Hi. This is just a quick follow-up video to the multimeter input protection video I did before and I'll link to it. uh down below. if you haven't seen that, it won't make any sense otherwise.

Now a lot of people asked and I think rightly so because it is a little bit confusing these this diode protection here. We're using the Fluke 27 as example. This is the schematic I used on the whiteboard in the previous video, but this is the actual schematic from the Fluke 27 Service manual and this diode Bridge Here and these four diodes here, which give a 3.6 volt nominal voltage drop um across the current shunt resistor. In this case, Up here, it's say 5 ohms up there for the Milliamp one, for example and they was, uh, asking, how do the manufacturer fluke in this case, ensure that these dodes won't blow before the fuse blows here I Mean the fuse is designed between these Here's the milliamps and microamps Jack here J2 And then the fuse is in series with Wp4 and Wp5 there.

Even though you can't see it, just assume that the fuse is in there like that. Now, how do we ensure that these diodes do not blow before this fuse blows? Because the fuse has a thermal Mass It takes time to heat up. It takes time to blow how, and so during that time that it blows. How do we ensure that these diodes don't go poof? The whole idea of this Circuit of course, is that the fuse is supposed to blow first and the diodes are supposed to just limit the voltage drop to protect the 5 ohm.

In this case, the 5 ohm uh, milliamp. Uh Current shut resistor up here. And if you do basic Ohms law, V^2 on R In this case, uh 3.6 Vol Squar on 5 Ohms is only 2 1/2 Wat Okay, so let's assume that you apply a voltage across the input. here.

it goes through the fuse a very low impedance Source By the way that's capable of you know, many tens of amps or even hundreds of amps or whatever. And and so you've effectively shorting out your input jack between J2 and J4, you've made a mistake: measure uh, volts on when you've got the multimeter lead plugged into the Jack amp. Oops, something's going to blow here. Is it going to be the fuse first? Or is it going to be these diodes or this resistor now ignoring the fuse for a second, The diode will limit that current and that voltage immediately to 3.6 volts across the current shunt resistor of uh, nominally 5 Ohms.

So that's only 2 1/2 Watts It's a 4 wat resistor here. As you can see, it's going to handle it. No problems at all now. I Mentioned that these diodes standard 1n 47s here I Mentioned that these are pretty slow types.

They're not shocky types. Now, that slow actually refers to the reverse recovery time of the diode. and in that case, we don't really care about that. That's why you don't need shocky diodes here.

all you care about is the switch on time, in which case, these diodes are effectively going to switch on instantly regardless of whether or not they're shock key or not. So there's no issue with the turn on time of these diodes here. They will switch on immediately, limit that voltage to 3.6 Vols across that 5 Ohm resistor, and we'll have many, many amps flowing through this fuse as well. Pretty much as much current as the voltage source you've hooked it up to can provide.

Now, Uh, these Dodes. They're only 1 amp dodes one in 47s, so you might think, well, one amp. That's not much at all. But aha, we'll go into the data sheets and we'll take a look at that Now this fuse.

We also need to know how long it takes this fuse to blow. So let's go into the data sheet for a fuse, shall we? Let's have a look here. Here's a typical Busman DMM multimeter fuse and these are the ones that fluke, uh, use and recommend as well as we'll look at ones from Little Fuse as well. The Flu series.

No surprises why it's called Flu. It's short for fluke because these are designed for fluke and other multimeters. It tells you. Designed for multimeters only.

Okay, and this is the F actual fuse specified for the Uh Flute 27. It's a 44 on 100 or 450 milliamp fuse HRC High rupture capacity fuse and notice that it is, uh, these are designed to. Here, it is intended to carry 100% of the rated current indefinitely. So that's just a little heads up on these fuses.

If it's rated to 440 milliamps, it's not going to blow at 440 milliamps. It's actually going to hold that current and never ever blow. It only blows when it's higher than that, and how much higher than that will determine how much time it takes. Now, if we go down here, we'll get some characteristic curves and these will tell us.

Now on the Y axis, we've got the time in seconds it takes to blow. So here's 1 second. This is the 1 second. Mark Here this is 1/10th of a second.

This is 10 seconds. This is 100 seconds. So let's actually this is the characteristic curve. We want the 44 on 100 The 440 Milliamp feuds.

Let's not worry about the 11 amps. it's going to be the same. So this video will just concentrate on this 440 milliamp fuse on the milliamp range. Now it'll blow in 1 second.

Here you go across and that's what the X axis here is. This is the current in milliamps. so this is 1 amp here. this is 100 milliamps down.

Here this is 10 amps here. this is 100 and 1,000 and so on. So let's see how long it takes this fuse to blow in 1 second time in seconds on the Y axis. Here extrapolate that we'll go up there.

There you go at 1.5 There it is. 1.5 amps. Uh, if you've got 1.5 amps flowing through this fuse, it will blow in one second. There you go.

Well, let's see what the lowest point on this characteristic curve is. It's down here At let's say that's 2.4 amps or something. So if you've got 2.4 amps flowing through that 440 milliamp fuse, then it's going to blow in 0.01 of a second or 10 milliseconds. It doesn't tell you anything faster than that, but of course you can see that the curve is slowly branching off like that.

Okay, so it's just going to get quicker and quicker. So if there's 100 amps flowing through that thing, it's going to blow. You know, practically instantaneously. Really? Okay, so there you go.

That is the fastest. Let's say the fastest. It's going to switch in 10 milliseconds or you know, at 2 and 1/2 amps. But let's say we've got say 5 amps flowing through that fuse.

Which is might be a reasonably, uh, you know, a good number to pick, a nice round number to pick. It's going to blow pretty quick. Right under 10 milliseconds, way under 10 milliseconds. It might be 1 millisecond or you know, 500 microc or something like that.

So aha, is that faster than our diode at that particular current? Now let's take a look at our diode here: 1 N47. In this case, we'll look at the bridge rectifier next. So we've got our 1 N47 diode here. and yes, it is a Bog Standard 1 amp diode, 1 amp.

Nor here it is average rectified output current at 75 C 1 amp. That's how these diodes are specified at their average rectified output current. So you might think this is absolutely useless if we got 5 amps flowing through this thing or more and is going to blow the ass out of this diode. And well, you know, um, buyby diode.

You have to repair your multimer. You can't just change your fuse. Aha, look at this. It also has a spec for nonrepetitive Peak Forward surge current specified at 8.3 millisecond single half sign wave superimposed on rated Uh load.

This is a pretty standard terminology which means half a sine wave at it can handle 30 amps. So this little piss and you know, weak 1 amp diode can actually handle 30 amps for 8.3 milliseconds without blowing. It can handle that single surge there. Not a problem, so easily handle 30 amps.

and for 8.3 milliseconds. We saw and now graph down here that even at 5 amps, it's going to blow in well under 10 milliseconds. Even at 3 amps here, it's going to blow in well under that 8.3 milliseconds. Bingo The fuse is going to blow first, and you can bet your bottom dollar when Fluke designed this multimeter, that's part of the design aspect.

The good engineering work gone into this. They would have looked at that diode data sheet and they would have went well. 30 amps? No problems where? Well, you know, we're an order of magnitude over where we need to be CU 30 amps. here.

look extrapolate that graph. It's ah, man. it's way down the bottom of the graph heaps quicker. So that fuse is definitely going to blow.

And we've only got a single figure there of 30 amps for a nonrepetitive, uh, single half sine wave? Well, what happens if you go over that? What if if you know you're AC source and you got multiple half sine waves there, it's it's It's certainly repetitive. What is the difference in the middle there? Well, you can go down to the characteristic curves down here. Here's the one we're interested at. this one here.

on the y-axis we've got the peak forward surge current in amps. There's the 30 amps that we had before number and the X-axis is a number of Cycles at 60 hertz Max non-repetitive forward Peak surge current. And there's 13 amps. So we got the single figure.

But that graph allows us to get more than that. So if you got 100 Cycles like that at 60 HZ so well, over a second, you're still talking 10. You know, almost 10 amps there. Sort of at that.

uh, sort of 1 second. Mark So that diode for a second is still going to hold 10. amps and it's a wimpy 1 amp diode. but aha, there's a diode bridge in there.

Well, this happens to be the same diode Bridge used in there. It's a Df2, so let's have a look. Once again, it's a nominal 1 amp rated diode. you know, pretty piss.

We kind of bog standard one you you'd use in your basic uh linear power supply. It's not a shocky type once again, but here you go look at this peak: forward surge current, single, half sine wave 50 amps. It's even better than the one in 4007 and that's for the Df2 which we're using here. Not a problem.

50 amps. We absolutely hit a home run there. This fuse is definitely going to blow first and if we want some, uh, double check on that. Here's the little fuse uh, brand one, the Flu series The Fluke series of course.

once again, we've got our nice characteristic graph here. Let's have a look at it. This one only goes down to 10 milliseconds as well. Doesn't go any faster the this one.

Let's have a look at: 1 amp, 2 amps, 3 amps, 4 amps, 5 amps. It's going to blow in 20 milliseconds there. Once again, if you were replacing this fuse and you weren't using the uh, the recommended fuse replacement, you might want to actually look at this these sort of Curves and see that you know, is this fuse suitable for my particular multimeter? I've got diodes that can handle you know, 50 amps for 8.3 milliseconds or something like that. Well, you look at these graphs, but pretty much all of these HRC fuses going to be within the same ballpark.

in terms of their uh, the average time versus current curve. here, they're all going to be pretty much the same. so you can go through the same thing with the micro AMP one and stuff like that because the 10 amp one pretty much isn't protected by this Uh Dio Bridge here because as you can see, the amps come ah sorry I can't really highlight this as I'm doing it, but the current comes directly in there, straight through there, straight back out. and really, it's only the sense line which goes off there.

So this D bridge is not applicable to the Uh 10 Amp current range. but there you have it. I Mean you've got other things that can blow in there, your PCB tracers, and stuff like that, your wiring, and all that sort of stuff. but usually they're going to handle pretty beefy amounts of current unless the multimeter is poorly designed.

So there you go. I Hope that uh, cleared it up and it was interesting. And if you want to discuss the video, jump on over to the Eev blog forum. and if you like it, please give it a big thumbs up.

Catch you next time.