Fuse(s) for my coil driver circuit?

Discussion in 'General Electronics Chat' started by Nicholas, Jan 30, 2016.

  1. Nicholas

    Thread Starter AAC Fanatic!

    Mar 24, 2005
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    Hi again guys!

    Once again :), this is the circuit(see below), but there are two of them side-by-side for driving two coils(or the like).
    The coils will not be the same, one might draw 10 amps, the other 5 amps. They will be different for sure.

    What should I do about fusing the circuit? and should I fuse it at all?

    Thanks,

    Nick
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  2. #12

    Expert

    Nov 30, 2010
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    What could go wrong?
    The mosfet shorts and the coil stays on? That won't pop a fuse.
    The coil (or the wiring to it) shorts and the mosfet gets in danger? Yep. A fuse would help that.

    Regard' My method for making decisions.
     
  3. Nicholas

    Thread Starter AAC Fanatic!

    Mar 24, 2005
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    So, since a mosfet is cheap it may be better to lose it than installing a fuse?

    Thanks,
    Nicholas
     
  4. #12

    Expert

    Nov 30, 2010
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    I dunno. Are mosfets cheaper than fuses?
    I think so. If you are the only person who has to use it, fine. If some non-electronics person needs to use it, install a fuse that can be replaced...or don't. Everybody else is making un-repairable boards to force new purchases. Why be different?
     
  5. hp1729

    Well-Known Member

    Nov 23, 2015
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    Self resetting fuses?
    https://en.wikipedia.org/wiki/Resettable_fuse
     
  6. Nicholas

    Thread Starter AAC Fanatic!

    Mar 24, 2005
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    Wouldn't a coil that stays on increase in heat and thus increase in resistance...and then draw more amps?

    Thanks:)
     
  7. hp1729

    Well-Known Member

    Nov 23, 2015
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    An increase in resistance would draw less current. Getting hot it might decrease in resistance and draw more current, yes. Typical for inductors.
     
  8. PeterCoxSmith

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    Feb 23, 2015
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  9. William Heacock

    New Member

    Oct 30, 2015
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    Hi all,
    One thing to remember, fuses are for safety, not to save components. Todays' semiconductors often require over current of a few milliseconds or less to die a violent death. Typical fuses are thermal devices that often take much longer than that to heat up and open.

    Make a list of the possible failure modes that could compromise safety. Look at the SYSTEM that includes the coil circuits. Have an understanding of what COULD happen and the probability. Then you must weigh the cost vs. the probability to decide where and what kind of protection to use.
     
  10. bertus

    Administrator

    Apr 5, 2008
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  11. #12

    Expert

    Nov 30, 2010
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    Totally backwards thinking.
    A coil that is designed to stay energized just sits there doing its job. If it is designed for pulse activation, it overheats and the resistance of the copper increases. Increased resistance to current flow means less current flow.
     
  12. wayneh

    Expert

    Sep 9, 2010
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    That's a good list of basics. Point 4 should point out more forcefully, in my opinion, that fuse ratings are easily misunderstood. For instance, if your circuit might be damaged by 1A for a few seconds, you might think a regular 1A fuse, not a "slo blo", should be about right. But if you look at the specs for the fuse and read the current/time chart, you'll see that a generic 1A fuse takes a long time to blow at 1A. It'll blow much faster at 2A, but will still take far longer than you might expect. It's eye-opening the first time you carefully read a fuse chart.
     
  13. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Although there exists Rectifier or Semiconductor fuses which are intended to save Semiconductor circuits and a have extremely fast sweep through when blowing.
    Max.
     
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