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Capacitors for voltage regulators — Parallax Forums

Capacitors for voltage regulators

kt88seampkt88seamp Posts: 112
edited 2009-12-09 00:42 in General Discussion
I am building a·5 and 3.3 volt power supply for a ft232rl chip and a propeller. The 3.3 volt·via the propeller will power a moc3010 triac driver along with the prop's eeprom. The·5 volt powers only·the ft232rl.·A transformer 120V to 12V (center·tapped)·via a rectifier will drive·both regulators in parallel.·What equation should I use to calculate the·regulator input capacitor(s) and also the caps off the output pins on each regulator·(if any are needed). The regulators I am using are these:

AME8800SEFTZ series·(5 volt and 3.3 volt)

Both are available on Mouser.

http://www.mouser.com/ProductDetail/AME/AME8800AEFTZ/?qs=sGAEpiMZZMuLLNXTG1MZalY17qJmshbySy2ZD%2f4V1iw%3d

http://www.mouser.com/ProductDetail/AME/AME8800SEFTZ/?qs=sGAEpiMZZMuLLNXTG1MZapp2Mh8OgVzFTOA7GVxV7cU%3d

Comments

  • Mike GreenMike Green Posts: 23,101
    edited 2009-11-20 03:33
    You have to read the datasheet for the specific regulator you intend to use. The datasheet always shows a sample circuit with recommended capacitor values and, if the capacitor selection is at all complicated, shows you how to figure it out. The capacitor needs varies widely from regulator to regulator, so you have to reference the datasheet.
  • kt88seampkt88seamp Posts: 112
    edited 2009-11-20 04:27
    I checked the spec sheet's sample circuit and its 1uF on the input and 2.2 on the output. In the past when I built regulated DC power supplies the datasheet capacitance was insufficient when·it·drew a lot of current. This can prove the datasheet wrong unless the datasheet is trying to say you need a·minimum of this capacitence for the regulator to work at all. Also in the·propeller lab circuit they have 1000uf caps on both the inputs and outputs. This seems a little much. Someone must have done something to calculate those values, unless they just felt like adding a bunch of capacitance just to be safe.
  • Mike GreenMike Green Posts: 23,101
    edited 2009-11-20 06:05
    Those values indeed sound like minimums for stable operation.

    The input capacitor provides local storage of energy when the regulator is at some distance from the power source and when there is sufficient resistance and inductance in the supply wiring that a sudden demand for power will be found wanting. I suspect that the Propeller lab circuit used a 1000uF capacitor at the input of the 3.3V regulator because it's really the output capacitor of the 5V regulator. There's not a lot of need for an input capacitor that large, even when the regulator is at some distance from the power source.

    The output capacitor provides local storage of regulated power and you can roughly compute the amount of capacitance needed from the amount of current drawn by the load (Propeller, peripherals, motors, etc.) and the amount of time you want to keep things running. A 1000uF capacitor is reasonable if you're going to try to drive a motor or solenoid or relay where the instantaneous current demand may be an Amp or more.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2009-11-20 06:28
    You need to read page 24 of the datasheet. That's where it explains about the external caps. Just referring to a sample circuit is never enough.

    -Phil
  • Cluso99Cluso99 Posts: 18,069
    edited 2009-11-20 07:33
    The 1000uF capacitor is for the rectification of the 12V ac signal and affects the ripple. There are equations for this but you will have to check the wiki. 1000uF is typical and if you are rectifying 12Vac then 25V or 35V would be recommended. BTW 12Vac is rather high.

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  • kt88seampkt88seamp Posts: 112
    edited 2009-11-20 12:20
    Just a few chips will not draw much current. I will not be running any solonids or anything of that magnitude. I do not imagine needing all that much capacitence. I will probably lower the 12 volt to 8 or 9 volts as the transformer manufacture offers many options. I also read page 24. It clues me in more, but does not tell me how much I need based on my load.
  • W9GFOW9GFO Posts: 4,010
    edited 2009-11-21 01:25
    I'm using a 5V, 1.5A regulator that specifies a .33uF cap on the input and a 10uF on the output. Would there be a problem if I used 47uF caps instead?

    attachment.php?attachmentid=65252

    Rich H

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    Caps.jpg
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  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2009-11-21 01:54
    Probably not. But it depends on the regulator. You have to read the datasheet to know for sure. It will also tell you what kind of cap to use (e.g. ceramic, tantalum, aluminum) and what ESR range is acceptable. Not all 47uF caps are created equal; there's a vast difference among them. And pay attention to the tolerances. If the regulator doc specs at least 10uF, say, a 10uF +/- 10% cap will not suffice.

    -Phil
  • Mike GreenMike Green Posts: 23,101
    edited 2009-11-21 01:59
    A 47uF output capacitor is probably just fine. You have to be careful with some older regulators and large output capacitors. The datasheet usually discusses the issue and what to do about it.

    A 47uF input capacitor is probably ok, but you may need the small (0.33uF) capacitor anyway. Again, the datasheet should discuss this if it's an issue.
  • W9GFOW9GFO Posts: 4,010
    edited 2009-11-21 02:46
    Thanks for the replies,

    Here is the datasheet for the regulator I'm using. I'm not seeing much discussion about capacitors in it.

    Rich H

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  • Mike GreenMike Green Posts: 23,101
    edited 2009-11-21 03:19
    You're right, there's not much discussion about capacitors. The main requirement is shown in Figure 18 where the output capacitor characteristics must fall in the shaded area. Pretty much anything with a capacitance > 3uF and an ESR > 2 Ohms should work. You may want to use an aluminum electrolytic for the 47uF output capacitor to keep the ESR > 2 Ohms for stability. Typically the ESR drops with increasing capacitance, so you may not want to use a capacitor significantly larger than 100uF.
  • W9GFOW9GFO Posts: 4,010
    edited 2009-11-21 03:37
    They sure don't make it easy. Here is the capacitors that I have, they don't say what the ESR is other than "low", but they are aluminum electrolytic.

    Rich H

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  • Mike GreenMike Green Posts: 23,101
    edited 2009-11-21 04:32
    Here's an article on ESR: en.wikipedia.org/wiki/Equivalent_series_resistance
  • sam_sam_samsam_sam_sam Posts: 2,286
    edited 2009-12-09 00:42
    This is from the data Sheet
    ·To me the part that are in blue are the important thing to keep in mind when using this regulator


    The AME8800/8811 is stable with an output capacitor
    to ground of 2.2mF or greater. Ceramic capacitors have
    the lowest ESR, and will offer the best AC performance.
    Conversely, Aluminum Electrolytic capacitors exhibit the highest ESR, resulting in the poorest AC response.
    ·Unfortunately, large value ceramic capacitors are comparatively expensive.
    One option is to parallel a 0.1mF ceramic capacitor with a 10mF Aluminum Electrolytic.
    The benefit is low ESR, high capacitance, and low overall cost.
    A second capacitor is recommended between the input and ground to stabilize VIN.
    ·The input capacitor should be at least 0.1mF to have a beneficial effect.
    All capacitors should be placed in close proximity to the pins.
    ·A "Quiet" ground termination is desirable.
    This·can be achieved with a "Star" connection.

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    Sam

    Post Edited (sam_sam_sam) : 12/9/2009 1:55:22 AM GMT
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