Step down Transformer 240v AC to 12v 100amp

  1. Hi All,

    I have a step down Transformer 240v AC to 12v 100amp output.

    My question is, can i use this as a step up transformer as well by connecting to the other side of it?

    My supply voltage is of course 240v AC, this can be controlled with a 240v 10amp Variac to take it easy on things.

    Not sure so i thought i had better ask first.

    Thank You
     
  2. jcsd
  3. Danger

    Danger 9,879
    Gold Member

    Re: Transformer

    As far as I know, any step-down transformer is also a step-up unit if fed from the other end. I'd seriously recommend that you check with the manufacturer to find out if the coils and insulation can handle the reverse of their original function. There's a serious fire/shock/generic **** hazard if you proceed without professional assistance.
     
  4. Re: Transformer


    Danger, no way of checking from the manufacturer, it's an older transformer.
    I am aware of the dangers involved, i was hoping to get the professional assistance from this forum.
     
  5. Danger

    Danger 9,879
    Gold Member

    Re: Transformer

    Sorry that I can't help you with the professional aspect; I'm not all that well educated. Since I have no idea of your background, I chose to go the cautionary route. Bear in mind that some of our members, let alone onlookers, are pre-teens. It's imperative that nobody is ever injured due to something posted here. I apologize if I offended you with my response.
     
  6. Re: Transformer

    Danger,

    No need to apologise as Caution is top priority when working with electricity & your comments are to be noted by all.

    No offence taken actually, you couldn't affend me with a brick, im Aussie hahaha!
     
  7. Danger

    Danger 9,879
    Gold Member

    Re: Transformer

    Well, good on ya, mate.
    You gotta hook up with Astronuc. He's been across the lake for a long time, but still a Foster's man. :biggrin:

    And just so's you know, I'm a Canuck, not a Yank. :wink:
     
  8. Re: Transformer

    Ok, looking further into it i see that transformers are bi-lateral & can be used as either a step up or step down.
    It has been advised that i use no more than 12v into the primary winding resulting in no more that 240v from the secondary winding using it as a step up transformer.

    Is there a reason or explanation why no more than 12v can be applied to the primary winding to achieve a higher secondary winding output?
     
  9. Re: Transformer

    The primary of a transformer is designed to handle the reflected load current and magnetization current.

    The secondary is designed to handle only the load current.

    If you turn it around, you need to derate it.
     
  10. MATLABdude

    MATLABdude 1,724
    Science Advisor

    Re: Transformer

    I can think of two possible problems with this:
    1) The secondary winding is made from wire that is of too small a gauge to carry the amount of current you're looking to feed in, and it burns up

    2) The voltage on the primary is so high that there's arcing through the insulation of the wires in the winding. For many transformers, the wire has only a very thin insulation which can be easily arced across.
     
  11. Re: Transformer

    Any xfmr can be reversed if you observe the voltage and current limitations. If the unit inputs 240V and outputs 12V, then it can be reversed, i.e. it will operate fine with 12V input across the low voltage winding, and output 240V across the high voltage winding.

    The current ratings for each winding remain as spec'd.
     
  12. Re: Transformer

    Not so fast. An unloaded transformer draws current. This is called the magnetization current. You might be hard pressed to actually come up with a transformer that will self distruct, but in principle the magnetization current could exceed the rated current.

    This leads to excessive heating of the core. This could cause the saturation flux density to rise past the saturation knee which leads to an increase in the magnetization current. The rising current adds to more core heating and could result in divergent thermal run-away.

    At a guess, I'd think a wall wart with AC output is a good candidate for this sort of failure. Hook two of them up, back to back, and see if the output is 120 VAC, and what gets hot over time.
     
    Last edited: Oct 9, 2008
  13. Re: Transformer

    ----- Phrak quote

    Not so fast. An unloaded transformer draws current. This is called the magnetization current. You might be hard pressed to actually come up with a transformer that will self distruct, but in principle the magnetization current could exceed the rated current.

    This leads to excessive heating of the core. This could cause the saturation flux density to rise past the saturation knee which leads to an increase in the magnetization current. The rising current adds to more core heating and could result in divergent thermal run-away.

    At a guess, I'd think a wall wart with AC output is a good candidate for this sort of failure. Hook two of them up, back to back, and see if the output is 120 VAC, and what gets hot over time.

    ----- end quote


    Begging to differ. First of all, I am well aware of what magnetizing current is. I designed xfms for a company well known for them. Just out of curiosity, have you designed xfmrs? If not, it's no big deal.

    When you say the Imag exceeds the rated current, my first reaction is "run that by me again please!" Imag is typically around 5% of the rated load current. If the mag current was excessive, which it isn't, how does runaway take place? An increase in current through the windings incurs a larger IR drop due to the winding resistance. This results in a lower voltage impressed across the primary. The flux will not run away, but decrease with this condition.

    If a xfmr is rated at 240V in, 12V out, then the high side, which has thinner wire, and carries lower current, carries an Imag around 5% of rated load. If we instead excite the low side, 12V as the primary, the exciting current times the turns changes very little for the same frequency. The flux is related to volts per turn so that 240V across 240 turns results in approx. the same flux as 12V across 12 turns. The N*Imag will be very close to the original in amp-turns. The core runs at the same temp, and the low side windings run around the same temp, as they are wound with heavier wire, carry more current, but again, sized for full load which is much greater than the Imag.

    Regarding the wall wart, I've never seen one with ac output. The output is always, AFAIK, rectified into dc. If you connect 2 units back to back, of course the 2nd unit gets hot. Connecting dc across the 2nd units' winding will burn it up. No debate there.

    This is pretty basic stuff. If I've erred I welcome and appreciate correction, but we're talking about transformers here. These are well documented and have been in use since the latter part of the 19th century. There are few mysteries involving xfmrs. BR.

    Claude
     
    Last edited: Oct 9, 2008
  14. Re: Transformer

     
  15. berkeman

    Staff: Mentor

    Re: Transformer

    cabraham is correct. The 240Vrms/12Vrms transformer can be run backwards, but with 12Vrms input on the 12V side, not 240Vrms input on the 12V side.

    I've seen AC/AC wall warts, but they're not very common. If you did hook them up back to back, you'd get close to 120Vrms (or whatever the input voltage was) out the far side, but it would be somewhat lower due to transformer losses.
     
  16. Re: Transformer

    Thanks to all for there input as i have a clearer picture of what i needed to know.

    Thanks Again
     
  17. Re: Transformer

    Unloaded, it would be. To my dismay, when searching for bootstrap methods to power a switch mode supply high side, I found that low current transformers---under 50 mA--are terribly lossy. I couldn't find a supplier that advertized anything less at the time. I needed something like 5 or 10 mA. Anyway, the lower the rated current, the worse they become. Half the wire crosssection must be taken up with enamel! That, and the apparent limit in wire size of #40, seems to be why they didn't exist.

    You can imagine what happens when the enamel thickness remains constant, while wire size decrease.
     
    Last edited: Oct 10, 2008
  18. Re: Transformer

     
    Last edited: Oct 10, 2008
  19. Re: Transformer

    Quick scan through the posts... I would agree with those who say it would not work... Real transformers have a complex equavlent circuits far removed form the simplistic voltage/current in/out governed by the turns ratio....due to winding resistance..eddy current losses.. They are designed for specific voltages in and out and should only be used outside that voltage with care.
     
  20. Re: Transformer

    I should do what I see fit. I don't tend to find comfort in when others tell me what can't be done. It usually just means a desire to maintaining a position of authority threatened by initiative.
     
  21. Re: Transformer

    Why ask for advice then?

    If you do try it feed it via a low value fuse or circuit breaker or both.

    I would think a 12 secondary used as a primary would have too few turns ..too low an inductance for 240 Volts.. The 240 primary used as a secondary may not handle higher voltages anyway. A proper step-up transformer would have a primary similar to your 240/12 step down but a secondary with many many more turns which could handle the high voltage....e.g..HT transformer used in a microwave oven.
     
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