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
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