Trying to find a device

Discussion in 'General Electronics Chat' started by DerStrom8, Jun 4, 2015.

  1. DerStrom8

    Thread Starter Well-Known Member

    Feb 20, 2011
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    Hi all,

    I have been racking my brain over the past 24 hours trying to figure this one out, but have had no success as of yet.

    I am in need of a device that operates much like a standard fuse--it passes current with very little resistance until it reaches a certain threshhold, and then cuts current off. However, the device I'm looking for must be surface-mounted and self-resettable. I have looked at PTC fuses, but they have a resistance that increases as current increases. The device I'm looking for keeps a very low resistance (preferably <0.1 ohms) until a current limit is reached (i.e. 50mA) and then it cuts out. When current is removed, it will reset. The closest I have found is the PTC fuse, but it doesn't quite work for me. I need the voltage on the line to remain nearly the same (a fluctuation in the millivolts range might be ok, but it should be as low as possible) and same with the resistance.

    Any thoughts?
    Matt
     
  2. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
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  3. dl324

    Distinguished Member

    Mar 30, 2015
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    PTC fuses don't interrupt current when they trip. Low current fuses have significant resistance; they even sometimes have resistors in series with the fuse wire... I have never heard about a self resetting fuse component that interrupts current.
     
    Last edited: Jun 4, 2015
  4. DickCappels

    Moderator

    Aug 21, 2008
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    A quick search for "Surface Mount Circuit Breaker" was disappointing.

    Have you considered making your own? Tiny relay, a little current detection and latching circuitry with a reset input?
     
  5. DerStrom8

    Thread Starter Well-Known Member

    Feb 20, 2011
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    As I mentioned before PTC fuses really don't do what I'm looking for. They have a (relatively) significant resistance even in normal conditions.

    Correct, they simply increase their resistance which reduces the amount of current flowing (preventing damage to the circuitry downstream). They reset when the current is removed. Still not what I'm looking for though.

    Unfortunately I don't have the space for that. The board I'm working on must be very small and I was hoping for a single component that could be connected in-line with one specific trace.
     
  6. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    I think it might be better just to limit the max drive current supplied to the circuit with a current limited supply line.
     
  7. ian field

    Distinguished Member

    Oct 27, 2012
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    Something somewhere has to sense the current - that almost always means a volt-drop somewhere in the circuit.

    AFAIK: the lowest drop you can get is across a few turns of thick copper wire with a hall sensor coupled into its field.
     
  8. DickCappels

    Moderator

    Aug 21, 2008
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  9. DerStrom8

    Thread Starter Well-Known Member

    Feb 20, 2011
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    I believe I've mentioned this three times now :D

    I have looked at polyfuses (PTCs) but they don't quite do what I need. They have a relatively high internal resistance even in normal operation.
     
  10. ian field

    Distinguished Member

    Oct 27, 2012
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    A polyfuse is probably as good as it gets unless you want to build some circuitry around a hall effect sensor.
     
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