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Typical bandwidth of 60hz wallwart transformers

Discussion in 'Electronic Design' started by Jim Flanagan, Apr 22, 2007.

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  1. Jim Flanagan

    Jim Flanagan Guest

    Hi -
    I am going to be designing a 400hz 3ph test generator
    (about 1W max) and want to use some 60Hz transformers
    to use in a DC-AC inverter (linear). Would these
    show a significant rolloff at 400Hz or is the bandwidth
    OK for this application? What about using some small
    audio transformers? I need the output to be around 160 Vrms
    and am not sure if a miniature audio transformer has sufficient
    dielectric properties to handle this. Obviously, I am trying
    to do this on the 'cheap'. Any suggestions?

    thanks for any input
  2. Bob Eld

    Bob Eld Guest

    Any 60Hz transformer should work well at 400Hz. The reverse is not true,
    however. This assumes a sine wave without a spray of higher harmonics from
    switching circuitry.
  3. SuperM

    SuperM Guest

    At a one watt draw? I hardly think you need to worry about your
    losses in such a circumstance.
  4. SuperM

    SuperM Guest

    Hand wound pot core transformers of about a 25 to 37 mm diameter
    should be much more efficient at your operating frequency, but at your
    power level, I hardly think worrying about a custom xfmr is needed.
  5. Small 60 Hz transformers work pretty well to at least a
    kilohertz, at no more power than their original rating.
  6. Barry Lennox

    Barry Lennox Guest

    No problem, I have used el-cheapo transformers (designed for 50 Hz)
    several times at 400 Hz (for aircraft test sets) and variable from
    about 100Hz to 3-4 Khz. They work fine, even though I'm sure the
    efficiency drops off. At the other end of the scale, they also work
    OK down to 16 Hz, again I'm sure the efficiency drops off
    dramatically, but for the power I required (and your 1 Watt) it did
    not matter.

  7. Avoid lamination transformers because they can
    have a poor output impedance due to their higher
    leakage inductance.

    Toroidal mains transformers will work ok.

    The flux density will be about 1/7th of what it
    was designed for, so you can run what was a 115V
    primary as a 160V secondary with no trouble.
  8. Tam/WB2TT

    Tam/WB2TT Guest

    I don't see any problem. I once measured the frequency response of a
    standard laminated core 12 V filament transformer, and the minimum loss was
    actually somewhere between 300 and 400 Hz. Also used the 12v center tapped
    secondary of a Radio Shack transformer as a 16 to 4 Ohm autotransformer for
    audio. It was quite usable, but sounded a bit mellow.

  9. MooseFET

    MooseFET Guest

    I assume the OP is making a sine wave with a linear drive to the
    transformer. The transformer may be enclosed within a feedback loop.
    This would make the output impedance low at the frequencies below the
    gain cross over point.

    If this is the case, the laminated transformer would not be a big
    issue. The gain cross over point could be at several KHz. He is only
    drawing 1W from the supply so the load impedance will be quite high.
    This would allow Xl to also be fairly high before it started to push
    down the practical limit on the gain cross over.

    The OP may want to consider placing a modest capacitor across the
    secondary windings. This would make the high frequency impedance
    lower at the cost of making the circuit resonate with the leakage
    inductance. with a good control loop design, this would be a low Q
  10. MooseFET

    MooseFET Guest

    I think it is good enough for voice and perhaps an AM radio but not
    really HiFi.

    IIRC, the cutoff frequency on the transformer I used was about 4KHz.
    It was a 110-220 transformer but I was using the two primaries to make
    a 1:1 isolation transformer.
  11. Phil Allison

    Phil Allison Guest


    ** Nonsense - a centre tapped or multi-tapped secondary winding ( when
    used as an auto-transformer) has wide bandwidth, more than the audio band
    for sure.

    The bandwidth of a transformer has little to do with the core and everything
    to do with the arrangement of the windings - ie side by side on a divided
    bobbin is the poorest.

    ........ Phil
  12. The small ones I tested as pretty good as audio output
    transformers, out to a few kilohertz were the side by side
    (split bobbin) EI core type.
  13. Phil Allison

    Phil Allison Guest

    "John Popelish"

    ** Not many consider a few kHz as "pretty good audio".

    Even the cheapest 70 volt line transformers for BGM and voice PA are not
    made that way.

    ........ Phil
  14. Fair enough. i meant their response was pretty good
    (amplitude and phase shift) out to that audio frequency.
    Yes, of course.
  15. MooseFET

    MooseFET Guest

    Tranformer of unknown make: Numbers 19966624-41965DJ printed on the
    A little larger than typical wall wart xformer about 1.75 by 1.25"

    6:1 ratio

    Generator resistance: 8.6
    Load resistance: 287

    Freq Vp-p
    100 2.0
    1K 2.0
    3K 1.80
    5K 1.41
    7K 1.30
  16. Phil Allison

    Phil Allison Guest

    "Phil Allison"

    ** Shame that is NOT an auto-transformer.

    This is a typical multi-tapped tranny, with divided bobbin construction.

    With the 0 volt used as common, the 7.5 volt terminal driven by an audio
    power amp and the 15 volt terminal loaded with 16 ohms - response tested
    flat from 15 Hz to 18kHz.

    The -3dB point proved to be at 35 kHz.


    Go away.

    ....... Phil
  17. Dr. Polemic

    Dr. Polemic Guest

    You said in another post that "The bandwidth of a transformer has little
    to do with the core...".

    Just how little would the bandwidth be affected if your measurement were
    repeated with the core removed?
  18. Phil Allison

    Phil Allison Guest

    "Dr. Polemic Fucking Idiot "

    ** So go find that other post, read it very carefully & take account of the

    Then go drop dead - you asinine prick.

    ......... Phil
  19. Dr. Polemic

    Dr. Polemic Guest

    Your statement appears to be context free. You didn't say:

    "The bandwidth of *an auto-transformer* has little to do with the core...".


    "The bandwidth of *a two-winding transformer* has little to do with the

    You just said:

    "The bandwidth of *a* transformer has little to do with the core...".

    The word "transformer" was unqualified.

    Anyway, you could just tell me what context you intended.
  20. Phil Allison

    Phil Allison Guest

    "Dr. Polemic Fucking Idiot "

    ** You clearly appear to be a grossly autistic moron.

    Spare the world from your congenital insanity - kindly top yourself now.

    ......... Phil
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