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Hobby electronics projects-- fire safety?

Discussion in 'Electronic Basics' started by [email protected], Jun 28, 2007.

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  1. Guest

    I'm a newbie to electronics, but I've come up with a project to cool a
    closed electronics cabinet. It uses an ac-dc 12V power (wall wart)
    adapter which would power a 500mA load. The adapter is rated for
    800mA.

    Are there any safety problems with this? Fire hazards? What would
    happen if a short occurred?

    The adapter is UL listed, but I've been unable to see what criteria
    they use for power adapters. Does the UL mark imply any level of
    performance or safety?


    Thanks,
    Ethan
     
  2. Eeyore

    Eeyore Guest

    It's basically up to you to ensure a short doesn't happpen but in the event it
    did such adaptors typically are protected by an internal thermal fuse.

    Graham
     
  3. Plugpacks are usually protected with an integral thermal fuse.
    But good practice would be to add your own fuse to your application.
    You could use an line fuse holder that can be wired in series with
    your the plugpack lead.

    Dave.
     
  4. Ken Moffett

    Ken Moffett Guest

    Interesting...about them having internal fuses. I've cut
    many, many of them open...to get out the small transformers
    for various projects...and have never encountered a
    fuse...thermal or current...on either side of the
    transformen, on the the little PCB's, or in the transformer
    windings. I'm talking about the straight
    transformer/diodes/capacitor versions...not regulated or
    switching. I thought they depended on crappy, easily
    saturated transformers for safety limiting.

    ???

    Ken
     
  5. Phil Allison

    Phil Allison Guest

    "Ken Moffett"

    ** Not all plug pack transformers have a thermal fuse. However, all must
    pass a safety test where the output is shorted until the unit fails, without
    any hazard to the user.

    ** Yep.

    ** Some transformers are "inherently short circuit proof " so will not heat
    the windings enough to burn even if shorted indefinitely.

    Others simply burn and go open - but because of the construction method,
    there is no risk of the primary and secondary becoming connected.



    ......... Phil
     
  6. Eeyore

    Eeyore Guest

    A thermal fuse would be 'buried' in the windings. Did you cut the transformer
    open ? Externally you wouldn't be able to distinguish a blown thermal fuse from
    a 'blown' transformer winding.

    Graham
     
  7. I don't know where all this talk of "thermal fuses" comes from. After
    all, a regular fuse has always sufficed to protect things.

    The only reason something gets too hot is because too much current
    is going through it, and a regular fuse can stop that.

    I was under the impression that a lot of those low current transformers
    were wound so that the primary would break if too much current went
    through them, and that was the "fuse".

    On the other hand, I had an old Sony ac adaptor that did get shorted,
    and nothing happened so it got really hot, enough to scortch the surface
    it was resting on. It was, though, one dating from about 1971, so one
    hopes such things don't happen nowadays.

    Michael
     
  8. Phil Allison

    Phil Allison Guest

    "Michael Black"

    ** Seems YOU don't know a whole bunch of things.

    ** Far from the truth.


    ** There are many circumstance where a wire fuse is not able to do that
    reliably or accurately enough.

    Happens when the maximum safe operating current and the unsafe or overload
    current values are similar.

    Happens when the overload current is only 50mA.

    Happens whenever the fuse is accessible to a user.


    ** You labour under many silly misconceptions, then.


    ** That can still happen.



    ........ Phil
     
  9. Eeyore

    Eeyore Guest

    That only illustrates how little you know about product safety.

    Graham
     
  10. Eeyore

    Eeyore Guest

    They have thermal fuses.

    Graham
     
  11. Jasen

    Jasen Guest

    a thermal fuse blows before the varnish ignites.
     

  12. Really? I've seen a lot of the thermal fuses brought out to separate
    terminals. One lead of the primary was soldered to one of the terminals,
    and the fuse could be replaced, or jumpered for testing to determine if
    the unit was worth repairing. Some transformers even have a notch in
    the nylon or plastic bobbin to make it easy to remove the fuse and
    replace it.


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  13. Eeyore

    Eeyore Guest

    The only transformers I've seen with the leads brought out were ones with resetting
    thermal switches. These are a bit more upmarket.

    I've also come across transformers with buried self-resetting thermal switches too.

    Graham
     
  14. Eeyore

    Eeyore Guest

    Or melts, causing a potentially dangerous fault condition. It's all about
    temperature rise. Most of the common insulations are rated for no more than 130C
    and the transformer winding itself won't fail open at those temps.

    Graham
     
  15. ehsjr

    ehsjr Guest


    I just opened one that had failed to get the wire from
    it. The fuse was not apparent, until I dissassembled
    the laminations to get to the bobbin, and removed the
    tape so I could unwind the primary. The fuse was
    between layers of tape.

    Ed
     
  16. Probably a thermal fuse placed to get blown if the windings overheated.

    Power supplies / "wall warts" that are UL listed as a "Class II
    transformer" or "class II power supply" are supposed to be reasonably safe
    even if the load malfunctions.

    However, I would use some common sense with the load. If the load is a
    prototype or an experimental device, I would add a fuse that is actually
    tested to reliably blow if the load fails short or encounters its most
    probable failure modes that result in increased current draw (component
    failure or shorting within the load). With the fuse reliably blowing even
    when the Class II wallwart is warmed up and its winding resistances are
    higher.
    Otherwise only operate the experimental/prototype device when you are
    around to unplug it if things start smoking. I know that Class II power
    supplies are supposed to be reasonably safe from shorts and overloads, but
    I suspect some barely pass certification and some may cut corners compared
    to the units that got sent to the certification lab. For example (with a
    different device), I have known a fire to start from a fluorescent lamp
    ballast overheating when a recently-failed lamp caused a starter to "get
    stuck" (fail short).

    If you see any fluorescent lamps blinking on and off or steadily glowing
    only at the ends, remove the lamp ("bulb") or the starter. If there is
    steady end glow, the starter failed short if there is a starter in the
    fixture.

    - Don Klipstein ()
     
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