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Thread: Capacitor Value... How close do you need to be?

  1. #1
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    Capacitor Value... How close do you need to be?

    I am looking to build another 12VDC PSU using a TI-5071. It requires a 100 uF capacitor on both the Input and Output lines.

    All I have right now if 220 uF caps.....

    In the past a guy I knew said exact match on capacitors was not the important, but since that was several years ago, I am concerned my memory is playing "happy smacks" with my brain.

    Does anybody know if using a 220 uF capacitor in place of a 100 uF will be okay?

    Here is a link to the tech sheet on the TI-5071

    http://focus.ti.com/lit/ds/symlink/pt5071.pdf

  2. #2
    Variable Bitrate markland556's Avatar
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    you have to use the right value. i can bet that something will blow up of you use a 220, LOL. But seriously in some situations you have to be exact, you might be able to get away with 110 or 90, but i dont think there is such things. But you wanna use the right value.
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  3. #3
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    Make sure you use low esr caps as per data sheet!
    I would be inclined to try the 220uf if the voltage rating was same and they are low esr

    edit - panasonic fc series look good to me btw, electrolytics are +/- 20% anyways

  4. #4
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    Input/Output Capacitors: The PT5071 regulator requires a 100μF electrolytic capacitor at the input and output for proper operation in all applications. The ESR
    (equivalent series resistance) of both capacitors must be less than 250mW @100kHz. In addition, C1 and C2 must be rated to a minimum of 300mArms ripple current.


    The capacitor value is not that important here as long as its equal or greater than 100uF. Whats more important is the voltage rating, ESR and ripple currents. Alot of cheap capacitor have high ESR so if you wanna use them you can parallel them up to lower the overall ESR, increase the ripple current as well as the capacitance. For the input capacitor use something like 25V and above so it can handle the rough voltage from the car battery. For the output use 16V rating to handle the 12V output.

    Use 0.25 ohm ESR or lower for the capacitors, notice you have to get high capacitance value to achieve the lower ESR. As I said parallel them up to also satisfy the ripple current.

    http://www.farnell.com/datasheets/21084.pdf


    Sanyo does the OSCON type capacitor, low ESR but very expensive.
    http://www.farnell.com/datasheets/36108.pdf

    Its probably worth it since you be using less capacitor in parallel. But choose the capacitor whatever suits your application best, space, price or simplicity.

  5. #5
    Maximum Bitrate kiltjim's Avatar
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    I don't think in this situation the value is all that important. It looks like they are just smoothing caps. You should probably do what Chris31 says. It doesn't look like the assembly is using it for timing, or anything, but you should try to get as close as possible.
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    Hmm weird I cut and paste and the OHMS symbol became a W

  7. #7
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    Wow, fast and helpful as usual.... Thanks a lot guys...

    Here is the Tech Data sheet for the caps I have

    http://www.panasonic.com/industrial/...BA0000CE22.pdf

    I measured the case and it appears I have ones that 8 x 15 (dia. x length), and looking on Page 2 of the Tech Data that gives a ripple current of 730 mA (which is good) and an Inpedence (assuming to be the same thing as ESR) of 0.085 ohms/100 kHz (which is also good...) Oh, they are 25V.

    So it looks like they are okay to use. Would you agree?

    Thanks again.

  8. #8
    Raw Wave Rob Withey's Avatar
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    Bigger is fine. I used bigger on my 5071 supply. Low ESR is very important.
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  9. #9
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    in the hifi world PSU circuits are often the best place to tweak for sound quality. As stated previously almost doubleing the electrical size for the input and output apacitors should improve the supply however there is a line of thorght that also adding (in addition) smaller capacitors (in this case .047uF)is good aswell for improved hi freq (audio) stability.

    Im sorry if this is a little too techy and boring but just my tuppence worth.

    regards Nigel

  10. #10
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    Capacitors typically have much looser tolerances than other components. Typical filter caps will have -20/+50% tolerances. Substituting a 220uF for a 100uF filter cap is generally not a problem. One thing to keep in mind is that larger caps have a slower response than smaller ones, so it is generally a good idea to use two (or even three) caps in parallel for best performance. For example, a 220 uF and a 1 uF cap in parallel would be *much* better than a single 100 uF cap in this application.

    -p.

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