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Where to I put a Fuse ??

Discussion in 'Electronic Basics' started by Trudeau, Feb 3, 2005.

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  1. Trudeau

    Trudeau Guest

    Im building a little 120 Volt panel (for lack of a better word) to do some
    experiments with 120 Volts. In the past Ive just connected up aligator clips
    etc and really is unsafe. So Im going to put a switch and outlet on a board
    to do it right.

    Im not sure exactly where to put the fuse however. To some degree I know it
    could go anywhere. But really if something goes wrong I suppose you want the
    whole thing to be shut down so all live wires are killed.

    However others may have some other advice. Here is a diagram that I think is
    the best way to set it up.

    To make it easier for discussion I have noted 3 points F1, F2 and F3 for 3
    possible fuse points. F1 Seems like the most likely place.

    Excuse my drawing. View in Notepad. I hope it shows up OK this is the first
    time I have put a diagram into the newsgroup.

    | |
    Small Fuse House | .-----. | .----------------.
    F1 Switch | || |o--' | |
    + _ _/ | .o| +| | |
    o-----o-o_/ \o--o/ o---. ||120AC| --------F3 -| Projects |
    || 0 | 120 V AC Out | |
    - |'-----' To Projects | |
    o-------------------------. | | |
    === | |
    120 House current GND | |
    (created by AACircuit v1.28.4 beta 13/12/04
  2. Looks good. However, what's the point of the fuse? Your house should
    have a circuit breaker.

    If you install a ground fault interrupter plug, that will give you some
    added protection should you touch the wrong wire while standing in a
    puddle of salt water connected to pipe leading to ground. It won't help
    you if you complete a circuit between hot and neutral, but if you keep
    one hand behind your back, the worst that will happen is you burn your hand.

    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
  3. The purpose of fuses and circuit breakers is to reduce fire
    hazard. By using a low amperage fuse, the OP can use
    smaller wire in his project than the 14 AWG wire that
    comes from a typical 15A breaker.
    GFI is a good idea for any workbench where open
    equipment will be used. It can save equipment, too.
    Melted scope probe ground wires and cables are a
    sad sight when probes cost $100 and up.
  4. But a fuse specific to this thing means that if you do short out the AC
    line, your lights won't go off, along with any other equipment. Sure,
    blowing the circuit breaker in the house is cheaper, but you then have
    to go and reset it, and if you had a computer running on the same AC
    circuit, you have to restart it.

  5. Trudeau

    Trudeau Guest

    Thanks for your comments.

    I suppose the point of the fuse (I would use one of those little 1Amp Glass
    fuses) is to give added protection. Have it blow at a lower level. Nothing I
    do is going to draw much power. I suppose I was also thinking that it might
    ?? save the device that I was connected to from blowing sky high. I know
    that 1 Amp in its self can KILL you and Im just trying to be cautious. Why
    put in a bigger fuse that you need?
    As well there are other things on that circuit. I would like to prevent
    everything on that house fuse from being shut down needlessly.

    Comments ??
  6. Art

    Art Guest

    FWIW: Have you even given an isolation transformer any consideration, at
    least for safety sake? The fuse or GFI is a good idea but isolating your
    test bench may be a bit more prudent, IMHO. BTW if you do use an isolation
    transformer, do not overload the circuitry or you will just toast it.

    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
  8. Trudeau

    Trudeau Guest

    Thnaks again for your suggestions.
    Could someone explain, ( Simple as you can ) what the advantage of an
    isolation transformer would be in this project. As I understand it basically
    you have a transformer that separates the 120 house into I suppose 120
    coming from the other half of the transformer. (Is this called a 1 to 1). Im
    not sure what the difference is. Unless as I come to think of it you are
    reducing the amps from 15 house to say 1 or 2. I suppose this would have
    some added safety.
    Please Carify.

  9. John Fields

    John Fields Guest

  10. John Fields

    John Fields Guest

    Well, only partly. The insulation on the magnet wire and the nylon
    walls of the bobbin separating the primary and secondary (if the
    transformer is wound that way) do the galvanic isolation.
    If you're working with mains hot and neutral directly, without going
    through a transformer and you get hot and neutral reversed, the
    chassis of what you're working on could be connected to mains hot with
    the embarrassing result that you could fry a scope probe as soon as
    you clip the probe's ground to chassis or to whatever you thought was
    ground in the circuit. Worse, you could be holding on to the chassis
    when you grab the alligator clip... Interpose an isolation
    transformer between the DUT (device under test) and the mains and
    those problems go away. Of course you'll still have 120V coming out
    of the transformer, but if you don't know enough not to grab ahold of
    that, you'll probably be a candidate for a Darwin award soon,
  11. Trudeau

    Trudeau Guest

    Thanks for that.

    I'm a little unclear on what the Galvanic barrier does.

    I ve done a little research and learned that a galvanic process is a
    process that creates electricity. eg electricity in frogs legs.
    Secondly I have always know that when you put certain metals together that
    you get corrosion. I have now learned the name for this is a Galvanic
    As far as the discussion at hand I have found that a Galvanic Barrier in a
    transformer :

    A. Galvanic barrier has no metal metal contact, Well Yes of course its a
    transformer. It uses fields to pass the current and air as the barrier. I
    guess that not being directly connected to the mains is good. But its still
    B. A Galvanic barrier cuts down on noise. Not sure its important in this
    C. That a G.B. prevents ground loops. This I can see as being important.

    Am I right here. Are there more reasons to use the isolation transformer?
    I guess my confusion is that if the voltage coming into the transformer is
    the same coming out accept for the fact that it is cleaner and perhaps
    exactly 120 Volts. Im not sure what more added safety it adds, accept for
    the things I noted above. Sorry to be dense. What am I missing?

  12. Trudeau

    Trudeau Guest

    Thanks for your patience and helping me to understand.

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