EEVblog #853 - How A Multimeter Works

hi in a previous video I showed that my fluke Stephanie and B multimeter which I basically have had just had sitting in a box here on the shelf I because it's not you know it's not something I'd typically use every day I've got a better nicer meters than this one but anyway um it looks like it is like died that I don't know what has gone wrong with it's just been sitting in the box check it out I mean you know and obviously like the the chipsets working you know display comes on it seems to do or you know it appears to work when you actually turn the thing on but like nothing look zippity-doo-dah and no it's not the test leads I've tried various functions it just does not recognize anything at all has the input blown or something like that I have no idea as far as I know I haven't used it for you know anything serious since I got it in a mail bag way way back quite a few years ago now and um yeah I think I did a quick teardown on it once but that was like it so anyway let's crack this sucker open and have a look and of course yes this is the made-in-china fluke and just like the original this actually reminds me of the fluke 19 which was further flukes our first foray into the chinese-made multimeter market just to test the waters here it was at the fluke 19 was a here's a photo of it I grabbed from the internet I don't have mine anymore but I bought quite a few of these for work because at the time they were a big deal they came out it was like I think it was sub hundred dollars and for a fluke that was absolutely incredible you could buy it in the local tricky dick store and it was flukes first entry into you know they'll just testing the market to see if people would want or like a chinese-made fluke and they only sold it in Australia a couple of other asia-pacific countries and that was it as far as I know and it was a complete and utter failure Oh don't almost every one of them like died they had this fault where the chipset would get killed I can't remember the details but every one of the ones I had died I've got tons of reports of him dead there were at the time this was probably back in the late 90s perhaps I think it was a long time ago in a galaxy far far away but yeah it was a miserable fire fluke 19 but obviously well yeah they did a second suck of the seven hey we can still do this the flukes 17 B just start out as the 17 then I went to the 70 MB I'm not sure anyway let's crack this thing open and dumb see if we can find out what's wrong with it and yes the good thing is we have the schematic awesome so if you still got a work in Flook 19 B well you're one of the real lucky ones you've got a rare instrument there because I think most of them on the market died I'd love to find out the exact reason it was something to do with the chipset ASD sensitivity or something weird like that anyway this one failing just sitting on the Shelf there does not instill a lot of confidence in me hmm now I don't know about you but that non-standard spacing on the amps jack there gives me the heebie-jeebies all right let's crack this thing open two double A batteries lots of Chinese writing on there thankfully you I can understand the fuse ratings and let's whip it open see if there's anything obvious in here give it the smell test nope nothing smells burnt and nothing right off the bat oh oh oh oh look can you see what I see or don't see don't well there's your problem that is hilarious the screws are not on there I think this is a massive peg kak I've as I said I think I've done a teardown on this before and obviously I did not put the screws back oh well there it is I fixed it I'm pretty sure it'll now work again yep no worries well I could have just deleted this video but and what funds that I've got a schematic I can salvage this I can salvage any video no matter how tragic it is alright let's just I don't know have a look at things why not me and I just checked in yep I have actually done a full teardown video of this episode number 344 to be precise no wonder I don't remember it I'm going to do this one day I'm actually going to you know produce the exact same video I've done like five years ago or something and I won't even realise anyway um yeah so I won't go into huge detail about the construction of this thing I've done that in the previous teardown unsure suffice it to say that that's one of the wimpiest tenets current shunts I've ever seen look at it little an EB thing and the other thing that strikes me about this is of course these adjustment pots little one two three four five six is there have I missed one seven eight eight adjustment pots in this thing they're adjusting for everything why are they doing that well you know like every modern multimeter you know no longer has they're all close Kate what's called closed case calibration you don't have there's no pots inside to actually trim but this thing has trimming for almost everything why is that the case well it's just using a single chip set here we'll have a look at the actual chipset itself in a minute but and so it's just a regular off-the-shelf multimeter chipset there's no secondary processor that actually because basically multimeters can come in two varieties well probably three varieties actually one is just like this one as simple as it gets one multimeter specifically purpose designed multimeter chipset there's you know three or four manufacturers on the market who make these and it does everything it does all the multimeter range switching functionality measurement ADC it drives the LCD everything else does the whole shebang the second one is to use a multimeter which is probably more popular these days especially in the mid to high range meters is to use a specific multimeter chipset but it's only a front end chipset so it only does the measurement hardware you know the front end the range switching the ADC and you know various generators and all that sort of stuff and that is just a front end it can't actually do anything itself it's not really a processor can't drive an LCD can't do anything else usually a serial output which then goes to a secondary our processor which then drives the LCD and everything else now we can actually do a comparison with the new eevblog me to the be m23 five here and have a look at this one and you'll notice this one actually uses two chipsets here but this one is a little bit unusual usually when you see two chipsets like this you'll have just a multimeter front end chipset like this which doesn't contain a processor as just as I said before like the range switch in everything else the ADC all that all the stuff you know the true rms converter every you know everything else all the other functionality required for a multimeter chipset I put a typical wire datasheet over here of a chipset which is are fairly common for example now this one you would think that that's the case and it interfaces with the processor over here for the serial but this one's a little bit unusual in that no this one is actually a processor and does everything just like this one up here but it obviously does it's designed for a rack close case calibration it's got a squared prom building because it's got to store the contents and it either has a squared prom built in for the to store the software calibration functions or can use external a square prom but this one actually is this secondary chipset here our link in the datasheet here and it's actually just an LCD controller chipset so obviously couldn't get enough pins on this thing they needed even though it's a complete multimeter chipset with processor and everything built in requires external LCD controller so this one's a little bit unusual in that respect but it's more common to find a multimeter chipset and then a secondary processor now there's nothing stopping a single chipset multimeter like this one up here from having closed case software calibration but the the actual processor inside here is not designed that and for that it's bare-bones it's designed not to have any of that closed case calibration I you know a squared prom built-ins I can store our you know calibration settings and things like that and compensate it's designed to use these external trim pots around here so in that case it's a it's a poor choice by fluke in a modern multimeter to have to require pots like this to trim it just from a long-term you know drift characteristic and everything else it's at this poor form not when you know modern processes can handle these sorts of things but hey that's typical of these really cheap are slow in chip sets like this they require these external pots for those who want to know what the front end multimeter chipset on the eevblog meter is sorry secret squirrel so anyway I think that's pretty poor having these and trimpots on a mold and mouldy meet up just dear fail anyway now let's take a look at the schematic now I'll link in the complete schematic down below it seems to be the official one so I'm not sure proprietary sorry fluke but once it's on the internet it's on the internet so yeah there you go 2009 it was generated anyway yet so it looks to be a genuine it's not like a reverse engineered or anything like that and I'll link in the PDF down below now the chipset in this thing they've mainboard somehow I don't know when they convert to PDF they're mangled that but it's actually an F s 97 21 the LP 3 version and I'll link in the datasheet for this puppy down below and yeah it's a complete multimeter chipset for TF SSR fortune semiconductor they're one of you know three or four different makers of multimeter chipsets that is still around these special chipsets and as you can see you know that pretty much handles everything it is a single chipset one chip to rule them all and this miscellaneous stuff around so let's actually just have a quick look at the schematic see what we can see here first of all take a look at the current jacks here and I have done a separate video on multimeter input protection so if you want to know all about diffusers and how they do things in this diode bridge here and things like that which by the way is just our clamping that's basically what they're doing they're clamping the voltage then I'll link that one in as well if you haven't seen it's well worth a look and look they're actually trimming these are the shunt resistors okay so this is the here's the 10 amp input here here's our common Jack okay so our 10 amp goes through our HRC fuse of course into our typical 10 milli ohm shunt resistor that's that real wimpy look at it it's real wimpy look sorry I've got fixed contrast that fixed exposure on the camera here that's why it's all dark it's very difficult when you're doing white paper like this you've got our set manual exposure on the camera anyway ten milli ohm current shunt which is all fine and dandy and then they're doing a trim across that sewing typically what which actually is okay you could probably argue that one's not that bad so they're doing a divider here they're doing a voltage divider there's 100k resistor 47 they're actually trimming that that's a large thats a large trim range that's absolutely massive so yeah I would have limited that I think that's a bit of poor design work there anyway usually they physically trim the the nichrome why our current shunt typically it's made out of nichrome wire the current shunt and they'll physically trim it either by taking a little chunk out of it or adding some solder you know getting some pliers on there and getting given a little crimp or something just you know change the value by you know half Abby's dick or something like that so they've decided to add a trim pot and that's rather unusual most companies are just decide to trim it some other way or with the modern chipsets are just software trim of course okay so on the milliamp range here here's part of the range switch they've got this range switch actually split all the way through this schematic as you'll see and what this is showing these are the physical contacts on the PCB so when you put it in the milliamp position down here it's shorting out these two contacts what does that do well it shorts out this resistor up here so here's our milliamp input jack so if this resistor is shorted out then this one ohm resistor here is going to be our current shunt resistor actually it's one ohm plus this 10 milli ohms down here and you'll note that they've got another they've got a 1k resistor in parallel with that so they're just you know trimming that down slightly because they want to get it down because you you want this this should be like point 9 9 ohms really so that's what they're trying to trim it to because it's in series with this 10 milli ohm 10 amp shunt down here and then in the micro amp range this contact is moved from here up to the top so then we've got this 1k resistor in series here and you'll notice this is 0.1 percent this one here was only one percent tolerance for the milliamp current shunt it didn't you know they once again it's all about the Temko it's not about the absolute tolerance because this thing is actually trimmed but you'll notice these ones a point one percent here because there is no trimmer for the micro amp range so that's why they're using precision resistors in here because it's actually not easy nor cheap to get a 10 milli ohm current shunt resistor my I do that I use a 10 milli ohm current shunt resistor on my micro current for example and it's an expensive resistor even in thousands and thousands of volumes it's you know upwards of four dollars per resistor right it is really expensive so yeah if you want the precision you know straight off the bat without having to trim the thing you know mine's up but what is it is it point oh five percent I can't remember yeah I think it's a point oh five percent or point one nine point one percent is it yeah that's actually a very expensive resistor hence it's actually cheaper per unit to just have somebody trim this trim pot here yeah labor takes time but Labor's not that expensive compared to a four dollar resistor it just depends on which way you want to do it so yeah they've decided note we don't want to trim Oh for that because the 1k ones these low values one these low values of 1 ohm and 10 milli ohms they're expensive to get in real precision values but 1k is not like you can get those for 10 cents in volume or something like that right real fairly cheap right compared to these so so they're able to use precision resistors for those and no trimmer and you'll notice the sense voltage here is actually tapped off different position depends on wherever where in the amps range or whether they're the milli a brain so they've got another set of contacts on the range switch here which in the amps range of course it taps off from this voltage divider across this 10 milli ohm current shunt resistor in the middle amp and micro amp range up here it's the same contact then it taps off the top here and when it's in milli app range here it actually taps through this resistor onto there so the current shunt resistor is 1k in parallel with a hundred and 10 ohms then in series with one ohm in series with the 10 milli ohms there but it's actually strictly not true to say that there's no trimmer for the micro amp range here it's not for these resistors but because it's still in series with this 1 ohm resistor in parallel with this 1k here if you actually have a look we bring this in here and we have a look what we've got we've got 1k in parallel with 110 up here so that's ninety-nine point nine nine and zero 999 but I'll live the rest off anyway it's that in series with so plus one on the 1k here but that 1k is a trimmer ok so that's actually 0.999 if it's just the 1k okay if you haven't trimmed it lower than that plus the 10 milli ohm current shunt resistor down here that's a total value of maximum value not including tolerance of a hundred point 108 so these two down here are this one down here it's it's way too small to affect it's up in the 47 K range cup you know orders and orders of magnitude higher than the 10 milli ohm so it doesn't affect it it's only for when you're out tapping off the amps range here so obviously they have to trim this 1k here it's it's going to make a difference they want that to be like bang on okay because this has no software compensation at all so they want that to trim this value right down here to exactly bang on 100 so that trim part actually will find actually affect both the milliamp and the micro amp ranges and then they've just got two high value input protection resistors here so because it's going directly into the chipset they're just for some current limiting protection for the input pins here in case there's a slight possibility of overload and they don't have to be precision of course because this is high input impedance that's why you've only got one percent tolerance they're just joe bloggs resistors now let's take a look at our volts and ohms input down here this is our input jack and then we've got five resistors in series like this and they are these puppies in there there they are why have they got five like that I've explained that in the previous video it's to get a high withstanding voltage so each resistor has a certain voltage you know maximum voltage time so you whack five in series and you can get a fact of Li a high voltage resistor there and that's cheaper five of those is cheaper than you know one big one which you'll typically find by the way tada in the eevblog meter there you go this is more expensive and notice the isolation slot under there that's a high compliant voltage ceramic resistor much nicer than just the five but you know they get away with it I mean you know nothing inherently wrong with that you'll notice that they've got 1.5 mm there and also here now a real educated guess is that this is actually a design note to the PCB layout person to saying we need 1.5 millimeters minimum clearance on these things that's what we need because these are the high voltage input so isolated from everything else and this one here nine millimeters input clearance thank you very much so assuming that we're in either the Mille volts DC range the ohms or the capacitance range then these two contacts on the PCB here are going to be shorted and we're going to be using these input v input resistors here for that particular mode and if we follow the yellow-brick road here let's go over let's go over another protection resistor here we've just got some diode clamping here what are they Bev 199's are they so got some dire clamping and it's also fairly typical especially old flukes I'm not sure if fluke pioneered it or not I was kind of gonna maybe do a separate video on this it's on try and find a another fluke meter schematic here which actually shows these and yes actually found it here's a schematic from the fluke 77 series 3 which are there when actually comes from way like even much earlier multimeters I think the fluke 45 had it and you know all sorts of real old-school flukes have this dual transistor arrangement where it's basically they're configured as back-to-back diodes so it actually uses the the breakdown voltage of the transistor to actually act as a Zener and then having the two actually acts as a back-to-back zina because one will have the forward drop and the other one will actually have the breakdown as the Zener depending on whether it's positive or negative input and it's a rather unusual configuration but very popular so I'm not if anyone knows the history of that and whether or not fluke actually pioneered that then I'd love to love to actually know details on that anyway back to the 17 being here yeah they've just decided to use some 599's Mir now this part down here is rather interesting it's it's a different configuration to what I was mentioning before with the back-to-back zener clamp in there but look if you put it in DC volts or AC volts mode then it's basically shorting out this right so there's nothing there effectively nothing there at all but what they're doing is actually tying VSS effectively this is where the coupling VSS into the common terminal the common is actually the terminal over here so the battery negative VSS is actually the battery negative if you go up here there it is so it's they're not directly shorted together they tie it through this one MIG resistor here and on the base of this transistor q1 here they've actually just got a reverse biased diode there on the base of that thing and they've got a PNP here so it's basically they're shorting out that this goes up to you follow it up yellow brick road again it goes up to the bottom of our resistor divider here you so these are our range divider resistors here handled by our multiplexing inside the chipset and basically the common of those is shorter down to ground we're measuring AC and DC but when we're measuring ohms it disconnects the ground and it's you know it's go it's activating this part on the lower end of this resistor network here in the ohms mode and the capacitance mode here in the volts DC mode they've got an AC coupling cap here they actually and a and a series resistor so it's almost like a little snubber there are shorting that out and then our input goes by the way through a PTC positive temperature coefficient resistor which will increase in value in overloads and that's that puppy down in there sorry silly thing there it is there we go there's our input PTC resistor and then our big 1k here we go they this is where they have a note saying install either one of these so install that or install that so there's 1k resistor here in series that's very common input protection for me you know it pretty much handles this is how why you can put mains on the Ohm's range for example because you've got the PTC resistor here which will increase in value with any overload got the big beefy 1k high voltage resistor here and no worries whatsoever and without clamping and everything else inside here then no worries whatsoever because when we're in the ohms range here by the way we're actually we need to force a current out into the then to the positive Jack out here so that's why it's switch in this range switching is both used for our DC and AC voltage ranges plus the owns range as well it's rather quite quite clever so they're actually switching that in this particular circuit in here when they're generating the ohm so you know it's like constant current type thing out for the ohms range and likewise for the capacitor too because basically when you measure again how they do the capacitance measurement they just basically have a constant current output and then they just time chipset just times how long it takes to charge up bingo you can work out the capacitance and then of course the way that they're tapping that off not in until they have to generate the constant current from here like this but they also have to read back off as well so that's why in the ohms range here also bingo it's tapping off like that so that's the voltage sense in there and the chipset can actually measure the voltage across the resistor under test so I think they're attempting to do some sort of clamp in here in the ohms and the capacitance mode but to what end I'm not entirely sure because at low voltages this circuit is not going to engage it's not going to do anything so you know add a couple of volts that we're talking about when you're operating the ohms mode then this is not going to do anything but a higher voltage is yet it's going to start to conduct so yeah presumably some sort of attempt at clamping and this is not a true RMS multimeter so there's no true RMS I converted chip Everest separate analog devices one which is quite typical or the eevblog meter for example is a true rms multimeter there it is but you won't find your traditional analog devices true RMS converter chip in here because it does some clever stuff using external components in the main multimeter chipset might have two separate video on that one day anyway this so this one's an average responding meter and here's the AC average responding circuit control via the chipset here and being out another trick another trimmer in there for the AC calibration bloody trimmers dodgy as and it looks like they've got a amplifier in here yep Oh P that'd be op amps so they've got some internal op amps in here so you'd have to look at the data sheet to get the internal arrangement for this but basically so there are feedback resistors for our non-inverting op-amp with the times 10 gain there now some multimeter chipsets they'll have a combination of standard op amps in there plus they might have a for amp in there as well that you can select with the internal MUX in so that's you know not uncommon because the chopper amp very precisely you know no bugger-all DC offset so you might find those on say a higher end four and a half digit multimeter chipset and there's our DC calibration trim pot looks like that's is that hooked up to two reference pins we really need to look at the data sheet for this getting an internal block diagram to get more fancy pantsy speaking of fancy pantsy look at this this is quite unusual you don't often see this in a multimeter so thumbs up to this we've got um cold Junction compensation and that's for the temperature mode so you'll notice that when we select our temperature on the range switch here bingo it goes up to here into this amp here but then that's offset with a cold Junction compensation I've done a video on cold Junction compensation we saw a Waco over it again but awesome that's what high-end our thermocouple amplifiers and our thermometers like hang on I've got one here somewhere like this fluke okay type you know a fluke f3000 I've done a teardown of the of this and various others and these have that cold Junction compensation on the input pins here they have a temperature sensor which takes into account the temperature of the dissimilar metals which can cause offset errors in your temperature so they've actually added that in here and you'll notice look there it is a temperature sensor and once again the the designer has put a note in here for the PCB person place near the inputs because it's got to be right near the input jacks brilliant I don't know why they bother doing that but it's excellent I mean this thing's you know it shows has tried to build it down on cost but hey cold Junction and compensation brilliant and you'll notice there it is you fall down there there's our little temperature sensor right down there but really you know in this case it doesn't like matter in your proper ones like that other fluke thermocouple meter I showed before it actually physically couples the chip through to the middle of the input jacks and this one's just physically close it's not doing that so it's and you know it's a little bit how you're doing but it's I guess it's better than halfway up the meter but yeah in the scheme of things doesn't really matter here but yeah they've gone to that effort so I'm very very surprised and they've got a offset trim pot in there as well just for the temperature so they you know they really did take this thing seriously I guess maybe not surprising I mean it does have a specific you know maybe that was a big selling point they wanted to do this thing does temperature and does it probably better more accurately than your average multimeter there's not much else doing here got some miscellaneous looks like some filter stuff maybe happening around here ah there's our main oscillator for megahertz and direct LCD drive up here and Bob's your uncle that's about it I'm some extra right you know the buttons and things like that some extra range stuff and well not much well what are we got here our MUX yeah there we go 7 4 HC 1 4 8 our priority encoder so they basically got some pull-ups and pull-downs here just the range switching so they're basically just detecting which so the chipset can detect which range is actually currently selected so there you go I hope you enjoyed that I hopefully I managed to salvage this video was supposed to be a repair of this thing and sorry fluke I I thought this thing had come a gutter and failed on me but don't it was a peb kak screws when I previously put the thing after the teardown are these those things happen you know ah geez so that was better than deleting the video I hope anyway hope you enjoyed it if you did please give it a big thumbs up catch you next time you you

Channel: EEVblog

Views: 119,431

Keywords: fluke, multimeter, how it works, tutorial, schematic, fluke 17b, fluke 19, transistor, zener, campling, diode, protection, esd

Id: yuCXsT3_WRE

Length: 31min 47sec (1907 seconds)

Published: Fri Feb 19 2016