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 cat ratings and Surge overloads and HRC fuses. And you know isolation slots which if you've seen me tear down a multimeter 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? Uh, Fluke 27? that I just we just had a look at in the previous video how the input protection on this works. So I thought we'd take the schematic for the Fluk 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 Dio Jack which also does capacitance and other things. And usually we've got a separate amps and a separate milliamps and microamps. Jack Now I Know some some multimeters, some cheap ones will actually combine milliamps and microamps with the volt. Ohm d 1 Don't worry about that is one will focus on separate amps and milliamps and you'll notice that they're also fused.

This one's fused at 320 milliamps maximum and well at 10 amps maximum. so the fuses are going to be slightly above that in this case, 440 mamps and 11 amps for the amps 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 out of your multimeter and you're still going to be able to measure volts, ohms, and capacitance, and dodes and all that sort of regular stuff except current and that has its own input protection circuitry as well. That's where the Ms and the Ptcs you've heard me talk of come into it. So let's get the Whiteboard 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 will show inside shots of the multimedia as well.

But I did the Uh tear down in the last video, so this is fairly typical. But I'll just say up front. the input protection of multimeters changes a fair bit from manufacturer to manufacturer, depending on how the Adcs out here work and the the requirements for protecting those and the network dividers they the input 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 Um Amps, input jack, our 10 amp input jack, we've got our Milliamps and microamp input jack, and we've got our Volts, Ohms and Diode jack up here. And as I said before, they are compl completely separate input protection circuit.

So basically let's look at the amps and the fuse protection first and forget all about this circuitry. Up 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 milliohms.

It's that little copper strap that you can see inside the multimeter and it then we goes through our 11 amp fuse. here. In this case, it's a HRC 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 amps flowing through this thing, this fuse is going to the element inside.

It's going 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 uh, input range and you've got an 11 amp 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 were actually saying the specs oh, it can measure 20 amps for, you know, 10 seconds or something like that.

So the fuse isn't going to blow and your uh input current shance not going to heat up too much and be damaged and it can, uh, absorb those minor overload conditions. But the idea with a 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 Main's system right at 2400 Watts For example, for our 240 volt Australian thing here, that's continuous energy 2400 Watts But it's actually capable of lot more energy than that instantaneously. And that's the point with using High rupture capacity fuses. If you connect, accidentally, connect this up to a high energy system.

energy with lots of jewels. Go look that up. Um, 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 wats, that's a lot of heat, right? All of that trying to be dissipated in your tiny little multimeter 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 Moldy meter 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 a bit of Wi fuse wire inside a glass tube and that can actually blast open and then Arc over and the energy continues to flow. But these HRC fusers 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 uh, the test leads you use in the test uh system can have inductance in it as well. so when the fuse 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 microamp range as well. They use a separate input jack and a lower 440 Mila fuse in this case, but it's also a HRC fuse cuz the same sort of gross overload conditions can apply, but in this case, the milliamp microamp range instead of flowing through straight through the high current shum like that actually. um, it goes through the range switch itself. Hence if you get a really gross overload on the microamps and milliamps Jack you might blow your range switch too.

Potentially if it's not designed well enough. cuz I'll have those you know PCB traces on your you know you know those rain switches can be designed on your PCB tracers. And if the contacts aren't designed right, bang, you can blow the ass out of your range switch too. 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 the case of the Flute 27, it's a 4995 ohm resistor. Why that ODB value? Why not five? Because it's in series with the5 ohm resistor here. so it's 5 ohms total.

Um, that's your shunt resistor for your milliamp range. and then, uh, when you switch to microamp range, it disconnects these two resistors here and connects a 500 ohm range. so your shunt resistor is higher on your microamp range like that. But in both cases.

okay, the voltage is tapped off here and that goes into your uh 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 Dodes doing around here? Well, if you've looked inside a lot of multimeters on their input protection circuitry, you may see a typical Diode Bridge there. It's not huge powered. Um, you know basic 1 and 47s or might actually have a you know, a four terminal uh Bio 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 uh, microa one, your millia 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 a 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 to one di, we explain how this works this complex Arrangement works at the 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 dodes 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 assume that we input a positive volage here, for example. and 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 shutt resistor is going to be, uh, quite small.

That's your burden voltage, you remember I've done videos on that. My microcurrent solves that sort of issue. 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 Mill volts or a volt? or you know, some one. 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. Dio Bridge plus four extra doodes in in here.

And let's have a look at what happens here. Okay, you got a positive input voltage here. So current's going to flow down here like this. It's not going to flow through there because that Diode is reversed biased.

It's going to flow through here and then it's not going to flow through there. CU That diode's reverse VI 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 cuz it's reverse biased. so it's going to go flu here and flow down there to ground. So we now have 1, 2, 3, 4, 5, 6 diodes in series across there. Bingo, We've just protected.

I Mean, you know, 6times? Uh, 6 volts? You know, 3.6 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: OHS Law: Work out the power maximum power in the resistor during the time it takes for the fuses to blow. That's how that's why they use a Diod bridge circuit like that.

And why do they use a diode? Bridge 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 leads around or you've got AC or something like that, then that is going to work either way In the CA In the case of negative here, and positive here, it'll just flow the just flow the other way through these dodes. And of course, you don't actually need these diodes here there.

You only add those extra diodes in if you want to increase. uh, that voltage because you've got a high burden voltage multimeter for whatever reason to De with your ADC 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 dodes don't particularly have to be all that fast or all that high power, you know. Standard 1 in 47 stuff is what's used inside the flute 27 perfectly. add liquid, uh to dissipate enough power and to be, uh, fast enough And you know 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 Dio 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 fusers blow all the time, right? right? People set it to the wrong current range to accidentally measure volts or whatever and you blow the ass out of your fuses. You know you should keep half a dozen of the things in stock just because it happens all the time.

And just adding this extra input circuitry could 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 Eh, 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 circuitry. In this case, it doesn't actually go to the signal ground inside uh, the multimeter because they're a differential input. ADC So it goes to the ADC low input pin like that.

So you've also got some extra I Mean you know there's only so much current that can flow through 100K resistor even after you've clamped all your voltages and stuff like that? Super duper safe. So now we're going to have a look at the Volts own S and Diode Jack and capacitance Jack for that matter. So all this no longer exists. These fusers 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 ad type of ADC its input uh, combination.

So these two do not exist. All we've got is these resisto 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 Flute 27.

it's a 5 a 3.5k wire W 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 in 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 Ptc's or the other name for them is or the more correct name is a theist and it's PT 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 themister is. If you see a resistor either a zigzaggy one or a square one with a little dot next to it, it means it's a themister. And what it does is if the temperature of that device Rises I.E 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 Ohms law, too much power dissipated in this resistor, then the resistance is going to go up. So it's a self-protecting mechanism, but these things just like 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 jack.

So this is for these are your very slow Rising inputs. You know if it's a, volt multimeter and you slowly ramp it up to you know, 2,000 volts or something, then this thing is going to eventually kick in. It's for that slow stuff. so if your multimeter doesn't have at least one PTC one themister in series with with the input usually 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 design multimeter, it's he a junk. You shouldn't touch it. Now the next thing we need are something to handle the transient.

So it's really fast inputs because this is something a lot of people don't understand with multimeters. They think, ah, it's you know I'm only measuring the mains, it's only 230, You know, 240 volts or something like that. You know my multimeter is rated to 1,000 volts. What's the problem? It's all about transience.

Very fast transients. Is there a lightning strike on the line? Is there? Uh, A? You know, a huge industrial motor on? There are other industrial Machinery that's causing inductive Kickback onto your line. You can get massive surges on your lines all the time and that's where the Cat ratings come into it. Um 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 Main's thing. So a Cat 2 is the minimum you need for that. Cat Three again would be say add so Cat Two might be your typical uh Main's Outlet Um, something like that.

Cat 3 would be your typical switchboard or something like that. and Cat 4 on top of that means your real you know your Uh 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 for 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. Kilts So your th000 volt rated meter If it's Cat 3, it's actually designed to survive 6 or 8 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 Ms metal oxide Vistas You've heard me mention these before.

I'll Point them out. They're usually these round radial devices inside the multimeter. You know, 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 I IE Back to the input jack. Here, it's not necess.

sorry. When I say ground, it's the input jack ground, not necessarily the logic ground inside the multimeter. Now, a Mau Metal Oxide varista is has a symbol like this: a standard resistor symbol with that little squiggly line going through it like that, and a metal oxide fista. They're normally open circuit, completely open circuit, so you can have one of these Usually just ignore that there's four there at the moment.

Let's assume that that one goes down to our ground Point down there cuz in theory you only need the one. Okay, and normally it's open circuit so that resistor doesn't ex doesn't exist. It doesn't affect anything at all. But if it exceeds its nominal R voltage in this case, in the flute 27 430 volts then it will very 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, micros seconds Nan that kind of stuff very quickly and then that causes a PTC to heat up. Which then let's have a look at.

Draw a quick little crude graph here. so this is I Okay, you got current like this and this is T for 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.

W High voltage resistor here High Power High temperature resistor in series with this cuz it needs to dissipate that heat as well. When that Mauve is switched on absorbing that, you know the M 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 there four 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 Fluke? 27 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 1,000 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 all physically in series. so the Gap might be you know 2 or 3 mm 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 you're dissipating energy in multiple devices. which is much much better than relying on a single move.

for, uh, both. your input creepage distance I mean you could have a single Mo and then you can cut an isolation slot under. You know, a physical isolation slot in the board between it like that and you know you can probably get away with that. Um, not a problem, but you can avoid having to do that and designing extra safety margin by having multiple ones in series.

In the case of the Fluke 27, they got four uh, 430 volt ones in series, so that's about 1,700 volts so the Fluke 27 won't start clamping until the input gets to about, 1700 volts, well above the rated th000 volt input measurement range. But of course you could bet your bottom dollar that the rest of the input circuitry all in here is going to survive that 1700 volts just fine. Its Uh measurement range is just limited to 1,000 volts. Now, of course, the rest of this input circuitry here.

Here's where your 10 Meg input resistor might might be, and sometimes you'll have a higher value input resistor in here as well. So that's why, um, your nominal 10 Megga Ohm input resistance multimeter might actually be 11 megga Ohms or something higher than 10 megga. OHS Because there's the 10 m resistor plus the input protection resistors as well, and a good multimeter will actually have additional clamping past this point as well. In the terms of the fleet: I Haven't drawn it here.

Didn't really have room in terms 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 voltage down so you can have or you can have extra.

You know, an extra M is something or some other extra input. Prot 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 it meets a certain input Uh 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 Fluke 27, they have a 1 Megga high voltage uh hybrid ceramic resistor there and they have another 430 volt move there which just uses that existing. It's just another path to protect another input. Over here to the ADC all multimeters they all they the Adcs and chipsets and voltage dividers all over here. We have lots of different complex configuration often and they will require other configurations so that's why I Just showed this one here because when we open the multimeter, we'll see this stuff.

so let's do that. Let's see if we can see all this stuff inside our multimeter and in 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 Uh current. Jacks They might physically have have a as we've mentioned, physically have a barrier uh, you know, underneath or between or around.

For example, they might you know physically have a barrier around all these Ms so it doesn't Arc over to other components like the nearby range switch, which is typically the closest thing to the Uh input protection circuitry and the Jacks. So um, you know that those high voltage isolation slots will be totally dependent upon the physical design uh, physical construction of the unit. If you've got enough room inside there, if you got a huge, big multimeter, you got enough room to lay it all out. You don't necessarily need any high voltage isolation slots, but when your multimeter get smaller and smaller, all that stuff sort of cramped in there.

Um, those high voltage isolation slots can be very important CU Remember high voltage? Um, uh, not only just DC, but high voltage impulses can actually jump across distances across your board. Got to be care 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 fluke added just to take the edge off input pulses. Presumably this is our voltage input jack.

This is our amps Jack and that's our milliamps Jack down there. So let's let's follow these through and see what we get. Here's our voltage input jack here. So our voltage input jack is going to go through a 3.5k wire wound resistor, then a 1K The where's that? Here it is.

It's connected directly to the 3.5k wire W 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 this I don't think there's any real markings on that one. but anyway, I think it's a Rome brand.

uh PTC But there it is and the trace from that PTC actually goes under the bottom to 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 1meg 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? Beautiful. And then that goes into a move that Uh goes down to these four five red devices. Here are the MS So it goes into the extra Mve there and you'll notice that a tap goes off there over into the range switch.

That's why they need the extra protection here. CU This bug is off to a range switch is 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 Ms.

Then of course come back here. and three, there's our three series Ms there plus our extra one there and they buger off back to return back to the grounded input. Actually, I'm not particularly keen on this fluke 27 with the Uh return path for these Ms it actually branches down here, goes through this via down here, and then snakes its way through a couple of Trace Trace down here, going back to the ground point which comes from The Wire Not not the best example I'm afraid in this particular case of a return path back to ground for the Ms 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 teral and this one.

and we've got more high voltage isolation slots up here cuz this is all past the input protection circuitry. this is still high voltage. In fact, these Uh caps here are rated the these little Uh trimmer caps here, rated to 1700 volts. No coincidence, we' got four 430 volt Ms in series there, or roughly 1,700 volt.

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 shum with its dual terminals here, which I've shown tapped off like that four terminal measurement technique on that current shutt.

but you can see that these this is probably not the best example of Fluke 27 in terms of physical input construction. I Chose it because of its uh, schematic input which I had available. Its physical construction is, uh, 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 amp. Well, the amps Jack goes complet 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. And let's take a look at the Fluke 28 Series 2 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: The Fluke 27 was designed a lot many decades ago, whereas this Fluke 28 very recent. Now here's our ground input jack.

Here's our voltage input jack. and um, by the way, I'll mention this one. This meter has actually had the snot blowing 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 MS are blowing here and the Ptcs are seen better days. But here we go. Here's our voltage input jack.

nice high voltage isolation slot. Look at that, they 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 Theist Rt1 there in series with a high wattage resist, 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. Ms So they've maybe got more, uh, specked in more modern Ms or something like that because this is no, 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 Ms because these are probably better rated. 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. uh, that same high voltage 1meg ceramic resistor here. They've done it exactly the same again.

and the Mau So they've only they've just got two here instead of uh, what instead of the four, which is what's on the 27. 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 fusers, 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 Uh four terminal connection technique there.

and what do you know, that bridge rectifier and it looks like a single died so 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 Um voltage protection on this input as they did on the Fluke 2 7. 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 multimedia not only on the Deep ribs that, uh, go in the case like that. So all the energy and any explosions are contained or try to be contained within the multimeter instead of when you're holding it bang and it blows your damn hand off. Um, for those high energy circuits, then they can be designed If they're designed nicely. not only the Deep walls there, but they'll have internal blast Shields as well.

Like this Fluke 28 Series 2 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 board there, 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 Cat 4 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 little bit of circuitry to ensure that the Fus is low instead of the shunt resistors on the voltage side. the PTC is one of the along with the Mve. so if your if the multimeter you open doesn't have a PTC a Mauve, 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 got some brand you've never heard of before, open it up. Check it out. Check out the high voltage isolation slots, the H fuses Bridge rectifiers Ptc's Ms if it doesn't have all that sort of stuff happening in uh, you know, clearage 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, jump on over to the Eev blog forum and if you like it, please give it a big thumbs up.

Two thumbs up I Don't think you can do that. Catch you next time.