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Hi, I have two question about transformer and regulator:1- When

  1. Oct 30, 2007 #1
    I have two question about transformer and regulator:

    1- When one refers to “isolation” with respect to a transformer, what does it mean?

    It related to the fact that the transformer blocks DC component of the signal?

    this question is related to the electronic regulator 7812.
    If the input line voltage decreases beyond a certain point the series regulator will stop regulating. Why?
    I have attach the circuit related to it.

    Thank you

    Attached Files:

  2. jcsd
  3. Oct 30, 2007 #2


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    I can't view your doc file but for the first question; A transformer couples AC voltage from the primary coil to the secondary coil which are isolated from each other except for the magnetic field that provides this coupling.

  4. Oct 30, 2007 #3


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    2) Electronic regulators like the 7812 have a minimum Vin about 2v greater than Vout. If Vin is less than required it will not regulate properly.
    For an exact value for Vin minimum look at the data sheet.
  5. Oct 30, 2007 #4


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    Thats a good question. When using these ICs, it is important to know or at least have an simple understanding of the internals. That threshold voltage that you're referring to is called dropout voltage. The dropout for the 7812 is about 2V. As you've discovered this is the voltage where the IC stops regulating. Quick examination of the 7812's schematic will reveal the reason. The 78xx series uses an bipolar output stage. In particular this output stage is a NPN darlington pair. It is such output stages that determine the dropout voltage for these kinds of ICs.

    I'm not sure how far your circuit analysis skills go, but if you pull up the datasheet for the 7412, you'll notice that the darlington pair is transistors Q15, Q16, and the output of an "error amplifier" drives Q11-1. The PNP Q11-1 transistor controls the input bias current to the NPN darlington pair. Well from the darlington pair, we see that Vdropout will at least be 2*Vbe which is approximately 1.4V. I think the Q11-1 plays a role here with its Vce, but don't hold me to that. The point of this output structure (error amp, PNP, and darlington pair) is to provide a relatively stable output voltage for a varing input voltages and load current.

    You can get voltage regulators known as LDO (low dropout) regulators that have dropout voltages in the millivolts. You can find schematics for such regulators and compare the output stage to the 7812. Its a great way to learn!
    Last edited: Oct 30, 2007
  6. Oct 31, 2007 #5
    I think it means they are physically isolated. That's why you can use a transformer to check line voltages with less risk of dying, there is no connection from the lines to your body, only the stepped down voltage or current.
  7. Oct 31, 2007 #6


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    Just note about transformers. Not all transformers have isolation between primary and secondary.
    These are the autotransformers
    Most transformers that are delivering service to you home are autotransformers.
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