Mains Transformers

Discussion in 'General Electronics Chat' started by jzq, Jul 30, 2008.

  1. jzq

    Thread Starter New Member

    Jul 28, 2008
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    I have a US transformer which has an input voltage of 120V but I live in the UK where the mains are 240V.

    Would it be feasible to use some kind of resistor network, like a potential divider to bring the 240V down to 120V without using a transformer?

    Of course, after I have taken the 240V down to 120V, I will then be able to connect the US transformer I have.

    This is along the lines of what is was thinking: I would have a simple voltage divider with two resistors, R1 and R2 would be the same values to half the voltage. But for some reason I think there may be a better way to go about this. Also with this arrangement, wont the resistors need to have a very high power rating?


    Basically I need a way to connect my US Transformer to the UK mains. Can anyone come up with some ideas?

    Thanks for all your help!
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,649
    2,348
    Hello,

    When you want to use two resistors, they must be of rather high power.
    Another effect is when the resistors are not calcutaled corrctly, the output voltage can vary depending on the load.
    It is much safer to use a tranformer that makes 120 V out of the 240 V.
    This will also be more power efficient.

    Greetings,
    Bertus
     
  3. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    Is your life worth the cost of a replacement transformer?

    If you seriously don't know enough to answer your own question don't play with the mains.

    The explosion you could cause could blind, burn or worse.
     
  4. jzq

    Thread Starter New Member

    Jul 28, 2008
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    It is possible for me to get a transformer that will do the job but they cost about £80 which is about $160...
    Rather expensive, so that is why im looking for a different way of doing it.
     
  5. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    So explain exactly what you are trying to do, we may be able to help.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    If you had two of the same transformers, you could run the primaries in series and the secondaries in parallel (the phases would have to be matched, of course.)

    Using resistors would be terribly inefficient, and your input voltage would vary widely as your output load changed - making your output voltage vary widely as well.

    You didn't mention what the VA or current or wattage rating was for the transformer. That's something that would need to be known in order to even start on a viable solution.

    What does the transformer power? Is it a power supply that puts out a DC voltage?

    The more information you can give us up front, the more likely that we can suggest a viable solution. Without all of the pieces of the puzzle so to speak, we're throwing darts in the dark.
     
  7. jzq

    Thread Starter New Member

    Jul 28, 2008
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    Well ive done some more searching and found that it is possible to bring the voltage down by using capacitor voltage dividers or even the zener diode and resistor method but ive also found that those methods are rather dangerous and have apparently been outlawed in europe (for direct mains circuits).

    So after some searching i managed to find this: http://www.airlinktransformers.com/transformer/at500-step-up-step-down-transformer.asp

    A toroidal transformer that allows you to either step-up (120 to 240) or step-down (240 to 120). Its quite cheap as well compared to some i have seen.

    The toroidal transformer is rated at 500VA, so i should be able to connect it to the mains, and the secondary to my step up transformer that is rated at a max of 450VA.
     
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