Connect with us

Electrical device safety, fusing, heat

Discussion in 'Electronic Design' started by Eel, Feb 11, 2004.

Scroll to continue with content
  1. Eel

    Eel Guest

    Hi, I got snuckered into putting together a diverse set of pc boards +
    devices as part of what is supposed to be a software project. I'm
    going to deliver with a big disclaimer that I ultimately don't know
    what I'm doing, but I was wondering if anybody had some quick answers
    for some hopefully simple questions.

    1. Are there any safety guidelines for hooking a device up to 110 VAC
    current? Is this a matter of just making sure the connections are
    insulated and properly grounding anything that might come into contact
    with a stray voltage? I need to hook wall current up to a 24 VAC
    transformer that will drive a stepping motor via a 3rd party pc board.
    There are a couple of other boards that get their power from a

    2. Any links or info on specifying a fuse? I have 24 VAC at 2 amps,
    so that would be something like .6 amps for 110 volts? When should I
    use slow blow fuse?

    3. Is it all right to mount everything on metal box with a few vents
    and close it up? 24 volts at 2 amps is about 50 watts. It seems like
    I could put a 60 watt light bulb into a box and close it up no
    problem. Is that the case?

    4. Any good books on this topic? When I was a student (a while back)
    "The Art of Electronics" by Horowitz and Hill was popular. It seemed
    a bit "off balance" in that in one sentence they were telling you not
    to use electrical tape to insulate connections and in the next
    sentence they were talking about quantum electrical theory. Is the
    new edition any good? Any better books?

  2. Just a few thousand. Try UL standard UL60950.
    And a few thousand other things, like creepage and clearance, fire
    enclosures, mechanical strength....
    The voltage is not even 110 V, it's 120 V. You are starting a long way
    back. You do need a slow-blow fuse to feed a transformer, but you can't
    calculate its value just by using the transformer current ratio. You
    have to take into account inrush current. Under certain circumstances
    the current at switch-on is limited only by the DC resistance of the
    primary winding of your transformer. Calculate that current and then
    look at fuse I^2t curves to choose one that will withstand that inrush
    current for half a period of 60 Hz, i.e. 8.3 ms. Check that it will also
    NOT withstand twice your 0.5 A normal operating current for more than a
    few minutes at most. This latter is a rough guide: there are tests that
    have to be done to determine whether the fuse rating is correct.
    UL 60950 will tell you all about that! There are strict limits on the
    permitted temperatures of various parts.
    How dreadful!
    I expect you would find it much the same.
    Not that will tell you the answers to your questions, AFAIK.

    There are few books that tell you about how to design products to meet
    safety requirements. One problem is that publishers would be reluctant
    to publish such books because of possible liability if blindly and
    inappropriately following any advice or recommendation in the book led
    to an accident. You can learn design requirements from the relevant
    standards, such as UL 60950, but it's a very steep learning curve.
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day