Voltage regulator - LM7805 on radiator, using wires, where put capacitors ?

Discussion in 'General Electronics Chat' started by alan1975, Feb 22, 2012.

  1. alan1975

    Thread Starter Member

    Jan 1, 2012
    42
    0
    Hello,

    I would use few 7805 voltage regulators to get more current.

    I will put them on big radiator using wires between PCB and regulator.

    My question is, should i put capacitors 0.33uf 0.1 uF at LM7805 legs (as close ass possible to voltage regulator), or at PCB ?
    Wire will be about 20 , 30 cm long ?

    I will use MKT capacitors.


    Second question

    I will use 4700uF capacitor at output.
    How calculate resistor for discharge that capacitor after remove power cord ?
    I think there have to be formula.

    Regards
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,650
    2,348
    Hello,

    With a radiator, you probably mean a heatsink.

    The 0.33μF and 0.1μF capacitors should be close to the regulator, as they should prevent the regulator from oscillating.

    Bertus
     
  3. alan1975

    Thread Starter Member

    Jan 1, 2012
    42
    0
    Thank You for reply.

    Yes, heat-sink, that's good name.

    Great, that's what i think. i will solder them to legs.



    Question about resistor for discharge capacitor still open.


    I have another question.

    How calculate cross-sectional wire ?
    How big wire should be ?
    Is there any easy formula ?

    For example i would like have 2A 12V.
    How calculate how thin should it be ?

    I just made up that values, i would like know how to count it.

    Regards
     
  4. soda

    Active Member

    Dec 7, 2008
    174
    13
    You don't need more regulators for more current. All you do, add a 2N3055 transistor to the output of the regulator. Connect the collector to the input of the LM7805, the output of the regulator go to the base of transistor via a 1R 1w resistor and the emitter is then the output.Please remember to mount the transistor on a heat sink about 100mm x 100mm.If the transformer you use is larger than 5Amp, say about 7Amp, you'll then get about 5Amp from your supply.

    I can't see why you need a discharge resistor because you don't need one, but if you want to add one, then use a 56k

    If this wire cross section means the thickness of the power supply wire's then use about 1mm up to 1,5Amp and 1,2mm up to 3Amp. To get more info about wire size's, you can Google for it.

    Please remember to always explain what exactly you mean with your questions mate. [​IMG]
     
  5. soda

    Active Member

    Dec 7, 2008
    174
    13
    Read this PDF for more info about wire sizes
     
  6. MrChips

    Moderator

    Oct 2, 2009
    12,452
    3,371
    You do not need to put a 4700uF capacitor on the output of the regulator. That is the purpose of the regulator, to keep the output voltage constant. The large capacitor should go on the input of the regulator to smooth the voltage from a full-wave rectifier circuit.
     
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