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Simple Capacitor question — Parallax Forums

Simple Capacitor question

BasilBasil Posts: 380
edited 2007-05-10 02:58 in Propeller 1
Hi All,

This is pretty embarassing asking this as its pretty simple electronics blush.gif

In the attached circuit for the voltage regulators on my device, I have a big cap in there (C14) to keep the power to the 5V and 3.3V regulators stable when large currents are discharged through the 9V output.

Do I need to put a resistor in series with the capacitor C14? Or is ok to leave the resistor out?
I need to keep the circuit powered for as long as pos. if the power source drops below Vmin for the regulators.

Thanks!

Comments

  • BTXBTX Posts: 674
    edited 2007-05-10 01:04
    Basil.
    Did you forget to attach the circuit ?

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Regards.

    Alberto.
  • BasilBasil Posts: 380
    edited 2007-05-10 01:07
    BTX said...
    Basil.
    Did you forget to attach the circuit ?

    I most certainly did lol here it is. Been a long day!
    PSU.JPG
    230 x 218 - 8K
  • Mike GreenMike Green Posts: 23,101
    edited 2007-05-10 02:08
    You don't need a resistor. Consider a supercap as a "large" capacitor.
  • BasilBasil Posts: 380
    edited 2007-05-10 02:11
    Supercap?
    This is a 12mm dia surface mount capacitor, not sure what you mean by supercap :\
  • Mike GreenMike Green Posts: 23,101
    edited 2007-05-10 02:22
    A supercap is a specially constructed very high capacitance device (1/10F to several Farads) usually used as a backup power source very rapidly chargable and with a potentially high discharge rate.
  • BasilBasil Posts: 380
    edited 2007-05-10 02:29
    Ahhh cool. Could you recommend where I could find one?
    The issue I have is it needs to be SMD and the largest footprint I can fit on my board is for a Sanyo E12 (which I assume means 12mm dia).

    4700uf would hold the circuit up for ~2 seconds which is enough.

    Post Edited (Basil) : 5/10/2007 2:35:50 AM GMT
  • BasilBasil Posts: 380
    edited 2007-05-10 02:52
    You have answered my 'simple' question by the way [noparse];)[/noparse]

    Thanks Mike

    I have an idea!

    I could change the power supplies from parallel to serial configuration (Ie 9V->Diode->5V->3.3V) and put the cap on the 5V line. That way I can get away with using a lower voltage rated cap. which would allow me to increase its capacitance while keeping the space requirements to a minimum [noparse]:)[/noparse]

    Is there any downside to doing it this way? In an earlier thread of mine I asked this same question.
    The power loss at the 3.3V regulator would not be so big either.

    At first I went with parallel as I thought noise on the power supply lines could be a problem, but in reality the parts I am using do not produce much noise anyway and there are quite a number of smoothing caps [noparse]:)[/noparse]
  • Mike GreenMike Green Posts: 23,101
    edited 2007-05-10 02:54
    Here's a sample of supercaps (under memory backup capacitors).
    pdf
    pdf
    P75.pdf
    121K
  • BasilBasil Posts: 380
    edited 2007-05-10 02:58
    Ahhh those [noparse]:)[/noparse] Didn't know they were called supercaps lol

    Thanks
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