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Which capacitor to chose

celestino_galancelestino_galan Posts: 44
edited 2005-05-30 15:50 in General Discussion
Could anybody tell us about wich capacitors are best for voltage regulators... in general?

I mean why 22uf instead 1000uf... and so on?




Thanks

Comments

  • KenMKenM Posts: 657
    edited 2005-05-30 14:48
    My understanding is to use two caps.

    A "big" one such as the 22 uF or 1000 uF as you mentioned, and a small 0.1 uF.

    Each serves a different purpose.

    The big cap is to supplyadditional current for very short periods of time, if/when the regulator cannot keep up (regulate) with·a sudden·large current request.

    The 0.1 uF is used to shunt (short out) high frequency noise.

    That is my general understanding of there uses. This information was taught to me by Dr. Donald Zimmerman, a MIT graduate and medical Doctor, for what that is worth.
  • Guenther DaubachGuenther Daubach Posts: 1,321
    edited 2005-05-30 15:50
    It's a good idea to check the regulator data sheets for more information because it also depends on the regulator type you are using.

    For example, the LM7805 data sheet specifies an input capacitor of 0.33µF when the power supply filter is located at an appreciable distance from the regulator, and an output capacitor of 0.1µF to improve stability and transient response.

    On the other hand, the LM2940 data sheet specifies a 0.47 µF input capacitor when the regulator is located far away from the power supply filter, and an output capacitor of at least 22 µF to maintain stability to be located as close as possible to the regulator where the capacitor's ESR (Equivalent Series Resistance) is critical. Therefore, a tantalum type, or a smaller tantalum cap in parallel with an aluminum electrolytic cap is recommended in the data sheet.

    You can always increase the size of the output capacitor - say to 100, or even 1000 µF but you should be aware of a "side effect": When turning off the power, with a fairly large output capacitor, it may happen that this capacitor forces a reverse current into the regulator's output which might damage it. Therefore, it's a good idea to connect a diode (like a 1N400x) between the input and output pins (cathode connected to the input pin). This diode bypasses the regulator.

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    Greetings from Germany,

    Günther
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