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"Serious doubts about transformers and their safety"

Discussion in 'General Electronics Discussion' started by barathbushan, Nov 17, 2010.

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  1. barathbushan


    Sep 26, 2009
    hi guys , i have some doubts , about transformers


    recently i bought a 12-012 centre tapped transformer , i know the basics of using it , but there are still some doubts crawling in my head

    1) i have read in some other thread at other forum , that when the mains power (230 v)
    is given to the secondary terminals of normal step down transformers like the one i have bought , the primary terminals give increased voltage of about 1000v (step up action) . [IN SHORT THE PRIMARY AND SECONDARY CONNECTIONS ARE REVERSED]

    2) also what would happen if the secondary terminals are accidentally shorted , is there a built in short circuit protection ??

    3) my transformer is rated at 1 amp , what if i try to pull more current from it ??

    4) how long can a transformer be operated at full load ??

    since we are dealing with electricals not electronics, "SAFETY" matters so please dont give answers if you are not perfecty sure about .
    Last edited: Nov 17, 2010
  2. Resqueline


    Jul 31, 2009
    I took the liberty of relaxing the heading a little. These are after all very old and tried issues.

    1) That may happen only to a limited voltage and time.
    The core will saturate (before the voltage even doubles) and thereby limit the voltage, while the current increases to a level that will blow fuses or even the transformer wire.
    The safety lies in the hands of the equipment designer. For example he should not use a contact on the secondary side that any mains wire could be plugged into.

    2) The current will go sky-high, limited only by the combined impedances of the primary & secondary. It can stand this condition for several seconds though.
    A few specially designed transformers have current limiting built onto their (magnetic) design. Others rely on external fuses and some even use built-in thermal fuses.
    If a fuse is not provided then ultimately the primary wire may act as a fuse and the transformer is d-e-a-d for good.

    3) Depending on its size, the start temperature, and the amount of overcurrent, it may work well for minutes. It's all about a gradual temperature increase & - failure.

    4) 100% of the time (indefinitely) unless otherwise stated by the manufacturer.: For example; solder guns & welding transformers often have a use/rest time period limit.
  3. barathbushan


    Sep 26, 2009
    in all of the above issues besides the transformer going dead ,is their any thing dangerous going to happen , like a explosion , catching fire etc

    actually by using the transformer as mentioned in this site

    i shorted my mains , luckily i got away , since i used a fuse , but still lot of sparks flew everywhere , obviously the idiot who wrote rhe article in the site i mentioned , had not tested his circuit , befor giving out his nonsense and reckless circuit .
  4. Resqueline


    Jul 31, 2009
    Sparks can fly, and if you have too much dust on the floor (or curtains or other flammables nearby) then for sure a fire can start.
    The transformer itself won't sustain much of a fire, it's made from fire inhibiting materials except for the magnet wire varnish, but still the mains packs a couple of kW's.

    That (laser tube power supply) is one of the worst designs I have ever seen, it's completely hilarious... Good thing you at least had the sense to put a fuse in there.
    You can use transformers in reverse, and you can squeeze a little more voltage out of them than intended, but you can't put 115V into a 9V winding and get away with it.
    I feel sorry for everyone actually trying to build it, and for the laser tubes being run by it should anyone ever be able to find a transformer capable of dealing with that abuse.

    Good utilisation of a transformer core involves running it as close to saturation as possible. That places a distinct limit on the overvoltage you can hope to put through it.
    Everyone wants to save size, weight, & costs on transformers, so naturally that is the way all manufacturers would choose to make it.
    If they didn't then the transformer in question would be big, bulky, & costly for its power rating, and it would have the potential of being rewound for much more power.

    The "correct" way of making a simple laser tube power supply that will actually work (and be gentle on the tube) is only a little more involved than that one.
  5. barathbushan


    Sep 26, 2009
    thanks a lot for your guidance , actually the editor of that article told me to use a series inductor with the primary , after i mentioned that i had shorted my mains . This clearly shows that the design has been untested , and god save those who were not so lucky in using a fuse .
  6. Resqueline


    Jul 31, 2009
    Yes I saw that comment, and although it's a step in the right direction it still says quite a bit about the design & the designer.
    Clearly he has no idea what he's doing, and wisely he does not try to specify the data of that inductor either.
    Notice also the absense (still) of a specification on the nominal mains voltage (& frequency) of the original transformer even though people have asked about it.

    A 240V 50Hz transformer might have a fighting chance of surviving that abuse for a short time on a 110V 60Hz mains since it would be further away from saturation.
    It would be important though to use a transformer with as bad regulation as possible since this also is an indicaton of how far away from saturation it is.
    For example a 9V 2VA transformer might have an unloaded voltage of 18V. Thus you'd get full mains (unloaded) out of the primary if you put 18-20V into the secondary.
    But in order to get full mains voltage out with a full load on the primary however I believe you'd have to put something like 36-40V into the secondary.
    The bigger a transformer is (more VA) however, the better regulation it has, and the worse it it suited to this kind of overvolting/abuse.
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