polarized capacitors, tantalum / electrolytic

bajji93bajji93
edited August 2017 in Build-it-yourself
I'm trying to fabricate OpenBCI myself for my University Project. I'm using gerber and BOM provided in the download section to build it. I wanted to know the polarity on the board of the Capacitors C2,C4 and C42 which is Item.No 8 in the OpenBCI_8bit_BOM.xls. I've attached the legend of top and bottom layers of the 4 Layer BCI Board. Could anyone please mark the +ve and -ve of the above mentioned capacitors .. please help me out

image<--------  bottom side C4



image<----------- TOP SIDE C2 AND C42

Comments

  • wjcroftwjcroft Mount Shasta, CA
    Joel @biomurph may comment further but I believe only electrolytic capacitors and tantalum capacitors are polarized. These are just ceramic caps with no polarization concerns.

  • bajji93bajji93
    @wjcroft can the tantalum capacitors replace the electrolytic capacitors? or at least in this case? we're planning to use this -> http://www.mouser.in/Search/ProductDetail.aspx?R=TLJA107M010R1400virtualkey58110000virtualkey581-TLJA107M10R1400 in place of the MLCC. 

    what can i expect if i replace with this component.
  • ecubeecube
    edited February 2015
    @bajji93  you can safely replace electrolytics with tantalum capacitors if there is no chance of reverse polarity.  Electrolytics are somewhat tolerant of reverse polarity for short time periods, but tantalum capacitors will short out and even explode under reverse polarity.  Using tantalum capacitors for power supply conditioning is fine.
  • wjcroftwjcroft Mount Shasta, CA
    edited February 2015
    These two articles imply there are no advantages in this case for using tantalum over MLCC,

    http://www.dataweek.co.za/news.aspx?pklnewsid=27008

    http://www.kemet.com/Lists/TechnicalArticles/Attachments/93/2008-11 Update - Ceramic versus Tantalum.pdf

    and this diagram,

    image
    from, http://en.wikipedia.org/wiki/Types_of_capacitor


  • bajji93bajji93
    thank you very much @wjcroft for this diagram if I can replace it with the tantalum capacitors, how should it be placed on the board?  and thank you @ecube for warning me about the explosion. i can wait for the MLCC to arrive, but that will take another 5 days :(
  • wjcroftwjcroft Mount Shasta, CA
    I suggest just waiting for the MLCCs.
  • bajji93bajji93
    @wjcroft thank you .. we'll go with your suggestion ;)

  • biomurphbiomurph Brooklyn, NY
    you can definitely use tantalum caps. they can be more expensive, and may not fit the footprint as it is currently designed. We use MLCCs in V3, and used tantalum in one of the earlier versions (V1, i believe). The orientation of the tantalum should be so that the '-' side of the cap is going to the most negative voltage. 
  • pabloviruspablovirus
    edited March 2015

    Hi everyone,

    I am designing a prototype circuit using the ADS1299 for EEG/BCI and I currently cannot obtain 100uF (nor 10uF) SMD tantallum or ceramic capacitors. Do you think there would be any problem in using SMD aluminium electrolytic capacitors as bypass capacitors in VCAP1, in VCAP 3 and between VREFP-VREFN? Has anyone tested this? The datasheet for the ADS1299 does say "ceramic or tantallum" for VCAP1 to 4 but I cannot get SMD MLCC of the suggested values where I live.

    Thanks in advance

  • wjcroftwjcroft Mount Shasta, CA
    Pablo, I merged your question into this existing thread. Your electrolytics should work, just be sure to align polarity as Joel suggests above. Curious what country are you in?

  • pabloviruspablovirus
    Woops, I think my question was appropiate for this thread. Thanks for merging it.

    I posted my question on the Texas Instruments forum and the designer of the ADS1299 said that SMD electrolytic capacitors will work as long as their equivalent series resistance is one Ohm or less. Mine's are about 2.5 Ohms (I measured them today) so I will look into getting better capacitors elsewhere.

    BTW, I live in Argentina, and the current policies on importing stuff (especially electronics) from outside is really dumb (prohibitively expensive) and it is also nigh impossible for a regular person to obtain US Dollars legally so going through the black market for that makes the whole deal even more expensive than it should be.


  • biomurphbiomurph Brooklyn, NY
    I think that it probably would 'work' with the electrolytics you have, but that said, the low ESR value becomes more critical when dealing with this kind of microvolt measurement.

    Both MLCC and Tantalums are widely available. I'm surprised that you can't find them. Do you have access to Mouser or Digikey down there under the equator?
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