What fuse for micro circuit?

Discussion in 'The Projects Forum' started by Art, Mar 5, 2012.

  1. Art

    Thread Starter Distinguished Member

    Sep 10, 2007
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    61
    Hi Guys,
    I'm using a 7805 regulator to provide a regulated 5 Volt supply for a circuit with two microcontrollers.
    The 7805 is rated to supply 1 Amp.

    It's down to placing an inline fuse in the 12 Volt positive line supplying the 7805.

    Could someone suggest an Amp value to use for the fuse?

    I hear the 7805 is inefficient, and likely draws significantly
    more power than it provides on the regulated side.
    Cheers, Art.
     
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    488
    It's a linear regulator, so the input current is (almost) equal to the output current. Voltage drop x current gives you the power that needs to be dissipated inside the regulator.
    This is meant by being inefficient.

    A buck converter (switching converter) has higher efficiency. There are drop-in replacements for the 78xx series.

    If you want to use the 7805 at 1A you need to mount it on an appropriate heatsink.

    The fuse value depends on the true output current, not the rated output current. The 7805 also has a thermal shutdown, it lowers it's output voltage when getting to hot (overload). Not sure if you even need a fuse.
     
  3. Art

    Thread Starter Distinguished Member

    Sep 10, 2007
    785
    61
    I'm not consuming 1 Amp, but if there's a short on the regulated side,
    I want the fuse on the unregulated side to blow.
    A 1 Amp fuse might do it.
     
  4. jimkeith

    Active Member

    Oct 26, 2011
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  5. DickCappels

    Moderator

    Aug 21, 2008
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    If you want a simple method, measure the current into the regulator and multiply it by two. That's the current rating of the fuse. If there is a lot of capacitance from the load side of the fuse to ground (many thousands of uF), then make it a Slo-Blo fuse.

    But do you really need a fuse? The LM7805 has internal current limiting and thermal shutdown, which us usually good enough, and it saves you the trouble of replacing a fuse every time you get an accidental short.


    Take it from somebody who has shorted dozens if not hundreds of 7805's: my suggestion is: Don't bother with a fuse.
     
  6. John P

    AAC Fanatic!

    Oct 14, 2008
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    I agree. No fuse.

    If you want to limit current to less than the 7805 would give, you could consider a 78L05, which only allows 100mA. For a lot of circuits, that's plenty.
     
  7. Art

    Thread Starter Distinguished Member

    Sep 10, 2007
    785
    61
    So the 7805 shuts down because there's a short and it got hot.
    Then what if I'm not around to solve the problem?
    Wouldn't it just provide power again when it got cold?
    I'm interested in protecting the devices on the other side which are soldered directly to a PCB,
    and not easily replaceable.
     
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