Fuse for Power Supply

Discussion in 'General Electronics Chat' started by yurgo, May 28, 2012.

  1. yurgo

    Thread Starter New Member

    May 28, 2012
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    I want to fuse the primary side of a 120vac to 12vac (300 mA) step down transformer ... the fuse will be first in the circuit (before an on / off switch) on the hot wire. A three pronged grounded plug (metal case / transformer / switch to be grounded) from the ac main will be used. Could anyone tell me what rating of fuse should be used?

    Thanks for helping on this.
     
  2. wmodavis

    Well-Known Member

    Oct 23, 2010
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    I think your best bet to determine the fusing current is to actually measure the AC primary current when there is maximum load on the transformer secondary. Allow some margin (but not too much) to select a fuse, probably the next size up standard rated fuse.

    Another option would be to estimate by calculation of primary current based on data you provided.

    You also need to decide on the type of fuse i.e. slo-blow/fast-blow.
     
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  3. bertus

    Administrator

    Apr 5, 2008
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  4. wmodavis

    Well-Known Member

    Oct 23, 2010
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    Excellent article linked by bertus above. It is always a good starting point to look at manufacturer's design literature. They often provide excellent help with the pertinent factors to properly use their products.
     
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  5. MrChips

    Moderator

    Oct 2, 2009
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    If the assumption is that your supply will demand 12VAC @ 300mA then the 120VAC will have to deliver 30mA minimum. Since no transformer is 100% efficient and you also have to consider transients, you can increase this to 100mA.

    A fuse rated at 250mA @ 250VAC would not be unreasonable for this application.
     
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  6. yurgo

    Thread Starter New Member

    May 28, 2012
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    Thanks again for the assistance ... the 30 mA minimum input confirms what I had reasoned ... nice to have that confirmed. I will fuse the input side with a 250 mA @ 250VAC fuse as suggested.
    Another question ... would a 500 mA @ 250VAC fuse on the output side then be reasonable for the safest possible circuit?
     
  7. MrChips

    Moderator

    Oct 2, 2009
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    I cannot think of a good reason to fuse the output side. Someone else enlighten me.
     
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