EEVblog #84

[safarir]

Blowing multimeters, you make my day !

I also like the new music at the beginning and at the end


(Yes, I am monitoring your video folder  )

Sorry again for my bad english

[EEVblog]

Yeah, that was fun. Shame Doug only had the baby 400J unit, we were hoping to see a meter catch on fire. But we did get one to fully explode which was pretty cool.
Next video is a quick explaination from Doug about building a high voltage probe.

Dave.

[safarir]

Cool, I will build my own one so I could blow my old multimeter as soon as I get my new one

[Anders]

Would have been interesting too try the Smart Tweezers at 400J especially since this is really a “hand held” instrument!
Any volunteers, I mean someone holding it during the test? 

[alm]

This is probably vaguely similar to the CAT I 1000V test (4kV, 20 repetitions, 30 ohms source impedance), although the source impedance and duty cycle might be different. So the bottom line is that you shouldn't use any of the meters that exploded violently (which probably includes the early Fluke 70-series, depending on the velocity of that range switch) for mains or CAT I high-voltage (eg. flyback transformer) use. I'd still consider them safe enough for low-voltage work behind a power supply (eg. wall wart / transformer / battery / lab supply). I tend to touch that stuff with my bare hands, and use uninsulated tools with some random 5V arduino circuit, so I don't see a DMM without input protection as a an extra risk.

Disclaimer: I actually don't own any of those cheap DMM's, although I do own some older name-brand meters which don't have CAT ratings.

[mausball]

Dave-

Excellent destruction videos!

I too would have liked to see the big daddy unit connected. I'd also have liked to see the results of longer duration surges, like you can get from a wonky flyback circuit (as Doug mentioned when he was talking about the flyback diodes). The short pulses are nice, but I've actually run into things (one of which killed my original Fluke 73) that jumped north of 1kV for something like half a second. 500mS is long enough for the HV to crawl around inside and do a lot more damage, and to seriously stress protection systems.

I'd also like to see the results of such a test on some of the medium priced meters, say the $75-$150US range (old WaveTeks, Amprobe, B&K, TPI, etc). A lot of advanced hobbyists and professionals like myself keep a few of these mid-range meters around, both for their durability and for their improved performance. I know a few of mine are actually CAT I certified, but I'm curious about some others, like the B&K and TPI. Heck, even one of your nondestructive comparison reviews would be nifty for the midrange 

[Simon]

It might be a good idea to have CAT ratings explained, maybe a video blog on them ?

[alm]

Note that Fluke has got the source impedance for the CAT ratings wrong in several appnotes, I believe I complained about this in the past, but apparently they haven't fixed it (or the error was in multiple documents). They do get it right in some documents, though. One of their appnotes lists a source impedance of 30 ohm for every category, another lists the same source impedance for CAT III 1000V and CAT IV 600V.

[Kiriakos-GR]

Here they are, CAT standards ..  http://zone.ni.com/devzone/cda/tut/p/id/2827

Well after this "presentation" I am still scratching my head from confusion.

I do not believe that this was what most people had in mind as review of multimeters.

Even the article above  , states clearly  that any standard was imposed by having in mind the proper isolation, and safe behavior at the occasion of an transient .

Under the above clear specifications ,  the cheap multimeter that got the strike of the transient ,
and did not blow ,  its a Damn perfect safe multimeter .. simple as that.

The Fluke got toasted , the PCB become carbon , and  soon some one will just say that it just needs a new calibration and it will be fine ..    

Nice video , it will do the counters to jump high, but nothing useful in it , just wild action and not meaningful conclusion.

Personally I do not give a shit about Fluke , I am interested to find from the lot of manufacturers,
the ones who own a decent brand or .. a decent model of DMM .
Simple as that.

And I am even prepared to get the "Highway"  in order to keep my "emotional little world" in balance.    
    

[Simon]

so basically the CAT rating specifies the percentage of over voltage relative to the max safe voltage, I don't really see the point of it, it is far easier to just state the max voltage, my Rigol scope is CAT I 300 V, well to me CATI means nothing the important thing is that it will resist 300 V,

I would hope that the CAT ratings specify something like duration of overload  and/or other parameters otherwise it's a useless term to me.

Well we have already seen a blog before on the build quality of these meters, I mean you can go on for ever, at the end of the day it seems obvious that if you spend less than £30 on a multimeter it will not be much good, the shootout video showed just what can happen to poor quality multimeters for those who may have crazy ideas of trusting a cheap meter to 1000 V just because the manufacturer who has no come back says it can

[Zad]

I have to say I was really quite surprised about the way the Flukes handled the overload, especuially given that it is way below the rated limit. Particularly the way the sacrificial components weren't contained (no blast walls in that part of the case) and seem to have caused internal stress fractures within the PCB. Also surprising how relatively 'well' some of the Won-Hung-Lo crap did. They will never meter again, but Im not sure the Flukes would either. When I say well, I mean "would only cause a minor fire and loss of fewer than 5 fingers".

[Pradipta]

It is reasonably surprising to have an interim episode before the Multimeter Shootout. I expected to see Amprobe and Extech on test grounds as depicted by the Tweets. When will we see that big day?

[alm]

One important parameter that is omitted from the above table is the source impedance, which determines how much current the source will be able to deliver, i.e. the size of the bang if something shorts/arcs. This is the important difference between CAT I and CAT IV. Compare touching the HV electrode of a large CRT (with 30kV acceleration voltage) with touching a 33kV high-voltage transmission line. The former will hurt but probably not kill you (most of the danger is actually from the muscle reaction, not the current), the latter might actually cook or vaporize you. The former is CAT I, the latter is CAT IV. This is why CAT I tests are conducted with a 30ohm source impedance (peak short circuit current for CAT I 600V is 2500V/30ohm = ~83A), and CAT IV with a 2ohm source impedance (peak short circuit current for CAT IV 600V is 8000V/2ohm = 4000A).

I believe IEC 1010 only requires that the equipment should not be dangerous to the user (explode or catch fire), not that it keeps working.

CAT I 300V on your scope means that it's rated for up to 300V on CAT I (i.e. residential/office equipment on the secondary side of the power supply), so 110V (or 230V) mains is outside the specification. So it actually means more than just '300V max'.

[Simon]

ah yes i was also thinking of the source impedence I must have misread that table as being the item impedence (I really need an early night) but that still leaves us with the pulse duration to define.

i agree that the idea of the protection is to save the user not the instrument, if it were made of solid metal with dedicated protection chambers and you could buy replaceble protection circuitry it would be an ideal world but the price might not be ideal. It was quite obvious though that the cheaper meters are not that resistant, the one that blew apart and even the one that bulged a bit could have cause severe injury, perhaps dave could post the CAT specs of each meter and its reaction to give us an idea of what he was dealing with

[Simon]

hang on, so if the scope says CAT I 300 V does that mean that it will take 300 V at most or that it is the value to look for in that table which at CAT I will take up to 1500 V ?

[kc1980]

Cool video.  I too would like to see what would happen to the Meterman/Amprobe as I own one. 

The coil discharge reminded me of a neat coin-shrinking trick done by some crazies on youtube.   I'm not sure how many Joules/Watts are needed to shrink a coin.  Wacky...

[Simon]

well the coin changed shape, the diameter reduce and the thickness increased, i expect it was stretched by the forces at work

[alm]

hang on, so if the scope says CAT I 300 V does that mean that it will take 300 V at most or that it is the value to look for in that table which at CAT I will take up to 1500 V ?

It's designed for CAT I circuits with a continuous voltage of 300V, and will not endanger the user in case of transients (IIRC 20 pulses with a certain duration, duty cycle and source impedance) up to 1500V.

[Simon]

thats comforting, so with a 10:1 probe attached thats 15'000 volts, now I'm feeling a bit happier about probing for back EMF 

[kc1980]

well the coin changed shape, the diameter reduce and the thickness increased, i expect it was stretched by the forces at work

yes, the volume is the same so technically it's not "shrinking".  here's the best explanation that I can find, though I'm still not 100% satisfied with it:
http://capturedlightning.com/frames/shrinker.html

In the video, I completely understand why the aluminum block is repelled.  The increase/change in magnetic field from the coil induces an eddy current that generates an almost-equal but opposite magnetic field from the block.  The magnetic repulsion sends the block flying in the air.

Now, the coin shrinking thing is a bit more tricky to understand.  The assumption is that this trick works on non-magnetic metals.  I'm wondering if the coin material needs to be magnetizable.

[alm]

thats comforting, so with a 10:1 probe attached thats 15'000 volts, now I'm feeling a bit happier about probing for back EMF 

Assuming the probe is CAT I 3kV rated, and only at low frequencies. Plus it doesn't have to survive, just not blow up in your face.

[Kiriakos-GR]

hang on, so if the scope says CAT I 300 V does that mean that it will take 300 V at most or that it is the value to look for in that table which at CAT I will take up to 1500 V ?

Yes thats the idea of this table.
So any test to it, it should limited to an 1500V transient .

Generally speaking , we are unable to predict the maximum voltage of an transient , so to feel safe 100% about our DMM .  
The transient in the Mains , could happen because of a lightning absorbed by a pole , or by an 20KV transformer who had sorted out.
Its an extreme situation , totally unpredictable.

Looks logical to us , to separate the DMM's   by deferent quality standards, still no one are able to offer protection , at the next step over the Mains of 220 V, that is the 20KV !!
So the 20KV its the next enemy who could get in the 220 lines , even by an accident.

The electrical distribution network , are build by professionals, so there is special systems ( in simplified  description "Fuses" ) , that can eliminate an such scenario from happening.          
 
An Industrial called DMM are always closer in the " line of fire " ,  If we need to compare apples with apples , we should zap only DMM's called as Industrials , and the upcome will save many life's of Industrial electricians that they using them ..

Zapping DMM's for just home use , makes a nice sound and sparks , but thats all about it.

I was hopping too, the test to be limited at the max rating as limit, set by the manufacturer.
An 600V rated DMM to stay on  measuring 600V for 24 hours , its an normal test to prove it as honest.

Any way , the web its an huge space , and any one who needs or seeks information about anything, he just have to combine any information available , and make his decision at the end.

Dave did his part , and its just another deferent view of things, Biased or not , extreme or not ,
its just another view.      

[EEVblog]

I was hopping too, the test to be limited at the max rating as limit, set by the manufacturer.
An 600V rated DMM to stay on  measuring 600V for 24 hours , its an normal test to prove it as honest.

I now have a 1200V power supply adjustable in 1V steps, so can now test meters at their maximum rated voltage for however long. Look for that in all future meter reviews.

Dave.

[Pradipta]

People might find these useful...

[Simon]

thats comforting, so with a 10:1 probe attached thats 15'000 volts, now I'm feeling a bit happier about probing for back EMF 

Assuming the probe is CAT I 3kV rated, and only at low frequencies. Plus it doesn't have to survive, just not blow up in your face.

yes true the probes are rated for 600 V but I'm only measuring transients or back EMF, if the probe was damaged I would not be so bothered, it is the scope i want to stay intact.

so does CATI 300 V mean that it will withstand a continuous voltage of 300 V even though the max input is 40 V ?