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Transformers to step up voltage?

  1. Aug 25, 2014 #1
    I was shown how to use two transformers to step up voltage and I am curious as to how it works.
    The low side of the transformers were connected to 120v and the high side was connected to a load at 240v.
    What amazed me was that the low sides connected in series could be connected to the 120v wall outlet(Hot Leg and Neutral) with no load on that side and not be a short circuit.
    Is this right? Was it only working because the high side was connected to a load?
    Would it not work or be dangerous if there was no load connected to the high side? Open voltage?
    Are the coils enough resistance to prevent a short or is it that until a load is on the high side, there's no draw?
  2. jcsd
  3. Aug 25, 2014 #2


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    I am having a hard time visualizing what you are talking about. Draw some pix maybe?
  4. Aug 25, 2014 #3
    When you say you are using two transformers to step up voltage, are you talking about using two in series? Is it two transformers stepping up voltage from a shared 120v source? I'm a bit confused as to why you are using two transformers to step up voltage. Your post needs more details about what is going on or a picture.
  5. Aug 25, 2014 #4


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    Connecting transformers' primaries in series is allowable, providing you are careful to operate well within their ratings, including their insulation ratings to ground. A transformer primary only draws current when there is secondary current, that's the way transformers work. If there is no load on the secondary there will be almost zero current in the primary. No need for resistance.

    I suspect you may have seen someone connect primaries in parallel but secondaries in series using 1:1 isolation transformers as a way of deriving 240V from 120V mains? The phasing needs to be right.
  6. Aug 26, 2014 #5
    After thinking about it, and at what has been replied, I think what I was looking at was a way to load balance some loads with step up voltage.
    Ok, that's not what I am interested in. I am interested in stepping up voltage.
    So, what I realize now is that I can simply take one transformer with a 1 to 2 step up ratio.
    Or a 2 to 1 step down ratio and reverse it's wiring. I think.

    A transformer primary only draws current when there is secondary current, that's the way transformers work. If there is no load on the secondary there will be almost zero current in the primary. No need for resistance.
    Is this what a voltage transformer is? There isn't any current in the wire until a load is on the secondary side?
    This is what I wanted to be sure of, was that there wasn't any current in the secondary side.
    I was afraid that if i didn't have a load connected to the secondary side, that the open voltage would begin to rise and rise since it's being stepped up.
    So, it's ok if I don't connect a load to the secondary side until after the primary side is hot?
    I won't do that, but I just wanted to know for instance if the load wire happened to disconnect or if there was a break in the wire on the secondary side or if a fuse on the secondary side tripped.
    I believe it's in a current transformer that will be a problem if the secondary side is open.

    So I just need a single transformer as I've described to step up the voltage and of course a verified 240v load. Just making sure I can add 120v on the primary side without issue.

    I'm trying to wrap my head around how the primary side isn't a direct short when voltage is added to it's terminals. With no resistor at the return side of it to keep it from shorting. Is there only voltage present because the coil wiring is so thin?
  7. Aug 26, 2014 #6


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    Some one will explain this better ( transformer theory isn't my strongest subject :wink: ). But basically because of the inductive and capacitive reactance's of the primary coil at the AC mains frequency in your country (50 or 60 Hz)
    The coil doesn't look like a short circuit even tho its reasonably low resistance

  8. Aug 26, 2014 #7
    One thing to consider, I think.
    If the 240v load will use 5 amps, then it will really be pulling 10 amps from the 120v side?
  9. Aug 26, 2014 #8


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    Of course.

    Power in = Power out , neglecting losses.
  10. Aug 31, 2014 #9
    One more question:

    Would it be wise to ground one side of the transformer?
    Last edited: Aug 31, 2014
  11. Aug 31, 2014 #10


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    The manufacturer has already decided whether the iron core and casing must to be connected to mains earth, assuming your electricity supply provides an earth.

    Whether you connect one end of the secondary winding to earth is up to you. The secondary windings are [almost always] floating and independent, and it is immaterial to them. If you are unsure about the winding interconnects, you can always test for continuity with an ohmmeter on the unpowered transformer before you connect it up. (If you are curious, watch for the inductive kick.) :smile:
  12. Sep 6, 2014 #11
    I’ll try to explain how a transformer works in a few words. In a solenoid [or coil] the electric current will provoke a magnetic field. If it is alternative current this alternative magnetic field will produce an EMF [Electro-Magnetic Force] - a potential difference between solenoid ends opposite to the supplied voltage. This EMF depends not only on current value but also on the solenoid core material. If it is only air the EMF is wick and the remaining circuit voltage will be high then the stabilized current will be also high. If the core is magnetic material made the magnetic flux is high then EMF is strong and remaining voltage [subtracting EMF from the supply voltage] is low and so the resultant current-no short-circuit phenomenon.
    If another solenoid is provided on the same magnetic circuit the same magnetic field will travers the new solenoid and the EMF produced here could be used to supply other installation. How high will be the current in the second solenoid the second magnetic field will be high and tends to reduce the initial magnetic flux. In order to compensate it the primary solenoid current will rise. This is the “loading” phenomenon in simply words.
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