Transformer impedance

Discussion in 'General Electronics Chat' started by erlend, Mar 9, 2015.

  1. erlend

    Thread Starter Member

    Feb 8, 2008
    Could someone please tell me the difference between these two transformers?
    1. Primary impedance of 200 ohms and a secondary impedance of 1 ohm,
    2. Primary impedance of 200 kOhms and a secondary impedance of 1 kOhm.

    In my understanding of a step-up/down transformer both of these should affect the voltage (and current) by the same amount, since their coils-ratio is the same. Is the difference just in the power?
  2. Papabravo


    Feb 24, 2006
    I would expect that one of them is physically larger and is wound with larger diameter wire. As I am sure you are aware inductance varies with frequency. The same is true of transformers and it would be nice if we could depend on the spec writers to give us the frequency at which the impedance was measured. Sadly you have to plumb the datasheet to find the answer.
    cmartinez likes this.
  3. alfacliff

    Well-Known Member

    Dec 13, 2013
    the same turns ratio, just a lot more turns on one than the other.
  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
    How are you measuring the resistance, with a dc ohm meter?
  5. crutschow


    Mar 14, 2008
    Is this resistance measured or a specified?
    Audio transformers often have a stated impedance, depending upon the type of circuit they are in. It determines the signal voltage they can tolerate based upon their power rating.
  6. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
    To render a sensible response we'd need to know the Z form (i.e. parallel equivalent or series equivalent)

    If, on the other hand, this is a theoretic question and we may substitute L (as derived from XL) for Z then for an inductance ratio=1:200 the transformation/turns ratio will be ~14:1 in both cases... Of course the external matching networks would be quite different (each transformer).

    Note also that (Re: electromagnetic transformers) power handling is, essentially, a structural characteristic.

    Best regards
    PS do I hear you crying "apples and oranges"? -- well, like I said, 'to render sensible responses...' :rolleyes:
    Last edited: Mar 9, 2015
  7. #12


    Nov 30, 2010
    I always try to adjust my response to the intellectual level of the question. Often enough, I fail, but often enough, one can imagine exactly which school course this is taught in and try to work at that level of understanding. If you unintentionally insult the Thread Starter, he usually lets you know. :D
    cmartinez and Hypatia's Protege like this.
  8. erlend

    Thread Starter Member

    Feb 8, 2008
    Thanks for all the replies.

    This is just a theoretical question, so apologies for the lack of information. The limit of my understanding is that a step-up transformer puts the voltage up (current down) by the turns ratio (or V1/V2 = N1/N2). Similarly we all know that N1/N2 = sqrt(Z1/Z2), which is why HP gets ~14:1 above.

    Imagine that I have two different transformers, 1) secondary impedance of 200 ohms and a primary impedance of 1 ohm and 2) secondary impedance of 200 kOhms and a primary impedance of 1 kOhm. Both will step-up the voltage by the same amount (~14:1) however the current-draw from each is different. The first one will draw more current (and there will be more current from the output. Is this correct?
  9. #12


    Nov 30, 2010
    This is true, assuming an equal voltage being applied. The 1 ohm primary would need something like a powerful audio amplifier to drive it while the 1K impedance can be driven with a single Class A transistor amplifier.