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Decoupling capacitor

Discussion in 'Electronic Basics' started by Ziguy, Jul 24, 2003.

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  1. Ziguy

    Ziguy Guest

    Hi,

    I currently design an small electronic circuit. The circuit is mainly a 5V
    regulator and an accelerometer connected to a comparator (LM393) that drive
    a MOSFET switch. Do I need decoupling capacitors for the accelerometer and
    the comparator? I thought that maybe just one 0.1 uF capacitor after the
    regulator would do the job... By the way, this is a automotive application,
    the output of the accelerometer is filtered to 1Hz and the board has about 1
    square inch.

    Thanks
     
  2. John Larkin

    John Larkin Guest

    One cap should be OK for a small thing like this, but be aware that
    many regulators will oscillate if they don't have enough output
    capacitance. See the reg datasheet.

    Other posters will give you the three-bears line: big cap plus medium
    cap plus small cap. Ignore them.

    John
     
  3. If there is no other sources of noise being powered from this
    regulator, there is little need for lots of capacitance on the 5 volt
    bus. Your idea of .1 uf sounds pretty good. The bigger problem will
    be noise from the 12 volt side that gets past the regulator. I would
    worry more about what you do about the + or - 50 volt spikes that will
    occasionally appear there. Perhaps, a diode to prevent reverse
    voltages from getting to the regulator follower by an RC or LC
    filter. This capacitor should be more like 10 or 100 uf in parallel
    with a .1 uf film or ceramic cpacitor.

    And make sure yout 1 second low pass filter is referenced to exactly
    the same node as the reference voltage it is compared against.
     
  4. Ziguy

    Ziguy Guest

    Before the regulator, I put in series a diode and a resistor, and in
    parallel a zener, a 100uF and a 0.33uF tantalium capacitors.

    Later,
     
  5. John Larkin

    John Larkin Guest


    I'd advise against the tantalum. They're expensive and have a bad
    habit of exploding when used across supply rails. In a car, where
    serious overvoltage spikes are likely, tantalums will likely die. A
    100 uF aluminum cap should be enough.

    I recently did a strain-gauge signal conditioner with eight excitation
    supplies, and foolishly bypassed each regulator output with tantalums.
    Even there, it had enough peak current to blow the caps occasionally.
    We had to replace them with aluminums, and fortunately found some that
    fit the tantalum surface-mount footprint.

    John
     
  6. Ziguy

    Ziguy Guest

    Do you suggest me to forget the 0.33uF cap at all? Not even a 0.33uF ceramic
    cap! Are they not needed for high frequency noise?

    Thanks again,
     
  7. Dan Fraser

    Dan Fraser Guest

    No matter what, put a .01 ceramic disk cap across every IC, as close to
    the pins as you can get. I have seen too many strange things happen
    without them that it is cheap insurance against power line induced
    noise.

    I'll spend the extra 5 cents per chip so I don't get hassled a year from
    now and have to retrofit a pile of units in the field.

    --
    Dan Fraser

    From Costa Mesa in sunny California
    949-631-7535 Cell 714-420-7535

    Check out my electronic schematics site at:
    http://www.schematicsforfree.com
    If you are into cars check out www.roadsters.com
     
  8. Cees Keyer

    Cees Keyer Guest

    The small cap mentioned above is a inrush C, the switching action
    (charging mos transistors) in the i.c. needs for a short time a lot of
    charge (current, due to the impedance of the power supply lines on the
    board you'll see a dip in the voltage, which might result in a reset of
    a piece of the circuitry. The small ceramic C will solve this problem.

    The problem with 78 or 79 series regulators is that they are quite noisy
    over a large piece of the spectrum up to 30MHz or even higher.
    A small 10nF ceramic capicitor close to the regulator, i.e. on the
    "legs" of it will solve this.
    It's good engineering practice to put small ceramic 10nF C close to non
    switching regulators both on input and output.

    In a car a load dump might occur which takes the voltage on the 12 V
    supply upwards to 80 volts or more for a few seconds.
    And the spikes from the ignotion are anoying as well.

    Cees.
     
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