EEVblog #373 - Multimeter Input Protection Tutorial

hi quite a few people have asked me to do a video on multimeter input protection and how it actually works and how it relates to Kat radians and surge overloads and HRC fuses and value no isolation slots which if you've seen me teardown I'm altimeter I'm explaining all this stuff all the time but a few people have asked can I actually explain how a typical multimeter like this fluke 27 that I just we just had a look at it in a previous video how the input protection on this works so I thought we'd take the schematic for the fluke 27 and we'd have a shot at explaining input protection let's go now as with any typical multimeter we've got a common jack we've got a volts ohms and diode jack which also does capacitance and other things and usually we've got a separate amps and a separate milliamps and microwaves Jack now I know some multimeters some cheap ones will actually combine milliamps and micro amps with the Volt ohm Dyer one don't worry about that is one will focus on separate amps and milli amps and you'll notice that they're also fused this one's fused at 320 milliamps maximum and well a 10s maximum so the fuses are going to be slightly above that in this case for 40 milliamps and 11 amps for the ABS input jack there now these fuses in multimeters we'll talk about what HRC fuses are but these fuses only protect the amps and milliamp ranges they have nothing to do with protecting the voltage input jacks and we'll take a look at the circuitry and see why so you can rip those fuses on your multimeter and you're still going to be able to measure volts ohms and capacitance and diodes and all that sort of regular stuff except current and that has its own input protection circuitry as well that's where the mob's and the PTC's you've heard me talk of come into it so let's get the white board and here it is we're going to have a crack at basic input protection circuitry on a multimeter and this is actually the exact input circuitry or pretty close to the exact input circuitry for the fluke 27 multimeter and I'll link in the service manual down there in the description and you can download it and have a look at it yourself so we'll be able to actually physically see these components and how they work and we'll show inside shots of the multimeter as well but I did the teardown in the last video so this is fairly typical but I just say upfront the input protection of multimeters changes are fair bit from manufacturer to manufacturer dependence on how the ad sees out here work and the the requirements for protecting those and the network dividers they put dividers they use and all sorts of things and the measurement ranges and types it varies a lot okay so this is typical of a basic multimeter volts ohms amps and that's you know pretty much it so let's have a look at what we have here we have our input jacks our four input jacks we've got ground down here we've got our amps input jack our 10 amp input jack we've got our milliamps and micro amp input jack and we've got our volts ohms and diode jack up here and as I said before they are completely separate input protection circuits so basically let's look at the amps and the fuse protection first and forget all about this circuitry at the top does not exist so let's look at the 10 amp input jack first and you're probably familiar with this and it works exactly as you'd expect we've got our ground input here and it goes through a current shunt resistor in this case 5 milli ohms it's that little copper strap that you can see inside the multimeter and it then where it goes through our 11 amp fuse here in this case it's a chassis or a high rupture capacity fuse so if you accidentally shot this to a power supply which is capable of delivering say 50 amps and you have 50 s flowing through this thing this fuse is going to the element inside it's got to heat up and it's going to blow fairly quickly in the case of 50 amps but if you only if a multimeter has say 10 amp input range and you've got an 11-hour fuse it's not going to suddenly blow at 11 amps it'll take quite a hell of amount of time to actually do that so some multimeters um say for example they'll actually say in the spec so it can measure 20 amps for you know 10 seconds or something like that so the fuse isn't going to blow and your input current shunts not going to heat up too much and be damaged and it can absorb those minor overload conditions but the idea with the HRC fuse like this high rupture capacity if you accidentally connect this amps jack across the mains for example 240 volt or 110 volt supply there's a lot of energy in the mains system right add 2400 watts for example for our 240 volt Australian thing here that's continuous energy 2400 watts but it's actually capable of a lot more energy than that instantaneously and that's the point with using high rupture capacity fuses if you can accidentally connect this up to a high energy system energy with lots of joules go look that up then it's capable of delivering a lot of energy into your multimeter and if you're familiar with like a bar radiator heater that uses those bars 2400 watts that's a lot of heat right all of that try to be dissipated your tiny little multi meter input circuitry something's going to go boom like that and it's just going to blow the crap out of your meter and blast everywhere sometimes flames will shoot out all sorts of stuff pretty horrible thing and the multimeter can catch on fire all sorts of stuff so what this HRC fuse does is tries to contain all of that energy with inside its body it's actually the fuse wire itself if you see a regular glass fuse it's only just a you know a bit of what a fuse wire inside a glass tube and that can actually blast open and then arc over and the energy continues to flow but these HR CF users have got sand inside them or other material that can absorb the energy and stops all of that arcing over so it's very important to have a multimeter with HRC fuses and also the test leads you use in the test system can have inductance in it as well so when the few is actually opens you can get an inductive kickback back into your multimeter which then can cause an extra voltage high voltage overload it can get really nasty now similar sort of things going to happen on your milliamp and your micro amp range as well they use a separate input jack at a lower 440 milliamp fusing this case but it's also HSE few is because the same sort of gross overload conditions can apply but in this case the milliamp and microwave range instead of flowing through straight through the high current shunt like that actually it goes through the range switch itself hence if you get a really gross overload the micro amps and milliamps jack you might blow your range switch to potentially if it's not designed well enough because I'll have those you know PCB traces on your you know those rain switches can be designed on your PCB traces and if the contacts aren't designed right bang can blow the arse out of your range switch to but basically on say the milliamp range what it does is just from a circuit topology point of view it actually puts in in this case in that case of the flu twenty seven it's a four point nine nine five ohm resistor why that oddball value why not five because it's in series with the point double O 5 ohm resistor here so it's five ohms total and that's your shunt resistor for your milliamp range and then when you switch to micro amp range it disconnects these two resistors here and connects a 500 ohm range so your shunt resistor is higher on your micro amp range like that but both cases okay the voltage is tapped off here and that goes into your ADC and your you know measurement circuitry and dividers and all sorts of stuff to measure your signal but what's all this weird-looking diode bridge and a whole bunch of diodes doing around here well if you've looked inside a lot of multimeters on their import protection circuitry may see a typical diode bridge there it's not huge power don't you know basic one in four SS sevens or might actually have a you know a 4 terminal diode bridge itself in there why is that there and how is it connected well it's actually connected directly across your shunt measurement input here be it your microwave one your milliamp one it's connected directly across your input circuitry and the reason it does that is because if you short out a power supply accidentally and there's huge amount of current flowing through here well these resistors here they can actually heat up as well if you get a large voltage connected directly across your shunt resistor in here then you can blow your shunt resistor before you blow your fuse because the fuse actually takes some time to heat up as we explained so what they do is they add this diode protection across here not only does it protect your input circuitry here by limiting it into one daya we've explained how this works is complex arrangement works at them in a minute but it basically clamps the voltage across here and hence the voltage across your shunt resistor to a low voltage and that will ensure that during that time current will flow through the diodes and then it will ensure that the fuse has time to blow so it's basically a protection mechanism not only for your input circuitry but also to ensure that the fuse blows instead of your shunt resistor so how does this work exactly and why are all these diodes here well let's that we input a positive voltage here for example air we've got a positive voltage here with respect to down here and it's an overload input condition okay and the fuse hasn't blown yet okay so let's say you know it's 10 volts or something right now normally the voltage across your shunt resistor is going to be quite small that's your burden voltage you remember I've done videos on that my microcurrent solves that sort of issue I've done a whole article on it in silicon chip if you want to go read that sort of thing your input shunt resistor on your current ranges is usually only going to drop you know a couple hundred millivolts or a volt or you know someone some really bad multimeters might be a couple of volts or up to 10 volts typically but the Fluke 27 they've decided that they need one diode bridge plus four extra diodes in here and let's have a look at what happens here okay you've got a positive input voltage here so currents going to flow down here like this it's not going to flow through there because that diode is reverse-biased it's going to flow through here and then it's not going to flow through there because that diode is reverse vice you remember follow the arrow on the diode that's how that's a great thing about the diode symbol so it's going to go flow through here through here through here through here through here it can't go back up there because it's reverse biased so it's going to go flew here and flow down there to ground so we now have one two three four five six diodes in series across there bingo we've just protected I mean you note six times 0.6 volts you know three point six volts or something like that drop so really that is a protection mechanism diode protection mechanism for the input voltage here is going to be limited to the drop across those diodes and hence the voltage across your shunt resistor is also going to be limited use Ohm's law work out the power at maximum power in the resistor during the time it takes for the fuses to blow that's how that's why they use a diode bridge circuit like that and why do they use a Brij because you might put a negative voltage in here and a positive voltage on here who knows what the idiot user is going to do they can swap their leads around oh oh you've got AC or something like that then that is going to work either way indicate in the case of negative here and positive here it'll just flow the this flow the other way through these diodes and of course you don't actually need these diodes here there you only add those extra diodes in if you want to increase that voltage because you've got a high burden voltage multimeter for whatever reason to do with your a DC's or whatever it is it doesn't matter so they've added so you could add in those extra diodes if you need to increase that for your burden voltage or you can simply short out your diode bridge like that if your burden voltage is under two diode drops or 1.2 volts and this diode bridge and these diets don't particularly have to be all that fast or all that high power you know standard one in for double-oh-seven stuff is what's used inside the flute twenty-seven perfectly adequate to dissipate enough power and to be fast enough and it doesn't have to be that quick because the fuse is going to take some time to blow anyway fuses don't blow instantly they take seconds to blow so any properly designed multimeter is going to have some sort of diode bridge protection on the current ranges like that it's just a belt and braces approach additional protection for your input circuitry and for your current shunt resistors over and above the fuses because fuses blow all the time right people set it to the wrong current range they accidentally measure volts or whatever and you blow the arse out of your fuses you know you should keep half a dozen of things in stock just because it happens all the time and just add in this extra input circuitry can ensure that the fuse blows and nothing else so if you see a multimeter without some sort of additional amps protection like this diode bridge and it's not designed that well and you'll notice there's one extra resistor down here 100k resistor and that goes off to the rest of the secretary in this case it doesn't actually go to the signal ground inside the multimeter because there a differential input ADC so it goes to the ADC low input pin like that so you've also got some extra I mean there's only so much current that can flow through 100k resistor even after you've clamped all your voltages and stuff like that SuperDuper safe so now we're going to have a look at the volts ohms and diode jack and capacitance jack for that matter so all this no longer exists that these fuses have nothing at all to do with protecting the voltage Jack common misconception out there so let's have a look at it and the input protection is actually pretty basic it's not as complicated as it looks because actually pretend that these two components don't exist these are just extra components that happen to be in the Fluke because of its a DS type of ADC and its input combination so these two do not exist all we've got is these resistor II looking devices here so let's take a look at them right our voltage input jack here we've got a standard resistor in the case of the fleet 27 it's a 5 a 3.5 k wire where our resistor it's going to be a high temperature one high power one you know it's going to be a pretty big input resistor there and then that's insert permanently in series with the input that's the first thing you're going to have then you're going to have what's called I often call them ptcs or the other name for them is all the more correct name is a thermistor and it's PTSD ptc stands for positive temperature coefficient it's a nominal 1k resistor hence the value and the little dot that's what the symbol for a thermistor is if you see a resistor over a zigzaggy one or a square one with a little dot next to it it means it's a thermistor and what it does is if the temperature of that device Rises ie positive temperature going up it's got a positive temperature coefficient so the resistance goes up so if you get an overload condition where there's too much current flowing through here and using Ohm's law too much power dissipated this resistor then the resistance is going to go up so it's self-protecting mechanism but these things just like a similar diffusers they act quite slowly they have that thermal inertia and require heating up that thermal mass inside in order for the value to go up so they're not an instantaneous device this is a slow effectively a slow blow protection device for want of a better term for your input jacks so this is for these are very slow rising inputs you know if it's a thousand volt multimeter and you slowly ramp it up to you know two thousand volts or something then this thing is going to eventually kick in for that slow stuff so if your multimeter doesn't have at least one PTC one thermistor in series with the input usually you can see it you can follow the trace in it's pretty simple and if it's not in series with the input in somehow and it's a piss-poor designed multimeter keep a junk you shouldn't touch it now the next thing we need are something to handle the transients there's really fast inputs because this is something a lot of people don't understand with multimeters they think it's you know I'm only measuring mains it's only 2:30 you know 240 volts or something like that you know my multimeter is rated to a thousand volts what's the problem it's all about transients very fast transients is there a lightning strike on the line is there a you know a huge industrial motor on there are other industrial machinery that's causing inductive kick back onto your line you can get massive surges on your power lines all the time and that's where the cat ratings come into it I'm I won't go into cat ratings fully here you can look that up but basically cat one you know the lowest rating means don't use it on anything to do with a high-energy circuit that can have these high-energy impulses on them potentially these low impedance circuits like a mains thing so a cat 2 is the minimum you need for that cat 3 again would be say a distance Oh cat 2 might be you pickle mains outlet something like that cat three would be your typical switchboard or something like that and cat four on top of that means your real you know your as substation you know your main distribution panel for a whole site because that's where the high energy spikes can be higher in energy big voltage spikes things like that so this is where we need some input protection circuitry these very fast transient pulses and those cat ratings will be defined by how many high voltage transient pulses they can survive and anywhere up to 8 kilovolts so your thousand volt rated meter if it's cat 3 it's actually designed to survive six or eight kilovolt transient voltages and the way it does that that PTC is not going to help you at all doesn't have time to heat up and raise the temperature so we use moves metal oxide varistors you've heard me mention these before I'll point them out they're usually these round radial devices inside the multimeter a really big and chunky usually and we'll get into that why that's important in the moment and you need one of those from here to ground when I say ground it's the internal ground of the multimeter it's ie back to the input jack here it's not necessary when I say ground it's the input jack ground not necessarily the logic ground inside the multimeter now a month or metal oxide varistor is has a symbol like this a standard resistor symbol with that little squiggly line going through it like that and a metal oxide varistor they're normally open circuit completely open circuit so you can have one of these usually it just ignore that there's four there at the moment let's assume that that one goes down to our ground point down there because in theory you only need the one okay and normally it's open circuit so that resistor doesn't exist it doesn't affect anything at all but if it exceeds its nominal rated voltage in this case in the flute twenty seven 430 volts then it will there we quickly clamp down hence sort of the hysteresis kind of symbol in there like that once it reaches that threshold bang it'll clamp down and go very low impedance and shunt all of the current down through there like that which then will cause the PTC to heat up relatively quickly so this absorbs all of that pulse energy like that and because it's very low impedance there's going to be a very low voltage across it to then go into the multimeter but these things can act you know extremely quickly you know microseconds nanoseconds that kind of stuff very quickly and then that causes a PTC to heat up which then let's have a look at sure a quick little crude graph here so this is I okay you got current like this and this is t4 time okay so your current is down like this and it suddenly goes wham straight up like that and then it's going to roll off something like that as that ptc heats up and that's also why you need a big high wattage wire round high voltage resistor here high-power high temperature resistor in series with this because it needs to dissipate that heat as well when that mod is switched on absorbing that you know the mob doesn't magically absorb the energy it's got to flow through these two resistors as well so this has to have adequate power dissipation as well during that time before the PTC goes way up in value and it goes up in the mega ohms range and there's no more current flowing through there and your multimeters protected but you're asking why is it for in series like this remember this doesn't exist yet we're not talking about that yet this is why are they using fluke in the flu 2007 using four of them in series instead of one well you can actually get away with using one you could have one a big one at a thousand volts but it's better to actually put multiple ones in series not only can you dissipate more power but then you get greater creepage distance by the physical gap so you'll see them off basically in series so the gap might be you know two or three millimeters between there that's going to have X amount of voltage breakdown and then you've got another couple of millimeters as you step up to the next one and the next one and the next one so you you're increasing your creepage distance so you don't get arc over the single move like that for high voltage transients and then you're dissipating energy in multiple devices which is much much better than relying on a single mode for both your input creepage distance I mean you can have a single mode and then you can cut an isolation slot underneath physical isolation slot in the board between it like that and you know you can probably get away with that not a problem but you can avoid having to do that and design an extra safety margin by having multiple ones in series in the case of the flu twenty seven they got four four hundred and thirty volt ones in series so that's about seventeen hundred volts so the flu twenty seven won't start clamping until the input gets to about seventeen hundred volts well above the rated thousand volt input measurement range but of course you can bet your bottom dollar that the rest of the input circuitry all in here is going to survive that seventeen hundred volts just fine it's the measurement range is just limited to a thousand volts now of course the rest of this input circuitry here here's where your 10 Meg input resistor might be and sometimes you'll have a higher value input resistor in here as well so that's why your nominal 10 mega ohm input resistance multimeter might actually be 11 Meg ohms or something higher than 10 mega is because there's the 10 mega ohm resistor plus the input protection resistors as well and a good multimeter will actually have additional clamp in pass this point as well in the terms of the flute I haven't drawn it here didn't really have room in terms of the Fluke 27 if you go look at the schematic diagram it's got actually extra transistors which switch on when the voltage level gets over a certain point so they switch on and clamp the is down so you can have all you could have extra you know an extra mullahs or something or some other extra input protection circuitry after this main one this one here what I've shown is the main one that's designed to absorb all of that input energy and meet that cat rating requirement that and get the certification for the cat rating so that meets a certain input our pulse in terms of voltage and energy level and time so that's the important stuff anything else over here is just bonus stuff the manufacturer will include just to add some extra belt and braces protection for the ADC and the rest of the circuitry and what's this I hear you ask well this is actually just a quirk due to the input switching requirements and stuff like that in the case of the flu twenty seven they have a 1 megaohm high voltage hybrid ceramic resistor there and they have another 430 volt mod there which just uses that existing is just another path to protect another input over here to the ADC all multimeters they all know the ADCs and chipsets and voltage dividers all over here I have lots of different complex configurations often and they will require other configuration so that's why I just showed this one here because when we open the multimeter will see these stuff so let's do that let's see if we can see all this stuff inside our multimeter any previous videos of multimeter tear downs you've seen me point out high voltage isolation slots and they're typically between input jacks between components so they might physically have a barrier between the voltage jack and ground or the current Jack's they might physically have a as we've mentioned physically have a barrier you know underneath or between or around for example they might you know physically have a barrier around all these moles so it doesn't arc over to other components like the nearby range switch which is typically the closest thing to the input protection circuitry and the jacks so you know that those high voltage isolate patience lots will be totally dependent upon the physical design physical construction of the unit if you've got enough room inside there if you're a huge being multimedia enough room to lay it all out you don't necessarily need any high voltage isolation slots but when you can multimedia gets smaller and smaller all that stuff sort of cramped in there those high voltage isolation slots can be very important because remember high voltage not only just DC but high-voltage impulses can actually jump across distances across your board got to be careful of that all right let's see if we can find all this stuff in our fluke 27 PCB here okay we've got our common Jack down here which has that little input choke down in there not all multimeters have that that's just something that's flukes added just to take the edge off input pulses presumably this is our voltage input Jack this is our amps jack and that's how milliamps Jack down there so let's follow this through and see what we get here's our voltage input jack here so a voltage input jack is going to go through a 3.5 KYR we're on resistor then a 1k thermistor where's that here it is it's connected directly to the 3.5 K wire round resistor there that's a high power high energy resistor probably high temperature as well and there is our PTC we can't actually see it there's I don't think there's any real markings on that one but anyway I think it's a Rome brand PTC but there it is and the trace from that PTC actually goes under the bottom two here this top part and then goes off to the rain switch and through all the rest of it and then we've also got our 1 Meg high voltage ceramic resistor here and here it is there's the input jack and there's the high voltage ceramic resistor check that out isn't that beautiful and then that goes into a mob that goes down to these four five red devices here are the mobs so it goes into the extra month there and you'll notice that a tap goes off there over into the rain switch that's why they need the extra protection here because this bug is off to a rain switch then ultimately goes off to the ADC somewhere so they're just protecting this extra input that they require for some reason over on the ADC input and range switching circuitry but then the other moves then of course come back here and three there's our three series mobs there plus our extra one there and they bugger off back to return back to the grounded input actually I'm not particularly keen on this fluke 27 with the return path for these mobs that actually branches down here goes through this via down here and then snakes its way through a couple of trees a trace down here going back to the ground point which comes from the wire not at not the best example I'm afraid in this particular case of a return path back to ground for the mobs and the high energy impulse but it's good enough to meet the cat ratings and here's an example of a little high voltage isolation slot there between this particular terminal and this one and we've got more high voltage isolation slots up here because this is all past the input protection circuitry this is still high voltage in fact these caps here are rated the these little trimmer caps here rated to 1700 volts no coincidence and we've got four 430 volt the moths in series there or roughly 1700 volts so there's still potentially some high voltage flowing around this part of the circuitry so they've whacked in a couple of high voltage isolation slots as required and it's not terribly easy to see down in there but you can see the 10 amp current shunt with its dual terminals here which I've shown tapped off like that for terminal measurement technique on that current shut but you can see that these this is probably not the best example the flew 27 in terms of physical input construction I chose it because of its a schematic input which I had available its physical construction is a quite old school but once again you can see some high-voltage isolation slots in there like that and discrete input wires say from the ABS well the amps jack goes can put straight into the fuse over there high rupture capacity fuse down over there into here back to ground through the shunt and ah it's hard to follow but you get the idea let's take a look at the flute 28 series to the modern replacement for the fluke 27 we've just looked at and it has a much tidier input circuitry here but let's see if it's similar after all these years I mean this is the fluke 27 was designed a lot many decades ago whereas this flea 28 very recent now here's our ground input jack here's our voltage input jack and by the way I'll mention this one this meter has actually had the snot blown out of it we hooked it up if you've seen my previous video I did with Doug we hooked it up to a real high energy machine and we blew the crap out of this thing that's why the moths are blowing here and the PTC's seen better days but here we go here's our voltage input jack nice high voltage isolation slot look at that then they've gone right to the edge of the board cut that out completely around so it doesn't interfere with the fuse over here and right around like that beautiful that's as good as you could expect and then look exactly the same input circuitry we have a thermistor our t1 there in series with a high wattage resistor and there it is I mean they've got them the other way around but it doesn't matter which which way you put them in it makes no difference and then we have this one actually has three moles so they've maybe got more SPECT in more modern mods or something like that because this is not yes this is actually a cat 4 rated device so this is actually a higher rated energy device than the Fluke 27 but it's got fewer moths because these are better raided and once again they've put a couple in series just to get the high voltage there and they have also have that additional one here's that same ceramic is that that same high voltage one makes ceramic resistor here they've done it exactly the same again and the more so they've only they've just got to here instead of what instead of the four which is what's on the twenty seven so there you go it's exactly the same high voltage cap here and that voltage input circuitry practically identical after all these decades fluke no that that is a good input protection scheme and as for the fuses again once again high voltage isolation slots around there so it protects that circuitry there from the physical jack itself high rupture capacity fuses of course you wouldn't expect any less there's the 10 amp current shunt with the four terminal connection technique there and what do you know that bridge rectifier and it looks like a single diodes it looks like they've got bridge rectifier with a single diode across there and there so they don't obviously don't need as higher voltage protection on this input as they did on the fluke 27 but it's all there it's exactly the same configuration and one thing you've seen me talk about before is blast protection inside the multimeter not only on the deep ribs that go in the case like that so all the energy and any explosions are contain or try to be contained within the multimeter instead of when you're holding it bang and it blows your damn hand off for those high-energy circuits then they can be designed if they designed nicely not only the deep walls there but they'll have internal blast shields as well like this flute 28 series to that just goes in there and actually fits between look it actually fits between these high-voltage isolation slots that that there matches up perfectly with that shape around there and it physically separates everything not only with the air-gap on the bald air the isolation slot but physically the blast protection between the plastic in the molded case that's a perfect example of a well-designed high-energy cap full rated input protected multimeter so there you have it there's actually not much to multimeter input protection it all pretty much comes down to HRC fuses isolation slots a little bit of circuitry to ensure that the fuse is low instead of the shunt resistors on the voltage side the PTC is one of the keys along with the mob so if you're if the multimeter you open doesn't have a PTC am of a bridge rectifier and HRC fuses then it's probably not designed as well as a quality fluke and it's probably a heap of garbage so this is a very good baseline to look at when you're evaluating a multimeter yourself to see if it's safe if you've got some brand you've never heard of before open it up check it out check out the high voltage isolation slots the HRC fuses bridge rectifiers ptcs mobs if it doesn't have all that sort of stuff happening and you know Claridge and creepage distances and all that sort of thing if all that's not going on then you know it's just a one hung low slap together cheapy multimeter and they don't really know what they're doing and it's unsafe and really should only be used for mucking around on low voltage stuff not high energy circuits so that's pretty much the basics of multimeter input protection I hope you enjoyed that and if you want to discuss it jumping over to the EUV blog forum and if you liked it please give it a big thumbs up - thumbs up I don't think you can do that catch you next time

Channel: EEVblog

Views: 96,621

Keywords: fluke multimeter, fluke 27, fluke 28, fluke, multimeter, input, protection, cat, rating, hrc, fuse, mov, thermistor, tutorial

Id: zUhnGp5vh60

Length: 39min 51sec (2391 seconds)

Published: Thu Oct 18 2012