LM 78XX Regulator

Discussion in 'General Electronics Chat' started by Lanz, Oct 3, 2009.

  1. Lanz

    Thread Starter Active Member

    Oct 30, 2008
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    Guys im doing a power supply circuit using LM78XX regulator.There are 3 output voltage 12V,9V and 5V.So im using LM7812,LM7809 and LM7805 for this.I use 12V 1A center tap transformer.As i know output of this regulators are 1A by default.My question is,can i use that transformer or need to use high current such as 12V 3A to supply smoothly all 3 regulators and what is the input voltage and current range of the regulators?


    [​IMG]
     
    Last edited: Oct 3, 2009
  2. Wendy

    Moderator

    Mar 24, 2008
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    Yes. The regulators will need heat sinks, but it works very well.

    Instead of C1,C5,C6,C7,C11,and C12, think one very large filter capactor. The smaller caps C2,C8, and C13 should be next to the regulators as shown.

    Concerning the smaller transformer, as long as the total current doesn't exceed 1A it would work too, the regulators don't pull much current themselves, but pass it on through. You might think about using a fuse common to all three.
     
  3. Lanz

    Thread Starter Active Member

    Oct 30, 2008
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    Of course with heatsinks.

    U mean the output of the transformer should be 1A n cannot exceed 1A.
    Fuse to the input or output of the regulator?
     
  4. hgmjr

    Moderator

    Jan 28, 2005
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    You should resize the current limiting series resistors for the LEDs across the 9V and 12V outputs to keep from over driving them.

    hgmjr
     
  5. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
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    The output of the transformer can not exceed 1A as that is the size you have currently chosen. If you would like to simultaneously pull 1A out of each of the regulators than you need a larger transformer, i.e. 3A. Right now you will be sharing the 1A current between all of your regulators.
     
  6. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    1,571
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    the first place to go to with these questions is the device data sheets. They will answer your questions and give you a chance to learn how data is presented in such a format.
     
  7. Audioguru

    New Member

    Dec 20, 2007
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    You cannot use a 12V center-tapped transformer. It has a ground wire (the tap) and two 6VAC wires. The unregulated DC to feed the regulators will be only 7.5VDC. But the 12V regulator needs at least 15V and the 9V regulator needs at least 12V.
     
  8. Wendy

    Moderator

    Mar 24, 2008
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    Good call AG. Replace the two diodes with a full wave bridge, and ignore the center tap on the transformer.
     
  9. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    You have to know the load current (maximum) for each regulator output to know what the transformer current rating has to be, as well as how much heatsinking is required. BTW: DC load current does not equal transformer current. Typically, you need around 1.5X transformer current rating comapred to DC load current.

    ALSO: 78XX regulators are not simply "1 Amp" regulators, the point where the current limiter cuts in is dependent on input=output voltage differential and it can be more or less than an Amp. Check the data sheets and see what I mean.
     
  10. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    True, you would need to use a full wave bridge with this transformer and the 12V reg might not get enough input voltage depending on the transformer's load regulation and how much current you draw from it.
     
  11. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Transformer RMS current (which is how they are rated) is not the same as the DC current after the regulators. I usually use a ratio factor of 1.5, but the exact value is complicate to derive and is different for FWCT and FWB. I think they run in the 1.4 to 1.6 range, but it's been a long time since I derived that.

    This subject is covered in an application document I wrote some years back. Go to APPENDIX A towards the back:

    EDIT TO ADD:

    My own text says this:



    So the 1.5X is an approximation, for a FWB circuit use 1.8X. In other words, if you need 2A of DC load current, the transformer must be rated for 3.6A RMS secondary current.
     
    Last edited: Oct 3, 2009
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