Measuring very low resistance of a fuse

Discussion in 'Test & Measurement Forum' started by smooth_jamie, May 16, 2018.

  1. smooth_jamie

    Thread Starter Member

    Jan 4, 2017
    89
    16
    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.
     
  2. crutschow

    Expert

    Mar 14, 2008
    18,634
    5,201
    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.
     
  3. RichardO

    Senior Member

    May 4, 2013
    2,130
    804
    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
     
  4. tindel

    Well-Known Member

    Sep 16, 2012
    624
    208
    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.
     
    Bernard likes this.
  5. ebp

    Active Member

    Feb 8, 2018
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    282
    The TS cares, and that is entirely sufficient to warrant getting answers helpful to his intent.
     
  6. tindel

    Well-Known Member

    Sep 16, 2012
    624
    208
    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.
     
    Last edited: May 16, 2018
    KeepItSimpleStupid likes this.
  7. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
    2,494
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    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.
     
  8. RichardO

    Senior Member

    May 4, 2013
    2,130
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    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. :oops:
     
  9. ian field

    AAC Fanatic!

    Oct 27, 2012
    6,037
    1,081
    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.
     
  10. ArakelTheDragon

    Active Member

    Nov 18, 2016
    540
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    Or you could check the datasheet, or send an email to the manufacturer.
     
  11. ian field

    AAC Fanatic!

    Oct 27, 2012
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    Unless the TS already has that spec and wants to verify compliance.
     
  12. ArakelTheDragon

    Active Member

    Nov 18, 2016
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    There is no if in my language, if wanted that he would have stated it in the first post!
     
  13. MrAl

    Distinguished Member

    Jun 17, 2014
    4,752
    1,037
    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.
     
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