to 300V on the 200mV scale
• A 1kohm PTC fuse that protects up to 300V in the resistance measuring range. Many Sears multimeters are known to use a transistor instead of a proper rectifier diode to power the resistance measuring function. This improper diode can't handle high Voltages and blows out but it can be easily fixed with a proper 1N4007.
• Bypass diodes protecting the resistors that are used in the current measuring function. This prevents the Voltage across the current sensing resistors from ever going above about 1.3 Volts.
• A fuse protecting the current measuring function. 2Amps is common.
• A ceramic fuse protecting all functions of the meter, including the 10A current measuring probe connection.

Answer 12

I believe the OP's edits posted while I authored my original response, but for the future, remember that your DT-830B meter has inputs and settings that specifically say "10ADC" and the range dial says "DCA". Your meter can only measure DC Current.

In any event, it may not be dead. Even cheap meters have overcurrent protection fuses, and that may be all that's wrong. (Last summer I bought some true cheapies from a street vendor in Serei Saophoan, Cambodia for a class I taught at Bantey Meanchey University)

PLEASE, do yourself a favor and read SparkFun's article about how to use a multimeter.

Finally, please read my warning to @Xen2050 about 220VAC safety.

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Finally, a note about 220V safety.

The USA uses 110V because it's safer than 220v-240v the rest of the world uses. It only takes 100mA to stop your heart and kill you, and 220v is plenty to overcome the resistance of your dry skin. If the current travels through your opposite arms (and thus, right across your heart, you can be killed. Here's a reference you asked for.)

I got a surprise 220v shock through the hand because I was testing one of these cheap chinese meters in the street market in Cambodia--with its back cover off. ($4 US, made in china, nice big analog meter!)

I didn't realize the probe tips were exposed on the backside. I tested it by plugging the test leads into an available outlet with one hand, while holding the meter in my other. Now Cambodia's hot in June, it was over 100F most days, and my hands were sweaty. Nice and conductive, far less resistance than normal American dry skin.

Even though I got shocked in one hand only (2" longitudinal gap across my thumb muscle, abductor pollicis brevis), it not only scared the willies out of me, my left elbow spasmodically tweaked and then hurt really bad for about three weeks.