Fuse value advice needed :)

Discussion in 'General Electronics Chat' started by Nicholas, Jul 17, 2013.

  1. Nicholas

    Thread Starter AAC Fanatic!

    Mar 24, 2005
    121
    1
    Hi all

    I have a set-up (made with help from you guys) where an IRLZ44
    MOSFET is switching around 40V and is turned on by a controller
    with a 5V signal. I made a mistake(!) and forgot to connect the
    ground between the MOSFET circuit and the PSU, which resulted
    in the IRLZ44 shorting and sending 40V into the controller. It smelled
    for DAYS, and was of course fried. So, I'm thinking about adding a
    fuse to take the chill instead of the controller if I mess up again.

    So my question is, what value should it be, and where is the best
    place for it? On the gnd wire from the MOSFET circuit going to the
    controller?

    Thanks a lot!
     
  2. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
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    Most of the time the fuse rating is based on the used current with a little head room in order to prevent the fuse from blowing because of a surge current ..
     
  3. tindel

    Active Member

    Sep 16, 2012
    568
    193
    Fuses - oh fuses... this is probably the single most agonizing part that I use. It depends a lot on your philosophy. Yes - size the part for the at least the nominal current plus some additional headroom. But also consider inrush currents and how that can effect the life of the fuse and if it will degrade the fuse over time.

    Also fuses have some resistance, so consider the power loss as well.
     
  4. Nicholas

    Thread Starter AAC Fanatic!

    Mar 24, 2005
    121
    1
    Thanks!

    So, in case of a normal controller, the voltage is 5V and the amperage
    should be about 30mA at max, yes? Does this mean I can use a fuse
    that is 250V 100mA and keep my controller pretty safe?
     
  5. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    A fuse will not work to do what you want. It takes a certain amount of time for a fuse to 'blow'. For a electronic part to fail can take a lot less time than the time a fuse takes. Fuses are usually only used on power to a circuit.

    A diode in the output from the controller would probably work though. The diode would stop any reverse flow of electric into the controller. A fast acting Schottky diode would be best.
     
  6. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,771
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    correct..

    And for reference a fuse is "typically" sized at 125% of max continuous current. So 100 Amp max continuous gets a 125A fuse.
     
  7. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,553
    2,375
    For semi protection, use a rectifier fuse.
    Fast sweep through!.
    M.
     
  8. Nicholas

    Thread Starter AAC Fanatic!

    Mar 24, 2005
    121
    1
    Shortbus, that sounds reasonable ! Any advice regarding the value of
    the fast diode? If a small diode is used, won't it just die and go
    open on reverse voltage? (like a fuse)

    Thanks!
     
  9. mank

    New Member

    Jul 19, 2013
    1
    0
    A fuse will not work to do what you want. It takes a certain amount of time for a fuse to 'blow'. For a electronic part to fail can take a lot less time than the time a fuse takes. Fuses are usually only used on power to a circuit.

    A diode in the output from the controller would probably work though. The diode would stop any reverse flow of electric into the controller. A fast acting Schottky diode would be best.
    __________________
     
  10. strantor

    AAC Fanatic!

    Oct 3, 2010
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    Thanks for the unprecedented information mank.
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,553
    2,375
    Rectifier or Semiconductor fuse for fast sweep through, but not cheap!
    Max.
     
  12. strantor

    AAC Fanatic!

    Oct 3, 2010
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    ....and again with the unprecedented information. What the heck is going on around here?
     
  13. Wendy

    Moderator

    Mar 24, 2008
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    Over the years I have become aware that transistors generally protect the fuses, not the other way around. The thing a fuse prevents is extra damage from shorts propagating through the circuit. A slow blow fuse is used when you know there are going to be current rushes, such as charging filter caps in a power supply.
     
  14. strantor

    AAC Fanatic!

    Oct 3, 2010
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    Well maybe that's the answer, a sacrificial "fuse" transistor. You could solder in a female header that the transistor plugs into, for easy replacement.
     
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