efficiency of fuses

Discussion in 'General Electronics Chat' started by h2opolo, Jun 19, 2014.

  1. h2opolo

    Thread Starter Member

    Aug 2, 2013
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    I have done a couple car audio installations and I know it is important to use a heavy gauge wire for high powered amps. One of the installations involved a 1/0 awg wire from the battery to a distribution block that went to 4 awg wires.

    in line fuses are always suggested in case one of the wires shorts. I installed a big 200 amp fuse a foot away from the battery on the 1/0, then I put 30 amp fuses on the 4 awg wire after the distribution block.

    For car audio, everyone always talks about how it's important to use a heavy gauge so the amplifier isn't starved of power.

    Does a fuse create resistance in the power wire? The fuses seem so small compared to the wire. How do the amps stay running properly when all of the current has to funnel through that tiny fuse? Does it reduce the power of the amplifier?
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    The conductor inside the fuse is made of a metal similar to solder. It has a lower melting point than the wire itself. The size of the conductor is calibrated very carefully so that when the rated current is reached, enough heat is generated to melt the conductor and so break the circuit.
     
  3. h2opolo

    Thread Starter Member

    Aug 2, 2013
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    I understand that thats how the fuse is supposed to function but when I look at the size of the conductor within the fuse, it is like the size of 14 awg wire. How does that small of a conduit for electricity not strain the system?

    The way I picture it is like a hose with a kink in it. The hose is capable of allowing the water to flow quickly but the kink only allows a tiny space for the water to flow. This would slow down the water in the entire length of the hose.

    Because the fuse is so thin does it add resistance to the wire like the kink in the hose?
     
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    It has little effect since while it is small it is also short. It has a higher resistance then straight wire but not very much.

    Those things have been keeping the world safe in an efficient way for over a hundred years.
     
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  5. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Fuses and circuit breakers are "little heaters"
    and yes they add resistance and dissipate power just like anything else. P=I^2R
    Their resistance changes quite a bit with temperature too..
    They run HOT!! but thats just how they work.
     
  6. Externet

    AAC Fanatic!

    Nov 29, 2005
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    Yes, fuses are resistive. That resistance is what allows to get warm in order to melt its fusing element. Warm becomes melting hot in overcurrent conditions.

    Once I found a PC that was behaving erratically. Its defective fuse was 2.5 Ohm. Too much !

    - "Expensive electronic equipment blows itself first in order to protect its fuse" -
     
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  7. alfacliff

    Well-Known Member

    Dec 13, 2013
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    the resistance of 20 amp fuses is too small to affect the system. search the fuse makers for information on fuses.
     
  8. inwo

    Well-Known Member

    Nov 7, 2013
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    +1

    Ohms per foot adds up. Ohms per 1/4"- not so much.

    Water analogy:
    There is a large pressure drop thru 100' of tubing, yet to get the same drop it takes a very small orifice (comparatively) or a valve turned almost completely off.

    That being said, small resistances do add up.
    Check the voltage drop under full load, with one meter lead on the source and one on the load.
     
  9. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    What I found surprising is that one would "expect" that a higher rated fuse dissipates more heat.. But thats not always the case..
    I do a lot of testing in that area and have found many cases where.. for example. a 3A fuse will dissipate more heat than a 20A fuse.
     
  10. inwo

    Well-Known Member

    Nov 7, 2013
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    Measure heat or power?

    Measuring the actual heat could get pretty complicated.

    Seems the 20 amp fuse must take more power to melt a larger element.
     
  11. crutschow

    Expert

    Mar 14, 2008
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    Not unexpected. Since the 20A fuse would have a lower resistance, it would dissipate less power at the same current. With both fuses of the same physical size at their rated currents, the power dissipation would be very similar.
     
  12. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Agreed. QFT.

    Stated another way you have to heat up the element to get it to melt. That means a higher resistance for a lower current to reach the melting point.
     
  13. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    yeah before I did the testing I would just assume a fairly steady slope if you graphed them all where as fuse size increased so would its dissipation.. But it was far from a steady slope.. It was a like a saw tooth when graphed out with some steady slopes then a few stragglers all over the board..

    Its actually partly due to the differing alloys used in the elements even within the same fuse type. Again many would think its simply a smaller wire (same alloy) for a smaller current fuse of the same fuse series but the alloy percentages change too.
    The ones I was investigating were mostly a combination of certain percentages of copper and nickel that composed the fusing element.
     
  14. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Typically the manufacturers have a chart of the "hot resistance" of the element.
    Then simply using I^2R I would get the expected dissipation.
    For those that didn't I would run the fuses at full load for a set time period and then measure the hot resistance (voltage drop method,etc...)
    Subtracting out known resistances of the other parts of the circuit,etc...

    It was all part of some UL testing.. UL just asked us to test with the largest fuses installed... Until I informed them how I could actually dissipate more heat using certain combinations of smaller fuses..
     
  15. MaxHeadRoom

    Expert

    Jul 18, 2013
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    There is also the Slo-Blo factor types and then fast sweep through types such as so called Rectifier fuses.
    Max.
     
  16. alfacliff

    Well-Known Member

    Dec 13, 2013
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    also, the nec says that the fuse should be rated to protect the wire, and his 1/0 wire would be fused at 150 amps. in all but automotive, where they seem to use their own code. a 150 amp fuse would have very low resistance.
    for anyone actually worried about their voltage drop, it is easy to measure, just hook yhour volt meter to the battery, and the other lead to the load. a lot easier than arguing about it interminably.
     
  17. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    The NEC does not cover automotive use.

    Just look at the "accessory plug" where you once put a cigarette lighter. No third prong for proper ground bonding, which is why you cannot use them in your house for a 12VDC solar system and still be within code guidelines. (Also they are not "rated" for that use.)

    Heck, hopefully an autombile doesn't have a ground; it only gets one when you "never see the sudden curve until it's way too late."
     
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