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Need to test some fuses

Discussion in 'General Electronics Discussion' started by keat63, Sep 10, 2015.

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  1. keat63


    Jun 8, 2011

    The story goes that we import vehicle fuses from the far east. (millions of them)
    Generally we have no problems, however, we have one customer complaining that a 3a fuse is blowing before it should.
    But the manufacturer says that it's within spec.

    So i've been asked to see if i can come up with something to put a batch to the test.

    My electronics knowledge is rubbish if i'm being honest, however from what I remember at school V / I * R
    should give me the resitance required to pop a fuse.

    So if i run a test rig at 12v, i should expect a 3a fuse to pop around 4 ohms resitance ??

    So in theory, i should be able to connect a multimeter, 12v battery, fuse and potentiometer in series ??

    If this is the case, what size pot would i need ?
    and if i were to cobble such a thing together, is it feasible that there's a pot out there that would perform for a range beween 12 ohms and 1.2ohms, so we could test a range of fuses.

    My apologies if this is schoolboy stuff, but it's been a while since my last confession.
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011

    Yes and no. While your math is right, your understanding of the workings of a fuse is not entirely right. A fuse rated 3A should not blow at 3A, only if the current is higher than 3A. When the current is higher than the rated current, the fuse will blow. How fast it will blow depends on the type of fuse (fast, slow...) and of the amount the overcurrent is higher than the rated current. Generally the higher the overcurrent, the faster the fuse blows. A fuse rated 3A may operate indefinitely at only a slight overcurrent (e.g. 3.1A), but will blow almost instantaneously at 10A.

    So in order to test the fuses you will need a power source that is capable of >3A.
    A load resistance of less than 4Ω on 12V will produce that much current, but at the cost of considerable wasted power (P=U²/R). A controlled way of setting up a test current would be by using a laboratory power supply with adjustable output current limiter. You'd set the output voltage of the supply to e.g. 2V and the current limiter to e.g. 3.3A (10% above the rated current should be o.k.). Then connect this setup to the fuse (no load resistor required) and check your fuses. By adjusting the currrent limier of the power supply you can easily and precisely find the point where the fuse trips.
    A potentiometer for adjusting this high current will be hard to find (I don't say it is impossible), an electronically regulated source is a much better solution. If you don't have such a power supply you can probably lease one or get one cheap on the internet (cheap only if you don't expect too much in terms of reliability and precision, but that's probably not an issue here).
    Alternatively you can do with a fixed voltage power supply and a range of load resistors that you switch between - you will have discrete currents values, not continuous ones as with a potentiometer.

    Remember to measure the real current and don't rely on calculations or the display of a cheap power source.

    Here is some good information on fuses, their operation and their characteristics.
    Old Steve likes this.
  3. keat63


    Jun 8, 2011
    So a lot more complex than I'd assumed then.
    Customer is saying that the fuses are blowing below the 3a threshold.

    I'll pass the info on and let the powers that be make the descisions.

    Thanks Harald
  4. Old Steve

    Old Steve

    Jul 23, 2015
    If the customer claims that they're blowing below 3A, you could simply connect one to a voltage source with a suitable resistance to allow 3A to flow and see if the fuse blows, as you originally suggested. You don't need to know the precise current that does blow it.
    Leave it connected for a bit, and if the fuse doesn't blow, the customer was wrong.
    For a quick crude way of testing this using a 12V lead-acid, (car), battery:-
    4R03 50W Resistor.JPG

    The big 10W resistors are necessary because there'll be almost 40W to dissipate.
    And the current will actually be just below 3.1A, due to the resistance of the fuse itself.

    N.B. For more exhaustive testing, and testing a range of fuse values, you'll need Harald's method.
    Last edited: Sep 10, 2015
  5. keat63


    Jun 8, 2011
    I was hoping that it would be as simple as hooking up a fuse, some power, a pot and ammeter, all inline and then cranking up the pot until 3amps was displayed waiting for the fuse to pop.
    I found a cheap lab psu on fleabay, for about £35, so i've passed this info to the people concerned.

    Just as a matter of interest though, how does the diagram compute to 4.03ohms
    See, this is where i should have taken more notice at school :)
  6. Old Steve

    Old Steve

    Jul 23, 2015
    1/Rt = 1/R1 + 1/R2 + 1/R3 + 1/R4 + 1/R5

    Or, for just 2 resistors in parallel:-
    (R1 x R2) / (R1 + R2)
  7. Arouse1973

    Arouse1973 Adam

    Dec 18, 2013
    If the PSU has a current mode, thats if you can limit the current and its constant current mode. Then just set the current to three Amps and connect the fuse across it. See what happens. Set the voltage to only few volts, that should be fine.

    CDRIVE Hauling 10' pipe on a Trek Shift3

    May 8, 2012
    Up until about the 70's it would have been that simple. Large high power (wire wound) Pots were easy to find. These days you'd have a better chance of digging up a Tyrannosaurus in your backyard. :D

    Arouse1973 and Martaine2005 like this.
  9. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    At which ambient conditions, specifically temperature?
    A classic fuse is a thermal device. It will blow when the filament gets too hot and breaks. Teh fuse doesn't care where the thermal energy (heat) comes from. If the ambient temperature is already high (e.g. when the fuse is placed in a closed location near or with an oven appliance), this heat adds to the thermal energy from the currrent and the fuse will blow earlier.
    Arouse1973 likes this.
  10. keat63


    Jun 8, 2011
    some very interesting stuff here chaps, and thanks for spending the time with me.
  11. duke37


    Jan 9, 2011
    It may be worthwhile to find out what the fuse is supplying. Equipment might be labelled as 3A but may need much more than this on start up. This is the case with electric motors and tungsten filament lamps.
    CDRIVE, Kiwi and Harald Kapp like this.
  12. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    it may also be worth looking at a datasheet for a presumably similar fuse. This should have a graph of current vs time. You could investigate a sample of your fuses and see how they match up.

    it is possible that your supplier is substituting a faster fuse and this could be a source of nuisance fuse failure.
  13. Kiwi


    Jan 28, 2013
    As duke37 says, you need to get some more info from your customer on what the fuse is supplying.

    Have they checked the actual current flowing through the fuse? They may have selected the wrong size fuse.

    Does the fuse blow on power up or after some time?

    What type of fuse are they, eg glass, blade, etc?

    Have you seen any of the blown fuses?
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