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7805 question?

Discussion in 'General Electronics Discussion' started by OlDrunk, Mar 25, 2014.

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  1. OlDrunk


    Feb 15, 2014
    Looking at 78xx circuits, just wanted to confirm if my power source is 12v switchmode regulated PSU rated output of 14.2A(old 360 PSU) and I want to use LM7805 (have many on hand) to get 5V , do I really need caps for filtering? can I just attach to heat sink and paralle them up for [email protected]+Amps or is there suggestion/need for some current protection, or a suggestion for better way to step down the voltage. My application is for powering a couple of USB hubs and Raspberry PI(5V needed) along side a USB2HDD adaptor([email protected] needed)

    Thank you for reading and any and all advice is very much appreciated!
    Last edited: Mar 25, 2014
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Always use capacitors. They are not only for buffering but also for stability of the regulator within the 78xx.

    You should not parallel voltage regulators of any kind without adding some means for equal current sharing. A small series resistor at the output of each regulator can do this, at the cost of a few millivolts voltage drop. If you don't do this, the regulator with the highest output voltage (within its tolerances) will have to supply the max. current. Current limiting will set in and the output voltage may become instabel.
    Better use a current booster.

    Note that you will generate a lot of excess heat: The 7805 drops 7V (12V-5V) which at 3A make for 21W of power lost to heating the regulator.
    You'd be better off using a switch mode regulator. There are module pin compatible to the venerable 7805, e.g. this one.
  3. davenn

    davenn Moderator

    Sep 5, 2009
    you cannot parallel 78xx series regulators up for higher current output
    you either use a higher current regulator or you use pass transistors onm the lower current regulator

  4. OlDrunk


    Feb 15, 2014
    Thank you very much for the replies! I'm going to look into switch mode regulators, the last thing I need is something blowing up on me.

    Thanks again
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