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transformer confusion

Discussion in 'General Electronics Discussion' started by [email protected], Aug 1, 2010.

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  1. brian@1234

    [email protected]

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    Aug 1, 2010
    Hello All,

    I purchased a transformer that has a primary winding rated for 230 volts and a secondary rated at 6 volts. I hooked this transformer up with the primary side receiving 120 VAC. I thought the secondary side would now be stepped down to around 3.0-3.5 VAC, but I found that the secondary side is measuring 6.2 VAC. Any thoughts as to why the voltage isn't lower?
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,411
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    Jan 21, 2010
    There could be several reasons:

    1) the transformer is actually a 120V transformer
    2) the transformer has very poor load regulation and the output voltage will fall substantially when a load is placed on it.
    3) you've actually got 220V mains.
    4) you're measuring peak rather than RMS voltage.
    5) your voltmeter needs calibrating.

    Having a no-load output twice the expected value is rather large. (2) is the normal reason, but I wouldn't think it would happen to this extent unless something else is going on.

    Is it a normal transformer? Or one designed for charging? It's possible that the secondary has a deliberately high resistance to limit the current (or even a resistor in series with it) so that the voltage under normal load falls a lot more than is typical.

    Do you have specs (or even a model number) for the transformer?
     
  3. brian@1234

    [email protected]

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    Aug 1, 2010
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,411
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    Jan 21, 2010
    Check the datasheet provided at that link.

    You'll notice that the specs give you a no-load voltage that is substantially higher than the rated voltage *and* that the transformer is "short-circuit proof". Both show that the regulation is poor (by design).

    I calculate, that all other things being equal that you should see a secondary voltage around 5.63 volts. You're seeing more than that, but it's only 10% high, which is not all that much.

    Depending on how the current is limited (by saturating the core or by winding resistance) you may find that the transformer may not maintain the same current rating as it would do at the rated input voltage.

    I'd probably try loading the transformer using a resistive load and graphing voltage against current to see if it does what you want it to at the current you require.
     
  5. brian@1234

    [email protected]

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    Aug 1, 2010
    yep, I assumed an ideal transformer. thanks for the help

    Brian
     
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