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hmm... fuses and power supplies

Discussion in 'Electronic Basics' started by Abstract Dissonance, Feb 6, 2006.

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  1. My transformer is rated at 25VCT at 2A's... but I assume this is 2A's from
    the secondary since its a step down transformer(hence the current on the
    secondary will be larger than on the primary)...

    So how do I choose the proper fuse? I was thinking at first it should be a
    2A fuse until I thought about what side I was doing and so the fuse would
    not blow at 2A on the secondary but the primary ;/ How do I get at the
    right fuse?

    Since its a 25V then I have about ~165/25 ~= 6.6 turns ratio so I need an
    amp about 2/6.6 ~= 300mA?

    If thats right then how do I adjust? Or does it really depend on the power
    going through the fuse? (which is what I would expect since that translates
    into heating which burns the fuse) If so then why the hell do they always
    put amps on the fuses instead of watts? If a fuse is rated at 250V/1A then
    it means it will burn up(open) at 250watts? So for my 25VCT/2A transformer
    would burn up at 50W and I'd need a fuse that would burn up before that(or
    just at that)... so 50W fuse would work? which means I could get any
    product of I and V that would give me that and it should be ok... but this
    isn't how its done it seems so whats going on?

    Thanks,
    Jon
     
  2. So how would you get on using a 240v fuse on say 230v or 250v ??
    The power changes but the amp rating is constant.
     
  3. Pooh Bear

    Pooh Bear Guest

    The 'right fuse' is often about avoiding fire or failure of insulation.

    Your transformer should supply the rated load current indefinitely.

    To cut a long story short - you need to experiment. Shorting the secondary is
    the normal method. The fuse should blow before the winding temperature rises to
    a dangerous level ( higher than its insulation rating ).

    It's all about electrical safety.

    Graham
     
  4. Pooh Bear

    Pooh Bear Guest

    Prety damn obvious.

    Not all fuses are installed on the same voltage circuits !

    Graham
     
  5. Phil Allison

    Phil Allison Guest

    "Pooh Bear"

    = Graham Stevenson = a know nothing pommy ****.


    ** NO it is FUCKING WELL NOT !!!!!!!!



    ** Got nothing to do with shorting the damn secondary.

    YOU FUCKING POMMY CRIMINAL LIAR !!!



    ** Another FUCKING LIE !!!

    Fuses only protect the damn equipment - ie damage limitation.





    ........ Phil
     
  6. The voltage and current ratings on fuses are really two independent
    ratings.

    A fuse will blow at (or really a little above) its advertised rating.
    It doesn't know anything about the power consumed by the load it is
    feeding.

    The voltage rating of the fuse indicates the maximum voltage that it
    can safely break when it blows. A 1 amp 32 volt fuse and a 1 amp 125
    volt fuse should blow at the same current, regardless of the circuit
    voltage, but if the voltage is 125 volts, the 32 volt fuse is likely
    to arc for some time, while the 125 volt fuse will blow cleanly.

    Sounds like you want at least a 1/2 amp 125 volt fuse on the primary
    of your transformer.
     
  7. ok. So does a fuse blow strictly due to current? If so, how? Doesn't the
    power determine when it blows... hmmm... P = I^2*R for fuses? So I could
    measure the resistance if the fuse and then compute its power rating and use
    that if I wanted? (and I know what my max power draw should be).
    ok, I see where you got the 1/2 amp I think. 125/25 ~= 4. Since my
    transformer is rated at 2A on the secondary it means that it will end up
    pulling 1/2 A on the primary.

    But I read in a book that you need to choose the fuse 50% to 100% greater
    than what you need? So should I go with a 1A slow blow fuse or play it on
    the safe side and use 1/2A slow blow?

    I'm not sure how well the transformer work and how long I can draw its
    maximum rated current before it screws it up... any ideas? (Obviously I
    can't have to high of a rating since then it won't do anything to protect
    the transformer)
    Thanks,
    Jon
     
  8. The fuse blows because of the power dissipated within it( from I^2*R,
    where R is the resistance of the fuse). The fuse does not know what
    power is consumed by the load, because it doesn't know the circuit
    voltage (until it blows).
    I think someone else calculated a primary current of 300 mA, so I
    allowed some safety margin - anything from 500 mA to 1 amp would be
    reasonable.
    If the transformer is rated at 2 amps, it should be able to deliver
    that current indefinitely without overheating.
     
  9. Ok. I think I got it.

    Thanks,
    Jon
     
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