Multimeter Glass/HRC Fuses being replaced by resistor?

Discussion in 'Test & Measurement Forum' started by Mr. Multimeter, Sep 17, 2018.

  1. Mr. Multimeter

    Thread Starter Member

    Sep 17, 2018
    61
    4
    Greetings,

    I've recently noticed some newer (albeit cheaper) multimeters, using some sort of fuse alternative to the standard glass/hrc fuse.
    I've included a pic from a new DMM below on the 250mA range.

    My question, .. Is this simply a 2 ohm resistor (as measured), or is there more to it than that?
    Also, other than cost, why utilize such a conduit mechanism ? Wouldn't a simple glass fuse be simpler to swap out from the user perspective?

    20180917_164251.jpg
     
  2. Hymie

    Active Member

    Mar 30, 2018
    683
    178
    Whether the fuse in question should be a HRC fuse depends on the maximum fault current which could flow. Normally meter HRC fuses are in the current/ammeter path where there is the possibility of a large fault current – but if the meter impedance limits the fault current to within the specification of the surface mount fuse, then an HRC fuse is not required.

    For most users, once the surface mount fuse has operated, the meter function requiring the fuse operational will be disabled. Therefore an easily replaceable fuse would be better for the end user.

    The surface mount fuse will be more than a 2 ohm resistor, having some I squared R limit which causes the fuse to operate.
     
  3. Mr. Multimeter

    Thread Starter Member

    Sep 17, 2018
    61
    4
    Thanks Hymie,

    I agree the accessibility of replacing a standard fuse is much better than having to desolder a burned out resistor!
    Hence, my original question, since this seems like a simple resistor, is that all it is or is there something more tangible to this surface mount component?
     
  4. rsjsouza

    Active Member

    Apr 21, 2014
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    The picture indicates this is a resettable fuse, a positive temperature coefficient (PTC) thermistor - i.e., the resistance increases with temperature, thus opening the circuit. Therefore it usually does not need replacement, unless its ratings are exceeded.

    This is usually something used in equipment exposed to lower energy circuits.

    In a distant past (antique vacuum tube radios) this used to be called a Fusistor.
     
  5. Hymie

    Active Member

    Mar 30, 2018
    683
    178
    Looking at the photo – the surface mount fuse does indeed appear to be marked as a 2 ohm resistor – if I’m reading the colours of the adjacent component correctly, this is a 56 ohm resistor.

    Therefore my statement re an I squared R limit might be nonsense, and they are trusting to luck that the component(s) will fuse/protect as required.

    Bear in mind that 1000V HRC fuses can cost more than many a cheap meter.
     
  6. Mr. Multimeter

    Thread Starter Member

    Sep 17, 2018
    61
    4
     
  7. Mr. Multimeter

    Thread Starter Member

    Sep 17, 2018
    61
    4
    Indeed, HRC fuses set me back a few pennies last week! These cheap meters are always interesting, you can never be quite sure what they deem as "input protection".
     
  8. Hymie

    Active Member

    Mar 30, 2018
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    Personally I doubt that it is a PTC, although they are available at higher voltages, most PTCs are limited to around 60V, and unlikely to be rated to 300V – hence my belief it is a 2 ohm resistor (as marked).
     
  9. Mr. Multimeter

    Thread Starter Member

    Sep 17, 2018
    61
    4
    Yes, I tend to agree...I think the voltage requirements exceed
    the PRG series (Resettable fuse) series of PTC thermistors.. at least I can't seem to find any datasheet with higher voltage ratings than 60V..
    Alas, if it is only a simple 2 ohm resistor, then what's the point? or am I missing something entirely?
     
  10. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
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    Might it be a 2Ω fusible resistor - 250mA would give 125mW?
     
  11. Hymie

    Active Member

    Mar 30, 2018
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    With sufficient current, the heating effects will cause the resistor to fail open circuit – sort of acting as a fuse.
     
  12. ebp

    Well-Known Member

    Feb 8, 2018
    2,332
    811
    The flat device is almost certainly a polymeric type self-resetting "fuse." I've used Bourns products that look nearly identical, right down to the small indentation in one of the terminals. I've never seen a surface mount resistor with notches in the ends or metalization that looks anything like that.

    MELF resistors are not exactly common, but they do exist and there are fusible types.
     
  13. Mr. Multimeter

    Thread Starter Member

    Sep 17, 2018
    61
    4
    Roger that, but if the resistor becomes degraded and does not automatically reset than the end user needs to desolder (preferable with a hot-air station) and install a new component. This to me seems fruitless for what is a cheap 20$ meter. I don't see why this approach is better than a cheap glass fuse that is easily replaceable ;) I do realize one reason for using a fusible resistor rather than a fuse, is the resistor will itself limit the current surge, whereas a fuse won't, I guess I don't know if the end justifies the means. BTW, I appreciate everyone's input!
     
  14. Mr. Multimeter

    Thread Starter Member

    Sep 17, 2018
    61
    4
    Thanks for the input, yes I've not seen a surface mount resistor with this kind of colouring before either.
    However, I'm not sure why it's marked as 2 and indeed has a value of 2 ohm... if it isn't a 2 ohm resistor?
     
  15. ebp

    Well-Known Member

    Feb 8, 2018
    2,332
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    Devices like that are often marked with a cryptic code and you need to have the appropriate datasheet to find out what it means, but I suspect it is actually rated for 2 ampere hold current. That it happens to be two ohms is simply coincidence - and unless you've use Kelvin leads or otherwise compensated for them, the 2 ohm measurement will include you test lead resistance. I've never seen a surface mount resistor marked with a single digit for resistance.
     
  16. Mr. Multimeter

    Thread Starter Member

    Sep 17, 2018
    61
    4
    I've just remeasured the SMD component in question using a Mastech MS5308 LCR Tester, it's giving me 1.97 Ohms via the Kelvin test leads, quite a coincidence indeed. Initially, I did REL my meter out to zero the test-leads for my first measurement (recently calibrated Fluke 83 with Probemaster Leads).
    I concur, usually there is a numeric identifier for smd resistors however it's usually encoded and does not give a verbose output of the resistor's value until you have inputted the resistor code on the component and generated the actual resistor output value. Okay, I'll sum it up as unique coincidence and I'll defer this as a Polymeric SMD type fuse in lieu of any further input.
    Appreciate everyone's help.
     
    Last edited: Sep 17, 2018
    rsjsouza likes this.
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