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[noob/idiot] Trying to understand an elementary breadboard power supply

  • 05-09-2010 7:28pm
    #1
    Eliot Rosewater
    Registered Users, Registered Users 2 Posts: 3,745 ✭✭✭


    Hey all!

    Just started learning electronics for fun. I can set up a power supply to convert 9v to 5v but I don't understand why it works, in particular I don't have a clue why there needs to be capacitors. Could anyone explain?

    In the attached images there is an extra LED and resistor (on the right of each image; parts 8 and 9) that aren't actually a part of the power supply, but which are just used to test if it's on.


    Circuit diagram:

    attachment.php?attachmentid=126627&stc=1&d=1283714691

    Pictorial representation:

    attachment.php?attachmentid=126626&stc=1&d=1283714691


Comments

  • #2
    5uspect
    Moderators, Arts Moderators Posts: 10,520 Mod ✭✭✭✭5uspect


    I'm not the greatest at electronics.

    The diagram refers to it as a regulator. The caps and I presume transistor, are probably there to clean up the input 9V.

    EDIT: Silly me. it's not a transistor it's a voltage regulator.

    Tho the caps do still appear to help damp out noisy voltage, but at both ends.


  • #3
    Eliot Rosewater
    Registered Users, Registered Users 2 Posts: 3,745 ✭✭✭Eliot Rosewater


    5uspect wrote: »
    Tho the caps do still appear to help damp out noisy voltage, but at both ends.

    Thanks - that makes sense (in principle)! I've taken out the caps and everything still works.

    I'm just curious as to how exactly the capacitors work in this circuit. Like, which way the current is flowing, etc.


  • #4
    FarmerGreen
    Registered Users, Registered Users 2 Posts: 1,134 ✭✭✭FarmerGreen


    Have a look at the data sheet :-
    http://www.national.com/ds/LM/LM340.pdf


  • #5
    Gurgle
    Closed Accounts Posts: 5,064 ✭✭✭Gurgle


    Eliot Rosewater wrote: »
    Thanks - that makes sense (in principle)! I've taken out the caps and everything still works.

    I'm just curious as to how exactly the capacitors work in this circuit. Like, which way the current is flowing, etc.

    Short answer:
    The caps remove noise from the power lines.

    Longer answer:
    The cap on the battery side is redundant as its a battery which won't generate high frequency noise. The cap on the LED side is redundant because you don't have a varying load.

    If e.g. you had an input supply from a rectified mains step-down transformer you would need the cap on the left. If you were driving a microchip that went through a process & sleep cycle the load would be varying and you would need the cap on the right.


  • #6
    Eliot Rosewater
    Registered Users, Registered Users 2 Posts: 3,745 ✭✭✭Eliot Rosewater


    Cheers for those posts fellers, it really cleared it up!

    I just didn't want to go further into the electronics without understanding, in effect, page 1 of the manual.


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  • #7
    titanium_x
    Registered Users, Registered Users 2 Posts: 22 ✭titanium_x


    The cap on the RHS could also be used to make the internal loop stable.
    It's possible your output might oscilate with out.

    Also the cap on the LHS might be required if they where long traces back to the battery. Not how both cap in your picture are connected to the point, this will create an ac gnd for any internal loops in the regulator.


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