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7805 regulator capacitors

Discussion in 'Power Electronics' started by HellasTechn, Nov 9, 2019.

  1. HellasTechn

    HellasTechn

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    Apr 14, 2013
    I have an 7805 regulator feeding a microcontroller for some very important project of mine.
    I have seen many different schematics of 7805 regulator circuits and i see that they use different capacitor values on the input and output pins of the 7805.

    My question is: how should i choose the input and output capacitor values ? I do need it to be as reliable as possible ant to last as long as possible. Also would it be a better idea to replace the aluminium capacitors with tantalum ?

    the Circuit is fed with 12V through a diode and a 20 Ohm resistor and the microcontroller consumes maximum 300ma of current. as input capacitor on the 7805 i use a 10uf and as output an 100uf aluminium electrolytic.

    Thank You.
     

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  2. Minder

    Minder

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    Apr 24, 2015
    See the ST app note on the IC, I always use the recommended .33uf on the input, and a .1uf on the output, if the circuit being fed is not near to the regulator, I add a 15uf to 20uf across the ,1uf.
    For 300ma you could use the 79L05 series.
    M.
     
  3. HellasTechn

    HellasTechn

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    Why would i use a negative Vreg over Positive one ? I Dont think its a good idea to put the supply line on the tab of the 7805 (to-220) together with the heatsink. I risk touching the ground plane.

    Any thoughts about tantalum caps ?

    Searching the web i came across the Solid aluminium electrolytic capacitors.
    I think the last can directly replace the traditional electrolytic having only pros. Right ?
     
    Last edited by a moderator: Nov 10, 2019
  4. Minder

    Minder

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    Apr 24, 2015
    Sorry, misprint, should be 78L05.
    Tantalum in this application should be OK. Solid Aluminium also, both typically smaller than the typical electolytic.
    M.
     
    Last edited by a moderator: Nov 10, 2019
  5. Hunter64

    Hunter64

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    Nov 20, 2018
    How can you use a 78L05 at 300mA?

    The 78L05 is rated for an output current up to 100mA.
     
  6. bertus

    bertus

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    Nov 8, 2019
    Hello,

    The 78M05 is capable of 500 mA, so that can be used for the mentioned 300 mA.

    Bertus
     
    narkeleptk likes this.
  7. HellasTechn

    HellasTechn

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    Thats right. the TO-92 package can deliver up to 100ma.
    The TO-220 package can deliver up to 1A if mounted on a large enough heatsink.
     
    narkeleptk likes this.
  8. Bluejets

    Bluejets

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    Oct 5, 2014
    Why R1..??
     
    davenn likes this.
  9. davenn

    davenn Moderator

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    Sep 5, 2009

    exactly :)
     
  10. Hunter64

    Hunter64

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    Nov 20, 2018
    Please take some time and read the correct datasheets, yes, that's plural. There is a significant difference between the 7805 and the 78L05, so please try to not mess up the numbers.
     
    Last edited: Nov 10, 2019
    HellasTechn likes this.
  11. HellasTechn

    HellasTechn

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    The 7805 comes in TO-220 package while the 78L05 comes in TO-92 package. They do the same thing but have different current and heat dissipation capabilities.

    Like i mentioned in post 1 i use the TO-220 version. R1 is there in order to drop the 12V down to about 10V thus reducing the thermal stress on the Vreg.

    Any ideas about the original question ?
     
  12. bertus

    bertus

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    Nov 8, 2019
  13. HellasTechn

    HellasTechn

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    Apr 14, 2013
    Thanks. I already know of them.

    My question is: how should i choose the input and output capacitor values ? I do need it to be as reliable as possible ant to last as long as possible. Also would it be a better idea to replace the aluminium capacitors with tantalum or with solid aluminium ?
     
    Last edited by a moderator: Nov 10, 2019
  14. bertus

    bertus

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    Nov 8, 2019
    Last edited by a moderator: Nov 11, 2019
  15. Hunter64

    Hunter64

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    Nov 20, 2018
    National Semiconductor doesn't recommend the way you want to use your circuit. I presume you already know that because it's in the application hints of the datasheet.

    If you really want to proceed this way, use a protection diode.
     
  16. Minder

    Minder

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    Apr 24, 2015
    You actually only need .1uf on the out pin if the rest of the circuit is close to it, and the rest of circuit fed has capacitive smoothing.
    The recommended on the input is 0.3Uf.
    M.
     
  17. bertus

    bertus

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    Nov 8, 2019
    Hello,

    Here is the schematic version of what @Minder said:

    3-pin_low_power_regulators_with_capacitors.png

    Bertus
     
    hevans1944, narkeleptk and Minder like this.
  18. Bluejets

    Bluejets

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    Oct 5, 2014
    So why not simply use a lower voltage supply to start with?
    9v is fairly common and cheap.
    Resistor defeats the whole purpose of using a regulator .
     
    davenn and Minder like this.
  19. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Not completely. In this circuit the resistor will help to dissipate power which then doesn't have to be dissipated by the regulator, thus keeping the regulator cooler. The output will still be regulated.
     
    HellasTechn likes this.
  20. bertus

    bertus

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    Nov 8, 2019
    Hello,

    The 20 Ohms resistor will be to large, as it will drop 20 X 0.3 = 6 Volts, leaving 5.3 volts at the input of the regulator when the input voltage is 12 Volts and the diode drop is 0.7 Volts.
    The regulator needs at least 2 volts overhead to be able to regulate.

    LM78XX_voltage drop.png

    Bertus
     
    davenn likes this.
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