[SOLVED !] Current Limiters


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Thread: Current Limiters5126 days old

  1. #1
    roger
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    Default Current Limiters

    I have an old but useful analogue meter instrument which essentially measures current on three switchable ranges, 5mA, 50mA, and 500mA.

    The ranges are protected from short circuit current by separate fuses.

    Can anybody suggest any ic's which can be used in place of the fuses to limit current on overload to, say, 150% of range selected.

    The limiter should have negligible effect until the overload occurs.

    I am happy to put a limiter on each range.

    Any advice welcomed.

    Thanks

    Roger

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  • #2
    Mark Rodgers
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    To claify the use of a fuse, its function is to provide an absolute protection againsed an errorr in a circuit. It will not limit an overload, rather stop a catastrophic level of overload from destroying a sesitive component(or circuit) by self destruction at a point that is lower than the level of catastrophic destruction that the primary component can withstand(in this case the primary component is your meter)
    In the ideal world(such as the perfect simulator) a 500ma fuse will allow 500ma to pass without worry, but will blow at 501ma and save the day!!! hoo rar for the ideal world.
    In reality you may not get any fuse to blow at much less than 10 times its rated value, ie 50ma will need 500ma to rupture the circuit.
    It will have a "time X current" to blow curve.
    if you pass 5amps through a 500ma fuse it may take 1escond to blow, and if the 5amps reduces to 100ma during this time the fuse will not blow.

    The best way to limit the overload current is not to let some idiot use the meter.

    I do have the reputation of being funny(so I have been told), with this in mind the term "idiot" is related to idiot and not "idiot", I have no wish to discriminate between the idiots of this world, they are all equal.

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  • #3
    Fanatical Contributor fanie's Avatar
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    Good Morning Roger and Mark,

    It is now 02h40 in the morning here and it's raining in Sunny South Africa. Man, I can almost hear the fishes grow!

    If you are only using the mA meter for low voltage applications say up to 40V and if you can live with a constant 1.2V drop I can recommend you use a circuit like attached. It is fairly easy to work R out for the correct current value. The circuit will allow current to pass untill the 1.2V limit over the R is reached and will then create a Vdrop across the 317 without increasing the max current. Take into consideration the qiescent current of the 317. Keeping the fuses in line is highly recommended. You can also put a switch over the circuit to cut it out if not desired.

    Another way of preventing stuff to make smoke signals (indicating "i am now stuffed") is to use a power supply with a current limit - but it will not be always ideal.

    If you have to have something that can handle a lot of misuse, I would suggest you use a high current shunt with a digital autorange meter measuring voltage as current, and retire the analog to the showcase.

    There are different types of fuses, slow blow, medium, fast and ultra fast. Each of them have their different uses in different aplications, replace with the same type.

    for what it's worth...
    Fanie

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