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Smoothing Capacitor Values

Discussion in 'Electronic Basics' started by Michael, Oct 1, 2006.

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  1. Michael

    Michael Guest


    I'm making a device for an ROV and need to bring the onboard power of 12V
    down to 5V.

    After a bit of research I can't seem to find any exact (or near enough)
    values for the smoothing capacitors once it's gone through a regulator (I'm
    planning on using a L78S05CV ) and just before
    (between the 12V and ground).

    Is it just a case of bigger is better?

    The other thing is some sites say to use film whilst other say I should use
    electrolytics and ceramics.....

    Can someone please explain?


  2. Chris

    Chris Guest

    Hi, Michael. The specs say you should have a cap at the input of the
    regulator if the power source is more than a couple of inches away,
    For being fed from a battery, 10uF should be plenty. For the output, a
    small cap helps for high frequency stability, and to keep the noise
    voltage within spec. You can either use another 10uF electrolytic, or
    an 0.1uF ceramic or film would be fine, too.

    When you lay it out, tie the GND pin of the regulator and the - pins of
    the electrolytics within an inch of each other if you can.

    Good luck
  3. Michael

    Michael Guest

    Thanks again Chris

  4. Eeyore

    Eeyore Guest

    No smoothing cap is needed because your input supply is DC already.

    You will however need some small caps to ensure stability located close to the
    regulator. See the manufacturer's data sheet for recommended values.

  5. Eeyore

    Eeyore Guest

    That regulator doesn't have a high enough input voltage spec to survive
    automotive supply voltage transients.

  6. Phil Allison

    Phil Allison Guest


    ** Errr - so is 35 volts DC max input is not enough ???

    What bollocks.

    ......... Phil
  7. Chris

    Chris Guest

    If you're still there, Mike, Graham has a good point. An automotive
    environment has high voltage transients of over +60VDC, which will
    eventually fry your regulator.

    For greater reliability, you might want to add a 24V Transzorb (Maplin
    N95CA part number 1.5KE24A, £0.46 ea. in stock) and a 1 ohm junkbox
    power resistor to your circuit like this (view in fixed font or M$

    | _____
    | B+ ___ | |
    | o----|___|-o-----o----|78S05|---o-------o
    | 1R | | |_____| | +
    | | +| | +|
    | D1| --- | ---
    | /-/ C--- | C--- 5VDC
    | ^ | | |
    | | | | |
    | | | | | -
    | o----------o-----o-------o------o-------o
    | GND
    (created by AACircuit v1.28.6 beta 04/19/05

    If a spike occurs on B+, the transzorb will act like a zener and clip
    the transient to a maximum voltage of around +24V. It's made to absorb
    brief, high energy spikes like this, and isn't too expensive for what
    you get. A quick scan didn't show any 1 ohm resistors at Maplin, but
    you'll probably have one (anything between 0.47 ohms and 2 ohms will
    do) in your junkbox. The 1 ohm resistor will limit the current during
    the spike, and keep the Transzorb from smoking. Actually, the resistor
    will also drop some of the voltage, which might mean you can use a
    smaller heat sink, too.

    This will make your automotive circuit a lot more reliable.

  8. Rich Webb

    Rich Webb Guest

  9. Chris

    Chris Guest

    There were a lot of burnt electronics in many vehicles -- in the 1960s,
    as the newfangled solid state electronics were being rolled out into
    cars. Everyone learned the hard way about load dump transients, which
    will frequently happen if the battery connection is intermittent.

    Usually some inductance is placed at the power input to automotive
    electronics, and there are various methods used to clip the amplitude
    of transients, many of them built into the boards in odd places. The
    junkbox resistor/Transzorb method is clunky, but it works.

    Either that, or if you'd like to use tubes in your car radio, they're
    very happy with voltage transients. ;-)
  10. Michael

    Michael Guest

    Yup I'm still here....havn't quite gathered up the courage to finally submit
    my order :)

    The circuit won't be connected up to a car though.....(although it will be
    connected to a car battery)

    What w rating should the resistor be in that circuit? As big as possible I'm

    I guess better safe than sorry is the key phrase here....


  11. Michael

    Michael Guest

    Don't know if it makes a difference......but the ROV is the underwater

    Example: ROV.gif

  12. Chris

    Chris Guest

    Hi, Mike. Load dump is a problem associated with alternators (and to a
    lesser extent, with battery chargers). If you're just running it off
    straight battery power (and there's zero chance that it will be powered
    when the battery is being charged) you won't need transient protection.

    If you decide you still need transient protection for your power
    supply, the value of series resistor just depends on the current
    requirement. P = (I ^ 2) * R. If you have a wirewound 1 ohm resistor
    in your junkbox, it will probably be 5 watts or more, which should be
    just fine for any current you might require (the 78S05 is good for 2
    amps max, which would mean 4 watts max).

    Good luck with your project, and feel free post back if you have any
    other questions.

  13. BobG

    BobG Guest

    Mike... linear regulators are not efficient... Look for a 12V to 5V DC
    to DC converter at Jameco or Digikey.. they can be 90% effcient!
    Batteries will thank you.
  14. Jamie

    Jamie Guest

    if that was the case, there would be a lot of burnt electronics in
    many vehicles.

  15. No, it isn't, because there are spikes in an automotive electrical
    system that go well above + 35 volts. If your weren't such a loser, you
    would have checked the facts BEFORE you dumped your usual load of crap.
    There are standards for automotive electrical and electronics hardware,
    including "Load Dump".

    Go back to molesting toasters, Phyllis, and leave the real
    electronics to the men.

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida

  16. Delco used a "Spark plate" which was a piece of thin double sided PCB
    material in their car radios, as well. It was soldered to the case on
    one side, and used as a tie point in the DC input and filtering.

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
  17. Phil Allison

    Phil Allison Guest

    "Michael A. Terrell

    ** Not if the battery is still functional -


    Go back to molesting little kids in Vietnam - like real US soldiers.

    ........ Phil

  18. Only in your small mind. There is a lot of stray inductance and
    capacitance in the wiring harness. Do you really expect anyone to
    believe that the arc when a starter solenoid opens doesn't impress a
    number of large spikes on the rest of the electrical system? Your
    glaring ignorance is showing, toaster molester.

    About damn time that you admitted it, Phyllis.

    First of all, i never served in Vietnam. Its you that got caught in
    those sexual acts, and real soldiers don't molest anyone, faggot.

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
  19. The photos from Abu Ghraib would seem to indicate otherwise.
  20. Phil Allison

    Phil Allison Guest

    "Michael A. Terrell"

    " Eyesore "

    " Actually, I was sent to Alaska INSTEAD of Vietnam. Two months after
    I arrived the base I was supposed to go to in Vietnam was overrun, and
    the entire staff was killed. "

    ** What a fucking shame YOU were not killed back then too !!!!

    Be one less TOTALLY INSANE Septic Tank **** polluting the earth.

    ** A 470 uF electro on the input of the voltage reg will suppress such
    transients - standard practice to use one.

    Most reg ICs have a similar input voltage limit to the one in question
    anyhow - so Eyesore has to say what ** HE ** meant by his asinine


    **** OFF - ASSHOLE !!!!!!!!

    ......... Phil
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