# Measuring very low resistance of a fuse

#### smooth_jamie

Joined Jan 4, 2017
107
Hi All,

I was wondering if anyone had any ideas about this. I need to measure the resistance of a fuse with precision, and it has a very low resistance (0.01 Ohm range). I have an LCR meter but it is not precise enough.

Typically, I would consider using the 4-wire method and use Ohm's law but since this is a fuse rated at 2A, the maximum voltage I can apply is around 2 mV to avoid blowing it. Can anyone recommend either:
1. Equipment that I can purchase
2. A circuit I can make that will allow a precise measurement
Any input is greatly appreciated.

#### crutschow

Joined Mar 14, 2008
30,801
The best you can do is to use the 4-wire method with about 1A of current through the fuse.
You will, of course, need a sensitive meter to measure the 10mV voltage drop across the fuse, and the meter's required resolution and accuracy are determined by the needed accuracy for the resistance measurement.

#### RichardO

Joined May 4, 2013
2,271
As you can see below, you can run the fuse at a very high current for a short time.
At 135% of rated current you could have up to an hour to make the measurement. It might be less, though...

Keep in mind that the fuse resistance is probably temperature sensitive so the current you measure at will likely effect the resistance. You could pulse a very high current through the fuse for a short period of time to get a more accurate measurement.

Table from attached Littlefuse data sheet:

% of Ampere Rating Ampere Rating Opening Time
100% 0.062A – 35A 4 hours, Minimum
135% 0.062A – 35A 1 hour, Maximum
200% 0.062A – 10A 5 sec., Maximum

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#### tindel

Joined Sep 16, 2012
900
4-wire measurement, possibly with a differential amplifier for some gain if you want to do it at DC. Richard is right about thermal variance better to pulse it. This is a difficult measurement, but this may get you into reasonably accurate and repeatable results.

I'm sure someone has a literal PhD in measuring fuse resistance.

Why on earth do you need accurate fuse resistance measurements? Doesn't the fuse you're using publish Rmax possibly even over temp? If you're just messing around in your basement - who cares.

@RichardO just because this person has a fuse does not mean they can pulse the snot out of it like that datasheet describes. We have no idea what the P/N of the OP's fuse is. There are all types of fuses, some are more sensitive than others.

#### ebp

Joined Feb 8, 2018
2,332
...
Why on earth do you need accurate fuse resistance measurements? Doesn't the fuse you're using publish Rmax possibly even over temp? If you're just messing around in your basement - who cares. ....
The TS cares, and that is entirely sufficient to warrant getting answers helpful to his intent.

#### tindel

Joined Sep 16, 2012
900
The TS cares, and that is entirely sufficient to warrant getting answers helpful to his intent.
There have been many times on this forum that an OP thinks they need some information, but once their reasoning has been questioned the OP either realizes they don't need that information, or that they can get the information they need in another manner. That's why I ask. I've been the victim of this many times in 10+ years of engineering work. A quick consult with another engineer often times gets me thinking in another way that I hadn't considered. Sometimes, I was on the right track and I need to know the fuse resistance too. It just depends on the situation.

Sorry if my original questioning seemed demeaning it wasn't meant to be.

In my years so far I have yet to need to measure a fuse resistance for anything more than to make sure it hadn't cleared. I have had to find out the Rmax from the datasheet several times though.

ETA: I also provided some agreement with the other two responders and an added amplifier suggestion, so I also answered the question at hand.

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#### KeepItSimpleStupid

Joined Mar 4, 2014
5,088
The only time I was misled by a fuse is when I had more than one 3AG series fuses, open when in circuit and was closed when out of circuit with an ohmmeter.

The equipment I had set up in the lab had a limit of about +-2 pA @ +-100 V and a max of 20 mA when the problem occurred. We did acquire two Keithley SMU's which may have been up to the task.

At home, II might have the equipment to do the job, but my current source is in need of repair and the nanovoltmeter needs a special cable.

#### RichardO

Joined May 4, 2013
2,271
@RichardO just because this person has a fuse does not mean they can pulse the snot out of it like that datasheet describes. We have no idea what the P/N of the OP's fuse is. There are all types of fuses, some are more sensitive than others.
Good point. I should not have implied that _his_ fuse was exactly the one I gave the information for. Fortunately, unless he has an "exotic" fuse such as a very fast blow or rectifier fuse he can probably use the Littlefuse spec's. If he blows his fuse then -- oops, he can't.

#### ian field

Joined Oct 27, 2012
6,536
Hi All,

I was wondering if anyone had any ideas about this. I need to measure the resistance of a fuse with precision, and it has a very low resistance (0.01 Ohm range). I have an LCR meter but it is not precise enough.

Typically, I would consider using the 4-wire method and use Ohm's law but since this is a fuse rated at 2A, the maximum voltage I can apply is around 2 mV to avoid blowing it. Can anyone recommend either:
1. Equipment that I can purchase
2. A circuit I can make that will allow a precise measurement
Any input is greatly appreciated.
The DSE ESR meter from Australia as a kit recommends using low value fixed resistors to calibrate it in the first place. assuming it has enough resolution - i'd suggest using it to compare fuses with standard fixed resistors.

the usual alternative would be a measuring bridge - but the specifications will need more than a quick knock up from the junk box.

#### ArakelTheDragon

Joined Nov 18, 2016
1,353
Or you could check the datasheet, or send an email to the manufacturer.

#### ian field

Joined Oct 27, 2012
6,536
Or you could check the datasheet, or send an email to the manufacturer.
Unless the TS already has that spec and wants to verify compliance.

#### ArakelTheDragon

Joined Nov 18, 2016
1,353
Unless the TS already has that spec and wants to verify compliance.
There is no if in my language, if wanted that he would have stated it in the first post!

#### MrAl

Joined Jun 17, 2014
9,549
Hello,

For resistances below 1 Ohm a Kelvin Bridge can be used.

You might have to play around a little to get a curve with current, as the temperature dependence will show up at some point, and the temperature is dependent on current and ambient temperature so you should get a set of resistance vs current and ambient temperature curves, so you've got your work cut out for you

You also need to use the intended fuse holder for the fuse.

#### ArakelTheDragon

Joined Nov 18, 2016
1,353
It will be better to use an instrumentarional amplifier or just transistor audio amplifier.

#### smooth_jamie

Joined Jan 4, 2017
107
Hello everyone,

Thank you for all your replies, there was some good answers. I managed to use a 4-wire method in the end and got a resistance very close to what was published in the datasheet. I validated my method using a low ohm resistor with a known (similar / close) value at room temperature, and I'm satisfied with the results, and that the method is precise enough.

I can see there has been a bit of discussion since my first post, just to clarify this was for work. The job was for a client who's design was dependant upon the fuse resistance which needed to be validated at a sub-zero ambient temperature (the datasheet only had values at 20°C).

The manufacturer didn't have the data, in these cases I always have to make a test.

#### kubeek

Joined Sep 20, 2005
5,776
...The job was for a client who's design was dependant upon the fuse resistance which needed to be validated at a sub-zero ambient temperature (the datasheet only had values at 20°C)
My guess would be a crowbar circuit supplied by capacitors, hence the requirement to exceed the I2t of the fuse over the temperature range?

#### ArakelTheDragon

Joined Nov 18, 2016
1,353
"Sub zero" means nothing to me. Give exact values.

#### smooth_jamie

Joined Jan 4, 2017
107
"Sub zero" means nothing to me. Give exact values.
I need to maintain a certain level of confidentiality. All you need to appreciate is that it was very cold (i.e. lower than 0°C) and the resistance at the temperature in question was not available from the manufacturer.

#### Janis59

Joined Aug 21, 2017
1,482
What You can boil over so simple measurement???
Wilson current mirror is rather accurate device so You may trust it will keep the current stable over the filament of fuse. Say 1 Amp.
Next, if say filament is damn thick, say 0,5 mm and 1 cm long and made of copper, then resistance will appear (ignoring a Focault effect) R=1,68E-8 (Ohm*m)*1E-2 (m)/pi*4*0,5E-3^2 (m)=0,855 miliohms.
Okay, thats no much, but where is said that 0,855 milivolts of DC or maybe even AC is difficult to measure?? I mean with straight milivoltmeter-on or oscilloscope-on methods. If want extremely accurate, please, apply the NE567 in AM detection mode, so You shall have a beautiful PLL. But I would vote for just fabrique made milivoltmeter and Wilson circuit instead of clumsy/unhandy/slow-cooking Kelvin bridge.
And if about Kelvin bridge, the simplest factory made Wheatstone bridge for sure well measures resistors until 0,04 Ohms (like mine, produced at 1947 russia), it is far cheaper and demand less cooking to get the reading.

And, if the due heating (similar to the real-work conditions) of filament is hardly prohibited, why not to shoot through a one thick pulse and measure the transient effect by writing oscilloscope. Inertia is one brilliant thing, rarely.

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